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The history of intertidal blue mussel beds in the North Frisian Wadden Sea inthe 20th century: Can we define reference conditions for conservation targetsby analysing aerial photographs?

H. Buttger, G. Nehls, P. Stoddard

PII: S1385-1101(13)00240-2DOI: doi: 10.1016/j.seares.2013.12.001Reference: SEARES 1174

To appear in: Journal of Sea Research

Received date: 30 December 2012Revised date: 22 November 2013Accepted date: 2 December 2013

Please cite this article as: Buttger, H., Nehls, G., Stoddard, P., The history of intertidalblue mussel beds in the North Frisian Wadden Sea in the 20th century: Can we definereference conditions for conservation targets by analysing aerial photographs?, Journal ofSea Research (2013), doi: 10.1016/j.seares.2013.12.001

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H. Büttgera, G. Nehlsa and P. Stoddardb

The history of intertidal blue mussel beds in the North Frisian Wadden Sea in

the 20th century: can we define reference conditions for conservation targets

by analysing aerial photographs?

Corresponding author:

Heike Büttger

BioConsult SH

Schobüller Str. 36

25813 Husum – Germany

Tel: +49 - 4841 - 66329 – 14

Fax: +49 - 4841 - 66329 – 19

E-mail: h.buettger@bioconsult-sh.de

Affiliation:

a BioConsult SH

Schobüller Str. 36

25813 Husum – Germany

b 66 Cavendish Avenue

Perth

PH2 OJU

Scotland

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Abstract

Conservation decisions often rely on defining a reference status for habitats and

species to enable targets to be set and progress measured. Long-lasting and

continual anthropogenic impacts on habitats and species make the setting of

undisturbed reference values such as diversity, distribution, population size or other

ecological characteristics, difficult. In turn this hampers assessment of ecological

status.

Within the Wadden Sea, intertidal blue mussel beds are important biogenic

structures which can be clearly defined from the surrounding flats. As mussel beds

are highly productive habitats, they are considered as biological quality indicators for

coastal waters. Nonetheless the reference status provokes controversy in

discussions between policymakers, stakeholders and researchers. In order to build

on existing knowledge of intertidal blue mussel beds in the North Frisian Wadden

Sea, we analysed aerial photographs from the 1930s, 1958, 1989, 1998 and 2010.

We supplemented this remote sensing data with annual monitoring data from 1999

to 2009 obtained from analysis of aerial photographs and field surveys.

Results show a generally high persistency of blue mussel beds likely over eight

decades, although sites were probably not permanent throughout the time period

and their areal extent had changed. Mussel beds occur mainly on the east side of

the islands which provide shelter against storms from the west. Studies of aerial

photographs for the 1930s and 1958 demonstrate the importance of historical data

to an assessment of the current status of the beds. In particular they help assess the

distribution and extent of mussel beds over time.

Keywords: Mytilus edulis, aerial photographs, Wadden Sea, GIS, long-term

development, mussel beds

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1. Introduction

An important tool within nature conversation is the definition of a reference status.

Assessing the current status of species or habitats demands reference data from

undisturbed (= pristine) conditions (Muxika et al., 2007), defined during times with

no or low anthropogenic impact (Vincent et al., 2002). Since most areas in Europe

are heavily affected by ongoing anthropogenic impacts (Borja et al., 2004), it is

extremely difficult to define a reference status that aligns to undisturbed conditions.

In particular the Wadden Sea, along the coast of the SE North Sea, is subject to

land reclamation, fishing, channel deepening, coastline modification and so on

(Reise et al., 2008; Lotze, 2005; Dolch, 2008). Climate change and rising water

temperatures (Reise, 2005; Martens and van Beusekom, 2008) affect the Wadden

Sea’s ecosystem, and with constant natural changes, the ecological baseline is

permanently shifting (Lotze et al., 2005). Knowledge of historical states improves

our understanding of the ecosystem and its present and future dynamics (Lotze et

al., 2005). For example, historical investigations on benthic communities by

Nienburg (1927) and Wohlenberg (1937) are important when evaluating the current

status in the northern German Wadden Sea (Reise and Beusekom, 2008). Within

the Wadden Sea, blue mussel beds are unique biogenic structures; they are

autogenic ecosystem engineers (Jones et al., 1994) and serve as habitat for various

animal and plant species, forming important food sources for birds.

