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Interannual variability in intertidal flat communities in Bridgwater Bay,

Hinkley Point

TR183 HP Interannual Variability in Intertidal Communities BPE ii

TR183 HP Interannual Variability in Intertidal Communities BPE iii

Interannual variability in intertidal flat communities in Bridgwater Bay,

Hinkley Point

Julie Bremner

TR183 HP Interannual Variability in Intertidal Communities BPE iv

TR183 HP Interannual Variability in Intertidal Communities BPE v

Version and Quality Control

Version Author Date

Draft 0.01 Julie Bremner 07 January 2011

Internal QC 0.02 Andy Payne 12 January 2011

Revision 0.03 Julie Bremner 21 January 2011

Executive QC & Final Draft 0.04 Brian Robinson 21 January 2011

Submission to EDF 1.00 24 January 2011

Approved without comment EDF / MH 22 June 2011

Declassified at EDF request 1.01 BEEMS admin 14 September 2011

TR183 HP Interannual Variability in Intertidal Communities BPE vi

TR183 HP Interannual Variability in Intertidal Communities BPE vii

Table of contents

Executive summary ......................................................................................................................................... 1

1 Introduction ................................................................................................................................................ 3

2 Methods ...................................................................................................................................................... 6

2.1 Sampling protocol............................................................................................................................ 6

2.1.1 Macrofauna sampling ............................................................................................................ 7

2.2 Assessment of interannual change ................................................................................................. 7

2.2.1 Analysis of difference ............................................................................................................. 9

2.2.2 Analysis of variability ............................................................................................................. 9

3 Patterns of change .................................................................................................................................. 11

3.1 Overview of the communities ........................................................................................................ 11

3.2 Multivariate community structure .................................................................................................. 15

3.3 Changes in ‘core’ taxa ................................................................................................................... 18

4 Importance of patterns in the context of the Bridgwater Bay ecosystem ......................................... 26

4.1 Caveats/limitations of the study .................................................................................................... 26

4.2 Interannual variability in Bridgwater Bay ....................................................................................... 27

4.2.1 The intertidal flat communities ............................................................................................. 27

4.2.2 The role of physical conditions ............................................................................................ 28

4.2.3 Patterns in relation to the wider ecosystem ......................................................................... 28

4.3 NNB in the context of interannual variability ................................................................................. 30

4.4 Overall proposals for future monitoring ......................................................................................... 31

References ..................................................................................................................................................... 32

Appendix A Supplementary information .............................................................................................. 34

A.1 Taxa encountered in the infauna surveys of Bridgwater Bay in summer 2008 and 2010. Taxa occurring in both years are marked with an asterisk and emboldened; taxa occurring only once overall (i.e. at only one station in only one year) are marked with a #. ................................................................. 35

A.2 MDS ordination of multivariate community structure across Bridgwater Bay in 2008 and 2010. The MDS uses modified (log 10) Gower dissimilarities on mean abundance values per station. ............ 37

TR183 HP Interannual Variability in Intertidal Communities BPE viii

List of Tables and Figures

Tables

Table 1 Soft sediment stations included in the analysis of interannual variability in the Bridgwater Bay intertidal soft sediment communities. ...................................................................... 7

Table 2 Two-way PERMANOVA of total abundance over the Bridgwater Bay communities. Significant effects are shown emboldened. .................................................................................... 13

Table 3 Two-way PERMANOVA of multivariate community structure over the Bridgwater Bay communities. Significant effects are shown emboldened. .............................................................. 16

Table 4 Mean abundance per 0.01 m2 (±95% C.I.) for ‘core’ taxa in the Bridgwater Bay communities of 2008 and 2010. Data are the means of all replicates in each year (17 stations × 3 replicates = 51 samples per year). Statistically significant differences between years are shown emboldened. ......................................................................................... 19

Table 5 Two-way PERMDISP of variability in abundance of ‘core’ infauna taxa over the Bridgwater Bay communities. Significant effects are shown emboldened...................................... 20

Figures

Figure 1 Intertidal flats in Bridgwater Bay, showing the characteristic mosaic of fluid mud, patchy sediments and ridge and runnel systems. ........................................................................................ 3

Figure 2 Statutory designations around Hinkley Point and Bridgwater Bay. The red line shows the original area of focus for the BEEMS intertidal and subtidal resources characterisation. ................................................................................................................................ 4

Figure 3 Soft sediment stations sampled for infauna communities in July 2008 or July 2010. The modelled extent of the current thermal plume is shown for reference. Only those stations outside the 1oC boundary of the plume were retained for the analysis. .............................. 6

Figure 4 Conceptual diagram of no difference (a) and differences in the location (b), dispersion (c) and location and dispersion (d) of groups of samples between two years (reproduced, with permission, from Anderson et al., 2008). ............................................................. 8

Figure 5 Spatial distribution of taxa across Bridgwater Bay intertidal in 2008 and 2010. Most taxa were found at just one station in each year. ............................................................................ 11

Figure 6 Distribution of abundance over the Bridgwater Bay intertidal communities in (a) 2008 and (b) 2010. Most taxa were, on average, of low abundance. Abundance per station is calculated as the sum of three replicates, giving a value per 0.03 m2. Mean abundance is the average of the n = 17 stations. ........................................................................... 12

Figure 7 Abundance occupancy over Bridgwater Bay intertidal infauna communities in (a) 2008 and (b) 2010. There is a significant positive relationship between the number of stations at which a taxon is found and their mean abundance (Pearson’s r = 0.823, p = 0.000 in 2008; r = 0817, p = 0.000 in 2010). Abundance per station is calculated as the sum of three replicates, giving a value per 0.03 m2. Mean abundance is the average of the n = 17 stations. ........................................................................................................ 14

Figure 8 Change in total abundance (left) and variability of total abundance (right) per station between 2008 and 2010. Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for detail). There were significant differences between stations and differential responses of individual stations between the two years. ................................................................................................................... 15

Figure 9 PCO ordination of multivariate community structure across Bridgwater Bay in 2008 and 2010. Axes 1 and 2 (a) account for 39.76% of the total variability, axes 3 and 4 (b) for

TR183 HP Interannual Variability in Intertidal Communities BPE ix

19.62%, and axes 5 and 6 (c) for 13.83%, giving a total of 73.21% across the first six axes. Vectors of Spearman’s rank order correlations between taxa and the respective axes are overlaid (correlations of >0.5 are shown). ...................................................... 18

Figure 10 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Nematoda, Nephtys hombergii (Nep hom) and Corophium volutator (Cor vol). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details). .................................................................. 22

Figure 11 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Hydrobia ulvae (Hyd ulv) and Macoma balthica (Mac bal). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details). .................................................................. 23

Figure 12 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Hediste diversicolor (Hed div) and Tubificoides amplivasatus (Tub amp). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details). ................................................... 24

Figure 13 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Pygospio elegans (Pyg ele) and Nephtys juveniles (Nep juv). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log. 10) Gower distance between replicates and the group centroid (see text for details). .................................................................. 25

Figure 14 Foodweb of the Bridgwater Bay ecosystem, as currently understood (from BEEMS Technical Report TR068). ............................................................................................................... 29

TR183 HP Interannual Variability in Intertidal Communities BPE 1

Executive summary

Bridgwater Bay is a system of intertidal mud- and sandflats and subtidal rocky and sedimentary seabed situated at the interface between the Severn Estuary and the inner Bristol Channel. The unique physical regime of the Severn Estuary leads to biological communities that are distinct from the other estuaries in the region. The selection of Hinkley Point, to the west of Bridgwater Bay, as a potential site for the construction of a new nuclear facility led EDF Energy to commission Cefas to examine the intertidal resources of the area. Initial work supplemented historical information with contemporary surveys, in order to characterise spatial patterns in the intertidal communities. Intertidal soft sediment benthos is variable over both spatial and temporal scales, but little information is currently available on how the communities across Bridgwater Bay vary over time. It is important to understand the temporal dynamics of the communities, in order for the potential impacts of new nuclear build to be placed into the context of background variability in the system.

This report presents an analysis of temporal variability in the infauna communities of Bridgwater Bay. The site was originally characterised in summer 2008, and some of the stations were revisited in summer 2010 and sampled for infauna. Differences in community structure and abundance of ‘core’ taxa were assessed, as were differences in levels of variability in these parameters. Such analyses allow an understanding of whether there are detectable changes in the communities (e.g. changes in species composition or relative abundance) and whether they become more, or less, variable over time (an indicator or stress in benthic communities). Temporal changes in sediment parameters are not assessed in this report, due to quality assurance issues. Sediment data will be included in a later version of the report.

