Vol. 463: 73–87, 2012 MARINE ECOLOGY PROGRESS SERIES Published August 30 doi: 10.3354/meps09858 Mar Ecol Prog Ser Species diversity, functional complexity and rarity in Arctic fjordic versus open shelf benthic systems Maria Włodarska-Kowalczuk1,*, Paul E. Renaud2, Jan Marcin We˛sławski1, Sabine K. J. Cochrane2, Stanislav G. Denisenko3 1Institute of Oceanology PAS, Powstańców Warszawy 55, 81-712 Sopot, Poland 2Akvaplan-niva, Fram Centre for Climate and Environment, N-9296 Tromsø, Norway 3Zoological Institute RAS, Universitetskaya nab. 1, 199034 St Petersburg, Russia ABSTRACT: We present a cross-system comparison of benthic species pools, species diversity, functional complexity and rarity in 2 typical Arctic coastal systems: an open shelf marginal sea and fjords (semi-enclosed, geologically younger basins remaining under strong terrestrial influences). A total of 388 van Veen grab samples were collected in the Barents Sea and 3 west Spitsbergen fjords (divided into inner and outer basins). Thirty percent of species were recorded only in fjords, questioning the common notion of the fjordic communities being merely subsets of the offshore species pools. Inner fjords, outer fjords and the open shelf hosted communities that differed signif- icantly in terms of species composition and diversity. Species richness, examined at the scales of both the ecological zone and an individual sample, was severely depleted in inner fjordic basins. The congruence of the patterns across taxonomic groups employing different life histories (ben- thic vs. pelagic dispersal) pointed to habitat deterioration rather than dispersal barriers as a factor responsible for the fjordic diversity clines. The between-habitat differences in functional diversity were expressed in decreased functional evenness and a decline in suspension-feeding, sedentary and large tube-dwelling species in fjords, affecting infaunal habitat complexity. Species redun- dancy declined across the shelf − outer fjord − inner fjord environmental gradients, suggesting a higher resilience of shelf systems. The environmental deterioration of fjordic basins did not result in the reduction of rare species. These results should be applied to strategies for managing open shelf and fjordic habitats in the Arctic. KEY WORDS: Biodiversity · Rarity · Functional diversity · Redundancy · Benthos · Arctic Resale or republication not permitted without written consent of the publisher INTRODUCTION (2009) found a positive relationship between the local and regional species richness, while Somerfield et al. The interplay between local and regional pro- (2009) showed that at different spatial levels, local cesses has been recognized as an important driver of communities were not random subsets of re gional diversity patterns at different spatial scales (e.g. Hus- species pools, and tended to be composed of species ton 1999, Ricklefs 1987), but these issues have re - more closely related (i.e. having similar tolerances to mained relatively poorly explored in marine systems environmental stressors and shared functional traits) (Smith 2001). Witman et al. (2004) showed that, on a than if selected randomly. global scale, local species richness is strongly related Fjords are semi-enclosed marine inlets that remain to regional species pools in epifaunal benthic com- under strong terrestrial influences and are consid- munities encrusting subtidal rocks. For soft-bottom ered to be strongly dependent on regional species North Atlantic benthic communities, Renaud et al. pools of neighboring open shelf seas (Pearson 1980). *Email: [email protected] © Inter-Research 2012 · www.int-res.com 74 Mar Ecol Prog Ser 463: 73–87, 2012 They are geologically young areas (produced after 2005). Concepts of complementarity and redundancy the last glacial maximum some 17 000 years BP), and have enhanced our understanding of the importance are regarded as ecologically immature, non-steady- of biodiversity in ecosystem stability and response state systems that have been evolving over relatively (Naeem 1998, Loreau et al. 2001). Embedded in this short time scales (Syvitski et al. 1987). Two mecha- discussion is the potential role of rare species. In nisms are usually cited as possible causes of the many systems, a relatively large fraction of the taxa fjordic diversity impoverishment: (1) the barrier present are rare, occurring at few stations and/or at hypothesis points to the geomorphologic barriers that very low abundances (Gaston 1994). Little attention prevent a number of open shelf species from coloniz- has been given to rare species in marine systems ing the fjordic habitats; (2) the habitat hypothesis with respect to what proportion of the total biodiver- states that lower diversity in fjords is due to the less