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*Corresponding author. Email: [email protected] 1
Running head: Monitoring of maerl beds 1
Mapping and monitoring the maerl beds in the Fal & Helford Special Area of Conservation: 2
a review of techniques 3
Katie Sambrook*, Trudy Russell 4
Falmouth Marine School, Killigrew Street, Falmouth, Cornwall, TR11 3QS 5
Abstract 6
Maerl is a generic term for several species of unattached, non-geniculated coralline red 7
algae. Maerl beds are considered to be of conservation importance due to their rarity, 8
environmental sensitivities and high biodiversity and are subject to both UK and European 9
legislation. The EU Habitats Directive resulted in the creation of a network of Special Areas 10
of Conservation, established to provide a degree of governance for key habitats and species. 11
The Fal & Helford Special Area of Conservation contains the largest live maerl bed in 12
southwest Britain. Conservation targets for maintaining the favourable condition of these 13
maerl beds include monitoring the extent, distribution, percentage of live maerl and species 14
composition. It is a requirement that these attributes are assessed every six years. This 15
review evaluates existing techniques used for subtidal benthic habitat mapping and 16
biodiversity surveys recommending suitable methods for monitoring maerl in the Fal & 17
Helford Special Area of Conservation. 18
19
Keywords 20
Maerl, habitat mapping, special area of conservation, biodiversity, monitoring, surveys 21
2
22
Introduction 23
24
Maerl is the generic term for unattached, non-geniculated coralline red algae (Birkett et al. 25
1998, Foster 2001). It has a hard calcium carbonate skeleton which research indicates 26
makes a significant contribution to carbon sequestration in the oceans (Canals & Ballesteros 27
1997, Birkett et al. 1998). Maerl beds form when living and dead maerl thalli accumulate, 28
with accretion occurring over long periods due to the slow growth rate of maerl, usually 29
between 0.05 and 1.0mm y-1 (Birkett et al. 1998, Foster 2001, Bosence & Wilson 2003, Grall 30
& Hall-Spencer 2003). Found in polar, temperate and tropical waters, maerl-forming species 31
are patchily distributed due to narrow environmental tolerances (Barbera et al. 2003, Grall 32
& Hall-Spencer 2003). Current flow is the primary abiotic factor that influences the 33
distribution of maerl as it is intolerant to smothering and burial (Birkett et al. 1998, Barbera 34
et al. 2003). Light, temperature and salinity also contribute to its spatiotemporal presence 35
(Birkett et al. 1998, Barbera et al. 2003, Wilson et al. 2004, Sciberras et al. 2009). Three 36
species of maerl are found to contribute to the majority of maerl beds in the UK, 37
Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum, the latter 38
being predominant (Birkett et al. 1998). 39
40
The nodular structure of maerl creates an interlocking matrix providing a habitat for a wide 41
range of infauna and epifauna (Birkett et al. 1998, Barbera et al. 2003). This lattice 42
formation is responsible for the high biodiversity found on maerl beds which is comparable 43
to other algal biotopes such as sea grass beds and kelp forests (Birkett et al. 1998, Kamenos 44
3
et al. 2004a). As well as exhibiting high biodiversity, maerl beds have been found to harbour 45
juveniles of commercially important species such as gadoids and the queen scallop 46
Aequipecten opercularis (Kamenos et al. 2004b, c, d). Studies in Scotland found that the 47
highest densities of juvenile cod (Gadus morhua), saithe (Pollachius virens) and pollack 48
(Pollachius pollachius) were observed between September and November (Kamenos et al. 49
2004b); and for A. opercularis between October and December (Kamenos et al. 2004b). 50
Although pristine live maerl beds (PLM) exhibit higher biodiversity, the long-term 51
accumulation of dead maerl deposits also represent an important habitat, particularly for 52
burrowing species (Birkett et al. 1998). Functional diversity on maerl beds is high with Grall 53
& Glémarec (1997) identifying eight trophic groups on a maerl bed in France. 54
55
Recognising the ecological importance of maerl, both L. corallioides and P. calcareum are 56
included in Annex V of the European Union’s Habitats Directive as species whose 57
exploitation is subject to management and are listed under the UK Biodiversity Action Plan 58
as priority species (Council Directive 92/43/EEC 1992). Maerl beds ares also protected under 59
Annex I of the Habitats Directive and appears on the Convention for the Protection of the 60
Marine Environment of the North-East Atlantic (OSPAR) list for threatened or declining 61
habitats and species. The Habitats Directive resulted in the creation of a number of Special 62
Areas of Conservation (SACs) established to provide a degree of governance and protection 63
to key habitats and species. 64
65
The Fal & Helford SAC is located in Cornwall and covers 6387.8 hectares (JNCC 2011). It 66
includes a range of Annex I habitats including ‘sandbanks which are slightly covered by 67
seawater all the time’ which contains the sub feature of maerl beds (JNCC 2011). The SAC 68
4
has the largest live maerl bed in southwest Britain, found on St Mawes Bank, and also 69
contains deep deposits of dead maerl gravel indicating that during the past live maerl was 70
much more prevalent than today (Birkett et al. 1998). 71
72
A number of studies have been conducted on the extent and biodiversity of maerl in the Fal 73
& Helford SAC. Blundel et al. (1981) recorded that live maerl was found at depths of 0-10m 74
