Effects of two decades of rising sea surface temperatures on sublittoral macrobenthos communities in Northern Ireland, UK

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Abstract

We examined whether two decades of rising sea surface temperatures have resulted in significant changes in the benthic community and frequency of occurrence of Northern and Southern species in three areas of Northern Ireland, using visual census data collected by SCUBA surveys undertaken during two periods: pre-1986 and post-2006. We found little evidence to suggest that rising sea surface temperatures have contributed to the changes in benthic assemblage structure between the pre-1986 and post-2006 surveys. However, there were slight but not significant declines in extreme Northern species at Rathlin Island, and increases in the mean number and frequency of occurrence of extreme Southern species in all three areas. There were also substantial declines in the spatial presence of 7 extreme Northern species and notable increases in distribution of 19 extreme Southern species. In contrast, there were no clear trends in the intermediate to Northern and intermediate to Southern species. These results suggest that rising sea surface temperatures have had significant impacts on the occurrence of rarer marine invertebrate species at the edges of their biogeographic range however the trends differed between areas in Northern Ireland.

Highlights

► We examine the effects of climate change on the marine benthos of Northern Ireland. ► We compared two SCUBA surveys: pre-1986 and post-2006. ► Significant effects for species with extreme southern/northern distributions. ► No significant effect on species with other distributions/community composition. ► Effects varied regionally within Northern Ireland.

Introduction

The earth's climate has warmed by approximately 0.6 °C over the past 100 years (Hansen et al., 2006; Walther et al., 2002), largely as a result of human activity (Bindoff et al., 2007). Much of this energy has been absorbed by the oceans (Hoegh-Guldberg and Bruno, 2010) and in the Northern Hemisphere averaged sea-surface temperatures have increased by approximately 0.6 °C ± 0.2 °C in the last 70–100 years with the most rapid increases occurring in the last twenty years (Hawkins et al., 2003; Hulme et al., 2002).

The ecological impacts of global climate change have been recorded in a wide variety of marine ecosystems (Hoegh-Guldberg and Bruno, 2010; Parmesan and Yohe, 2003; Walther et al., 2002). Rising temperature can influence physiological processes, gene expression, body form, behaviour, and the phenology of species and individuals (Angilletta, 2009; Durant et al., 2007; Portner et al., 2006; Somero, 2010), disrupt competitive interactions (Poloczanska et al., 2009), alter food webs (Philippart et al., 2003) and result in major shifts in the structure and function of communities, e.g. from algal to grazer dominated ecosystems (Hawkins et al., 2009; Wernberg et al., 2011a).

The vulnerability of species to rising sea surface temperatures will be influenced by a range of factors including their ontogenetic stage, and physiological and genetic adaptability (Brierley and Kingsford, 2009; Harley et al., 2006; Somero, 2010). The most vulnerable species are likely to be warm-adapted eurytherms living near the extent of their thermal range and extreme cold-adapted stenothermal species because, for these groups, any increase in the maximum temperature experienced is likely to be lethal (Somero, 2010). Sedentary benthic stenotherms such as corals for which the rate of local change may be greater than biological or evolutionary response times, are also particularly vulnerable to rising sea surface temperatures (Hoegh-Guldberg et al., 2008). In contrast, mobile species are likely to respond to rising sea surface temperatures by shifting their distribution ranges (Walther et al., 2002) and warming is expected to lead to the poleward movement of these species (Philippart et al., 2011).

Studies have demonstrated that rising sea surface temperatures have induced changes in marine assemblages around the UK, including plankton (Beaugrand et al., 2002; Southward et al., 1995), fish (Genner et al., 2004; Graham and Harrod, 2009; Perry et al., 2005; Stebbing et al., 2002), algae (Hiscock and Maggs, 1984) and intertidal marine invertebrates (Hawkins et al., 2008; Mieszkowska et al., 2005). However, there have been limited studies on the effects on the subtidal macrobenthos, mainly because of the lack long-term and broad spatial scale datasets (Birchenough and Bremner, 2010; Halpern and Cottenie, 2007; Hawkins et al., 2003; Richardson and Poloczanska, 2008). Where long-term benthic data are available, there is evidence to suggest that rising sea surface temperatures have contributed to changes in the abundances of benthic invertebrates in the North Sea (Frid et al., 2009) and benthic macrofauna in the English Channel (Hinz et al., 2011). However studies on other subtidal organisms and in other areas of the UK are lacking.

Sea surface temperatures around the UK are projected to increase between 1.5 and 4 °C by 2098 (Lowe et al., 2009). However, the rate of warming will be highly variable between regions, with effects being more pronounced in shallower areas such as the English Channel, Irish Sea and the southern North Sea (Belkin, 2009; Hulme et al., 2002; Lowe et al., 2009; Philippart et al., 2011). Northern Ireland is at the interface of a biogeographic boundary between the cold Arctic-Boreal waters from the North and the warmer Lusitanean waters from the South (Forbes and Godwin-Austen, 1859; Hiscock, 1998; Hiscock et al., 2004). This region has experienced a warming trend averaging about 0.3–0.5 °C between 1850 and 2007 (Cannaby and Husrevoglu, 2009; Young and Holt, 2007; Fig. 1); the strongest warming between 1982 and 2007 has occurred since 1994, the warmest years were 2005, 2006, and 2007 (Cannaby and Husrevoglu, 2009; Young and Holt, 2007; Fig. 1). Several benthic species found in this region are at the edges of their northern or southern distributional range (Forbes and Godwin-Austen, 1859; Goodwin and Picton, 2007; Goodwin et al., 2011a; Hiscock et al., 2004). Therefore Northern Ireland is an ideal region for studying the effects of rising sea temperatures on the subtidal macrobenthos communities.