The ‘core’ taxa (i.e. those that were present at the site in 2008 and 2010, abundant and widespread) were Nematoda, Hediste diversicolor, Nepthys (juv.), Nepthys hombergii, Pygospio elegans, Tubifidoides amplivasatus, Corophium volutator, Hydrobia ulvae and Macoma balthica. Community structure varied between the two years, partly as a consequence of changes in some of the ‘core’ taxa, but also, in part, to changes in the presence of the suite of rare taxa that dominated spatial structure across the site. Individual stations varied in their response over time, with some differing, but not others, and both increases and decreases in taxon abundance. There was no clear trend of increasing or decreasing variability between the two years.

The patterns were grossly similar to the view of the communities in the vicinity of Bridgwater Bay provided by the results of studies made in the early 1970s and late 1980s, suggesting some degree of stability in the communities over the past 40 years. However, caution should be exercised in comparing time-points decades apart, because nothing is known of potential changes in the intervening period. Patterns driven by the appearance or disappearance of rare taxa must be interpreted with caution because, when small areas of the total available habitat are surveyed (as is common in marine benthic studies), the presence of rare taxa can sometimes be overlooked.

The varied temporal responses seen across the site are likely attributable to the unique physical conditions operating in the estuary. Bridgwater Bay mudflat elevation is spatio-temporally variable, with simultaneous, patchy erosion and deposition across the shore. This takes place within the context of high turbidity and long-term erosion over the site as a whole. Such extreme conditions create infauna communities responsive to fluctuating sediment conditions. The communities are locally adapted to these conditions, but it is unclear whether they would remain so if sediment depths narrow as a consequence of continued surface erosion; species inhabiting the surface layers may not be affected, but fauna burrowing into deeper sediments may be compromised.

Intertidal flats are important components of marine ecosystems, because they buffer the coasts from wave action, support bird populations and provide nursery grounds for offshore fish and invertebrates. In Bridgwater Bay, the intertidal flat invertebrate communities are intricately linked to other components of the marine ecosystem, such as microbes, plankton, birds and fish. Several other ecosystem components exhibit temporal variability and it is likely that changes in one component will have implications for others.

The impacts of a new nuclear build at Hinkley Point will be related primarily to increased local seawater temperature, resulting from the discharge of cooling water into the bay. This is not likely to directly affect the prevailing sediment transport processes driving local invertebrate community structure, but it may facilitate


the localized introduction of warm-water species tolerant of extreme physical conditions, or negatively impact thermally sensitive species. The knock-on effects of such changes are difficult to predict in the absence of clear understanding of the background community dynamics.

Long-term monitoring is vital for understanding the temporal dynamics of marine ecosystems. Such monitoring is likely to be required for some ecosystem components under the regulatory framework surrounding applications for new nuclear development. As the intertidal flats are protected under EU legislation and play a key role in the ecosystem, annual monitoring of the infauna communities would be a productive endeavour that, in addition to meeting regulatory requirements, would provide a wealth of useful information on local dynamics. Ideally, such monitoring would be incorporated into a wider ecosystem monitoring strategy that addressed the other core components of the Bridgwater Bay trophic web, such as birds, fish, hyperbenthos and microphytobenthos.


1 Introduction

Bridgwater Bay is a system of intertidal mud- and sandflats and subtidal rocky and sedimentary seabed situated at the interface between the Severn Estuary and the inner Bristol Channel. The funnel-shaped Severn Estuary has an immense tidal range (in the region of 15 m) and fast tidal currents (up to, for example, 5.5 m s–1; see Langston et al., 2007). The extreme hydrographic conditions present in the area subject Bridgwater Bay to strong tidal-scour effects and high turbidity, and the intertidal areas are characterised by patchy sediments, expanses of fluid mud and sedimentary ridge and runnel systems (Figure 1). The unusual physical regime of the Severn Estuary leads to biological communities that are distinct from the other estuaries of southwest Britain (Boyden and Little, 1973; Warwick et al., 1991).

Figure 1 Intertidal flats in Bridgwater Bay, showing the characteristic mosaic of fluid mud, patchy sediments and ridge and runnel systems.

Bridgwater Bay is protected under several layers of national and international legislation (Figure 2). These include designation as part of the Severn Estuary Special Area of Conservation (SAC) under the EU Habitats Directive and as a wetland of international importance under the Ramsar Convention on Wetlands of International Importance (see BEEMS Technical Report TR068). These designations protect some of the main features of the bay, including the extensive mudflat system of Stert Flats. The mudflats are protected


both in themselves (as a primary qualifying feature of the SAC) and as a feeding and roosting habitat for internationally important bird species (under Ramsar).

EDF Energy operates a nuclear power-generating facility at Hinkley Point, to the immediate west of Stert Flats. The site is a potential location for a new nuclear build (NNB), to house twin EPR reactors and associated facilities and infrastructure. As part of the NNB development, EDF Energy commissioned Cefas to characterise the environmental resources in the Hinkley Point area and to assess the sensitivity of key physical, chemical and biological features (the British EDF Estuarine and Marine Studies, or BEEMS, project).

Existing studies of the local ecology, conducted mainly with reference to the Hinkley ‘A’ and ‘B’ reactor developments and the proposed Severn Estuary barrage (see BEEMS Technical Report TR068 and Warwick and Somerfield, 2010, for summaries of the relevant literature), were supplemented with an intertidal characterisation survey to document contemporary patterns in spatial variability of macrofauna communities and associated sedimentary parameters (BEEMS Technical Report TR029). Several potentially sensitive features were identified, including the intertidal mud- and sandflats of Bridgwater Bay.

Figure 2 Statutory designations around Hinkley Point and Bridgwater Bay. The red line shows the original area of focus for the BEEMS intertidal and subtidal resources characterisation.

Intertidal flats are important components of marine ecosystems, because they buffer coasts from wave action, support bird populations and provide nursery grounds for offshore fish and invertebrates. They are home to a variety of marine and estuarine species, although they generally support lower diversity than subtidal sediments. Intertidal flat communities vary both spatially and temporally on a range of scales, being patchy over metres to kilometres and varying in structure through seasons, years and decades.

Studies of the Severn Estuary in the 1970s and 1980s highlighted patchy and species-poor intertidal sediment communities with relatively high local species density (Boyden and Little, 1973; Little and Boyden, 1976; Warwick et al., 1991). Relatively slight differences in community structure were noted in sedimentary communities at intertidal sites around the estuary between 1972/73 and 1973/74 (Little and Boyden, 1976),


although medium- to long-term temporal patterns remain poorly understood. There have been, as yet, no studies of interannual patterns in the communities of the Stert and Berrow Flats areas that make up the majority of the Bridgwater Bay intertidal resource. This lack of information on temporal patterns prevents a solid understanding of the dynamics of the bay’s intertidal system.

The aim of this report is to assess variability in the infauna communities of Bridgwater Bay over a two-year period, between summer 2008 and summer 2010. This information can then be used to develop understanding of the temporal stability of the communities and, ultimately, to place any future responses to NNB activities in the context of ‘background’ variability within the system. For the purposes of this report, ‘background’ variability is defined as patterns of variability at the site in the absence of exposure to the thermal plume. In theory, background variability should be synonymous with natural variability, which should be assessed in communities not exposed to any form of anthropogenic disturbance. However, it is extremely difficult to find any examples of such conditions in marine benthic environments in the UK or, indeed, much of Europe. The Severn Estuary and Bristol Channel, in particular, are subject to a variety of marine activities that could potentially impact the biological communities (such as aggregate extraction, power generation, port development, and shipping), and an in-depth discussion of the issues associated with identifying baseline or background conditions is outside the scope of this report. The definition selected is considered to be a pragmatic solution under the current circumstances. The need for baseline information will be addressed in future work, once the timescale for development is finalised.


2 Methods

2.1 Sampling protocol

The site was originally surveyed in July 2008, as part of a large-scale characterisation of the intertidal resources at Hinkley Point (see BEEMS Technical Report TR029). In all, 40 stations around Bridgwater Bay were sampled for macrobenthic infauna and environmental parameters between 1 and 11 July 2008 (Burdon et al., 2009). Between 27 and 30 July 2010 (see BEEMS Technical Report TR154a), 34 stations were sampled, 21 of those stations sampled in 2008, plus 13 new ones added to assess seasonal variability and bird feeding activities.