in the Fal & Helford SAC; Davies & Sotheran (1995) mapped the extent of live and dead 75
maerl as part of the BioMar project; Perrins et al. (1995) compared the percentage of live 76
and dead maerl on St Mawes Bank between 1982 and 1992; Frau-Ruiz et al. (2007) surveyed 77
Falmouth Bay to establish the presence of maerl to inform decision-making on scallop 78
dredging and ship anchoring and Axelsson et al. (2008) undertook an ecological survey as 79
part of an appropriate assessment for the Port of Falmouth Development Initiative which is 80
currently seeking to dredge a part of the SAC in order to deepen the channel and increase 81
ship access. 82
83
The morphology and community structure of the maerl is likely to have changed since 84
Davies & Sotheran (1995) mapped the biotopes in the estuary. At this time both aggregate 85
dredging and scallop dredging were allowed within the site, although both activities were 86
banned in 2005, with the scallop dredging ban extending into Falmouth Bay in 2008 (Hall-87
Spencer 2005, legislation.gov.uk 2008). Both practices are known to be destructive with 88
research into maerl beds in other regions showing that scallop dredging can lead to >70% 89
reduction in live maerl coverage with no indication of recovery during the subsequent four 90
years (Hall-Spencer & Moore 2000). Due to its slow growth rates and intolerance to 91
5
activities that create sediment perturbation and siltation, maerl is now recognised as a non-92
renewable resource (Barbera et al. 2003). 93
94
In the UK to date, most surveys on maerl have been in relation to disturbance activities with 95
limited time-series data collected (OSPAR 2010). However, in order to maintain the integrity 96
of the SAC designation, it is fundamental that a monitoring programme is implemented to 97
conduct regular assessments of the protected habitats or species enabling early detection of 98
changes such as a decline in live maerl, issues with invasives, effects of climate change or 99
regime shifts (Birkett et al. 1998, Hiscock 1998, Sciberras et al. 2009). Conservation targets 100
under OSPAR and the Habitats Directive for maintaining the favourable condition of maerl 101
beds include monitoring the extent, distribution, percentage of live maerl, species 102
composition as well as recording the abiotic factors that could impact the health of the 103
maerl (OSPAR 2010). These checks are required at six-yearly intervals (Malthus & Karpouzli 104
2003, OSPAR 2010). 105
106
This paper seeks to evaluate current methodologies for obtaining this information, review 107
examples of good practice and make recommendations on the techniques that would work 108
best for the collection of data pertaining to the maerl in the Fal & Helford SAC. It does not 109
aim to provide detailed methodologies on how the surveys should be carried out. While 110
each site should be reviewed separately to assess potential threats specific to its location 111
and activity levels, it is a long-term aim that a standard protocol could be implemented 112
across other SACs containing maerl to establish a comprehensive network of data 113
contributing to its long-term sustainability. 114
115
6
Methods 116
117
Mapping the extent and distribution of maerl 118
The Fal & Helford SAC contains a busy port, has many recreational users and is home to the 119
only oyster fishery worldwide still fished under sail using traditional methods of dredging 120
(Challinor et al. 2009). An understanding of the extent and distribution of the live and dead 121
maerl is essential for informing decision-making surrounding the management and 122
conservation of this site. 123
124
Remote sensing 125
Recent technological advances have seen the development of remote sensing techniques 126
such as airborne sensing (aerial photography or video and hyperspectral data), satellite 127
imagery and acoustic sensing to support biotope mapping (Held et al. 2003, Diaz et al. 2004, 128
Godet et al. 2009). The use of remote sensing in the marine environment is advantageous 129
over traditional approaches such as grab sampling or dredging as it allows large areas to be 130
mapped over short timescales (Brown et al. 2002, Sciberras et al. 2009, Simons & Snellen 131
2009; Brown et al. 2011); provides more accurate spatial discrimination due to virtually 132
continuous sampling (Brown et al. 2002, Godet et al. 2009) and is non-destructive which is 133
important when surveying protected habitats and species (De Backer et al. 2009). 134
High costs and limited depth range restrict the use of both airborne sensing and satellite 135
imagery in marine habitat mapping at present although advances in hyperspectral mapping 136
7
should be reviewed in the future (Kenny et al. 2003, Brown et al. 2011). Acoustic sensing 137
has been used extensively in subtidal environments as a predictive tool, validated by 138
rigorous groundtruthing to confirm the results (Birkett et al. 1998, Hiscock 1998, Brown et 139
al. 2005, Ehrhold et al. 2006). There are four broad categories of acoustic mapping device: 140
(i) broad acoustic beam systems such as side scan sonar (SSS); (ii) acoustic ground-141
discrimination systems such as RoxAnnTM and QTC-ViewTM (iii) multiple narrow-beam swath 142
bathymetric systems and (iv) multiple-beam side scan sonar (Kenny et al. 2003). 143
144
Main types of acoustic survey method for benthic habitat mapping 145
146
Side scan sonar 147
Broad beam swath systems such as side scan sonar can produce an almost photographic 148
representation of the seabed (JNCC 2001, Kenny et al. 2003, Georgiadis et al. 2009). 149
