Trophic interactions in Zostera marina beds along the Swedish coast

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.

Seasonal variation in carbon sources of ­mesograzers and small predators in an eelgrass community: stable isotope and fatty acid analyses

“…The trophic importance of microalgae in benthic macrophyte systems was confirmed in recent stable isotope studies (Créach et al 1997, Connolly et al 2005, Douglass 2008, Jaschinski et al 2008. Our results support the assumption that fresh macrophyte leaves are of minor relevance for the nutrition of mesograzers in temperate seagrass systems (Lepoint et al 2000, Moncreiff & Sullivan 2001, Douglass 2008, Jephson et al 2008. Throughout the year, the fatty acid 18:4(n-3), which is characteristic of living eelgrass (Khotimchenko 1993), was only present in negligible amounts in Idotea baltica, Littorina littorea and Rissoa membranacea.…”

“…For example, the omnivorous crustacean Palae mon spp. switched to the qualitatively inferior macroalgae if the preferred amphipods became scarce (Jephson et al 2008, Persson et al 2008. The cause of the limitation of preferred food sources can be natural prey-consumer cycles as seen in our study, seasonal nutrient limitation, overfishing, which can promote competitors via a trophic cascade, or even hypoxia as a consequence of eutrophication, which reduces the number of small invertebrates available as potential prey for omnivores (Jephson et al 2008, Fox et al 2009).…”

δ15N and δ13C reveal differences in carbon flow through estuarine benthic food webs in response to the relative availability of macroalgae and eelgrass

“…This is similar to what has been observed for small grazers in other systems. For example, Jephson et al (2008) reported a similar apparent lack of fractionation for crustacean (Gammarus locusta, Idotea baltica, and Microdeutopus gryllotalpa) and gastropod (Rissoa spp., Littorina spp., Radix baltica, and Theodoxus fluviatilis) grazers in the Baltic Sea. It seems that N fractionation in the 2 species chosen is smaller than the 3 to 4 ‰ per trophic level that is commonly accepted for most organisms (Minagawa & Wada 1984, Michener & Schell 1994, Post 2002.…”

“…We therefore modeled δ 13 C mixing in the estuarine consumers in IsoSource, since δ 13 C signatures are the most useful for identifying C flow from producers to consumers in food webs (DeNiro & Epstein 1978, Fry & Sherr 1984. This approach has been successfully used in previous studies of benthic consumer diets where consumer δ 15 N signatures did not reveal clear enrichment to any predictable food sources (Jephson et al 2008). The model calculates all possible diet source combinations using diet composition increments of 5% where the predicted and observed mixture δ 13 C signatures matched within ± 0.1 ‰.…”

Effects of Altered Offshore Food Webs on Coastal Ecosystems Emphasize the Need for Cross-Ecosystem Management

“…Cage experiments also demonstrate that the black goby indirectly increases the biomass accumulation of ephemeral algae in seagrass patches up to five times by controlling the most efficient grazers: adult ([9 mm) individuals of the amphipod Gammarus locusta . Today, gammarid and isopod mesograsers occur in very low abundances in eelgrass beds, where they were abundant in the 1980s (Jephson et al 2008;Moksnes et al 2008;Baden et al 2010). Seagrass beds decrease turbidity by stabilizing sediments, and act as nursery ground for a number of commercially important fishes in the area (such as cod; Pihl et al 2006).…”