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Mar Biodiv DOI 10.1007/s12526-012-0139-y

ORIGINAL PAPER

Diversity of demersal and megafaunal assemblages inhabiting sandbanks of the Irish Sea

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Keywords Wind farms . Megafauna . Demersal fish . Irish Sea . BTA . St George’s Channel

Introduction

The EU Habitats Directive (93/43/EEC) lists sandbanks as candidate SACs (Special Areas of Conservation) in Annex I. Sandbanks consist of mixed coarse sandy sediments that are permanently covered by shallow seawater, typically at depths of less than 20 m. Sandbanks in the western Irish Sea provide diverse habitats for fish and invertebrate communities. These communities are characterised by low species diversity as a consequence of their high physical disturbance regime (Kaiser et al. 2004; Mackie et al. 1995). Fauna inhabiting sandbanks are strongly associated with sediment type and other factors, including water temperature, wave exposure, topographical structure, depth, turbidity and salinity. In the Irish Sea, there are several sandbanks that sustain a number of productive and profitable fisheries (e.g. whelks, seed mussels) and sensitive habitats (e.g. Sabellaria reefs). Identification of ecologically distinct assemblages and biologically sensitive areas is important for appropriate conservation and management plans to be implemented in marine ecosystems. The identification of climate change as a global threat to biodiversity has led to increased pressure on nations to reduce their carbon emissions, which in turn has led to increasing investments in the renewable energy sector (Ellerman and Buchner 2007; UNFCCC 1998). Many European countries are intensifying the development of offshore wind farms with plans to generate âˆź50,000 MW of power by 2030 (Shaw et al. 2002). There are currently plans to develop offshore wind energy in Irish Sea by erecting a further 150 turbines in the area of the Kish bank (Johnston et al. 2002). The presence of wind turbines on sandbanks cause a shift from soft to hard bottom habitat that

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Abstract Sandbanks are marine habitats of conservation importance under the EU Habitats Directive. These habitats are becoming subject to impacts of several human activities including fishing, aggregation extraction, and construction of offshore wind farms that may have detrimental effects on their structure and functioning. We characterised and compared the diversity and biological traits of demersal fish and megafaunal invertebrate assemblages inhabiting three sandbanks, one in the vicinity of a small existing wind farm and two which are proposed sites for future wind farm installations. Samples in the vicinity of the offshore wind farm were compared with two control sites on the same sandbank. There were significant differences in mean number of taxa, abundance and structure of assemblages between sandbanks. However, biological traits analyses (BTA) showed no differences in the functional traits of assemblages among sandbanks, suggesting functional redundancy. Despite a significant spatial variation in structure and Shannon diversity of assemblages between sites within sandbanks, fish and megafaunal assemblage did not differ between sites near wind turbines and the controls. The natural spatial variability in the diversity and biological traits of demersal and megafaunal assemblages inhabiting this naturally highly disturbed environment is larger than any changes associated with the presence of the wind turbines. This study provides important baseline data against which potential future impacts of human activities can be tested. J. Atalah : J. Fitch : J. Coughlan : J. Chopelet : I. Coscia : E. Farrell School of Biology and Environmental Science, Science Centre West, University College Dublin, Belfield, Dublin 4, Ireland Present Address: J. Atalah (*) Cawthron Institute, Private Bag 2, Nelson 7010, New Zealand e-mail: javier.atalah@cawthron.org.nz

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Received: 28 December 2011 / Revised: 17 November 2012 / Accepted: 20 November 2012 # Senckenberg Gesellschaft fĂźr Naturforschung and Springer-Verlag Berlin Heidelberg 2012

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Javier Atalah & Jayne Fitch & Jennifer Coughlan & Julien Chopelet & Ilaria Coscia & Edward Farrell


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particular ecosystem. It is a biodiversity measure based on the biological traits of the species present in a community and reflects the ecological complexity of an ecosystem (Diaz and Cabido 1997). Biological traits are those that define species in terms of their ecological roles in interacting with the environment and other species (Bremner et al. 2006). Protecting such biological traits protects many of the species that exhibit them (Duffy and Stachowicz 2006). This present study aims to: (1) characterise and compare diversity and assemblage structure of megafaunal invertebrates and demersal fish inhabiting three sandbanks in the western Irish Sea, and (2) compare the structure, biological traits and diversity of megafaunal invertebrate and demersal fish assemblages between sites near a wind farm and control sites.