As the aim of the study was to investigate ‘background’ variability across the site, only those stations not estimated to be affected by the cooling water discharge from the existing Hinkley Point ‘B’ reactor were considered. The spatial extent of the discharge plume has been modelled by Cefas using the Generalised Estuarine Turbulence Model (GETM, see BEEMS Technical Report TR177). The model was used to simulate the shape and structure of the thermal plume, and the mean excess seabed temperature outputs were mapped over the station positions (see Figure 3). Only stations outside the 1oC contour of the plume were retained for the analysis. In all, 17 stations met the criteria of being sampled in 2008 and 2010 and falling outside the 1oC plume and were included in the biological analysis (Table 1).

Figure 3 Soft sediment stations sampled for infauna communities in July 2008 or July 2010. The modelled extent of the current thermal plume is shown for reference. Only those stations outside the 1oC boundary of the plume were retained for the analysis.


Table 1 Soft sediment stations included in the analysis of interannual variability in the Bridgwater Bay intertidal soft sediment communities.

Station number Locality Latitude Latitude

HP10 Stert Flats, low shore 51.22144 -3.084904



HP16 Gore Sands (sand spit) 51.239408 -3.085353

HP18 Stert Flats, high shore 51.20612 -3.077432

HP28 Brean Down, high shore 51.28011 -3.020134

HP29 Brean Down, mid shore 51.281496 -3.030193

HP30 Brean Down, low shore 51.281705 -3.042883

HP31 Brean Down, high shore 51.258165 -3.018416



HP34 Brean Down, low shore 51.256181 -3.060942

HP35 Stert Flats, mid shore 51.229629 -3.048705

HP36 Stert Flats, high shore 51.220866 -3.038445



HP48 Western Stert Flats, high shore 51.204376 -3.084541

2.1.1 Macrofauna sampling Macrofauna samples were taken using a standard 0.01 m2 corer, pushed into the sediment to a depth of 15 cm (BEEMS Technical Report TR154a). Fauna were fixed in 4% buffered formalin with Rose Bengal and stored in alcohol. Organisms were identified to the lowest possible taxonomic level, enumerated and weighed (aggregated per taxon per sample). Biomass was recorded as wet weight (WW) for the 2008 survey, but as ash-free-dry-weight (AFDW) for the 2010 survey.

Three replicates were collected at each station in 2008 (as is common in site characterisation surveys), and five replicates were taken in 2010. Only the first three replicates from each station of the 2010 survey were used in the statistical analyses, to ensure comparability with the 2008 data.

2.2 Assessment of interannual change

The analysis addressed two main questions:

1) Do the communities differ between years?

2) Do the levels of variability in the communities differ between years?

The first question considers whether the structure of the communities changes between years (changes in data location), and the second considers whether the communities become more or less variable over time (differences in data dispersion).The answers to these questions help us to understand patterns of variation in


the benthic communities across the site and will, ultimately, allow the effects of any future NNB effects to be placed in the context of ‘natural’ or ‘background’ variability.

The distinction between differences in location and dispersion are illustrated in Figure 4. Samples in Figure 4a differ in their location but not their dispersion, so community structure differs between the two years, but the structure within years is equally variable. In Figure 4b, samples do not differ in location, but they differ in dispersion (i.e. community structure does not differ between years, but it is more variable in one year than the other). In Figure 4c, both location and dispersion differ (i.e. the communities are more variable in one year than the other and they also differ in structure). The distinction between these two types of change in community structure are important, because increasing variability in community structure is an indicator of increasing stress in benthic communities (Warwick and Clarke 1993).

Figure 4 Conceptual diagram of no difference (a) and differences in the location (b), dispersion (c) and location and dispersion (d) of groups of samples between two years (reproduced, with permission, from Anderson et al., 2008).

The following variables were assessed within the statistical framework:

Total abundance Abundance of ‘core’ taxa Community structure (based on taxon abundance)

Biomass was not assessed during the analyses because the biomass values for the July 2010 dataset were not available in their entirety at the time of writing.

Assessment of taxon richness and diversity (alpha, or point-diversity) can yield useful summary information. However, richness measures can be confounded by differences in identification levels (for example, richness values will be inflated when both juvenile and adult forms of the same species are described, or when some individuals within a genus can be identified to species, whereas others cannot be identified below the level of genus). In addition, although they give information on changes in the number of species found at each station, they tell us little about whether the identities of these species have changed, i.e. they do not tell us whether the same species are still found in the next sampling period. For these reasons and because our primary interest is in the structure of the communities and the distribution of abundance over the site, taxon richness and diversity measures were not included in the statistical analyses.

Patterns of change in benthic communities are often driven by a ‘core’ subset of taxa that are widespread, abundant, or persistent over time (Magurran and Henderson, 2003). Investigation of these taxa provides


understanding of large-scale or long-term dynamics of the communities. Taxa were considered to be ‘core’ members of the communities if they met the following three criteria:

1. present at 5 (approximately 1/3) or more stations in either year [widespread];

2. comprise 1% or more of total abundance across the site in either year [abundant];

3. present in both 2008 and 2010 [persistent].

2.2.1 Analysis of difference Tests of difference in total abundance, ‘core’ taxa and multivariate community structure were obtained using PERMANOVA (Anderson, 2001), a non-parametric equivalent of traditional ANOVA that uses permutations to generate p-values. It can be calculated using any measure of dissimilarity among samples, which makes it useful for examining biological community data (Anderson et al., 2008). PERMANOVA tests the null hypothesis of H0, no differences in community structure/taxon abundance among groups.

Although the primary interest of the study is in year-to-year differences in the communities, these are likely to be affected by spatial patterns, so two-way PERMANOVA was utilised, with year and station as factors. Dissimilarities among the samples were calculated using a modified (log 10) Gower measure. This measure allows an order of magnitude change in abundance the same weight as a change in species composition (for multivariate data) in determining the dissimilarity between any two samples (Anderson et al., 2006).

A crossed (mixed) ANOVA model was applied, with year as a fixed factor and station held random. Type III (partial) sum of squares and permutation of residuals under a reduced model were used. A posteriori pairwise comparisons were applied where significant year effects were identified.

For the multivariate data, principal coordinates analysis (PCO) was used to investigate further any significant differences in community structure identified by PERMANOVA. It is a flexible ordination tool that projects the distances from any dissimilarity measure into Euclidean space, allowing relationships among samples to be visualised (Anderson et al., 2008). Non-metric multidimensional scaling (MDS) is often utilised for the purpose of ordinating benthic community data, because it is well suited to the analysis of such data. However, PCO is better suited to the overall analytical strategy when PERMANOVA and PERMDISP (see section 2.2.2.) methods are utilised (Anderson et al., 2008). The PCO was initially undertaken using replicate-level data. However, the large number of points on the resulting ordination plot made it difficult to interpret, so the ordination was repeated using the mean abundance values per station (which remain consistent with the principles of PERMANOVA). Maps of mean abundance were used to investigate any significant differences in the univariate variables, as well as maps of change in mean abundance between 2008 and 2010.

2.2.2 Analysis of variability The PERMDISP test for homogeneity of dispersions was used to assess changes in variability for total abundance, ‘core’ taxa and multivariate community structure (Anderson, 2006). The PERMDISP test is an extension of Levene’s test of homogeneity of variance that is applicable to multivariate data (Anderson et al., 2008). The test compares, for a number of groups, the distance between observations and their group centroid. This is similar to, in the univariate context, the spread of individual points around the group mean. Values of p can be calculated using either traditional tables or permutations. PERMDISP tests the null hypothesis of H0, no differences in within-group dispersion among groups (= no differences in variability among groups).

As with the PERMANOVA, two factors (year and station) were tested in order to tease out the effects of spatial patterns and temporal change. Dissimilarities among the samples were calculated using a modified (log 10) Gower measure. The test was run using distances to centroids and permutation to generate the p-values, with 9999 permutations.


PRIMER 6 + PERMANOVA+ (PRIMER-E Ltd) and Minitab 16 (Minitab Inc.) were used for the statistical analyses.


3 Patterns of change

3.1 Overview of the communities

In all, 34 taxa were encountered in 2008 (Appendix A.1), including 12 crustaceans (9 species and 3 genera), 14 annelids (11 species, 2 genera and 1 family) and 5 molluscs (4 species and 1 superfamily), as well as nemerteans, nematodes and an arachnid. Juvenile Nephtys, Gammarus, Idotea and Tellinoidea were present over the site. The tellinid juveniles are most likely to be Macoma balthica, because this is the only tellinid encountered to date in the EDF-commissioned surveys of the Bridgwater Bay intertidal (another tellinid, Abra alba, was found subtidally, but only in very low numbers – see BEEMS Technical Report TR067 Appendix C, Table 2).