To meet various EU conservation objectives, shellfish stocks need to be maintained

as food for migratory birds (Essink et al., 2005). To protect blue mussel eating birds,

in particular eiders (Somateria mollissima), the Netherlands and Denmark

implemented a policy that protects some mussel stock for birds and excludes some

beds from fishery activities (Laursen et al., 2010; van Stralen, 2010). Implementing

that approach and measuring its effects relies on having set reference values or

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thresholds, such as mussel bed area or biomass. Historical data on mussel stocks in

the Wadden Sea may improve our understanding of the ecosystems and future

possibilities in conservation (compare Lotze et al., 2005). Besides food policy

requirements intertidal blue mussel bed are biological quality elements for example

within the EU Water Framework Directive and potential indicators within the Marine

Strategy Directive (de Vlas et al., 2005; BMU, 2012).

Dijkema et al. (1989), Nehls and Thiel (1993), Brinkman et al. (2002), Hertweck and

Liebezeit (2002) and Herlyn et al. (2008) analysed aerial photographs, older

publications and mussel bed layers in a sediment profile to glean more information

about bed distribution, site stability and area coverage back to the 1960s and 1970s.

Analysing further older data, including aerial photographs from the 1930s and 1958

will enhance the knowledge. The 1930s data are of particular value because there

was almost no mussel fishery (Ruth, 2004; Reise, 2005) and no eutrophication in

pre-industrial times (Beusekom, 2005). We analysed aerial photographs from the

1930s, 1958, 1989, 1998 and 2010 with these objectives:

1. to assess how accurate aerial photography was in estimating location and

extent of blue mussel beds and to analyse the method’s difficulties and/or

limitations and if this method could be applied to historical photographs

without ground truth.

2. to evaluate the development of blue mussel beds over 80 years and identify

changes to the size and location of beds over the last eight decades, in the

process assessing whether photographs from 1930s and 1958 provided

sufficient detail to define reference conditions.

3. to examine whether reference values within the continuously changing

Wadden Sea, with its high inter-annual fluctuations, could be defined.

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2. Methods

2.1. Study site

This study focused on the tidal flats of the German North Frisian Wadden Sea, from

the Eiderstedt peninsula in the south to the Danish-German border in the north (Fig.

1). The survey area of 1,688 km² included the entire List tidal basin (not completely

displayed in Fig. 1) and all the islands. The intertidal zone amounted to 962 km²

(Spiegel, 1998). The Wadden Sea’s regular tidal change, alternating every six hours

(tidal range about 2 – 3.2 m) creates highly variable, constantly changing

environmental conditions. Salinity in the survey area ranges seasonally between 25

psu in winter and 32 psu in summer, although variations within the tidal circle are

only 1 psu (Becker, 1998a). Water temperatures range from winter values of -1.5

and -1.9 °C to around 23°C in summer (Becker, 1998b). Particular influences are

severe ice winters with ice rearing, storms with strong westerly winds and storm

surges (Nehls and Thiel, 1993).

2.2. Aerial photographs

We analysed aerial photographs from the 1930s, 1958, 1989, 1998 and 2010 (Tab.

1). Three sets of images from the 1930s covered only parts of the overall intertidal

North Frisian Wadden Sea study area (Fig. 2). All other aerial imagery covered the

entire area. All photographs were black-white except the 2010 colour series. For the

1930s and 1958 imagery, digital black-white georeferenced ortho-photomosaics

were available.

Aerial photographs from May 1989 were scanned and processed with Erdas

IMAGINE 8.3.1 software (Stoddard, 2003). Analogue pictures from 1989 were

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enlarged to 1:10.000, and mussel beds drawn on an overlying sheet. All were

scanned and merged in IMAGINE. A reflecting stereoscope was used to produce a

3D picture of areas that were difficult to interpret, with mussel beds again drawn on

a top sheet and scanned and merged in IMAGINE.

1998 aerial photographs were taken at 1:15000 scale, using a Zeiss RMK top

camera with a 153mm Pleogon A3 lens and Agfa Avipot Pan 400 panchromatic

black-white film. The photos overlapped 60% along and 30% transverse to the flight

direction. To ensure consistent georeferencing, IMAGINE analysed the scanned

prints’ central coordinates from the differential geographic position system (DGPS),

flight height and lens focal length. The analysis corrected only camera-specific

distortion, not angle. The mussel beds in 1998 were digitised with IMAGINE 8.3.1.

The colour photographs from 2010 were produced as a georeferenced ortho-

photomosaic supplemented by prints of each picture. Mussel beds were digitised

with ArcMap 10 by ESRI.

2.3. Quality and analysis of aerial photographs

Mussels were recorded by digitising the outer border of the intertidal beds. Mussel

beds form close to the low water line (McGrorty et al., 1993), so aerial photographs

had to be taken at low tide, optimally between one hour before and one hour after

low tide (Millat, 1996). Conditions had to be cloudless since clouds or their shadows

hinder bed identification. Furthermore, spring tides and prevailing winds can affect

low tide levels (Millat, 1996).