Methods

Study area

This study focussed on three sandbanks located in St George’s Channel in the western Irish Sea (ICES division VIIa): Kish, Arklow and Blackwater (Fig. 1). The sandbanks are located approximately 10 km offshore, parallel to the southeast coast of Ireland. They stand in 20–30 m of water and rise to within a few meters of the water surface. The sandbanks are quasi-stable features in dynamic equilibrium with tidal and wave conditions. They form a natural barrier to the strong tidal streams which occur in the southwest Irish Sea, and subsequently they are subject to tidal scour (Wheeler et al. 2001). Currently, at the Arklow sandbank, there is a wind farm consisting of 7 wind turbines operating in the central part of the sandbank. Each turbine consists of a monopile foundation approximately 6 m in diameter and driven 30 m into the seabed.

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can promote important changes in associated communities and can provide additional habitat for sessile fauna inhabiting local boulders and cobbles (Airoldi et al. 2008). The introduction of man-made structures, such as piers, gas platforms and wrecks, has been shown to lead to rapid colonisation by hard substrata fauna, even in areas which naturally have soft bottoms and where hard substrata is spatially remote (Bacchiocchi and Airoldi 2003). It has been suggested that wind turbines can play the role of artificial reefs and can support communities of fish and invertebrates not previously seen in high numbers at the site (Wilhelmsson et al. 2006). However, the ecological impacts of offshore wind farms are often poorly understood (Gill and Kimber 2005; Petersen and Malm 2006; Garthe and Huppop 2004) and may be of great importance in coastal zones, where anthropogenic pressures on the local flora and fauna are already high (Halpern et al. 2008). Ecological effects from offshore wind farms are expected to be most severe during construction and decommissioning (Gill and Kimber 2005). The potential effects during the construction period include habitat loss, displacement and burial. These impacts are considered to be temporary and can be minimised if care is taken to avoid areas containing rare or sensitive habitats or species (Petersen and Malm 2006). The potential impacts involved during the much longer operational phase include alterations in the local hydrodynamics, resulting in localised sediment scour that is likely to affect benthic communities (Airoldi 2003; Hiscock et al. 2002). Furthermore, changes in habitat structure and quality, particularly increased heterogeneity resulting from the “reef effect”, which will alter the biological communities, are possible (Clynick et al. 2007). The “reef effect” can be caused by the presence of the wind turbine monopile which can support fouling communities, including species not previously associated with the area (Petersen and Malm 2006). Additionally, the noise, vibrations and shadows generated by the turbines together with electromagnetic fields from the electric cables, may disturb organisms both below and above the water surface. Changes in electromagnetic and noise characteristics of the water column are of particular concern to taxa such as elasmobranchs and cetaceans, which rely on their ability to detect magnetic and acoustic stimuli for a number of behaviours including navigation and prey detection (Gill and Kimber 2005). Around the turbine blades, noise, vibrations and shadow may impact birds (Garthe and Huppop 2004), as they are at risk of collision with the turbines or displacement from migration routes for feeding and yearly breeding (Barrios and Rodriguez 2004). The conservation and monitoring of marine habitats not only requires an understanding of their biological diversity and structure but also of their functioning (Bremner 2008). Traits diversity refers to the variety of biological processes, functions or characteristics of a

Sampling design Demersal fish and megafaunal invertebrates (>2 cm max. length) were sampled in the Kish, Arklow and Blackwater sandbanks in St George’s Channel during a survey aboard the RV “Celtic Voyager” in August 2007. Two sites were located at the Kish sandbank; one at the northern end and one at the southern end. Three sites were located at the Arklow sandbank; one ‘wind farm’ within 200 m of the wind turbines, and two positioned ∼8 km to the north and south of the turbines, identified as ‘control’ sites (north and nouth, respectively). In the Blackwater sandbank, two sites, one at the northern end and one at the southern end, were sampled (Fig. 1).


