Temporal variability of a benthic food web: patterns and processes in a low-diversity system

Trophic interactions are not static links, but are dynamic both in space and time. In temperate environments, seasonal patterns in primary production may be pronounced and, hence, the zoobenthic community and the trophic couplings within it change during the main production season, We assessed temporal variability in a benthic food web using stable isotope ratios of C and N. Organisms belonging to 3 trophic levels, producers (e.g. macroalgae and vascular plants), primary consumers (macrozoobenthos) and secondary consumers (epibenthic fishes and Crangon crangon), were sampled monthly in a shallow soft-substrate habitat in the northern Baltic Sea. The study focused on intra-seasonal (main production period from June to September 2005) and inter-annual (comparing data collected in June of 2 consecutive years) variability of trophic interactions involving phyto- and zoobenthos. Intra-seasonal changes in stable isotope ratios of food web components occurred both on a horizontal (within a trophic level) and a vertical scale (between trophic levels). Generally, unimodal patterns were evident, with enrichment of C-13 and depletion of N-15 occurring during the production period. The fluctuation in stable isotope values over the season decreased with increasing trophic level. Predator stable isotope values are probably affected by changes at the base of the food web and changes in consumer diet. The intra-seasonal pattern, together with limited interannual variability, indicates a temporally stable food web structure.