Plankton food webs of the Gulf of Mexico spawning grounds of Atlantic Bluefin tuna

Abstract

We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic Bluefin Tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show that the food web is dominated by the microbial loop (>80% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). In contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process <4% of net primary production in the mixed layer. Nevertheless, ABT larvae feed preferentially on calanoid copepods and other suspension-feeding zooplankton that in turn derive much of their nutrition from diatoms and mixotrophic flagellates. This allows ABT larvae to maintain a comparatively low trophic level ([~]4.0 for pre-flexion larvae; [~]4.2 for post-flexion larvae) that increases trophic transfer from phytoplankton to larval fish.