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Fig. 1 The location of the three study sandbanks, Kish, Arklow, and Blackwater, in relation to the island of Ireland. The position of the seven wind turbines is denoted by crosses (+) and sites are shown as circles. At each site, 3 replicated trawl samples were made (n03)

At each site, three replicated trawls were conducted parallel to each other approximately 50 m apart. Fishing gear consisted of a 2.9-m beam trawl with 3 tickler chains and a 40-mm mesh cod end liner. Trawls were conducted for 15 min at a depth of âˆź10 m and covered âˆź15,000 m2 at an average cruise speed of 4 knots. The entire catch was weighed and sorted, except when the catch weighed more than 200 kg, in which case conspicuous, rare and large species including fish and elasmobranchs were collected and a random subsample of a quarter of the weight of the remaining catch taken, with abundances of organisms in the subsample extrapolated to the total catch.

Most fish caught were identified onboard to species level except gobies (Gobiidae), which were grouped by family. The abundance of each fish species caught was recorded and total length was measured to the cm below. Megafaunal invertebrate taxa were identified on board to the lowest possible taxonomic level and their abundance recorded. Taxa not identified on board were preserved in 5 % formaldehyde and returned to the laboratory for identification. Encrusting organisms and benthic macrofauna were omitted from the analyses because trawling does not sample them adequately (Kingsford and Battershill 1998).


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Biological traits modalities Maximum relative size

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Longevity (years)

R

Reproductive method

A

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Large <2 2–5 >5

Asexual (budding) Sexual (broadcast spawner) Sexual (egg lay/brood—planktonic larvae) Sexual (egg lay/brood—mini-adults) None

A M

A

Relative adult mobility

Attachment

MV

F

FO

FD

SX

SC

Movement method (adult)

MI

Low Medium High None Temporary Permanent Sessile Swim Crawl Burrow

Flexibility (degrees)

High (>45)

Body form

Medium (10–45) Low(<10) Flat

Feeding method

Mound Erect Deposit

Sexual differentiation

Filter/suspension Opportunist/scavenger Predator Gonochoristic

Sociability

Synchronous hermaphrodite Sequential hermaphrodite Solitary

Migration

Gregarious Colonial Non-migratory

Biological trait analysis

Biological Trait Analysis (BTA; Bremner et al. 2006) was used to investigate changes in the ecological features of assemblages. For this purpose, 13 biological traits were used to describe life history, morphology and behaviour of the faunal groups, reflecting their involvement in ecosystem processes (such as bioturbation, carbon storage and nutrient cycling) and perceived sensitivities to environmental disturbances. The 13

Small Small–medium Medium Medium–large

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S

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Differences in assemblage structure were tested using distance-based permutational multivariate analysis of variance (PERMANOVA; Anderson 2001a) based on Bray– Curtis similarity of the abundance data. The two-factorial nested design included ‘Sandbank’ as fixed factor and ‘Site’ as random factor nested in Sandbank. Factor Sandbank had three levels: Kish, Arklow and Blackwater. Factor Site had two levels for both the Kish and Blackwater sandbanks, and three levels for the Arklow sandbank. The three sites in Arkow Bank were the ‘wind farm’, and the two ‘control’ sites, north and south. Abundance data were log (x+1) transformed prior to analysis to balance the contributions of common and rare species (Clarke 1993). Each term in the analyses was tested using 4,999 permutations of the correct relevant permutable units. Because there were not enough permutable units to get a reasonable test by permutation for the factor ‘Sandbank’, P values were obtained using a Monte Carlo random sample from the asymptotic permutation distribution (Anderson and Robinson 2003). Significant terms were then investigated using a posteriori pairwise comparisons with the PERMANOVA t statistic and 999 permutations. This included a planned comparison between the wind farm and the control sites at the Arklow sandbank. Multivariate patterns were visualised by non-metric multidimensional scaling (nMDS). The similarity percentages procedure (SIMPER; Clarke 1993) was used to identify the percentage contribution that each species made to the observed value of the Bray–Curtis similarities. Univariate permutational analysis of variance (Anderson 2001b) was done on several variables: total abundance, number of taxa and Shannon diversity index (H′). Levene’s test was used to check the assumption of homogeneity of variances and the assumption of normality was checked by visual inspection of residual plots. Total abundance data was log (x+1) transformed. Univariate analyses were achieved using a distancebased approach as described above for the multivariate analysis by choosing to use Euclidean distances for a single response variable when running PERMANOVA. This is preferable to traditional ANOVA, because PERMANOVA calculates P values using permutations, rather than relying on tabled P values, which assume normality.