sity can be expected to be rare (but see Ellingsen et favorable environment relative to offshore habitats al. 2007), or how these taxa may operate in the com- (Buhl-Mortensen & Höisæter 1993). The barrier hypo- munity. The little evidence that exists, however, sug- thesis assumes that shelf seas serve as local species gests that rare species may provide the buffer of pools for fjordic biocenosis. Fjord communities are functional redundancy and offer some indication of often regarded as range extensions of shelf commu- community stability (Death 1996). Rare species can nities that are filtered by barrier or habitat filters. play a major role in driving temporal changes in In Arctic fjords, the main environmental gradients/ assemblages, for example under climatic or anthro- barriers are produced by the inflows of glacial melt- pogenic changes, because they appear to be much waters that are usually located in inner basins (Syvit- more sensitive to fluctuations in environmental con- ski et al. 1987). Meltwaters are loaded with high lev- ditions than abundant species (Benedetti-Cecchi et els of particulate mineral material, much of which al. 2008). Documenting this element of regional bio- sediments close to the glacial or glaciofluvial inflows diversity, therefore, may be important for both eco- (Syvitski et al. 1996, Zaja˛czkowski & Włodarska- logical under standing and ecosystem management. Kowalczuk 2007). In inner basins of west Spitsbergen Because of intensified impacts of climate change in fjords, the sedimentation rates in the water column Arctic regions (ACIA 2006), and the increase in can reach 2000 g m−2 d−1 on average, whereas the human activities (e.g. tourism, petroleum explo- sediment deposition at the sea bottom was estimated ration, fishing) driven by reduced ice cover, there is to be up to 6−9 cm yr−1 (Trusel et al. 2010). High sed- an urgent need for development of management and imentation results in the formation of unconsolidated monitoring strategies that take into account an sediments that are easily eroded, frequently resus- understanding of biodiversity and ecosystem func- pended and redeposited and can be disturbed by ice- tion. It is unclear whether traditional indicators of berg scouring or catastrophic gravity flows (Syvitski system status and our understanding of ecological et al. 1987, Zaja˛czkowski & Włodarska 2007). Longer response are relevant in Arctic regions (e.g. Olsen et persistence of fast ice cover and high water turbidity al. 2007). Further, it may be necessary to manage in the inner fjordic basins re duce pelagic primary shelf and fjordic regions differently, and these con- production, and organic matter that sediments to the trasts must be accounted for when developing indi- bottom is diluted by the high inorganic component. cators (taxa, indices, etc.) and target values that are As a result, the organic matter content in sediments valid in this regional context. decreases along the gradient from the open shelf to Here we present a study based on an extensive the inner fjord glacial bays (Włodarska-Kowalczuk & data set including 388 samples collected in 3 high lat- Pearson 2004, Winkelmann & Knies 2005). itude Arctic fjords (west Spitsbergen) and in the Functional diversity is based on understanding neighboring open Arctic shelf sea (Barents Sea). The what organisms do in ecosystems rather than on evo- Barents Sea is noted as having the greatest biodiver- lutionary history as reflected in taxonomic relation- sity of all Arctic marginal seas (Sirenko 2001), per- ships, and is proposed as a better determinant of the haps because of the strong influence of both Arctic ecosystem processes than the traditional measures of and Atlantic waters and the high productivity of the species diversity (Petchey & Gaston 2002). The rela- marine frontal systems. Macrobenthic communities tionships between species and functional diversity of the Barents Sea have been thoroughly described across different systems are poorly understood, and (e.g. Denisenko et al. 2003, Denisenko 2004, Coch - they are not necessarily strongly correlated. Thus, rane et al. 2009). Benthic communities of fjordic taxonomic diversity cannot serve as a universal sur- waters of the nearby Svalbard archipelago have also rogate for functional diversity (Micheli & Halpern been subject to considerable study (e.g. Włodarska- Włodarska-Kowalczuk et al.: Fjords and shelf benthic diversity 75 Kowalczuk & We˛sławski 2001, Włodarska-Kowal- management and conservation of the marine benthic czuk & Pearson 2004, Renaud et al. 2007). No attempt systems in Arctic open sea and semi-enclosed marine has been undertaken, however, to compare these 2 inlets. neighboring marine systems. In this paper we specif- ically ask: (1) Are fjords