Identifying Bottlenecks to Energy Circulation in the Bay of Biscay Pelagic Food Web: Key Species Under the Spotlight

Abstract

Understanding the functioning and resilience of marine ecosystems requires identifying the main energy flow pathways. Key trophic groups occupy strategic positions in the trophic interactions network, acting as hubs that control the energy distribution across the ecosystem. This study examines the Bay of Biscay's pelagic food web using stable isotope analysis with stomach content data, creating a network of 38 trophic groups and 125 interactions. Both annual-weighted and seasonal (spring and late summer) networks were constructed. The analysis of unweighted and weighted annual networks found that low-trophic level epipelagic fish (European anchovy, Engraulis encrasicolus; sardine, Sardina pilchardus; and sprat, Sprattus sprattus) is a key trophic group displaying higher scores in many centrality indices. These forage fish play a central role in facilitating energy transfer across trophic levels, thus representing a critical link between the planktonic food web and higher trophic level predators and fisheries. Overall, annual networks showed that phytoplankton-dominated grazing chains support a higher diversity of predators compared to chains originating from particulate organic matter (POM). The analysis of weighted networks accounting for seasonal variations in trophic interactions revealed that, during late summer, predators occupy more vulnerable positions than in spring. Changes in feeding preferences cause blue whiting to shift from mostly depending on grazing chains during spring to occupying a position along POM-dominated chains in late summer. These findings highlight the need for fisheries management strategies to prioritize the conservation of key trophic groups supplying energy to predators while considering seasonal shifts in the structure of the energy flow network.