Comprising of an underwater transducer connected by a cable to the towing vessel where 150
data is recorded, an acoustic signal is emitted from a single beam or multiple beams rapidly 151
returning echoes which are transmitted to the recording device for later analysis (JNCC 152
2001). As the sonar is towed at a fixed height above the seabed, it casts relatively large 153
acoustic shadows enabling the detection of variations in sediment structure (Kenny et al. 154
2003). Side scan sonar provides continuous coverage of the area being surveyed (Brown et 155
al. 2011). The introduction of digital side scan sonar devices has increased mapping 156
capabilities with object detection possible down to tens of centimetres (JNCC 2001). To 157
some extent, the quality of the images generated can be evaluated manually with features 158
such as sand ripples identifiable by eye (JNCC 2001). The REBENT monitoring network in 159
8
France has adopted side scan sonar as part of its benthic mapping programme which 160
includes maerl beds (Ehrhold et al. 2006). Georgiadis et al. (2009) used side scan sonar to 161
map coralline algae in the southern Aegean Sea. Side scan sonar has also been used in 162
studies by Brown et al. (2002, 2005) and Freitas et al. (2006) to identify different seabed 163
assemblages alongside groundtruthing exercises and OSPAR recommends its application 164
where maerl beds are thick and extensive (OSPAR 2010). 165
166
Acoustic ground-discrimination systems (AGDS) 167
Acoustic ground-discrimination systems (AGDS) operate using a hull-mounted single-beam 168
echosounder which detects differences in the seabed using acoustic reflection properties 169
(Greenstreet et al. 1997, Kenny et al. 2003). A single pulse is emitted from the device which 170
is directed straight down towards the seabed producing a footprint of the area directly 171
below the vessel (Brown et al. 2005). On reflection, the echo is returned to a transducer 172
(JNCC 2001). Due to the focused pulse emitted from the sounder, acoustic ground-173
discrimination systems do not produce continuous coverage maps like side scan sonar, but 174
instead produce a track covering the area beneath the vessel (Wilding et al. 2003). Gaps 175
between the tracks must be interpolated which can result in inaccurate assumptions about 176
the seabed habitat (Kenny et al. 2003, Brown et al. 2005). This means that AGDS should be 177
considered a predictive tool rather than a definitive view of the seabed (JNCC 2001, Wilding 178
et al. 2003). There are two systems commonly in use in the UK - RoxAnnTM and QTC-ViewTM 179
(JNCC 2001, Foster-Smith & Sotheran 2003). RoxAnnTM derives its values from the 180
interpretation of two elements of the returning echo: the first echo (E1) is related to 181
seafloor roughness, while the second multiple return echo (E2) is related to the hardness of 182
the seabed (Kenny et al. 2003, Wilding et al. 2003). Supplemented by groundtruthing, the 183
9
returned responses are classified into seabed characteristics (Kenny et al. 2003, Wilding et 184
al. 2003, Brown et al. 2005). QTC-ViewTM converts the echo into a digital form using the first 185
returning signal only and organises the seabed into acoustic classes (JNCC 2001, Ellingsen et 186
al. 2002). It can used in ‘supervised’ or ‘unsupervised’ modes, the first requiring 187
considerable groundtruthing using calibration sites, the second relying on post-processing 188
analysis (Ellingsen et al. 2002). The use of AGDS has become widespread in SACs in the UK 189
(Brown et al. 2005) with Davies & Sotheran (1998) adopting this approach when evaluating 190
biotopes in Falmouth Bay and the lower Fal Ruan Estuary. The JNCC Marine Monitoring 191
Handbook (2001) recommends the use of AGDS where broad-scale surveys are required and 192
as a tool for locating sites of particular interest, thus reducing survey time. The cost of AGDS 193
in comparison to other acoustic devices is relatively cheap and integration with other 194
onboard systems such as GPS is straightforward (Birkett et al. 1998, Kenny et al. 2003, 195
Wilding et al. 2003). However, concerns have been raised around the unpredictability of 196
responses, resolution at varying depths, levels of interpolation required and accuracy when 197
determining habitat boundaries (Kenny et al. 2003, Wilding et al. 2003, Brown et al. 2005; 198
Freitas et al. 2011). 199
200
Multibeam echosounders 201
Multibeam echosounders (MBES) were originally an extension of single-beam 202
echosounders, transmitting multiple beams which are capable of covering a broad swath 203
either side of the vessels track (Brown & Blondel 2009, Simons & Snellen 2009). Multibeam 204
echosounding is becoming increasingly utilised within the field of seabed habitat mapping 205
due to its collective ability to obtain bathymetry and backscatter data concurrently (Brown 206
& Blondel 2009, Brown et al. 2011). The return signal can provide details of the geoacoustic 207
10
properties of the sea bed including grain size and porosity, both of which could be useful 208
features for identifying maerl beds (Brown & Blondel 2009). With the introduction of faster 209
processing capabilities and quality improvements to the backscatter images which are now 210
comparable to those collected through side scan sonar, studies using multibeam 211
echosounders have rapidly increased (Brown et al. 2011). Like side scan sonar, continuous 212
coverage maps can be created in conjunction with groundtruthing (Simons & Snellen 2009). 213