Table 1 Biological trait variables and categories used to describe functional diversity in the demersal and megafaunal assemblages inhabiting sandbanks of the Irish Sea

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Data analysis

Seasonal migration H

Living habit

Life stage migration Tube-dweller Permanent burrow dweller Crevice dweller Free living


Mar Biodiv Table 2 List of taxa recorded on the trawl samples from each site at three sandbanks in the Irish Sea: Arklow, Kish and Blackwater sandbanks Arklow

Kish

N

Wind Farm

S

Blackwater

N

S

N

S

PORIFERA Halichondria panicea CTENOPHORA Ctenophora indet. CNIDARIA

P P P

P

Hydractinia echinata Hydrallmania falcata Alcyonium digitatum

P

P

P P

P P P

P P

Anomiidae

P P

P

P P P

P P P

P P P

P

P P

P

P P

P P P P

P

P

P

P

P

P

P

P

P

P P

A

Buccinum undatum Calliostoma zizyphinum Colus gracilis Donax vittatus Ensis sp. Euspira catena

P

P

P P R

Sabellaria spinulosa MOLLUSCA Aequipecten opercularis

P

P

Adamsia carciniopados Urticina felina ANNELIDA Aphrodita aculeata Harmothoe imbricata Pomatoceros triqueter Nereis fucata

P

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Actinea fragacea Tubularia indivisa Nemertesia antennina Nemertesia ramosa

P

P

P P

P

P

P P

P P

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Mactra stultorum Mytilus edulis Neptunea antiqua Onchidoris bilamellata

P

P

Spisula elliptica Tritonia hombergi CRUSTACEA

Pagurus bernhardus Palaemon serratus Maja esquinado

P

P

Lepidonotus squamatus Liocarcinus holsatus Hyas coarctatus

P P P

P P

P

P P

P

Pisidia longicornis Semibalanus balanoides Pandalus montagui Macropodia tenuirostris

L

P

A

Abietinaria abietina

P

P

P

P

P

P

P

P

P P

P P

P P P

P

P

P

P P


Mar Biodiv Table 2 (continued) Arklow

Kish

N

Wind Farm

S

Blackwater

N

S

Eurynome aspera Palaemon sp. Macropodia rostrata Cancer pagurus Inachus sp. Liocarcinus depurator

P

P

P

P

P

P P P

P

P

P

P

P

P

P

P

P

P

A

Raja montagui Scyliorhinus canicula OSTEICHTHYES

P

P P P

Corella parallelogramma CHONDRICHTHYES Raja brachyura Raja clavata

L

P

P

P

P P

A

P

Astropecten irregularis Crossaster papposus Echinocardium cordatum Ophiothrix fragilis Ophiura ophiura Psammechinus miliaris TUNICATA

P

P

P

P

P

P

P

P

P

P P

FO R Pleuronectes platessa Solea solea Echiichthys vipera Trisopterus minutus

P P P

P

P P

P P

P

P

P P

P P

P P P

P

P

P

P

P P

P

P P P P

P

P

P

P

P

P

P

P

P

P P P P

P P P

Melanogrammus aeglefinus Merlangius merlangus Platichthys flesus

P P P

P denotes the presence of a given taxa in the sample, N north, S south

P

P

Agonus cataphractus Ammodytes tobianus Arnoglossus laterna Callionymus lyra

Eutrigla gurnardus Gobius spp. Limanda limanda

P P

P

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ECHINODERMATA Amphiura chiajei Asterias rubens

P

P P P

P

P P R

Alcyonidium diaphanum Membraniporidae Flustra foliacea

S

P

Inachus leptochirus Crangon crangon Atelecyclus rotundatus Balanus crenatus BRYOZOA

N

P

P P P

P P P P P P P P

P

P

P P

P


Mar Biodiv Table 3 PERMANOVA of Bray–Curtis similarities among trawls based on the abundance (multivariate data), biological traits (BTA) and permutational ANOVAs for mean number of taxa, total abundance and Shannon diversity Multivariate