The number of taxa increased to 36 in the 2010 survey. Of the taxa present in 2008, 14 (41%) were not recorded in 2010, and and additional 16 taxa were encountered (Appendix A.1). Nine crustacean species were recorded, as well as 17 annelids (12 species, 3 genera and 2 families), 5 molluscs (4 species and 1superfamily), nemerteans, nematodes, an arachnid, a collembolan and a cnidarian. Juvenile Nereididae, Nephtys, Arenicola, Mytilus edulis and Tellinoidea were found at the site in 2010 (with the tellinoids, again, most likely to be Macoma balthica).

Figure 5 Spatial distribution of taxa across Bridgwater Bay intertidal in 2008 and 2010. Most taxa were found at just one station in each year.

Most taxa were rare in both 2008 and 2010, with 18 taxa (52%) found at only one station in 2008 and 17 (47%) at just one station in 2010 (Figure 5). Very few taxa were widespread in either year, with only Macoma balthica (in 2008) and Hydrobia ulvae (in 2010) present at every station. Much of the change in taxa present between the two surveys was attributable to the appearance or disappearance of rare taxa; ~93% of the taxa lost or gained between 2008 and 2010 were found at just 1 of the 17 stations (13 of the 14 taxa lost and 15 of the 16 taxa gained).


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Figure 6 Distribution of abundance over the Bridgwater Bay intertidal communities in (a) 2008 and (b) 2010. Most taxa were, on average, of low abundance. Abundance per station is calculated as the sum of three replicates, giving a value per 0.03 m2. Mean abundance is the average of the n = 17 stations.

The spatial distribution of taxa appeared to be linked to abundance in both years. Most taxa occurred at a mean of 5 or less per station (Figure 6), with rare taxa being generally of low abundance and a positive association between the number of stations at which a taxon was present and its mean abundance (Pearson’s correlation; r = 0.823, p = 0.000 in 2008; r = 0817, p = 0.000 in 2010; Figure 7).

(a)

(b)


Table 2 Two-way PERMANOVA of total abundance over the Bridgwater Bay communities. Significant effects are shown emboldened.

Source d.f. Sum of squares

Mean sum of squares Pseudo-F p(perm) Unique

permutations

Total abundance

Year 1 0.64572 0.64572 1.9101 0.1885 9854

Station 16 21.753 1.3596 25.422 0.0001 9919

Year*Station 16 5.409 0.33806 6.3214 0.0001 9918

Residual 68 3.6366 0.053479

Total 101 31.444

There were significant differences in total abundance among the stations (Table 2). However, there was no significant overall difference in total abundance between the years, because stations varied in their response over time, with abundance decreasing on average at some stations (particularly towards the west of Stert Flats), but increasing at others (Figure 8). Assessment of change in variability among stations and over time yielded similar patterns, with no significant effect of year overall, but differences among stations (PERMDISP station comparison; F = 5.3725, p(perm) = 0.001) and increases in variability at some stations over time, but decreases in others (PERMDISP station by year comparison; F = 4.3609, p(perm) = 0.0102; Figure 8). Therefore, although there was no overall change in total abundance or variability in total abundance between 2008 and 2010, individual stations may have experienced some change.

PERMANOVA is sensitive to differences in dispersion among groups (Anderson et al., 2008). Therefore, significant results from both the test of location (PERMANOVA) and dispersion (PERMDISP) mean the differences among stations and differing temporal responses among stations could be attributable to differences in dispersion, differences in location, or both. Comparisons of changes in total abundance and variability in total abundance at each station suggest that it is likely to be a combination of both, because the stations differ in the nature of the change along a gradient from large positive to small negative changes for the two variables (Figure 8).


Figure 7 Abundance occupancy over Bridgwater Bay intertidal infauna communities in (a) 2008 and (b) 2010. There is a significant positive relationship between the number of stations at which a taxon is found and their mean abundance (Pearson’s r = 0.823, p = 0.000 in 2008; r = 0817, p = 0.000 in 2010). Abundance per station is calculated as the sum of three replicates, giving a value per 0.03 m2. Mean abundance is the average of the n = 17 stations.


Figure 8 Change in total abundance (left) and variability of total abundance (right) per station between 2008 and 2010. Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for detail). There were significant differences between stations and differential responses of individual stations between the two years.

To summarise, the communities seem to have been quite similar overall in 2008 and 2010. The number of taxa increased slightly between years, with several taxa lost between 2008 and 2010 and several other taxa gained. Most taxa were spatially rare and of low abundance in both years, with a small number of abundant and widespread taxa present (mainly Hydrobia ulvae and Macoma balthica). Total abundance did not alter significantly overall between the years, nor did variability in total abundance. However, differential responses among stations mean there may have been some change over the period at individual stations.

3.2 Multivariate community structure

Community structure differed among the stations and there was an interaction between year and station, with stations displaying a variety of responses over time (Table 3). However, there was still a significant difference in structure overall between 2008 and 2010, with the average distance among stations highest between 2008 and 2010, less in 2008, and lowest in 2010 (Table 3).

PCO on the replicate data produced an ordination plot that was difficult to interpret. The PCO was, therefore, repeated on the mean abundance of taxa per station. The first axes of the ordination did not explain a large part of the variability in the data, with the first two axes explaining ~40% and the first four 60% of the total variation in community structure (Figure 9).

The patterns in these axes were determined more by spatial differences across the site with, on the whole, little difference between years at any one station (i.e. the two time-points from each station are mainly close together on the plot). Some degree of separation between years begins to show along axes 3 and 4 (Figure 9b), with separation between the two years clearer on axes 5 and 6 (Figure 9c). That the ordination did not reveal obvious temporal patterns indicates that it is of limited value in elucidating the difference in structure between years in this instance. MDS on the same data reveals a little more distance between years for many of the stations, but also fails to visualise an obvious overall temporal pattern (Appendix A.2).

There were marginal differences in variability in community structure among the stations (PERMDISP F = 2.4127, p(perm) = 0.0505), but the stations did not respond differently from each other over time and there were no significant differences between 2008 and 2010. This means that the differences in community structure between 2008 and 2010 highlighted by the PERMANOVA were the result of changes in relative species abundance or composition, not to increases/decreases in variability over the site. This can be seen quite clearly on the PCO ordination plot, where there are no obvious differences in dispersion between the 2008 and 2010 samples on any of the first six axes (Figure 9).


Table 3 Two-way PERMANOVA of multivariate community structure over the Bridgwater Bay communities. Significant effects are shown emboldened.

Source d.f. Sum of squares

Mean sum of squares Pseudo-F p(perm) Unique

permutations

Year 1 3.3437 3.3437 3.6007 0.0013 9919

Station 16 42.792 2.6745 10.76 0.0001 9793

Year*Station 16 14.858 0.92863 3.7359 0.0001 9706

Residual 68 16.903 0.24857

Total 101 77.897

Average distance between/within groups (pairwise comparisons)

2008 1.2033

2010 1.1718

2008-2010 1.2391

Vector overlays were plotted on the ordination, to visualise potential relationships between the taxa and the axes of variation. Spearman’s rank order correlations of >0.5 were visualised (Figure 9). Nephtys (juv.), Nephtys hombergii, Hydrobia ulvae, Macoma balthica, Hediste diversicolor, Pygospio elegans, Nemertea and Nematoda were the taxa most associated with patterns along the first two axes, Tubificoides amplivasatus on axes 3 and 4 and Nephtys homgergii on axes 5 and 6. These overlays will identify potential linear or monotonic relationships between taxa and the axes of variation, but they will not highlight Gaussian, unimodal or multimodal associations. Therefore, care must be taken in drawing conclusions about causative factors for the variation seem in the PCO plots and the vectors used as an indication of potentially determinative taxa. Patterns of abundance in some of these taxa are explored further in section 3.3.

To summarise, spatial patterns contribute most of the differences in community structure. These masked the temporal change to some extent, although there were significant differences in overall community structure between 2008 and 2010. There were no significant differences in variability over the site between the years. Taxa potentially responsible for these differences in structure are investigated further in section 3.3.


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Figure 9 PCO ordination of multivariate community structure across Bridgwater Bay in 2008 and 2010. Axes 1 and 2 (a) account for 39.76% of the total variability, axes 3 and 4 (b) for 19.62%, and axes 5 and 6 (c) for 13.83%, giving a total of 73.21% across the first six axes. Vectors of Spearman’s rank order correlations between taxa and the respective axes are overlaid (correlations of >0.5 are shown).