Brown & Blondel (2009) observed that multibeam echosounders are now able to provide as 214
much if not more information than side scan sonar. However, a high level of understanding 215
is required to establish the most appropriate analytical approach and significant expertise is 216
needed to interpret the data (Brown et al. 2011). To date no studies have been published 217
demonstrating the successful application of multibeam echosounders to identify maerl. 218
A more comprehensive review of acoustic techniques used for seabed habitat mapping can 219
be found in Brown et al. (2011). 220
221
Groundtruthing 222
While acoustic surveys are increasingly used to map and classify seabed habitats, 223
groundtruthing is still necessary either for calibration with acoustic analytical tools or to 224
enable classification of the discrete characteristics identified through acoustic imaging (JNCC 225
2001, Brown et al. 2011). Grab samples, towed or drop-down video and remote operated 226
vehicles (ROVs) are common methods of groundtruthing. 227
228
Grab samples 229
11
Grab sampling involves the use of a two shelled steel device that is lowered open to the 230
seabed and digs into the sediment, bringing the two shells together to retain a sediment 231
sample which can be analysed at the surface. There are a number of grab samples in 232
standard use: the van Veen which is most appropriate for soft sediments; the Day Grab 233
which can be used on a variety of sediment types and the Hamon Grab which can be used to 234
collect samples on coarser sediments including cobbles (JNCC 2001). Both the van Veen and 235
Day Grab devices can be used from a small vessel with two operators where the Hamon 236
Grab sampler can only be launched from a larger vessel and requires a minimum of three 237
operators (JNCC 2001). When using grab sampling to groundtruth acoustic data, 238
consideration should be given to the method of sampling arrays. The JNCC Marine 239
Monitoring Handbook (2001) has details on random, stratified random and systematic 240
approaches to sampling. Analysis of the contents of the grab can be used to assist with the 241
classification of seabed characteristics found during acoustic surveys (Greenstreet et al. 242
1997, Davies & Sotheran 1998, Brown et al. 2002, Foster-Smith & Sotheran 2003, Freitas et 243
al. 2003, Ehrhold et al. 2006, Georgiadis et al. 2009). The number of samples and replicates 244
collected should take into consideration the area of the survey and the variations detected 245
from the acoustic mapping exercise (JNCC 2001). 246
247
Underwater video imaging 248
Boat operated underwater video imaging systems include drop-down video, towed sledge 249
video and remotely operated vehicles. Drop-down video involves the use of a video camera 250
attached to an umbilical cord being lowered over the side of the survey vessel either when 251
stationary or at low speed (JNCC 2001). This method gives the operator a limited degree of 252
control about the location studied and can be easily deployed to survey as many areas as 253
12
required (JNCC 2001). Brown et al. (2002) and Ehrhold et al. (2006) used drop-down video 254
to validate acoustic survey data collected for biotope mapping. Towed sledge video involves 255
a camera and lights mounted onto a rigid sled supported by buoyancy devices to keep the 256
image stable as the gear is towed (JNCC 2001). Large areas can be surveyed rapidly using 257
this approach and the clarity of the image enables accurate identification of biotopes (JNCC 258
2001). To reduce positional errors when comparing acoustic data to the video footage, a 259
transponder should be ideally fitted to the sledge to pinpoint its location (JNCC 2001). 260
Without a transponder, manual calculations will have to be performed prior to comparison. 261
Davies & Sotheran (1998) and Ruiz-Frau et al. (2007) used towed video when mapping 262
biotopes in the Fal. Remotely operated vehicles (ROVs) are also linked to the research vessel 263
by an umbilical cord but can be manoeuvred remotely by an operator onboard. ROVs can 264
survey large areas and focus on precise points of interest and are useful at sites with rapid 265
depth changes (Moore & Bunker 2001). However they are expensive and difficulties can be 266
encountered with recording the devices position. Georgiadis et al. (2009) used ROV drops in 267
conjunction with side scan sonar to map coralligène formations in the eastern 268
Mediterranean Sea and Thomson (2003) used an ROV as part of project to develop 269
autonomous sensors for marine resource mapping. All of these approaches provide a 270
permanent record of the seabed which can be stored, edited and reused. They are also 271
non-intrusive methods for identifying different substrate types. Successful use of any image 272
recording device is highly dependent on underwater visibility. 273
274
Evaluating proportions of live and dead maerl 275
276
13
Both live and dead maerl are considered important habitats for a diverse range of organisms 277
although the biodiversity on live maerl beds is considered greater (Birkett et al. 1998). The 278
slow growth rate of maerl means that any detrimental influences such as eutrophication, 279
invasive species, dredging or pollution is likely to have long-term implications for maerl and 280
its inhabitants (Hall-Spencer & Moore 2000, Barbera et al. 2003, Grall & Hall-Spencer 2003). 281
By establishing and monitoring the location and extent of live maerl beds, managers of the 282
Fal & Helford SAC will be able to observe any reduction in area. 283
284
There are a number of methods that could be used to collect this information. Grabs or 285
