Upwelling-like bottom intrusion enhances the pelagic–benthic coupling by a fish predator in a coastal food web

Abstract

Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ13C and δ15N for a demersal fish predator, Acropoma japonicum. This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ13C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ13C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.