On aerial photographs intertidal blue mussel beds look darker than surrounding

areas. They exhibit a typically orthogonal structure (Millat, 1996; Dolch, 2008,

compare Fig. 3). Some mussel heaps are elongated and run parallel to each other.

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The algae Fucus vesiculosus forma mytili, can cover mussel beds and show darker

on photographs, making bed identification easier. Mussels completely overgrown

with barnacles could show up brighter on photographs. Defining a border between

mussel and seagrass beds could be difficult. Seagrass beds have higher coverage

and smoother structure, often transected by tidal channels (Fig. 3). Seagrass can

also fill internal gaps within mussel beds. Variations in shell accumulation (from

different mussel species) could hamper identification (Millat, 1996). Mussel beds

covered with a thin layer of mud are more difficult to spot. Areas with sparse mussel

cover (< 10 % coverage) are difficult to identify. Young beds are also difficult

because the resolution of 1:25.000 and 1:15.000 photographs is insufficient (Tab. 1;

Herlyn, 2005).

For the 1930s and 1958 photographs, no ground truth data was available so

analysis was solely on the photographs.

Analysis of 1930s photographs was partly hampered by low contrast or poor

definition but original hardcopy images from 1935, 1936 and 1937 supported the

analysis. We decided to combine the 1930s data, although photograph quality

differed between years. The low number of days of ice coverage 1934-1937 (Fig. 4)

indicated that the mussel bed area was not significantly affected by ice rafting and

so we summarized those years to one reference in the 1930s. We also compared

results with the analysis of seagrass to minimise misclassifications (Dolch et al.,

2012).

Identifying mussel beds on photographs from August and September 1958 was

more difficult because tidal flats were also covered by algae and seagrass. Aerial

photographs from 1989 were analysed by Stoddard (2003). The quality of aerial

photographs from 1998 and 2010 was good enough to analyse mussel beds.

Analysis since 1998 has been accompanied by annual field surveys.

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To minimise potential bias caused by analysing photographs without ground truth

data, we correlated field surveys with photographs (see 2.4; statistic software R

version 2.13.2; R Development Core Team, 2011). We incorporated results from

Stoddard (2003) using a correction factor based on a linear regression (y = mx +b)

between field surveys and photograph results. The median of the coefficient (m)

from 1998 and 2010 was used as the correction factor for the 1930s and 1958

images. It was not used on 1989 data (Stoddard, 2003) as the analysis was based

partly on a reflecting stereoscope which made mussel identification more accurate

(Herlyn, 2005).

2.4. Additional monitoring data

The Wadden Sea Ecosystem Research Project surveyed North Frisian blue mussel

stocks 1989-1994 (Ruth, 1994; Stoddard, 2003). Annual monitoring of beds between

1998 and 2010 gave a comprehensive picture of recent mussel bed changes

(Büttger et al., 2011b). The data were based on aerial photographs and field surveys

using GPS, carried out using standard methodology (TMAG, 1997). In general, the

bed areas recorded in the field surveys were combined with the results from the

aerial photographs. However, in some years the aerial surveys covered only parts of

the North Frisian Wadden Sea and in 2006 and 2009 no surveys were conducted.

We reviewed the mussel bed area data obtained from field surveys between 1989

and 1994 and between 1999 and 2009 to support our evaluation of data from the

photographs. Since 2004, the balance of blue mussels and Pacific oysters

(Crassostrea gigas) in a bed has been analysed through the different live wet

weight/m² of the two species. Where no biomass samples are available, the

classification is worked out on visual appearance (< 30% = pure mussel bed with

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scattered oysters, > 30% - 60% = balance between mussel and oyster patches,

> 60% = oyster bed with mussel patches; Nehls et al., 2009a).

2.5. Considering ice winter effects and storms

Intertidal mussel beds can be strongly affected by ice winters and storms (Nehls and

Thiel, 1993; Steenbergen et al., 2006). Ice drift can remove mussels and alter the

extent of beds. These effects cannot be seen on aerial photographs. Furthermore,

cold winters stimulate blue mussel population increases, encouraging enhanced egg

production and reduced predation (Strasser et al. 2001; Beukema and Dekker 2005,

2007). To evaluate from historical photographs ice winter effects on mussel bed

areas, we applied a correlation (Spearman rank correlation) between changing

mussel bed area (area t – area t-1, t = year) and days with ice coverage in the two

preceding winters (applicable for years 1989-1994 and 1999-2010). The Federal

Maritime and Hydrographic Agency (BSH, Germany) provided the data, using the

arithmetical mean of days with ice at 13 stations along the German coast. It

characterises the extent and duration of ice occurrence.