BTA

Mean no. of taxa

Total abundance

Shannon diversity

Source

df

MS

F

MS

F

MS

F

MS

F

MS

F

Sandbank Sites (Sandbank) Residual

2 4 14

10600 3173.6 1448.4

3.34** 2.19***

2620 961 393.2

2.73 2.44*

499 63.4 40.2

7.87* 1.58

21.1 2.8 1.4

7.61* 2.03

0.56 0.8 0.17

0.69 4.74*

Transformation

log (x+1)

log (x+1)

None

log (x+1)

None

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Table 4). The common starfish (Asterias rubens) was highly abundant at all sandbanks, with over 14 % contribution to the overall similarities among samples. At the Kish sandbank, there was a high abundance of the hermit crab (Pagurus bernhardus), the plaice (Pleuronectes platessa) and dead

Stress : 0.14

a Stress : 0.09

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biological traits used in the analyses were: individual/colony size (relative weight), adult longevity (years), reproductive method, relative adult mobility, degree of attachment, adult movement, body flexibility (°), body form, feeding habit, sexual differentiation, sociability, migration and living habit (Table 1). Taxa found in >50 % of the trawls or in the top 90 % of abundance at any individual trawl were retained and used for the BTA analysis (i.e. 26 taxa in total). Individual taxa were then scored for the extent to which they displayed the categories of these traits using a ‘fuzzy coding’ procedure (Chevenet et al. 1994). The scoring range of 0–3 was used, with 0 being no affinity to a trait category and 3 being high affinity. The abundance data were log10 (x+1) transformed prior to the analyses. The information contained in the biological traits matrix and the abundance matrix were then combined using a weighting procedure, by multiplying both matrixes, resulting in a sample by trait matrix containing the overall frequencies of occurrence of the biological traits at each trawl station. The generated matrix represents the biological traits of the assemblages and was analysed using multivariate techniques in the same way as described above for the original species list. Information on biological traits was obtained from J. Bremner (personal communication), while for the full list of traits and more details on the method, see Bremner et al. (2003, 2006).

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*P<0.05, **P<0.01, ***P<0.001

Results

Assemblage structure

A total of 83 taxa of fishes and invertebrates were recorded from the trawl samples from the three sandbanks (Table 2). Assemblage structure differed significantly among sandbanks and among sites within the different sandbanks (PERMANOVA; Table 3). The nMDS plot (Fig. 2) illustrates that assemblages are distinguishable among the three different sandbanks. These differences were due to changes in the relative abundance of the dominant taxa (SIMPER analysis;

b Fig. 2 Non-metric multidimensional scaling ordinations on the basis of Bray-Curtis dissimilarities of a assemblages in each sandbank of log (x+1) transformed abundance data, and b biological traits modalities in each sandbank. Kish (circles), Arklow (squares), and Blackwater (triangles). White, black and grey symbols denote north, south and wind farm sites, respectively


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Av.Sim

Sim/SD

Percent. Cont.

Asterias rubens Pagurus bernhardus Pleuronectes platessa

233.1 156.5 40.0

14.6 10.2 2.2

1.7 1.84 0.75

44.21 30.77 6.53

Alcyonium digitatum Eutrigla gurnardus Inachus dorsettensis Limanda limanda

117.2 14.2 21.8 14.2

1.0 0.9 0.8 0.5

0.6 0.88 0.45 0.65

3.08 2.65 2.39 1.47

Arklow Bank Pagurus bernhardus Asterias rubens Raja montagui

24.2 9.4 3.9

19.4 4.8 1.9

1.33 0.8 0.45

59.85 14.77 5.74

7.8 1.7

1.6 1.1

0.7

0.8

Liocarcinus holsatus Blackwater Bank Pleuronectes platessa Asterias rubens Limanda limanda

