Changes in food web structure of fish assemblages along a river-to-ocean transect of a coastal subtropical system

There is an urgent need to understand how food web structure changes along environmental gradients. In this study we investigated changes in trophic organisation and the relative importance of autotrophic sources sustaining fish assemblages along a transect from river to ocean. In order to address these topics, we analysed fish stomach contents and isotopic composition of consumers and food sources. The findings revealed a greater number of autotrophic sources sustaining fish assemblages in the continental systems (especially in the river) than in the adjacent marine system. Bipartite networks depicting trophic relationships between fish and prey also changed along the transect, showing comparatively higher complexity in the estuary. These findings could be explained by the greater number of food web components (autotrophic sources, fish trophic guilds and prey) associated with pelagic and benthic food chains within the estuary compared with the adjacent systems studied. The findings of this study highlight the need to take into account river-to-ocean changes in food web structure of fish assemblages in management plans to mitigate human impacts in coastal systems.