Severe storms might have similar negative effects to ice on mussel beds (Nehls,

2000). However, precise historical data of storm events were lacking so their effects

are only discussed in general.

2.6. Effects of land claim measures and fishery

We checked the aerial photographs from the 1930s and 1958 to see if changed

hydrodynamics caused by embanking, dike and groyne work and by fishery

activities such as dredging impacted on numbers of mussel beds. We also

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considered general information about the blue mussel industry including data on

mussel landings between 1930 and 2010, provided by the Ministry of Energy

transition, Agriculture, Environment and Rural Areas Schleswig-Holstein (Ruth, pers.

comment).

2.7. Spatial distribution and steadiness of mussel beds

The spatial distribution of blue mussel beds was analysed by calculating the amount

of mussel bed area in relation to the intertidal area in each tidal basin. The

calculations took into consideration the fact that land reclamation over recent

decades has altered the intertidal area. From 1998 onwards, we were able to use

the same intertidal area for calculation. The tidal flats in the Wadden Sea change

from year to year. The intertidal areas for the 1930s, 1958 and 1989 were calculated

by using the recent data and adding areas that have not yet been embanked (Fig.

1).

Mussel bed stability was calculated as the number of years that sites were

populated. We used photographic data from the 1930s, 1958, 1989, 1998 and 2010

to calculate this. A mussel bed could achieve a maximum stability rating of 5, where

the bed appeared in all 5 photograph series. We did not include all monitoring years

between 1999 and 2009 because this would have resulted in an overweighting for

this period. But we analysed sites that had existed in 6 or more years between 1998

and 2010.

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3. Results

3.1. Mussel bed area

The aerial photograph analysis for the 1930s series revealed about 6.6 km² of

intertidal blue mussel beds (Fig. 5), increasing to 9.75 km² when the correction

factor was applied. The total area could have been slightly higher, as the 1930s

photos did not cover the entire study area (e.g. south-west of the island of

Pellworm). The area in 1958 extended to 7.3 km², plus 3.1 km² with correction. The

largest bed area to date (about 15.0 km²) occurred at the end of the 1980s. Between

1990 and 1994, the area stabilised at around 10 km² (Fig. 5). In 1998 the area

decreased to only 5.3 km². In the following three years, the area increased to levels

seen at the start of the 1990s. Between 2002 and 2005, the total area dropped to

2.3 km², the lowest value recorded, and then increased again to about 5.8 km²

(2009). Since 2004 oyster stocks have increasingly come to dominate blue mussel

beds and totally colonized many former blue mussel sites. In 2009 half the area was

dominated by oysters while the other half was still blue mussel dominated. Due to

ice rearing in the ice winter of 2009/2010, several mussel beds disappeared or

diminished and the total area dropped to less than 3 km².

Maps with the interpretation results of the aerial photographs from the 1930s, 1958,

1989, 1998 and 2010 can be provided electronically as supplementary material (->

Appendix).

3.2. Correlation between interpretation of aerial photographs and field

surveys

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The linear regression calculated from 1998-2010 indicated that the aerial

photographs analysis comprised between 45% only in 2001 and 92% at best in

2010 of the results gained by filed surveys (Fig. 6). The median 68% from 1998 and

2010 was used as the correction factor for the results of the 1930s and 1958.

Correlations were worse in 2001, 2004, 2007 and 2008 when results from single

mussel beds influenced readings. In the latter two years decreasing mussel bed

area associated with worse bed structures hampered photograph analysis. Only four

years deliver a correlation lower than 60%. Otherwise, the aerial photographs

reflected 66% to 92% of the field survey results.

3.3. Effects of ice winter and storms

Severe ice winters like 1946/1947, 1962/1963, 19969/70, 1978/1979 and 1995/1996

exhibited more than 40 days’ ice coverage along the German North Sea coast (Fig.

4). We found no significant positive or negative correlation (p > 0.05) between

changes in mussel bed area and the days with ice coverage in the two previous

winters. Between 1930 and 1939 each year had less than 20 days’ ice coverage, but

conditions differed strongly between the southern and the northern tidal basins in

the Wadden Sea of Schleswig-Holstein (Fig. 4, data for different tidal basins were

provided by BSH but are not shown here). The winter of 1955/1956 saw about 30-

40 days of ice coverage. The following two winters had less than 15 days each and

the effect of this on intertidal mussel beds is assumed to have been low. The mussel

beds in 1989 seemed unaffected by an ice winter, since the only incident occurred

during 1986/1987, when the area close to Husum experienced at least 39 days of

ice coverage. This probably led to a high mussel bed area reading in 1988/1989.