Between 1982 and 1986 (hereafter referred to as pre-1986) SCUBA diving surveys (Northern Ireland Sublittoral Survey) were conducted in 201 sites in 5 main areas, Rathlin Island, Skerries, Belfast Lough, Strangford Lough and Carlingford Lough located around the coast of Northern Ireland (Erwin et al., 1986, 1990). The overall objective of these surveys was to identify and describe the benthic community in relation to the substratum types and environmental parameters (Erwin et al., 1986, 1990). Between 2006 and 2009 (hereafter referred to as post-2006) 139 sites in 3 main areas, Rathlin Island, Skerries and Strangford Lough were resurveyed (Sublittoral Survey of Northern Ireland SSNI) using similar methodology (Goodwin et al., 2011a) to examine any long-term and broad-scale patterns of change in the subtidal macrobenthos communities. We pooled these data into 2 time periods (pre-1986 and post-2006) as a previous study demonstrated there was very little variability between years within the survey periods (Strain et al., 2012).

Sea surface temperatures have not been consistently recorded in at the 5 survey areas in Northern Ireland between 1986 and 2009 however we collated data from Malin Head, Ireland located 50 km west north west of the Skerries and 80 km west north west of Rathlin Island in the Northern Atlantic Ocean and Port Erin, Isle of Man located 55 km east-south-east of the mouth of Strangford in the Irish Sea and these data show a strong trend of warming (Fig. 1). We formulated the following specific hypotheses to test whether rising sea surface temperatures have directly altered the subtidal macrobenthos community in Northern Ireland using presence/absence data collected in the pre-1986 and post-2006 surveys:

Hypothesis 1

There will be significant differences in Northern Ireland subtidal assemblage structure between the pre-1986 and post-2006 surveys which are driven by changes in the frequency of occurrence of species near the Northern and Southern edge of their distribution.

Hypothesis 2

There will be significant declines in the mean number and frequency of occurrence of species near the Northern and Southern edge of their distribution in Northern Ireland between the pre-1986 and post-2006 surveys.

Hypothesis 3

There will be significant increases in the mean number and frequency of occurrence of species near the Northern and Southern edge of their distribution in Northern Ireland between the pre-1986 and post-2006 surveys.

Section snippets

Study areas

The 3 study areas, Rathlin Island, Skerries and Strangford Lough chosen in the post-2006 surveys are highly representative of Northern Ireland's diverse flora and fauna and cover the entire biogeographic and temperature range of the region (Erwin et al., 1986, 1990) (Fig. 2). All 3 areas are subject to extremely strong tidal streams and are classified as moderately to strongly exposed to waves (Erwin et al., 1986, 1990). Rathlin Island is the most northerly area (Fig. 2, Appendix 1a). It is a

Benthic assemblage structure

There were significant differences in benthic community between the pre-1986 and post-2006 surveys at all three areas (Rathlin Island T = 3.00, P = 0.001, Skerries T = 2.220, P = 0.001, and Strangford Lough T = 2.460, P = 0.001, Bonferroni adjusted α = 0.020). The nMDS plot clearly showed that the benthic assemblage structure had changed between the pre-1986 and post-2006 surveys (Fig. 3). It also demonstrated that Strangford Lough had become more similar to the Skerries and Rathlin Island in

Effects of rising sea surface temperatures on the frequency of occurrence of subtidal macrofauna

Long-term datasets are important for assessing the impacts of rising sea surface temperatures on shallow (>100 m depth) subtidal communities both in the UK and elsewhere (Birchenough and Bremner, 2010; Callaway et al., 2007; Genner et al., 2004; Hawkins et al., 2003; Hinz et al., 2011; Hiscock et al., 2004; Wernberg et al., 2011b). In this study we found very little evidence to suggest that rising temperatures have contributed to overall changes in the subtidal assemblage structure along the

Acknowledgements

Collection of the recent data and analysis was funded by Northern Ireland Environment Agency under the Centre for Environmental Data and Recording (CEDaR) partnership initiatives Sublittoral Survey Northern Ireland and Nationally Important Marine Features. We thank Julia Nunn (CEDaR) for facilitating access to the data and Mark Wright, Damian McFerran, Peter Crowther and Marianne McKeown for their support during these projects. Many people were involved with the SCUBA diving fieldwork above and

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  • Cited by (5)

    1

    Current address: Marine Division, Department of the Environment, 17 — 25 Great Victoria Street, Belfast BT2 7BN, United Kingdom.

    2

    Research associate: Queen's University Marine Laboratory (QML), 12-13 The Strand, Portaferry, Co. Down, BT22 1PF, United Kingdom.

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