5.4 4.8 1.8

Ophiothrix fragilis

5.2

A

Spisula elliptica Scyliorhinus canicula

L

Kish Bank

0.48 0.83

5.05 3.25

0.78

2.44

13.8 10.1 6.1

1.54 1.71 1.13

40.93 30.03 18.15

2.1

0.26

6.16

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Av. Abund. average abundance, Av. Sim. average contribution to overall similarity among samples, Sim/SD the ratio of the average contribution to the overall similarity among samples to the standard deviation of the average contribution to the overall similarity, Percent. Cont. percentage contribution to the overall similarity among samples

Av.Abund

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Table 4 SIMPER results for contributions from the most important taxa towards the Brayâ&#x20AC;&#x201C; Curtis dissimilarity distinguishing each of the sites (cut-off set at 90 % contribution to total similarity)

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manâ&#x20AC;&#x2122;s fingers colonies (Alcyonium digitatum). At the Arklow sandbank, P. bernhardus was the most abundant taxon. Additionally, at this sandbank, a considerable proportion of the fauna comprised the spotted ray Raja montagui, bivalves Spisula elliptica and lesser spotted dogfish Scyliorhinus canicula. The Blackwater Bank was dominated by the flat fish P. platessa and Limanda limanda, and the ophiurid Ophiothrix fragilis. Length frequencies of the three most common fish species, namely L. limanda, P. platessa and R. montagui, presented the same overall pattern with a high number of juveniles and few adults. However, important differences were measured among banks with a particularly high occurrence of small juveniles at Kish Bank. At this sandank, 100 % of L. limanda and 99 % of P. platessa were <25 cm with a majority of small juveniles (<15 cm). In contrast, on the Arklow Bank and Blackwater Bank, individual juveniles were larger, but less abundant. Similarly, only juveniles of R. montagui were recorded (<55 cm) with a high frequency of small individuals on the Kish Bank (the majority <24 cm), but also on the Arklow and Blackwater Banks. There were no evident differences in the assemblage structure between the wind farm site and controls (pairwise comparison P>0.05). The nMDS plot (Fig. 2) shows no clear separation between assemblages from the wind farm site and the controls. Assemblages differ significantly between sandbanks in terms of mean number of taxa and total

abundance, but not in Shannon diversity (Table 3; Fig. 3). Pairwise comparisons revealed significantly higher number of taxa and abundance in the Kish sandbank, compared to the Arklow and Blackwater Banks (pairwise comparison, P< 0.05). Furthermore, there was a significant variation in Shannon diversity between sites within sandbanks (Table 3; Fig. 3). Pairwise comparison did not detect any significant difference in mean number of taxa, total abundance and Shannon between the wind farm site and the controls (pairwise comparison P<0.05; Fig. 3).

Biological trait analysis BTA showed no evidence of significant variation in the functionality of the assemblages present at the different sandbanks (Table 3; Fig. 3b), although there was a significant variation between sites within sandbanks (Table 3; Fig. 3b). However, there was no significant difference between the traits exhibited by assemblages at the wind farm and control sites in the Arklow sandbank (pairwise comparison P<0.05; Fig. 3b). SIMPER analysis showed that the dominant biological traits in the assemblages were: free living habit, gonochoristic sexual differentiation, longevity over 5 years, none migratory, crawling movement method and scavenger feeding method. Figure 4 confirms this pattern, indicating little variation in the percentage in the dominant traits among sandbanks. For example, assemblages in all three sandbanks had a range of maximum relative size


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Discussion

a Number of taxa

30

20

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10

0

A

8

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6

4

2

P P R

Total abundance (log)

b

0 2.5

A

2.0

1.5

1.0

FO R

Shannon diversity (H')