3.3 Changes in ‘core’ taxa

Nine taxa met each of the three criteria set (widespread, abundance dominant and persistent) and were therefore selected as ‘core’ taxa. These were:

Nematoda Hediste diversicolor Nepthys (juv.) Nepthys hombergii Pygospio elegans Tubifidoides amplivasatus Corophium volutator Hydrobia ulvae Macoma balthica

These taxa are the same set identified as potentially correlating with the greatest degree of variation in the multivariate community structure visualised in the PCO (with the exception of Nemertea, which were also identified as correlating with the first two ordination axes, see section 3.2 above). This suggests that they have some importance in determining patterns of variation among stations and, to some extent, between years.


Table 4 Mean abundance per 0.01 m2 (±95% C.I.) for ‘core’ taxa in the Bridgwater Bay communities of 2008 and 2010. Data are the means of all replicates in each year (17 stations × 3 replicates = 51 samples per year). Statistically significant differences between years are shown emboldened.

Taxon Mean abundance 2008

Mean abundance 2010

Change 2008 to 2010

Annelida

Hediste diversicolor 5.33 (3.36) 1.33 (0.96) -4

Nephtys (juv.) 0.16 (0.11) 4.96 (1.83) 4.80

Nephtys hombergii 0.70 (0.33) 1.20 (0.40) 0.49

Pygospio elegans 5.45 (3.52) 1.88 (1.37) -3.57

Tubificoides amplivasatus 1.10 (0.63) 1.96 (1.47) 0.86

Mollusca

Hydrobia ulvae 7.49 (3.48) 10.80 (5.64) 3.31

Macoma balthica 5 (1.52) 7.12 (3.26) 2.12

Crustacea

Corophium volutator 3.70 (3.82) 0.72 (0.74) -2.98

Other phyla

Nematoda 1.29 (0.98) 0.37 (0.23) -0.92

Abundance of Nematoda, Nepthys hombergii, Corophium volutator, Hydrobia ulvae and Macoma balthica followed similar patterns to total abundance (see section 3.1), in that they exhibited significant differences among the stations and no significant overall differences between years, but varied responses over time at individual stations, for both abundance and variability in abundance (Tables 4 and 5, Figures 10 and 11). Therefore, as with total abundance, although there was no overall change in abundance or variability in abundance between 2008 and 2010, individual stations may have experienced some degree of change and this was likely to be attributable to a combination of both parameters.

Hediste diversicolor followed a similar pattern to the others except that, whereas stations responded differently over time in terms of abundance (Table 4), they did not do so for variability in abundance (Table 5). In this case, individual stations may have experienced some degree of change over time in H. diversicolor abundance, but variability in abundance was similar over all stations between the years (Figure 12). There were differences in abundance and variability in abundance between stations for Tubificoides amplivasatus, but no differences between years (Tables 4 and 5). Although the stations responded differently over time in variability in abundance for this species, there was no significant interaction between station and year for abundance (Tables 4 and 5, Figure 12). T. amplivasatus was found at just 6 stations overall, so the analyses are likely to have been affected by a large number of zero values.


Table 5 Two-way PERMDISP of variability in abundance of ‘core’ infauna taxa over the Bridgwater Bay communities. Significant effects are shown emboldened.

Taxon Source F d.f.1 d.f.2 p(perm)

Nematoda Year 6.202 1 100 0.1955

Station 9.7117 16 85 0.0001

Cells (Year*Station) 7.8906 33 68 0.0001

Hediste diversicolor Year 7.4563 1 100 0.1562

Station 11.204 16 85 0.0001


Nepthys (juv.) Year 41.318 1 100 0.0001

Station 13.951 16 85 0.0001


Means and Standard Errors

Mean Standard error

2008 0.24702 0.037236

2010 0.7009 0.059995

Nephtys hombergii Year 1.2255 1 100 0.3162

Station 5.8258 16 85 0.0001


Pygospio elegans Year 9.6608 1 100 0.0645

Station 7.5464 16 85 0.0001


Tubificoides amplivasatus Year 0.017217 1 100 0.9423

Station 5.1887 16 85 0.0386



Table 5 (contd.) Two-way PERMDISP of variability in abundance of ‘core’ infauna taxa over the Bridgwater Bay communities. Significant effects are shown emboldened.

Taxon Source F d.f.1 d.f.2 p(perm)

Corophium volutator Year 9.9454 1 100 0.1135

Station 23.375 16 85 0.0001


Hydrobia ulvae Year 0.21427 1 100 0.6595

Station 4.1031 16 85 0.0009


Macoma balthica Year 2.19 1 100 0.1925

Station 5.8438 16 85 0.0001



Figure 10 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Nematoda, Nephtys hombergii (Nep hom) and Corophium volutator (Cor vol). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details).


Figure 11 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Hydrobia ulvae (Hyd ulv) and Macoma balthica (Mac bal). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details).

There were significant differences in abundance of Pygospio elegans between stations (Table 4). Individual stations responded differently over time, but a significant overall change between years was also evident. That there was no significant difference in variability in abundance between years (Table 5), suggests that changes between the two years were attribuTable to changes in abundance, not an increase or decrease in variability. There appears to have been an overall reduction in abundance of P. elegans across the site between 2008 and 2010 (slight increases at two stations, decreases at six stations; Figure 13).

Nephtys juveniles showed a similar pattern to that of P. elegans (Tables 4 and 5), although significant differences in variability in abundance between the years means the changes may be attribuTable to differences in abundance or variability in abundance between 2008 and 2010. Examination of the plots of changes in the variables suggests a role for both parameters, with increases (or no change) in mean abundance across all stations (Table 4), as well as differences in levels of variability (Figure 13). The change in abundance of Nepthys juveniles was attributable to both increases in abundance at individual stations from 2008 to 2010 and the appearance of individuals at new stations in 2010.


Figure 12 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Hediste diversicolor (Hed div) and Tubificoides amplivasatus (Tub amp). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log 10) Gower distance between replicates and the group centroid (see text for details).

To summarise, Nematoda, Hediste diversicolor, Nepthys juveniles, Nepthys hombergii, Pygospio elegans, Tubifidoides amplivasatus, Corophium volutator, Hydrobia ulvae and Macoma balthica were core members of the communities over the two years. Neither abundance nor variability in abundance of Nematoda, N. hombergii, C. volutator, H. ulvae, M. balthica, H. diversicolor and T. amplivasatus changed significantly overall between 2008 and 2010. However, differential responses among stations mean that there may have been some changes over the period at individual stations in either abundance, variability in abundance, or both. Significant reductions in abundance of P. elegans were noted across the site between the two years. There were also changes between the two years for Nepthys juveniles, which were a result of both changes in abundance (increases in abundance or colonisation of new stations) and variability of abundance.


Figure 13 Change in total abundance (left panels) and variability of abundance (right panels) per station between 2008 and 2010 for Pygospio elegans (Pyg ele) and Nephtys juveniles (Nep juv). Red indicates a reduction, green an increase and a cross no change. Total abundance values are means of three replicates per station and variability values are means of the modified (log. 10) Gower distance between replicates and the group centroid (see text for details).


4 Importance of patterns in the context of the Bridgwater Bay ecosystem

4.1 Caveats/limitations of the study

Although this study has yielded useful information on changes in the Bridgwater Bay intertidal flat communities between two years, the observations should be considered preliminary, rather than providing definitive answers. Studies of temporal change in benthic communities are generally of several years’ duration. This study consists of only two time-points, summer 2008 and summer 2010. This may be problematic for two reasons; (i) the surveys were two years apart and we have no knowledge of the communities in the intervening year and (ii) little knowledge of temporal dynamics can be gained by looking at changes over such a short duration.

The first point is not of major importance. Benthic infauna tend to have short life cycles and a two-year period between assessments is not the most effective way to achieve understanding of fine-scale detail (such as changes in cohorts or recruitment patterns). However, at the larger scale of multi-community assessment, it does provide a level of detail useful for gaining an overall understanding of major change at the site. The second point is more problematic; trying to understand the boundaries of ‘natural’ variability in communities over such a short period is difficult because one or both of the time points may reflect unusual conditions (e.g. responses to a particularly extreme storm event, or an unusually calm winter), which may inflate (or deflate) the patterns of change. Although unusual events form a component of natural conditions at a site, it is only by looking over a range of conditions (over a number of years) that one can gain a fuller understanding of the dynamics of a system.