core samples (discussed above) could be used but would only provide data on a small 286
proportion of the survey area. They would also be unable to provide a quantitative 287
measurement as the grabs take a significant ‘bite’ from the seabed and the proportions of 288
sediment may not represent the actual quantity of live maerl present (Ruiz-Frau et al. 2007). 289
The use of drop-down or towed sledge video means that large areas can be surveyed 290
relatively quick to assess the extent of a live maerl bed but quantification of results would 291
again be difficult (Davies & Sotheran 1995, Ruiz-Frau et al. 2007). Thomson (2003) found, as 292
part of the SUMARE programme (Survey of Marine Resources) to establish autonomous 293
sensors for identifying maerl, that the use of a greylevel histogram applied to video footage 294
identified unique signatures for live and dead maerl. Although time consuming, direct diver 295
observation using quadrats or transect lines could be employed and would provide 296
quantifiable data on percentages of dead and live maerl (Axelsson et al. 2008). In the 297
Republic of Ireland, SCUBA divers use direct propulsion vehicles to rapidly assess 298
percentages (OSPAR 2010). All these methods could be interpreted and used in mapping. 299
300
14
Assessing community structure / biodiversity 301
302
The complex structure of maerl brings a number of challenges when surveying the 303
biodiversity of maerl beds. Steller et al. (2003) classified the type of organisms associated 304
with maerl into three main categories: 305
Epibenthic: motile or sessile organisms living on the seabed. 306
Cryptofaunal: those organisms living within the natural cavities created by the lattice 307
network of maerl. 308
Infaunal: those individuals living buried in the substrate. 309
The small size of many organisms found on maerl beds has implications for their 310
classification and any detailed studies require a high level of taxonomic expertise. Birkett et 311
al. (1998) recognised this as an issue and observed that identifying down to genus level 312
could still provide enough information to assess the health of the biotope. The size of 313
organisms also affects the types of survey that can be selected to gain sufficient quantitative 314
information on biodiversity. Diver surveys, hand-held video and still photographs can 315
provide data on conspicuous species but may miss smaller organisms. These methods 316
cannot be used to survey the cryptofauna and infauna. In order to obtain results for these, 317
samples must be collected and analysed ex situ. As maerl is slow growing, the quantity of 318
samples required when employing any intrusive survey techniques such as grab samples and 319
cores needs to be carefully considered. Maggs (1983) suggests that the minimum sub-320
sample size should be where a 10% increase in the number of species in the sub-sample is 321
derived from a 10% increase in the area. Processing of samples must be appropriately 322
15
designed to ensure the preservation of organisms for analysis and successful capture across 323
the size range of organisms. 324
325
In situ diving surveys 326
Diver quadrat surveys can be used for recording epibenthic species found on maerl. 327
Quadrats come in a variety of sizes, commonly 1m2, 0.25m2 or 0.1m2 (JNCC 2001). For maerl, 328
a 0.25m2 grid quadrat would be a sensible choice for the size of organisms under survey. A 329
minimum of two suitably qualified divers descend a weighted shot line, which can be used 330
by the survey vessel to record location, and lay a transect in a pre-agreed direction. This 331
avoids potential bias by the divers once underwater to select areas that look ‘most 332
interesting’ (Eleftheriou & McIntyre 2005). The quadrat and transect line should be slightly 333
negatively buoyant so that it can rest on the seabed while the divers complete the survey 334
(Axelsson et al. 2008). Divers should have a standard survey form to record species and 335
abundance. The JNCC Marine Monitoring Handbook (2001) recommends conducting a pilot 336
survey of the location to familiarise the divers with the species likely to be encountered 337
during the survey. Once the survey is complete, the dive team should use a delayed surface 338
marker buoy to inform the survey vessel of the end location. This will enable the results to 339
be mapped. Collecting data in this way is a non-destructive technique; provides quantitative 340
data which allows changes to species composition to be monitored over time; is a simple 341
method for recording conspicuous species without damaging the maerl and is easily 342
repeatable (Eleftheriou & McIntyre 2005). The divers need to be confident of their buoyancy 343
control to avoid kicking up sediment or causing damage to the surrounding maerl; should 344
have an understanding of the biotope and potential species and have the appropriate 345
16
qualifications for conducting underwater surveys (JNCC 2001). Percentages of live and dead 346
maerl could also be recorded as part of this survey. Steller et al. (2003) used transect 347
surveys to estimate species richness and abundance on maerl beds in the Gulf of California. 348
Hand-held video surveys carried out by qualified divers can provide information on the 349
conspicuous benthos in the chosen study site (JNCC 2001). A video camera, mounted in 350
underwater housing and with lighting to improve image quality is the standard equipment 351
required (JNCC 2001). It provides a permanent record of the site surveyed and analysis can 352