c

0.5

0.0

The fish and megafaunal invertebrate assemblages recorded in the different sandbanks are commonly found around the east coast of Ireland and were similar in terms of composition and abundance to those identified by previous studies (Ellis et al. 2000; Kaiser et al. 2004; Mackie et al. 1995; Roche et al. 2007). There was large spatial variation in assemblages, at both sandbank and site scale. Additionally, there was a north to south gradient in total abundance and number of taxa of the assemblages associated with the three banks. These variations may be explained by changes in the substrate. Previous studies have shown a large variability in the granulometry and organic content of the sediment among these sandbanks, ranging from gravel to very fine sand with relatively low organic content (Roche et al. 2007; Mackie et al. 1995). For example, the sediment of Arklow Bank consists predominantly of sand, cobbles, shells and pebbles on the northern end tending towards fine sand at the southern end (Fehily & Timoney & Co 2001). The Blackwater Bank substratum comprises fine sand with very low organic carbon, and supports a relatively impoverish community. The Kish sandbank, where higher abundance and number of taxa were recorded, is characterised by medium size sand and gravelly sediment (Wheeler et al. 2001). The characteristics of sedimentary environments are closely related to the prevailing oceanographic conditions. All three banks surveyed are subjected to high velocity tidal currents, which can reach 2 msâ&#x2C6;&#x2019;1. This is particularly evident at the Blackwater Bank, where hydrodynamic conditions are a considerable source of physical stress for organisms and are unfavourable for the accumulation of organic matter. Association between substrate characteristics and the structure of megafaunal communities has been previously described for the Irish Sea, where communities are considered to be regulated by physical, rather than biological, processes (Mackie et al. 1995; Ellis et al. 2000). BTA acts as a proxy for the ecological functions delivered by the assemblages inhabiting sandbanks. The most frequent traits (e.g. free living habit, crawling movement, scavenger feeding method, longevity over 5 years) are common to most of the dominant taxa (both invertebrates and fish) found at the different sites. These traits have been linked to key aspects of functioning of these habitats, including: energy and nutrients cycling, food and propagule supply/exports and adult and juvenile migration/emigration (Frid et al. 2008; Marchini et al. 2008; Neumann and Kroencke 2011). Despite the structural changes of the assemblages among sandbanks, there were no differences in the biological traits exhibited, indicating no changes of the ecosystems processes across these assemblages. This suggests a redundancy in the traits exhibited by the dominant organisms and in their involvement in the maintenance of

Kish

Arklow

Blackwater

Fig. 3 Mean (+ 1SE) a number of taxa, b total abundance and c Shannon diversity of fish and invertebrates on trawls at each site within the Kish, Arklow, and Blackwater Banks (n03). Black, white and grey bars denote north, south and wind farm sites, respectively

categories. Similarly, all reproductive methods were represented in assemblages from all three sandbanks; however, asexual reproduction was the less common category. In terms of sociability, assemblages were mainly characterised by solitary and gregarious organisms, with a small proportion being colonial (Fig. 4).


P P R

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A

L

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Fig. 4 Mean percentage of categories of trait variables maximum relative size, longevity, reproductive method, movement method, feeding method, sexual differentiation, sociability, migration and living

FO R

ecosystem processes. This functional redundancy may be an indirect measure of the resistance or compensation potential within the assemblages to changes in their structure (Clarke and Warwick 1998; Frost et al. 1995). While environmental conditions broadly limit the range of possible trait values in an assemblage, a large proportion of common traits can be present within a single location or community (Richardson et al. 2012; Westoby and Wright 2006). A consequence of high local trait diversity is that turnover in functional traits among communities can be low and substantially less than the turnover of species diversity (de Bello et al. 2009). However, it is important to consider that only the most dominant taxa (26 in total) were included in the BTA analysis. Presumably, sandbank assemblages may have higher trait diversity because rare species often have distinctive or atypical trait values, making a large contribution to the breadth of traits in a community, despite their small contribution to community biomass (Richardson et al. 2012). The high abundance of juvenile plaice, dab and spotted rays in the study area suggests the presence of nursery areas.

habitat for each site within the Kish, Arklow, and Blackwater sandbanks. N north, S south, W wind farm

Plaice have discrete spawning grounds (Dunn and Pawson 2002), low fecundity and slow growth rates (Nash et al. 2000). Juvenile plaice utilise demersal habitats in sandy beach areas as nursery grounds and may spend up to 2 years in these grounds before joining the parent stock. These areas, typified by high food provision and low abundance of predators, form a vital habitat for the young plaice and year class strength is determined during this phase of the life cycle (Nash and Geffen 2000). Furthermore, the reproductive potential of most elasmobranchs, including the spotted ray, is known to be lower than in most teleost fish, a factor that increases their vulnerability to anthropogenic disturbances such as commercial fishing or habitat degradation. Indeed, many large demersal elasmobranch species, including the spurdog (Squalus acanthias) and thornback ray (Raja clavata), both of which were historically significant to the fisheries in the study area, have been seriously depleted. Some, including the common skate (Dipturis batis), have disappeared from the Irish Sea (Brander 1981; Ellis et al. 2005). Therefore, the large proportion of juvenile spotted