The mussel bed area in 1998 was affected by the ice winter in 1995/96 which

destroyed most beds (Strasser et al., 2001). However, it also resulted in a strong

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spatfall repopulating several sites. The final series of aerial photographs, from 2010,

followed an ice winter with more than 35 days coverage which wiped out several

beds.

Storms can destroy intertidal mussel beds as per events documented by Nehls,

2000. The winter gale “Anatol” in December 1999 swept mussels out of established,

populated heaps and beds and dispersed them (“Streufelder”). Nehls, 2000,

estimated a total loss of 30% due to the storm. Information about storm frequencies

in the German Bight (Schmidt, 2001) indicates that storm gales (12 Bft) did not

occur during the second half of the 1930s. In the 1940s and 1950s storm gales

occurred more frequently.

3.4. The effects of land reclamation and fishing

Land reclamation and diking date back centuries. Since the 1930s about 7,107 ha of

the North Frisian coast have been embanked (Kunz and Panten, 1997; Fig. 1).

Basing on available data, mussel beds were affected at two sites. A bed of 1.75 km²

that had existed in the 1930s in the Hörnumtief was lost following later construction

of a dike and embankment at Friedrich-Wilhelm-Lübke-Koog. Whether the loss was

due to the embankment is unknown but re-establishment was probably prevented.

The embankment of Beltringharder Koog (northeast of Nordstrand) destroyed 3.350

ha of intertidal area and with it several intertidal blue mussel beds (0.2 - 0.3 km²,

Reise, pers. comment) documented by Reise (1979).

Mussel fishery expanded in the 20th century and might have affected intertidal beds

(Seidel, 1999). At the beginning of the 20th century blue mussels were harvested

with rakes and forks during low tide (Seaman and Ruth, 1997). In the 1920s annual

landings reached 2000 to 3000 t (Ruth, 1994). In 1919, the mussel fishery in the

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study area collapsed due to fishing pressure and a severe ice winter (Seaman and

Ruth, 1997). Dredging started in 1934, with two or four dredges (1.60 m width; Ruth,

pers. comment), and blue mussel fishery proliferated in the 1940s (Reise, 2005) but

landings remained less than 1500 t per year until 1943. An increasing market in the

1950s prompted development of the overall fishery. Before 1958, mussel landings

were low with 2244 t (1956) and 2640 t (1957; Ruth, pers. comment). Projecting

from the blue mussel monitoring data approximated values for coverage with 25%

within the mussel bed area and about 10 kg LWW/m² (mean values 1998-2010) to

calculate a rough value for intertidal blue mussel biomass, the landings in the 1930s

refer to about 2 to 6 % of the intertidal stock. In 1958 landings rose above 6000 t but

data might include harvests after the aerial photographs were taken in August and

September 1958 (Tab. 1). Landings in 1958 and the two previous years amounted

to between 11% and 34% of the intertidal stock, according to the approximated

values above. But the photographs from the 1930s and 1958 contain no signs of

damage caused by fishery activities. Dredges with an opening of 1.6 to 2 m would

leave detectable signs on photographs with a pixel size of 60 x 60 cm (Tab. 1).

Since 1995 no seed mussel fishery has taken place in the intertidal, and since 1997

harvesting has been prohibited in the Wadden Sea of Schleswig-Holstein (CWSS

2002).

The photographs from 1989 were taken in May and intertidal seed mussel fishery in

that year was conducted later in summer, so these data were not directly affected by

the fishery. Intertidal seed fishery in the years before 1989 totalled about 20.000 t

annually and concentrated mainly on subtidal seed (Nehls et al., 2009b).

3.5. Spatial distribution and stability of mussel beds

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Mussel bed distribution and density varies. Most are found close to the low water

line. Tidal basins with high concentrations of beds exhibit the highest site stability

over decades (compare Fig. 6 and Fig. 7). The most stable occur east of the islands

(Fig. 6). Most sites that showed in all the aerial photographs were populated for

more than six years during the monitoring. Intertidal areas exposed to south-west

weather patterns along the main tidal channels (Hörnumtief, Süderaue and

Norderhever) were always sparsely and irregularly populated. Single mussel beds

remained in the same location over decades and translocation incidence was low

(Fig. 9).

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4. Discussion

The results reveal a consistent bed distribution pattern throughout the study area.

Sites exposed to the dynamics created by westerly winds are less stable. However,

in sheltered areas even individual beds and mussel clusters are stable. While sites

can persist for at least 80 years, ice and storms can destroy them, but data indicate

that beds tend to re-establish at the same sites.