be conducted ex situ. Poor underwater visibility; scaling of the images recorded; the ability 353
to collect quantitative data that can be readily related to GPS points and consistency of 354
recording means that this method may be more appropriate for collecting footage that can 355
be used for educational purposes rather than monitoring. Axelsson et al. (2008) conducted 356
diver video surveys in the Fal Estuary in conjunction with in situ diver observations. 357
Still photography of maerl beds involves the use of an underwater high-resolution camera 358
with lighting. In order to obtain quantitative data, the camera should be mounted onto a 359
reference frame or a quadrat used to provide a scale to the photograph (JNCC 2001). In the 360
same way as video, photographs provide a permanent record and with greater resolution 361
capabilities than video, photographs can enable more detailed images to be collected. 362
Analysis can be carried out ex situ meaning that the divers do not need to be taxonomists. 363
Most commonly used for collecting fixed point data, there are currently no examples of this 364
method being used for random sampling (JNCC 2001). However, this method could be used 365
in conjunction with a diving quadrat survey to record unidentified specimens or get close up 366
shots without removing specimens. 367
368
17
Sample collection techniques 369
Hand-held cores can be used to collect cryptic fauna, infauna and sediment samples 370
(Bordehore et al. 2003, Moore et al. 2004, Axelsson et al. 2008, Sciberras et al. 2009). These 371
cores are operated by divers who manually push the core into the sediment. Axelsson et al. 372
(2008) found that the substrate in areas of the Fal made the extraction of sediment samples 373
using hand-held cores quite challenging. On reaching the appropriate depth, the top and 374
bottom of the corer are sealed with caps and it is placed into bags for secure storage and to 375
prevent any organisms escaping (Axelsson et al. 2008, Sciberras et al. 2009). Ideally on 376
board the survey vessel and within 24 hours of collection, the samples should be sieved and 377
fixed in 10% formal saline to ensure the samples are preserved for laboratory analysis (JNCC 378
2001, Axelsson et al. 2008). The small nature of some species found on maerl means that 379
the mesh size should be below 1000μm to ensure robust analysis (Moore et al. 2004, 380
Axelsson et al. 2008). 381
To collect cryptic fauna, Steller et al. (2003) used divers to collect random samples of maerl 382
thalli by hand, which were then preserved and the cryptic and burrowing species were 383
extracted in the laboratory for analysis. 384
Grab samples (discussed in the mapping section) can be used for biological sampling as well 385
as groundtruthing (Sciberras et al. 2009). A combined approach to groundtruthing and 386
infaunal sample collection can involve skimming off the top layer of the contents of the grab 387
sample for sediment analysis and fixing the rest for biological analysis in the laboratory 388
(Moore et al. 2004). Ruiz-Frau et al. (2007) used a 0.1m2 Day Grab for collecting sediment 389
and biological samples in Falmouth Bay. Grab samplers usually require deployment from a 390
research vessel with space to winch the samples in. 391
18
392
Measuring abiotic factors that could impact the health of maerl beds 393
Environmental tolerances of maerl are still poorly understood (Birkett et al. 1998). However, 394
any robust monitoring programme should include abiotic measurements so they can be 395
factored in if any unexplained degradation of the maerl beds occurs. 396
Temperature is known to affect the geographic distribution of maerl beds with L. 397
corallioides absent in Scotland due to cooler temperatures but present in southwest Britain 398
(Birkett et al. 1998, Wilson et al. 2004). The species composition of maerl beds is influenced 399
by temperature, so monitoring the water temperature on maerl beds could track changes 400
that may occur as a result of climate change (Wilson et al. 2004). Maerl beds are normally 401
found in fully saline conditions but the impact of variable salinity conditions is not 402
understood (Birkett et al. 1998). Temperature and salinity can be measured together in situ 403
using a temperature-salinity probe (Sciberras et al. 2009); through the use of a CTD 404
(conductivity-temperature-depth) package or by using a niskin bottle to collect water 405
samples for analysis on the surface. 406
Maerl is a coralline algae and therefore needs to photosynthesise in order to grow, however 407
irradiance requirements are not understood for maerl (Birkett et al. 1998). In the UK, maerl 408
beds rarely exceed 30m (Wilson et al. 2004). Turbidity can affect the amount of light 409
penetrating to the maerl bed and can be measured simply using a secchi disk (Sciberras et 410
al. 2009). 411
The nutrient requirements of maerl are not known, however with a rigid skeleton of calcium 412
carbonate maerl does have a requirement for calcium (Birkett et al. 1998). In laboratory 413
19
experiments, King & Scramm (1982) found that the calcium ionic concentration affected 414
maerl growth, with an optimum uptake of 30 ‰. Under laboratory conditions, Martin & 415
Gatuzzo (2009) conducted experiments to assess the effects of ocean acidification on 416
coralline algae and found that based on current projections, net dissolution was likely to 417
exceed net calcification in Lithothamnion cabiochae by the end of the century. pH therefore 418
should be monitored and can be tested in situ using an underwater housed pH meter or on 419
