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Marine Ecology Progress Series

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MEPS 369:63-76 (2008)  -  DOI:

Trophic interactions in Zostera marina beds along the Swedish coast

Therese Jephson1,*, Per Nyström2, Per-Olav Moksnes3, Susanne P. Baden4

1Department of Ecology, Marine Biology, Campus Helsingborg, Lund University, Box 882, 25108 Helsingborg, Sweden
2Department of Ecology, Limnology, Ecology Building, Lund University, 22362 Lund, Sweden
3Department of Marine Ecology, University of Gothenburg, Box 461, 40530 Göteborg, Sweden
4Department of Marine Ecology, University of Gothenburg, The Sven Loven Centre for Marine Sciences—Kristineberg,
45034 Fiskebäckskil, Sweden

ABSTRACT: We compared eelgrass Zostera marina communities in 3 regions in Sweden believed to be affected by eutrophication and overfishing, to determine whether bottom-up or top-down processes control the biomass of epiphytic macroalgae and grazers. Nitrogen and carbon isotope signatures were analyzed to explore the food webs and to identify the grazing species feeding on filamentous algae and/or eelgrass. Mixing model (IsoSource version 1.3.1) analysis of the isotope signatures indicated that the amphipods Gammarus locusta and Microdeutopus gryllotalpa fed primarily on filamentous algae and that only 2 small gastropod species consumed eelgrass. Moreover, the grass shrimp Palaemon elegans and P. adspersus were ca. 1 trophic level above amphipods and algae, but according to the mixing model played different trophic roles in the different areas. The highest biomass of filamentous algae was found in the west coast beds housing grazers with the lowest biomass and mean size (predominantly G. locusta and M. gryllotalpa, 0.5 to 3 mm). In contrast, the Baltic Sea beds had low algal biomass, but the grazers (mostly G. locusta and Idotea baltica) had higher biomass and were significantly larger (mean size ca. 10 mm). An overall negative correlation was found between grazer biomass and biomass of filamentous algae. The significantly smaller grazers and absence of isopod grazers on the west coast may be due to substantial consumption by small predatory fish. This supports the suggestions that, in Swedish eelgrass beds, grazers are top-down controlled, and overexploitation of large predators and eutrophication play an important role in the recent increases in algal biomass.

KEY WORDS: Seagrass · Eelgrass · Zostera · Stable isotopes · Mixing model · Trophic interactions

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Cite this article as: Jephson T, Nyström P, Moksnes PO, Baden SP (2008) Trophic interactions in Zostera marina beds along the Swedish coast. Mar Ecol Prog Ser 369:63-76.

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