First we will discuss whether intertidal beds can be identified effectively and

secondly discuss the spatial distribution of beds, their stability and their main

structuring factors. Finally, we will discuss whether analysis of historical aerial

surveys can deliver reference values for conservation targets.

4.1. Reliability of mussel bed identification from aerial photographs

The areas of blue mussel beds covered by field surveys and aerial photographs can

differ, depending on the quality and the scale of the aerial photographs and the

extent and circumstances of field work (Millat, 1996; Herlyn, 2005). Single beds

might be too small to spot and beds may go undetected if characteristic features are

not obvious on photographs. Conversely, some structures on photographs can be

mistaken for mussel beds. However the correlation from monitoring data

documented up to 92% of mussel beds, even where as few as 45% were detected

on photographs. Analysing beds on photographs without any ground truth data has

its limitations (Michaelis, 1987; Herlyn, 2005). Effective monitoring has to combine

aerial photography and field surveys, as recommended by TMAG (TMAG, 1997).

In correlating field survey and aerial photographs a median 68% correction factor,

was applied to the 1930s and 1958 results. In the absence of ground truth for these

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data sets, this median was applied as an additional area estimate, but this figure

may have been smaller. The correlation results in Fig. 6 indicate that most results

match better than 65%. Therefore we assume that the 1930s and 1958 results

obtained from the aerial photographs probably reflect the truth much better than the

applied correction factor suggests.

4.2. Area, spatial distribution and stability of mussel beds

Blue mussel beds covered 8-11 km² in the 1930s, 1958 (using the correction factor),

1990-1994 and 1999-2001. The largest reading, 15 km², documented in 1988 and

1989, originated from a good spatfall in 1987. Since 2002 blue mussel stock has

declined significantly due to low spatfall (Nehls et al., 2009a; Fig. 5). Densities of

mussels measuring < 20 mm in autumn (Sept./Oct.) were low between 2004 and

2009 with values between 313 ± 249 mussels/m² (2008) and 817 ± 493 mussels/m²

(2005). Numbers were up to three times between 2000 (879 ± 1175 mussels/m²)

and 2003 (2985 ± 4208 mussel/m²). In 1999 12 new beds established. However, in

2003 only four new beds were formed and 2005 just six. Furthermore, since 2004

mussel beds in the List tidal basin and between the islands of Amrum and Föhr have

been taken over by Pacific oysters and in 2009 56% of the total area was dominated

by oysters. The ice winter of 2009/2010 removed a high number of blue mussel

beds south of Langeness and only 0.62 km² was left in 2010. While oyster beds

suffered less mechanical destruction, mortality rate of oysters was 90% (Büttger et

al., 2011a). Oyster dominated sites cannot be distinguished from blue mussel sites

on aerial photographs.

The results show a generally high site persistency of blue mussel beds in the North

Frisian Wadden Sea, probably spanning more than eight decades. However, these

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sites had probably not existed permanently and site areas would have changed. The

general distribution pattern and overall stability of the mussel beds remained more

or less constant. Mussel beds are located mainly to the east of islands. Nehls and

Thiel (1993) demonstrated that the islands provide shelter against prevailing

westerly storms. When storms and ice remove mussel beds, they can take years to

recover (Nehls, 2000; Nehls and Thiel, 1993; Dankers et al., 2001; Strasser et al.,

2001). But severe winters stimulate bivalve recruitment success (Beukema and

Dekker, 2005, 2007; Strasser et al., 2001, Dare and Walker, 1993), mainly on

former mussel bed sites (Dankers et al, 2001; Brinkman et al., 2002), and especially

where remains of shells provide the foundations for new settlements (Herlyn et al.,

2008). Our analysis of the correlation between days with ice coverage and changes

in mussel bed area revealed no significant results. Since our 1999-2009 analysis

(Fig. 4) involved mild winters (compare Folmer et al., submitted), we were unable to

establish whether temperature changes affected mussel bed area. Since 2004 the

spread of Pacific oyster has increased mussel bed areas. Knowing the number of

days with ice coverage does not help predict mussel bed losses or gains. However,

the few ice days and storm gales preceding the aerial photographs in the 1930s and

1958 (Schmidt, 2001) show that beds were unaffected in ice-free conditions and

non-storm weather.

Stable distribution patterns are also documented in the Dutch Wadden Sea and in

the Wadden Sea of Lower Saxony (Dankers and Koelemaij, 1989; Brinkman et al.,

2002; Hertweck and Liebezeit, 2002; Herlyn et al., 2008). Low orbital velocity is the

main requirement for mussel bed habitat (Brinkman et al., 2002).