the research vessel if a water sample has been collected. 420
421
Discussion 422
423
The three-dimensional structure of maerl brings additional complexities and considerations 424
on top of those encountered during other subtidal surveys. The current deficiency in 425
standardised protocols for assessing maerl communities combined with rapid advances in 426
the technology used for mapping benthic habitats makes difficult work for conservation 427
managers when establishing a monitoring programme. There is no single method that can 428
monitor all the conservation objectives set for maerl within the Fal & Helford SAC. Instead 429
the solution requires a suite of approaches that will satisfy the requirements and provide 430
the management team with a robust dataset that can inform decision-making. The 431
recommendations discussed in the proceeding sections are based on the knowledge gained 432
through studies on maerl in other regions, the use of standardised methodologies where 433
possible and factor in resource and cost considerations. Due to the environmental 434
sensitivities of maerl, the recommendations exhibit a preference towards non-destructive 435
techniques where feasible. Table 1 summarises the recommendations discussed below. 436
20
437
Recommendations for mapping 438
While the importance and potential application of acoustic devices for seabed habitat 439
mapping is widely recognised, the difficulties with accurately differentiating maerl habitats 440
from other mixed sediments is still problematic and can currently only be used if validated 441
with groundtruthing. All three forms of acoustic sensing discussed have been used to map 442
maerl biotopes but the most recent research shows a preference towards the use of side 443
scan sonar in conjunction with a multibeam echosounder (Kenny et al. 2003, Diaz et al. 444
2004, Ehrhold et al. 2006, Brown et al. 2011). Side scan sonar has the highest definition of 445
seabed features amongst the techniques in use and this combined with the bathymetric and 446
backscatter imaging provided by multibeam echosounders will generate a robust baseline 447
for the monitoring programme (Diaz et al. 2004). The two devices are also capable of 448
providing 100% coverage unlike AGDS which needs interpolation between the tracks. From 449
previous studies of maerl in the Fal Estuary, it is apparent that maerl occurs in fairly shallow 450
waters (Blundel et al. 1981, Hiscock 1998). Acoustic surveys therefore need to be carried 451
out during high tides for maximum accuracy. Provision will need to be made within the 452
budget for a mapping expert to advise on the survey design and interpret the outputs. Due 453
to the area the Fal & Helford SAC covers, the quickest and most cost-effective option for 454
groundtruthing is drop-down video. For deeper parts like the channel in Carrick Roads, 455
towed sledge video may be required but will give comparable results. 456
457
While these surveys can be carried out at any point during the year, it is recommended that 458
mapping is carried out during the summer months when visibility, light and weather 459
21
conditions are optimum. For consistency, future surveys should be carried out across the 460
same period. Surveys lasting longer than an hour should have tidal corrections applied 461
(Brown et al. 2005). 462
463
Recommendations for evaluating proportions of live and dead maerl 464
An initial baseline assessment should focus on establishing the location and dimensions of 465
the live maerl beds. Drop-down video is a practical way to collect this data and would work 466
well with the groundtruthing exercise. With drop-down video, GPS points can be easily 467
collected when live maerl is observed and later plotted onto a GIS map. Where live maerl 468
beds are identified surveys should be conducted to identify the perimeter of the bed so that 469
a true area can be calculated. While the primary goal is to obtain a map showing the 470
locations, another important measurement is to identify the proportion of live and dead 471
maerl present within the live maerl beds. The best method for obtaining quantitative data 472
is to conduct diver quadrat surveys along a series of transects. Procedural Guideline 3-7 473
from the JNCC Marine Monitoring Handbook (2001) covers the methodology in more detail. 474
Measurements should be taken as percentages not abundance scales to enable quantitative 475
analysis. It should be noted that these survey methods will only measure the surface layer 476
of the maerl. 477
478
Recommendations for biodiversity 479
Apart from the maerl species there are no key indicator species associated with maerl at 480
present so it is necessary to ensure a robust biodiversity survey is conducted. Collecting data 481
22
on the number and type of species found on maerl beds will require the application of 482
several methodologies and is likely to be the most time-consuming part of the monitoring 483
programme. Both live and dead maerl deposits should be surveyed. Multiple diver quadrat 484
surveys along a transect line are recommended for surveying the epifauna and flora 485
following the sample size recommendations proposed by Maggs (1983). This approach will 486
require at least one diver to be a trained biologist with an understanding of the taxonomy of 487
organisms associated with maerl. Organisms should be identified down to species level 488
where possible but genus is acceptable. The second diver should have a high-resolution 489
underwater camera with a macro setting and strobe lighting. Each quadrat should be 490
