Trophodynamics and functional feeding groups of North Sea fauna: a combined stable isotope and fatty acid approach

The trophodynamics of pelagic and benthic animals of the North Sea, North Atlantic shelf, were assessed using stable isotope analysis (SIA) of natural abundance carbon and nitrogen isotopes, lipid fingerprinting and compound-specific SIA (CSIA) of phospholipid-derived fatty acids (PLFAs). Zooplankton (z), epi- and supra-benthic macrofauna were collected in the Southern Bight, at the Oyster Grounds and at North Dogger, 111 km north of the Dogger Bank. The study included 22 taxonomic groups with particular reference to Mollusca (Bivalvia and Gastropoda) and Crustacea. Primary consumers (Bivalvia) were overall most N-15 enriched in the southern North Sea (6.1aEuro degrees) and more depleted in the Oyster Grounds (5.5aEuro degrees) and at North Dogger (2.8aEuro degrees) demonstrating differences in isotopic baselines for bivalve fauna between the study sites. Higher trophic levels also followed this trend. Over an annual cycle, consumers tended to exhibit N-15 depletion during spring followed by N-15 enriched signatures in autumn and winter. The observed seasonal changes of delta N-15 were more pronounced for suspension feeders and deposit feeders (dfs) than for filter feeders (ffs). The position of animals in plots of delta C-13 and delta N-15 largely concurred with the expected position according to literature-based functional feeding groups. PLFA fingerprints of groups such as z were distinct from benthic groups, e.g. benthic ffs and dfs, and predatory macrobenthos. delta C-13(PLFA) signatures indicated similarities in C-13 moiety sources that constituted delta C-13(PLFA). Although functional groups of pelagic zooplankton and (supra-) benthic animals represented phylogenetically distinct consumer groups, delta C-13(PLFA) demonstrated that both groups were supported by pelagic primary production and relied on the same macronutrients such as PLFAs. Errors related to the static categorization of small invertebrates into fixed trophic positions defined by phylogenetic groupings rather than by functional feeding groups, and information on seasonal trophodynamic variability, may have implications for the reliability of numerical marine ecosystem models.