Flood facilitates cross‐ecosystem trophic subsidies to marine animals

Abstract

Trophic subsidies are resources that cross ecosystem boundaries, influencing the structure and function of recipient food webs. Widespread regulation of rivers for human consumptive use has profoundly altered natural flow regimes, decoupling donor‐recipient dynamics between freshwater and marine ecosystems. We explored trophic subsidies to the Southern Indian Ocean from a highly regulated river during a 1 in 66‐year flood by comparing δ ¹⁵ N, δ ¹³ C and δ ³⁴ S values of three marine trophic guilds sampled across the flood plume and at geographically isolated reference locations. Secondary and tertiary consumers collected within the flood plume were δ ¹⁵ N enriched compared to reference animals and isotopic niche overlap declined across higher trophic levels (24% and 6% respectively). For both yellow‐eye mullet ( Aldrichetta forsterii —a secondary consumer) and Australasian snapper ( Chrysophrys auratus —a tertiary consumer), there was no overlap in isotopic niche between flood plume and reference animals. Terrestrial contributions to marine animal biomass in the flood plume varied by trophic level and subsidies were highest among secondary consumers (35% compared to 22% for reference animals). Terrestrial contributions to tertiary consumers (29%) did not significantly diverge from reference estimates (21%), as evidenced by overlapping 95% credible intervals. This modest difference likely stems from a temporal mismatch between the flood peak and slower isotopic turnover rates among large predators. Our results emphasize that dietary integration reflected in secondary consumers provided the most reliable subsidy estimates due to exposure duration and representative body mass. Here, we demonstrate significant trophic subsidies resulting from the temporary re‐establishment of river‐marine coupling in a highly regulated river. Our findings provide quantitative empirical support for trophic ecological theory and underpin the importance of managing freshwater‐marine linkages in the face of anthropogenic pressures.