Plasticity of trophic interactions in fish assemblages results in temporal stability of benthic-pelagic couplings.

This study addresses the temporal variability of couplings between pelagic and benthic habitats for fish assemblages at five periods in a shallow epicontinental sea, the Eastern English Channel (EEC). Organic matter fluxes fueling fish assemblages and the relative contribution of their different sources were assessed using stable isotope analysis and associated isotopic functional metrics. Couplings between benthic and pelagic realms appeared to be a permanent feature in the EEC, potentially favored by shallow depth and driven by the combination of two trophic processes. First, trophic interactions exhibited plasticity and revealed resource partitioning. Second, changes in the composition of fish assemblages did not impact benthic-pelagic couplings, as most dominant species were generalists during at least one time period, allowing complete use of available resources. Examining both unweighted and biomass-weighted indices was complementary and permitted a better understanding of trophic interactions and energy fluxes.

Seasonal and ontogenetic variation of whiting diet in the Eastern English Channel and the Southern North Sea.

An accurate description of trophic interactions is crucial to understand ecosystem functioning and sustainably manage marine ecosystems exploitation. Carbon and nitrogen stable isotopes were coupled with stomach content analyses to investigate whiting (Merlangius merlangus, Linnaeus, 1758) feeding behavior in the Eastern English Channel and Southern North Sea. Whiting juveniles and adults were sampled in autumn and winter to investigate both ontogenetic and seasonal changes. In addition, queen scallops (Aequipecten opercularis) samples were collected along with fish to be used as isotopic benthic baseline. Results indicated an ontogenetic diet change from crustaceans to fish and cephalopods. In autumn, δ15N values generally increased with fish size while in winter, a decrease of δ15N values with fish size was observed, as a potential result of spatial variation in baseline δ15N values. In winter, a nutrient-poor period, an increase in feeding intensity was observed, especially on the copepod Temora longicornis. This study provides further insights into whiting trophic ecology in relation to ontogenetic and seasonal variations, and it confirms the importance of combining several trophic analysis methods to understand ecosystem functioning.