The impacts of storms and ice and habitat suitability for spatfall lead to a distinct and

long-lasting bed distribution pattern (Nehls and Thiel 1993; Dankers et al., 2001;

Brinkman et al., 2002). As no significant changes in storm frequency have been

recorded and rising temperatures reduce ice incidence (Fig. 4), it is thought that

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destructive events may occur less frequently. Total mussel stocks, however, seem

more affected by annual variation in spatfall (Beukema and Dekker 2005, 2007;

Strasser et al. 2001; Nehls et al. 2006) and the recent decline of mussel stocks and

subsequent replacement by Pacific oysters provide some insights into how the

changing climate may induce substantial changes in the ecosystem.

Hydromorphological changes (dislocated tidal gullies) or anthropogenic intervention

(e.g. diking) fundamentally affect the distribution of mussel beds and may

permanently alter an area’s suitability for mussels. However, these factors have had

only small impacts on mussel beds. Dislocation of tidal channels affected mussel

beds on very local scales, but the general distribution pattern in the study area was

not impaired over eight decades.

4.3. Can we define reference conditions for conservation targets?

Blue mussel beds have a high intrinsic value as biogenic structures. Mature beds

serve as habitat and food source for numerous species, stabilize substrates,

improve water quality and support the spat settlement (Asmus, 1987; Dittmann,

1990; Seed and Suchanek, 1992; Gosling, 1992, Dankers et al., 2001). Recent

substantial decreases are of high concern (Nehls et al., 2009a). Knowledge about

long term distribution, stability of sites and total area coverage forms a valuable

background for setting conservation objectives and may help with evaluation of

ecological status, impact assessments and management decisions, for example

concerning fisheries.

Historical publications about blue mussels stocks are scarce and focussed mainly

on culture aspects of European oyster (Ostrea edulis) and blue mussels (Möbius,

1977; Hagmeier and Kändler, 1927; Hagmeier, 1941). European oysters occurred in

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the shallow subtidal (Möbius, 1877) while blue mussel beds occur in the lower

intertidal and upper subtidal (Reise, 2005), often settling beside oyster beds in the

intertidal (Hagmeier, 1941). The 1930s aerial photographs revealed for the first time

the large-scale distribution of mussel beds when there was low or nil eutrophication

and they enabled direct comparison with recent times. Even taking into account

limitations in accuracy and completeness of the 1930s and 1958 aerial photographs,

and the lack of ground truth data, they can be accepted as reference points for

identifying both stable mussel bed sites and the total area of beds. Both parameters

should be considered in defining a reference status for conservation. Mussel bed

area and total biomass (appraisal value) readings are vital in formulating how much

mussel stock should be set aside for birds (Nehls et al., 1997; Essink et al., 2005;

Laursen et al., 2010). Our results indicate that 8-11 km² are realistic in the North

Frisian Wadden Sea. The biotope “intertidal blue mussel bed at ‘stable sites’ ”

should be a biological quality element for coastal waters within the EU Water

Framework Directive (de Vlas et al., 2005). The analysis of historical aerial

photographs allows identification of stable sites on a long time scale (compare Fig.

6). These can be used as references. Changes in total mussel bed area may have

different causes than losses of individual stable sites. Both have to be evaluated in

order to distinguish between natural fluctuations and fundamental changes.

More recent changes like the spread of the Pacific oyster cannot be documented by

aerial photographs alone as oyster beds cannot be distinguished from blue mussel

dominated sites. This classification needs field surveys.

The analysis of historical aerial photographs provides a regional reference for

mussel beds and is a good tool to compare their development over different sub-

regions. This will increase understanding of why development of mussel beds varies

regionally (increasing or at least stabilised stocks in the south-western Wadden Sea

in the last decade and in contrast declining stocks in Denmark and the Wadden Sea

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of Schleswig-Holstein, compare Nehls et al., 2009a; Millat et al., 2012; Folmer et al.,

submitted).

Acknowledgements:

We thank the Schleswig-Holstein Agency for Coastal Protection, National Park and Marine

Conservation (LKN-SH, National Park Authority) for their support. The 1930s pictures were handled by

Niels Reinecke and those from 2004 by Vincent Sohni. Thanks to Tobias Dolch for checking difficult

seagrass/mussel bed sites. Martina Löbl did the encompassing for 1958. Thanks to Maarten Ruth for

discussing site by site in front of the screen and information about mussel fishery in the last century.