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The results of the present study provide baseline information on the invertebrate and fish communities inhabiting these environments against which potential impacts of future wind farms development can be tested. This is of particular relevance since further turbines and associated cable routes are planned for these sandbanks. Because of the natural spatial variation in the structure of the assemblages among and within sandbanks shown here, and the much larger scale of wind farms to be installed, it is uncertain how they may impact the local fauna. This highlights the importance of long-term environmental monitoring during all phases of the development of wind farms (i.e. pre-construction, construction and operational phases). If baseline data are available before the construction of a wind farm, such as that provided by this study, an appropriate sampling design will allow effective identification of any environmental impact, through measuring changes at the impacted site from before to after the construction as opposed to several unaffected control areas (Underwood 1992). If all the necessary precaution measurements are taken to minimise the impact during all phases of the development of offshore wind farms, together with the closure to all other activities (e.g. fishing) inside the wind farm area (i.e. the creation of no-take zones), this renewal energy industry could present an opportunity for the conservation of the marine environment through creating marine protected areas.

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ray, dab and plaice found highlights the potential sensitivity of the study site to future development. The importance of the site to elasmobranches should be determined prior to any further planning or development. The presence of the wind turbines was not associated with alterations in the diversity and structure of the megafaunal invertebrate and demersal fish assemblages. None of the response variables examined (total abundance, number of taxa, Shannon diversity, assemblage structure or biological traits) showed significant variation due to the presence of the wind turbines. It has been suggested that offshore wind farms can cause alterations in the diversity and structure of communities in the vicinity of the turbines (Wilhelmsson et al. 2006). These potential alterations and their implications will depend largely on the specific local conditions and also on the distance of the turbines to other reef or hard substrate communities (Petersen and Malm 2006). The systems examined in the presented study were high energy sandbanks, subject to intense disturbance from tidal and marine currents that cause erosion, scouring and abrasion of the megafaunal communities. Thus, any potential impact associated with the construction and decommissioning of wind farms, such as temporary habitat loss, nutrient re-suspension, burial and changes in hydrodynamics caused by the presence of the turbines is likely to be obscured by the natural disturbance to which this environment is subject. Furthermore, the distance of sampling from the turbines may have been too great to detect any potential impacts acting at a smaller spatial scale. The study itself took place during the operational phase, 7 years after construction in 2000. Thus, any impact which had occurred may no longer be evident. Only a few studies have investigated the impacts of offshore wind turbines on the diversity and structure of megafaunal invertebrate and fish communities. Wilhelmsson et al. (2006) studied the influence of an offshore wind farm in the Baltic Sea at a micro- (on the turbines themselves) and mesoscale (0–20 m from the turbines). Differences in fish community structure, greater abundance and lower diversity in the vicinity and surrounding of the turbines than in areas 20 m from the turbines were identified (Wilhelmsson et al. 2006). They suggested the turbines act as artificial reefs, where the submerged parts were primarily covered by blue mussels and barnacles (Wilhelmsson et al. 2006). This study focused at the macroscale (i.e. the whole wind farm), rather than the microor mesoscale; and an artificial reef effect around the wind turbines was not detected. The lack of differences detected in the present study suggests that any effect of the wind turbines may have on local fauna is restricted to within meters around the structures themselves. This may be only true for high energy environments, such as the sandbank examined here, so any extrapolation of these results to other study systems must be done with caution.

Acknowledgments This project (Grant-aid Agreement No. RV/ Bright Sparks-07–08) was carried out with the support of the Marine Institute’s under the Sea Change Strategy and the Strategy for Science, Technology and Innovation (2006–2013), funded under the National Development Plan 2007–2013. Thanks to Tasman Crowe for help with the experimental design and helpful comments on the manuscript. Thanks to Julie Bremner for kindly providing information for the BTA analyses.

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Diversity of demersal and megafaunal assemblages inhabiting sandbanks of the Irish Sea  

Diversity of demersal and megafaunal assemblamblages

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