photographed with a unique id. Any unknown organisms should be photographed for 491
identification ex situ. The photograph should include a scale for reference. A standard form 492
should be generated for the survey team to complete including depth, temperature and 493
percentage of live and dead maerl. Any diving surveys must comply with the Diving at Work 494
Regulations 1997 and adhere to the Scientific and Archaeological Approved Code of 495
Practice. 496
497
There are currently no non-destructive methods for surveying the infauna. The use of a 498
diver hand-held corer is considered a suitable method for obtaining samples. A number of 499
objectives can be achieved through collecting core samples. The species of maerl will be 500
easier to identify in the laboratory, the underlying sediment can be measured and cryptic 501
fauna and infauna can be surveyed at the same time. Samples should be processed and 502
fixed within 24 hours of collection for later analysis in a laboratory. Comprehensive 503
methodologies for collecting, processing and analysing cryptic fauna and infauna can be 504
found in Steller et al. (2003) and Sciberras et al. (2009) respectively. 505
23
506
Due to the seasonal variations experienced on maerl beds, two sampling seasons are 507
recommended to gain a greater understanding of community change however this will be 508
constrained by the budget allotted to the monitoring programme. Consideration should 509
also be given to avoid extensive destructive survey techniques around any peak recruitment 510
periods. If only one survey is practicable, then July is advised when species richness is at its 511
peak yet recruitment and settlement of known commercially important species is low 512
(Kamenos et al. 2004b, c, Peña & Bárbara 2010). 513
514
Recommendations for measuring abiotic factors 515
Although OSPAR (2010) states that monitoring should be conducted as a minimum at six-516
yearly intervals for SACs, it is recommended that abiotic factors should be collected twice 517
yearly during Spring and Autumn. Plankton levels are generally high in Spring while water 518
temperature is at its annual lowest, conversely in late Autumn, annual water temperature 519
reaches its maximum and plankton is on the decline (Miller 2004). Temperature, salinity, 520
pH, turbidity and dissolved oxygen levels should be recorded. These are relatively simple 521
and quick measurements to collect and may be important if any change to the extent and 522
distribution of the maerl is observed. It is important that the same equipment is used and 523
calibrated each time thus avoiding potential discrepancies between devices. 524
525
Further recommendations 526
24
Continuing advances in the technology surrounding benthic habitat mapping and our 527
increasing understanding of maerl biotopes mean that the techniques recommended in this 528
review will need to be reassessed at appropriate junctures. 529
While not within the scope of the review, it is worth emphasising the importance of the 530
supporting systems that will need to be established in order to manage and store the data 531
collected through the monitoring programme. A suitable format will need to be selected 532
which is likely to include GIS functionality and scope for statistical analysis of the data. 533
534
Conclusions 535
536
The high biodiversity associated with maerl beds has long been recognised. However for 537
many years maerl has been subjected to damaging anthropogenic activities such as 538
aggregate extraction and dredging. Investigations on maerl have largely focused on 539
assessing the implications of these destructive processes or sought to gain a greater 540
understanding of the biology and ecology associated with maerl. As such, there have been 541
few monitoring programmes established to look at the long-term dynamics of maerl bed 542
communities. 543
Valuable contributions have been made by the scientific community to expand our 544
understanding of maerl. These studies have shown that maerl grows exceptionally slowly, 545
has narrow environmental tolerances and is highly sensitive to disturbance resulting in the 546
acknowledgement that maerl is a non-renewable resource. They have also identified its role 547
in carbon sequestration and as a nursery ground for commercially important species. 548
25
As a consequence, a growing emphasis is being placed on the conservation importance of 549
maerl beds, with local and European protection designations at both species and habitat 550
level. In order to manage maerl beds effectively, it is important to establish a 551
comprehensive baseline from which to monitor long-term trends. 552
Those parties involved in the management of the Fal & Helford SAC should find this review a 553
useful summary of the techniques most suitable for surveying maerl beds. While it shows 554
that establishing a maerl monitoring programme for the Fal & Helford SAC is a complex 555
process that will involve significant time, cost and planning, it also demonstrates that there 556
are multiple methods that can be adopted in order to achieve the primary conservation 557
objectives. 558
559
Acknowledgements 560
561
This review was kindly supported by the Falmouth Harbour Commissioners. This manuscript 562
benefited from comments by C. Eatock, A. O’Brien, T. Russell, and A. Thornton. Particular 563
thanks go to thank N. Woods for his assistance with testing ROV and multibeam 564
echosounder methodologies and the use of the research vessel RV Ann Kathleen. 565
566
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