We thank Natalie Schmelzer (Federal Maritime and Hydrographic Agency of Germany) for providing

straightforward information about ice coverage in the North Frisian Wadden Sea. Lutz Christiansen

(Schleswig-Holstein Agency for Coastal Protection, National Park and Marine Conservation) kindly

provided the aerial photographs from the 1930s and 1958/1959 and offered the opportunity to sort

through the original pictures from 1958. The Alfred Wegener Institute (List/Sylt) kindly provided their

aerial photographs from the List tidal basin in 2005 and 2008 which supported the analysis of our

mussel data. We appreciated the linguistical proof reading by Seabury Salmon. And we are grateful to

Ansgar Diederichs, Tobias Dolch and three anonymous reviewers for valuable comments on the

manuscript.

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Figures

Fig. 1: Overview of the Wadden Sea (small map, trilateral map of the Wadden Sea

by the CWSS, Wilhelmshaven) and the area of study in the Wadden Sea of

Schleswig-Holstein (large map, both maps were provided by The National Park

Administration of the Wadden Sea Schleswig-Holstein; dark grey = islands and

mainland, light grey = intertidal, white = subtidal areas). Land reclamation since the

1930s is also plotted.

Fig. 2: Coverage of aerial photographs in the 1930s. The underlying map shows the

coastline and morphology in the North Frisian Wadden Sea in 2005 (dark grey =

islands and mainland, light grey = intertidal, white = subtidal areas).

Fig. 3: Example of the structural differences between mussel beds (above, dark

patches are mussel heaps) and seagrass beds (below). Seagrass areas have higher

coverage and smoother structure intersected by tidal channels on aerial

photographs from 2005 in the North Frisian Wadden Sea. In both images, bright

grey areas indicate bare mudflats and the borders of mussel bed and seagrass

areas are indicated by the white line.

Fig. 4: Number of days with ice coverage at 13 stations along the German North

Sea coast. Data were provided by the Federal Maritime and Hydrographic Agency of

Germany (BSH 1961, Schmelzer pers. comment). Data refer to the previous winter

in relation to the year indicated at the x-scale.

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Fig. 5: Total area of intertidal blue mussel beds in the North Frisian Wadden Sea

from the 1930s to 2010. Results based on analysis of aerial photographs, field

surveys and the correction factor as explained in the text (including data from Ruth,

1994 and Stoddard, 2003).

Fig. 6: Coefficients (m) of the linear regression (y = mx +b) between results of aerial

photograph analysis and field surveys with GPS of intertidal mussel beds in the

North Frisian Wadden Sea between 1998 and 2010 (no aerial photographs in 2006

and 2009).

Fig. 7: Stability of intertidal mussel bed sites in the North Frisian Wadden Sea based

on site specific data from the 1930s, 1958, 1989, 1998 and 2010. Areas where

several mussel beds consistently appeared are circled. White dots indicate mussel

sites which existed for six or more years during mussel monitoring 1998-2010.

Fig. 8: The relative area coverage (%) of intertidal mussel beds in four tidal basins in

the North Frisian Wadden Sea in the 1930s, 1958, 1989 and 1998-2010 (including

data from Stoddard, 2003).

Fig. 9: Stability of mussel bed structure in the List tidal basin. The four pictures show

details of a bed in 1936, 1958, 1989 (the beds were drawn on an overlaying folio

which was later scanned and processed; Stoddard, 2003) and 2002. The elongated

structure in the left of the pictures is a former dam.

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Table

Tab. 1: List of the analysed series of aerial photograph surveys to identify intertidal

mussel beds in the North Frisian Wadden Sea. Pixel size in cm of the edge lengths.

Appendix - electronic supplementary material

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Appendix A1-A5: Intertidal mussel beds in the North Frisian Wadden Sea digitised

from aerial photographs on the 1930s (A1), 1958 (A2), 1989 (A3), 1998 (A4) and

2010 (A5). The underlying map shows the coastline and morphology in the North

Frisian Wadden Sea in 2005 (dark grey = islands and mainland, light grey =

intertidal, white = subtidal areas, black = mussel bed). In map 5 most mussel beds in

the List tidal basin, Hörnumtief and between the islands Amrum and Föhr are

dominated by Crassostrea gigas.

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H. Büttgera G. Nehlsa &. P. Stoddardb

The history of intertidal Blue mussel beds in the North - Frisian Wadden Sea in

the 20th century: can we define reference conditions for conservation targets?

Highlights:

Historical aerial photographs were analysed to determine intertidal Blue

mussel area.

Results reveal high site persistency of blue mussel beds.

Results of 1930s and 1958 can be accepted as reference points of mussel

bed distribution (stability of sites) and total area.

Results are useful to define regional reference values for intertidal Blue

mussel beds.