One further problem encountered when assessing benthic communities is the issue of sampling effects. Benthic communities are dominated by rare taxa (Gray et al. 2005). Such rare taxa are notoriously difficult to sample with traditional benthic survey methods (they are spatially patchy and benthic sampling only covers a fraction of the area of interest, in general (see Reichert et al. 2010)). In the case of temporal surveys, this can result in potentially misleading information. In the current study, several taxa appear in one year, but not the other. It may be the case that they have genuinely disappeared from the site, or it may be that the sampling programme has not detected their presence in that year. For example, the amphipod Bathyporeia pilosa appeared to be present in 2008, but not in 2010. However, only three of the five replicates taken at each station in 2010 were analysed (to ensure comparability with the 2008 survey) and, if the full suite of five replicates is examined, it becomes clear that B. pilosa was actually present at the site in 2010.

This is a well-known issue in benthic ecology; taxon rarity is linked to sampling resolution. It is impossible to conduct a comprehensive benthic survey, so survey designs become a trade-off between resolution/effort and return. Cefas generally consider three replicates to be sufficient for characterisation studies (such as that of 2008) and five replicates to provide a useful approximation of the benthic communities for the purposes of monitoring and impact assessment (this level of replication is also used elsewhere in monitoring programmes). Benthic sampling is expensive and there are likely to be diminishing returns for increasing levels of replication. Therefore, the best option is to exercise caution in interpreting the appearance/ disappearance of individual taxa in a particular year and to place greater emphasis on changes that are persistent through a number of years. This is particularly important in the present study, where the analyses include only two time-points.

The exclusion of biomass and sediment particle size data from the analysis reduces the level of understanding that can be gained from the results. Biomass accounts for taxon body size, so is a better descriptor of the presence of an individual in the community than abundance, when considering ecosystem dynamics and function. Sediment data provide environmental context for any patterns of change seen in the communities. Unfortunately, these data were not available at the time of writing. However, they should be included in future analyses.

Although the above issues necessitate a level of caution in interpreting the results of the study, they do not preclude an initial understanding of the dynamics of the system, and some useful information has emerged that can provide context for the NNB application process and future monitoring strategy.


4.2 Interannual variability in Bridgwater Bay

4.2.1 The intertidal flat communities Community structure varied between 2008 and 2010, partly as a consequence of changes in some of the ‘core’ taxa over the area, but also, in part, because of changes in the presence of the suite of rare taxa that dominated spatial structure across the site (though see the caveats in section 4.1). Individual stations varied in their response over time, with some differing, but not others, and both increases and decreases in taxon abundance. There was no clear trend of increasing or decreasing variability between the two years.

Although the study covered a short period of sampling just two years between samplings, some useful insight into longer-term temporal stability can be gained by comparison of the patterns identified with those of previous studies in the area. Boyden and Little (1973) surveyed transects from high to low shore at 17 sedimentary sites from Epney to Minehead. Their sites 14 (Steart) and 13 (Berrow), sampled in July 1972, are in the vicinity of the area covered in the current study. Although the different resolution of the surveys prevents a comparison of the overall numbers and types of taxa identified, or those apparently absent in 1972 but present in 2008–2010 (two transects over the area in 1972 compared with 17 stations spread across the area in 2008/2010 is likely to mean that a wider range of taxa were identified in the contemporary surveys), taxa present in both surveys can be usefully compared.

A broadly similar picture emerges. The dominant taxa at the two sites were also identified as ‘core’ taxa over the period 2008–2010, with Nephtys hombergii and Macoma baltica dominating both transects and Hydrobia ulvae and Corophium volutator common at the Steart site. Two taxa dominant at the Steart site in 1972, Diastylis rathkei and Retusa obtusa, were not selected as ‘core’ taxa in 2008–2010. Both were present at the site in 2008 and 2010, but neither was widespread (D. rathkei was present at two stations in 2008 and one in 2010, R. obtusa at four in 2008 and three in 2010). These taxa may still be locally important members of the communities, although they may have specific environmental requirements that prevent them from attaining a wider distribution across the bay.

Boyden and Little (1973) noted that the majority of N. hombergii across the 17 sites surveyed were very small and that the M. balthica were predominantly juveniles. Recruitment is temporally stochastic for many marine benthic fauna; Tellinoidea were not particularly abundant in 2008 or 2010, although there appears to have been a large spatfall of Nepthys juveniles in 2010. Little can be said about the dynamics of species cohorts without evidence from longer-term studies. What is more interesting is the observation of mainly small N. hombergii. Studies undertaken during 2010 as part of the wider BEEMS programme have signalled that M. balthica are also generally very small on Stert Flats, compared with other locations (unpublished data). This may be a general phenomenon, resulting from the extreme physical conditions in Bridgwater Bay and the Severn Estuary/Bristol Channel. Individual size measurements (biomass and/or length) for key taxa including M. balthica and H. ulvae are being undertaken as part of the BEEMS trophic web studies and will be available in future to shed more light on this issue.

Warwick et al. (1991) surveyed 12 sites in the Severn Estuary in September and October 1987. Their site 29 (Stolford), surveyed at the beginning of October, is comparable with the 2008–2010 surveys. In all, 15 stations were surveyed at the site, set in a grid system designed to give an even representation of the area. They identified eight taxa at the Stolford site, with C. volutator, H. ulvae and N. hombergii dominating abundance, followed by M. balthica, D. rathkei, Spionidae, Bathyporeia sp. and Hediste diversicolor.

These findings are in line with those of Boyden and Little’s (1973) earlier study and the 2008–2010 survey. Therefore, it appears that the core makeup of the Bridgwater Bay communities has remained broadly similar over the past 38 years. These taxa are all common members of UK mudflat communities and their dominance in the communities is, therefore, unsurprising. In terms of understanding the patterns of ‘background’ variability over Bridgwater Bay, the temporal dynamics of the core taxa (recruitment patterns, size-composition etc.) would benefit from more attention, as would further investigation of long-term variability within the communities. It is also worth remembering that background physical conditions may vary in future from that experienced in the past (e.g. the thermal or sea-level effects of climate change) and, as such, the past may not remain a sound model for background conditions in future.


4.2.2 The role of physical conditions The intertidal mud- and sandflat communities of the Severn Estuary/Bristol Channel are quite different from those in the neighbouring estuaries of southwest England (Boyden and Little, 1973; Warwick et al., 1991). Soft sediment intertidal communities in other estuaries tend to be structured, in general, by ‘static’ variables such as sediment grain size and organic matter content (Warwick et al., 1991). In the Severn, the communities are controlled to a much greater extent by ‘dynamic’ variables related to tidal scour and wave climate that produce a high level of turbidity and large, often sudden, changes in sediment stability (Warwick and Somerfield, 2010).

The concave nature of the shore in Bridgwater Bay identifies it as liable to surface erosion (Kirby, 2010). An 11-year study of elevation changes on Stert Flats revealed a site subject to large fluctuations in bed sediment dynamics. Apart from diurnal and seasonal changes in bed height, there has been long-term erosion, set within the context of spatio-temporal variability in elevation.

The spatio-temporal elevation changes help to explain the variable patterns seen in the benthic communities on the flats. Under the prevailing conditions, erosion and deposition can take place simultaneously at different locations, as fluid mud and sand patches respond differently to hydrodynamic conditions and local ridges and runnels migrate (Kirby and Kirby, 2010). This short-term noise is likely to elicit a response from the benthic communities, with displacement of individuals as sediments are resuspended and the creation of a mosaic of assemblages. Most of the core taxa in Bridgwater Bay are mobile enough to recolonise patches relatively quickly in the short term (e.g. Nephtys and Hediste species can crawl over and through sediments, Macoma balthica can drift on byssus threads). Long-term colonisation is facilitated by the broadcast spawning common to many of the taxa. Species that are present in the bay are adapted to these disturbed conditions (see, for example, Pearson and Rosenberg, 1978). Therefore, such interannual variability results in changing dynamics within localised patches, but does not appear overly to influence the ability of species to persist across the site. Although sediment mobilisation may kill individual organisms, the macrofauna as a whole may in effect (and after a time-lag) follow the sediments.

Given the occurrence of net erosion at the site over at least the past decade (and likely much longer, see Kirby, 2010), the communities need also to be adapted to this phenomenon. The difficulty, in this instance, is estimating at what point, if at all, the extent of erosion will render the environment inhospitable to the infauna communities. Removal of surface fine sediments can lead to two potential situations of concern for macrofauna, (i) reduction in the depth of available sediments and (ii) change in sediment size distribution. Macrofauna distributions are closely linked to sediment grain size, so changes in size distribution will lead to changes in the type of species inhabiting the area. However, as long as sufficient patches of variable sediment type remain, communities associated with these patches can persist at a site-scale. Long-term thinning of the sediment is potentially more problematic, if it takes place over the site as a whole. Species have variable sediment depth requirements. For example, while Hydrobia ulvae lives primarily at the sediment surface, Hediste diversicolor and Arenicola marina can burrow to a depth of up to 20 cm, or greater (Petersen et al. 1998). Long-term sediment thinning may exclude the deeper-dwelling species from the site.

