Community dynamics of intertidal soft-bottom mussel beds over two decades

Helgol Mar Res (2008) 62:23–36 DOI 10.1007/s10152-007-0099-y ORIGINAL ARTICLE Community dynamics of intertidal soft-bottom mussel beds over two decades Heike Büttger Æ Harald Asmus Æ Ragnhild Asmus Æ Christian Buschbaum Æ Sabine Dittmann Æ Georg Nehls Received: 1 August 2007 / Revised: 30 November 2007 / Accepted: 4 December 2007 / Published online: 10 January 2008 Ó Springer-Verlag and AWI 2007 Abstract Macrozoobenthos communities in the North Sea showed pronounced changes over the past decade in relation to an increasing number of invasive species and climate change. We analysed data sets spanning 22 years on abundance, biomass and species composition of intertidal soft bottom mussel beds near the island of Sylt (German Bight) in the Northern Wadden Sea, based on surveys from 1983/ 1984, 1990, 1993 and from 1999 to 2005. Mussel bed area and blue mussel biomass decreased, and a change in the dominance structure in the associated community comparing 1984 to mid-1990s with the period from 1999 to 2005 was observed. Coverage of the mussel beds with the algae Fucus vesiculosus decreased since the end of the 1990s. Within the study period biomass and densities of the associated community increased significantly. Dominance structure changed mainly because of increasing abundances of associated epibenthic taxa. Apart from the Pacific oyster Crassostrea gigas all other alien species were already present in the area during the study period. Community changes already started before Pacific oysters became abundant. An attempt is made to evaluate effects on the observed changes of decreasing mussel biomass, ageing of mussel beds, decreasing fucoid coverage and increasing abundances of Communicated by J. van Beusekom. H. Büttger (&)
G. Nehls BioConsult SH, Brinckmannstr. 31, 25813 Husum, Germany e-mail: H. Asmus
R. Asmus
C. Buschbaum Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, 25992 List/Sylt, Germany S. Dittmann School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia invader. All four factors are assumed to contribute to changing community structure of intertidal mussel beds. Keywords Mytilus edulis
Crassostrea gigas
Ecosystem engineers
Mussel bed community
Macrozoobenthos
Wadden Sea Introduction Worldwide, coastal systems have undergone dramatic changes in recent decades, in particular due to eutrophication, climate change and introduced species (Valiela 2005; Philippart et al. 2007). Eutrophication often increases primary production and climate variations may enhance or depress recruitment success (Hagberg et al. 2004; Philippart et al. 2007). Global warming is expected to accelerate the establishment of non-indigenous species in North Sea waters (Stachowicz et al. 2002; Diederich et al. 2005). Surveys in the Wadden Sea and adjacent North Sea documented major changes of macrobenthic communities in the past decades (Reise 1982, Beukema 1991, 1992). Kröncke et al. (2001) described how climate oscillations influence the occurrence and density of subtidal macrozoobenthos in the southern North Sea, with significant increases in species number, abundance and biomass after mild winters. Franke and Gutow (2004) and Reichert and Buchholz (2006) documented changes of the inter- and subtidal hard-bottom macrozoobenthos community at Helgoland since 1984, recording losses of species as well as the immigration of southern species. Invasive species contribute significantly to community changes in the Wadden Sea (Nehring and Leuchs 1999; Reise et al. 2005). The introduced Pacific oyster (Crassostrea gigas) and the American slipper limpet (Crepidula 123 24 Helgol Mar Res (2008) 62:23–36 fornicata) have become abundant on blue mussel beds (Mytilus edulis) in the entire Wadden Sea (Thieltges et al. 2003; Diederich 2005; Smaal et al. 2005), particularly in the past 5 years. An increasing number of former mussel beds are now overgrown by oysters forming massive reefs (Diederich 2005, Nehls and Büttger 2007). At the same time, mussel bed area has decreased strongly, because of a lack of recruitment in some areas (de Vlas et al. 2005, Nehls et al. 2006). Mussel beds are complex entities that provide refuge and suitable habitat for an array of associated organisms (e.g. Suchanek 1980; Tsuchiya and Nishihira 1985, 1986). Blue mussel beds on soft bottoms can be classified as autogenic ecosystem engineers, being a habitat distinctly different from the surrounding tidal flats (Jones et al. 1994; Buschbaum and Saier 2001; Crooks 2002; Gutiérrez et al. 2003). Soft bottom intertidal mussel beds in the Wadden Sea are centers of high diversity (Asmus 1987; Dittmann 1990; Albrecht and Reise 1994), providing substrate for attachment and refuge as well as habitat for many endobenthic species. Biomass and species richness are usually much higher than on surrounding tidal flats. The associated fauna, as well as the mussels themselves, form a rich food source for many predators, such as crabs, starfish and birds (e.g. Beukema 1991; Thiel and Dernedde 1994; Hertzler 1995). Most field studies and experiments on community structure and dynamics of mussel beds have been short Fig. 1 Study area in the Wadden Sea near the island of Sylt (German Bight). Inset shows location of mussel beds, site 1 (Uthörn Außenwatt) and site 2 (Odde Watt), in 1989 (dark grey = islands and mainland, light grey = intertidal, white = subtidal areas) term. Yet, long-term variations and gradual changes in the environment, erratic spatfalls and strong effects of storms or ice cover during winter (Nehls and Thiel 1993; Reise et al. 1994; Strasser et al. 2001) also need to be considered. In this paper, we present for the first time a long-term study analysing the community structure of intertidal mussel beds in the Wadden Sea, relying on a series of studies from 1983 to 2005. This data set allowed us to raise the following questions: (1) Did the physical structure of the mussel bed change in terms of area or mussel density and biomass? (2) Did the associated community change in terms of composition and/ or abundances? and (3) Did the community respond to structural changes or introduced species? Methods Study site All data analysed in this long-term comparison were obtained in Königshafen, a shallow tidal bay comprising about 4.5 km2 of intertidal flats near the island of Sylt in the northern Wadden Sea (south-eastern North Sea, German Bight; Fig. 1). The tides are semi-diurnal with an average range of about 2 m. Salinity remains close to 30 psu and mean water temperature is 15°C in summer and 55°66’N Denmark 500 m N Königshafen site 1 Sylt site 2 = Blue Mussel bed North Sea The Netherlands 4°49’E 123 Germany 52°93’N 9°43’E Helgol Mar Res (2008) 62:23–36 25 4°C in winter. Detailed information on hydrography, geology, sediments and biota of the area are available from Reise (1985), Austen (1994a, b) and Gätje and Reise (1 (...truncated)