Interestingly, models of seabed stress have been used in the Severn to predict future gross-scale subtidal benthic community distributions in response to environmental change (the Severn barrage scheme, in this instance; Warwick, 1984; Warwick and Somerfield, 2010). If a long-term monitoring programme is initiated in Bridgwater Bay and some additional years of biological data are obtained, these could be coupled with hydrodynamic and sediment-transport models in an attempt to predict future biological conditions at the site.

4.2.3 Patterns in relation to the wider ecosystem The intertidal flat communities are intricately linked to other components of the Bridgwater Bay marine ecosystem (Figure 14). They are also fundamentally important as a feeding resource for local bird populations (see BEEMS Technical Report TR068a). Therefore, temporal changes in the communities are both a cause and a consequence of the dynamics of other ecosystem components.


Figure 14 Foodweb of the Bridgwater Bay ecosystem, as currently understood (from BEEMS Technical Report TR068).

Little is known of the microbial communities of Bridgwater Bay. Like the macrofauna, microphytobenthos (MPB) communities are reduced in the Severn and experience temporal variability, most likely because of the extreme physical conditions resulting in changes in bed height and rapid changes in biomass (Underwood, 2010). The difference between the two is that MPB biomass recovers rapidly when conditions become more favourable (Underwood, 2010), whereas macrofauna are likely to require a longer recovery period. Interannual variability in MPB biomass will directly affect the species that feed on them, such as surface grazers such as Hydrobia ulvae.

There is also evidence of temporal change in the fish and hyperbenthos communities of Bridgwater Bay (Henderson and Bird, 2010). Approximately one new species of fish is thought to have entered the system each year since the early 1980s and, like the intertidal infauna communities, different species have shown variable responses over time. Whereas some species (such as whiting, Merlangius merlangus) show no long-term trend and little year-to-year variability, some have increased (e.g. e.g. the five-bearded rockling Ciliata mustela), others have decreased (e.g. the sea snail Liparis liparis), and some exhibit large variability between years (e.g. the plaice Pleuronectes platessa). The situation is similar for the hyperbenthos communities, with some crustacean species increasing over the period (e.g. the pink shrimp Pandalus montagui), although none have significantly decreased. There is large interannual variability in abundance for some species, such as the white glass shrimp Pasiphaea sivado and changes in the level of variability for others (e.g. a decrease in variability after the 1980s for the edible crab or brown crab Cancer pagurus and an increase in variability during the past decade for the velvet swimming crab Necora puber). Many of these species utilise the intertidal flats as a food source (see BEEMS Technical Report TR068), so changes in the infauna communities may be reflected, at least in part, in the dynamics of their predators.


There have been changes in the waterbird communities of the Severn Estuary over the last two decades, with declines in grey plover (Pluvialis squatarola) and dunlin (Calidris alpina) and an increase in the winter shelduck (Tadorna tadorna) population (Burton et al., 2010). Like the fish and hyperbenthos, waterbirds feed on the Bridgwater Bay intertidal infauna communities, so the dynamics of the bird populations are linked to patterns of change in the infauna, with a two-way relationship of food limitation on the birds and predator control on the infauna.

Elucidating the relative contributions of physical and biological factors to long-term patterns in the intertidal flat communities, as well as the consequences of these changes for other ecosystem components, will require concerted effort in monitoring temporal change in the various ecosystem components.

4.3 NNB in the context of interannual variability

The Bridgwater Bay intertidal flat communities appear to be fairly stable, at least at a coarse scale. The communities are structured by the prevailing scour, wave and turbidity climate, with the taxa present adapted to the extreme physical conditions. If the prevailing physical conditions are altered in future, so may the nature of the infauna communities.

The construction of large installations, such as the much-discussed Severn barrage, would alter the hydrodynamic properties of the area; with changing sediment transport pathways and turbidity levels leading to less extreme conditions conducive to a broader range of benthic taxa (see the papers in the Marine Pollution Bulletin Volume 61 Severn Estuary special edition The Severn Estuary and Bristol Channel: A 25 year critical review. Marine Pollution Bulletin Volume 61, Issues 1-3). The most important impact of the proposed NNB at Hinkley Point on the intertidal marine ecology of the bay will be the discharge of cooling water into surface waters. Thermal disturbance of this kind is not likely to alter the sedimentary environment directly experienced by the infauna communities, so it is not considered likely that conditions would be any more favourable to the sensitive or suspension-feeding species thought to be excluded from the area by the sedimentary regime (see, for example, Warwick and Somerfield, 2010). Any changes attributable to increasing water temperatures would be likely to operate within the sedimentary constraints already imposed on the area.

There are two possibilities:

Increasing temperatures would allow colonisation of the site by warm-water species tolerant of extreme sedimentary conditions;

Increasing temperatures would reduce the abundance of thermally sensitive species.

It is very difficult to predict which, if any, warm-water species could invade an area, as there are many marine species in western Europe and any with the means to arrive at a site (e.g. migration, dispersal) could potentially colonise. Constraints related to the ability to exist in extremely turbid waters and mobile sediments would reduce the potential list somewhat, which would leave the complex task of assessing biological interactions between invaders and receiving communities. Some marine invasions appear to result in little appreciable effect on receiving communities (Reise et al. 2006), but the effects may not be so benign on a species-poor system, such as Bridgwater Bay.

Deleterious effects on thermally sensitive species are likely to result in a change in community structure across the bay. Current work in the wider BEEMS programme is assessing the likelihood and extent of temperature limitation for Macoma balthica populations. There is evidence of thermal sensitivity in M. balthica populations elsewhere in Europe (BEEMS Technical Report TR134). Local reductions in fitness or abundance, coupled with the lack of biological ‘insurance’ (see, for example, Lawton and Brown 1993) as a consequence of low species richness in the communities, would likely have knock-on effects on the species’ predators and competitors, both within the communities and in the wider ecosystem. However, if increasing temperature facilitates the invasion of new species at the same time as reducing thermally sensitive taxa, the system-level effects may be ameliorated somewhat. For example, Warwick et al. (1991) note that Scrobicularia plana is ecologically similar to M. balthica. The species inhabited the Severn at some point in the past, but is a warm-water species thought to be at the northern edge of its range in the southern UK


(Warwick et al., 1991). If the only factor keeping S. plana out of the Bridgwater Bay communities at present is temperature, and the species is equally palatable to the predators of M. balthica, it could potentially replace M. balthica in future.

Although the mode of impact on benthic communities is simpler for NNB than other marine activities, much less is known about the complex possible outcomes of a thermally altered environment. In this context, understanding the boundaries of background variability could provide important information to support future predictions of NNB effects.

4.4 Overall proposals for future monitoring

There is some evidence of change in intertidal flat community structure between 2008 and 2010, driven by some changes in core taxa, as well as the large number of rare species found across the site. There is a great degree of spatial variation in response over time, with abundance increasing at some stations and reducing or remaining unchanged at others. Levels of variability do not appear to have altered over the period. The short duration of the study, however, makes it difficult to obtain a definitive view of temporal change in the bay, although comparison with studies from the early 1970s and late 1980s suggest some coarse-level stability over time.

The difficulty with such comparisons is that they provide no information on changes that may have occurred over the intervening period – there is a 10-year gap between the first and second studies and a 20-year gap between the second and contemporary surveys. Long-term monitoring programmes have been successful for other components of the Bridgwater Bay ecosystem, such as beach profile monitoring (e.g. Kirby and Kirby, 2008) and the PISCES fish and hyperbenthos entrainment screen monitoring (Henderson and Bird, 2010). Regular, long-term monitoring of the intertidal infauna communities would yield information useful for understanding levels of ‘background’ variability.

Annual surveying is needed, in line with the relatively short life cycles of the species concerned. Monitoring of this nature is considered to be a likely regulatory requirement of the NNB construction and operation process, at least in the initial years. Additional benefits can be gained by monitoring other system components thought to be directly responsible for infauna community structure, such as beach structure (e.g. profiling and sediment properties) and infauna food sources (e.g. microphytobenthos, organic matter or meiofauna). Such components could be monitored at relatively little cost, provided there is careful selection of survey stations and methodologies.

Extension of the programme to include wider ecosystem components linked to the infauna (such as sublittoral communities, fish, birds, hyperbenthos, microphytobenthos and plankton) could potentially provide a robust understanding of change over time and context for NNB impacts, allow in-depth parametrisation and testing of the Bridgwater Bay trophic model currently under development by Cefas and, potentially, facilitate prediction of future species distributions. Although there is a high cost for such wide-ranging studies, some of these components may be required under conditions of approval for NNB. Development of an integrated monitoring programme that utilises the latest and most appropriate techniques and sampling strategies is the most cost-effective and efficient way to organise monitoring at the site.


References

Anderson, M.J. 2001. A new method for non-parametric multivariate analysis of variance. Austral Ecology, 26: 32-46

Anderson, M.J. 2006. Distance-based tests for homogeneity of multivariate dispersions. Biometrics, 62: 245-253

Anderson, M.J., Ellingsen, K.E. and McArdle, B.H. 2006. Multivariate dispersion as a measure of beta diversity. Ecology Letters, 9: 683-693

Anderson, M.J., Gorley, R.N. and Clarke, K.R. 2008. PERMANOVA+ for PRIMER: Guide to software and statistical methods. PRIMER-E: Plymouth, UK

BEEMS Technical Report TR 029. Ecological Characterisation of the Intertidal Region of Hinkley Point, Severn Estuary: Results from the 2008 Field Survey and Assessment of Risk. Cefas, Lowestoft

BEEMS Technical Report TR 067. Hinkley Point nearshore communities: Results of the day grab surveys 2008-2010. Edition 2. Cefas, Lowestoft

BEEMS Technical Report TR 068. The effects of new nuclear build on the marine ecology of Hinkley Point and Bridgwater Bay. Cefas, Lowestoft

BEEMS Technical Report TR 068a. The effects of new nuclear build at Hinkley Point on intertidal food availability for birds. Cefas, Lowestoft

BEEMS Technical Report TR 134. Review of existing literature on temperature sensitivity in the Baltic tellin, Macoma balthica. Cefas, Lowestoft

BEEMS Technical Report TR 154a. Hinkley intertidal survey and analysis report July 2010. Cefas, Lowestoft

BEEMS Technical Report TR 177. HP Thermal Plume Modelling: Stage 2 Review Initial evaluation of the Stage 2 models.

Boyden, C.R. and Little, C. 1973. Faunal distributions in soft sediments of the Severn Estuary. Estuarine and Coastal Marine Science, I: 203-223

Burdon, D., Dawes, O., Eades, R., Leighton, A., Musk, M. and Thomson, S. 2009. BEEMS WP6 Intertidal Studies; Hinkley Survey - Report to Cefas. Institute of Estuarine and Coastal Studies, Report ZBB716-D1-2008.

Burton, N.H.K., Musgrove, A.J., Rehfisch, M.M. and Clark, N.A. 2010. Birds of the Severn Estuary and Bristol Channel: Their current status and key environmental issues. Marine Pollution Bulletin, 61(1-3): 115-123

Gray, J.S., Bjorgesaeter, A. and Ugland, K.I. 2005. The impact of rare species on natural assemblages. Journal of Animal Ecology, 74 (6): 1131-1139

Henderson, P.A. and Bird, D.J. 2010. Fish and macro-crustacean communities and their dynamics in the Severn Estuary. Marine Pollution Bulletin, 61(1-3): 100-114

Kirby, J.R. and Kirby, R. 2008. Medium scale stability of tidal mudflats in Bridgwater Bay, Bristol Channel, UK: Influence of tides, waves and climate. Continental Shelf Research, 28: 2615-2629

Kirby, R. 2010. Distribution, transport and exchanges of fine sediment, with tidal power implications: Severn Estuary, UK. Marine Pollution Bulletin, 61(1-3): 21-36


Langston, W.J., Chesman, B.S., Burt, G.R., Campbell, M., Manning, A. and Jonas, P.J.C. 2007. The Severn Estuary: Sediments, Contaminants and Biota. Marine Biological Association of the UK Occasional Publications 19, Marine Biological Association of the UK, Plymouth

Lawton, J.H. and Brown, V.K. 1993. Redundancy in ecosystems. In Schulze, E.D. and Mooney H.A. (Eds) Biodiversity and Ecosystem Function. Springer, pp. 255-270

Little, C. and Boyden, C.R. 1976. Variations in the fauna of particulate shores in the Severn Estuary. Estuarine Coastal and Marine Science, 4:545-554

Magurran, A.E. and Henderson, P.A. 2003. Explaining the excess of rare species in natural species abundance distributions. Nature, 422(6933): 714-716

Pearson, T.H. and Rosenberg, R. 1978. Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanography and Marine Biology: an Annual Review, 16: 229-311

Petersen, K., Kristensen, E. and Bjerregaard, P. 1998. Influence of bioturbating animals on flux of cadmium into estuarine sediment. Marine Environmental Research, 45 (4-5): 403-415

Reichert, K., Ugland, K.I., Bartsch, I., Hortal, J., Bremner, J. and Kraberg, A. 2010. Species richness estimation: Estimator performance and the influence of rare species. Limnology and Oceanography – Methods, 8: 294-303

Reise, K., Olenin, S. and Thieltges, D.W. 2006. Are aliens threatening European aquatic ecosystems? Helgoland Marine Research, 60 (2): 77-83

Underwood, G.J.C. 2010. Microphytobenthos and phytoplankton in the Severn Estuary, UK: Present situation and possible consequences of a tidal energy barrage. Marine Pollution Bulletin, 61(1-3): 83-91

Warwick, R.M. 1984. The benthic ecology of the Bristol Channel. Marine Pollution Bulletin, 15: 70-76

Warwick, R.M., Goss-Custard, J.D., Kirby, R. George, C.L., Pope, N.D. and Rowden, A.A. 1991. Static and dynamic environmental factors determining the community structure of estuarine macrobenthos in SW Britain: Why is the Severn Estuary different? Journal of Applied Ecology, 28: 329-345

Warwick, R.M. and Clark, K.R. 1993. Increased variability as a symptom of stress in marine communities. Journal of Experimental Marine Biology and Ecology, 172: 215-226

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Appendix A Supplementary information


A.1 Taxa encountered in the infauna surveys of Bridgwater Bay in summer 2008 and 2010. Taxa occurring in both years are marked with an asterisk and emboldened; taxa occurring only once overall (i.e. at only one station in only one year) are marked with a #.

MCS code Taxon 2008 2010

D0764 Edwardsia (sp) ≠

G0001 NEMATODA * *

HD0001 NEMERTEA * *

P0117/0118 Eteone longa/flava * *

P0430 Sphaerosyllis taylori≠

P0458 Nereididae (juv)

P0462 Hediste diversicolor * *

P0475 Eunereis longissima≠

P0494 Nephtys (juv) * *

P0498 Nephtys cirrosa * *

P0499 Nephtys hombergii * *

P0672 Scoloplos armiger

P0677 Aricidea minuta

P0693 Levinsenia gracilis≠

P0754 Polydora cornuta≠

P0776 Pygospio elegans * *

P0799 Streblospio shrubsolii

P0864 Psammodrilus balanoglossoides≠

P0906 Capitella (sp. complex) * *

P0919 Mediomastus fragilis≠

P0929 Arenicola (juv.) ≠

P0931 Arenicola marina≠

P1489 Tubificoides amplivasatus * *

P1498 Tubificoides pseudogaster (agg) ≠

P1501 Enchytraeidae * *

Q0053 Acarina (sp) * *

R0068 Elminius modestus≠

R0078 Balanus improvisus≠

S0044 Gastrosaccus spinifer≠

S0133 Pontocrates altamarinus≠


S0451 Bathyporeia (indet.) ≠

S0456 Bathyporeia pelagica≠

S0457 Bathyporeia pilosa≠

S0458 Bathyporeia sarsi * *

S0462 Haustorius arenarius≠

S0471 Gammarus (juv.) ≠

S0616 Corophium volutator * *

S0805 Cyathura carinata≠

S0870 Lekanesphaera monodi

S0934 Idotea (juv.) ≠

S1188 Cumopsis goodsir * *

S1253 Diastylis rathkei * *

S1385 Crangon crangon≠

W0305 Littorina saxatilis≠

W0385 Hydrobia ulvae * *

W1077 Retusa obtuse * *

W1695 Mytilus edulis (juv) ≠

W2007 Tellinoidea (juv) * *

W2029 Macoma balthica * *

N/A Collembola


A.2 MDS ordination of multivariate community structure across Bridgwater Bay in 2008 and 2010. The MDS uses modified (log 10) Gower dissimilarities on mean abundance values per station.


Year20082010

10

11

15

16

18

28

2930

31

323334

35

36

38

47

48

10

11

15

16

18

28

29

30

31

32

33

34

35

36

38

47

48

2D Stress: 0.19

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