Modelling oligotrophic zooplankton production: seasonal oligotrophy off the Azores

One of the critical issues in large-scale physical/biological coupled models is the survival of zooplankton in a water column circulating an anticyclonic gyre. Survival is most at risk in regions where the phytoplankton food supply is low due to environmental stress by light-limitation (deep mixing in winter) or nutrient limitation (oligotrophy). To investigate this problem we simulated the ecosystem in a 1 m2 cross-section water column, using the Lagrangian Ensemble method in which plankton are treated as particles following independent trajectories through the changing environment. In this first part of a two-part article we report the results of simulating the ecosystem in a water column located off the Azores, where winter mixing reaches 200 m and there is seasonal, but not permanent oligotrophy. The model features diatoms and herbivorous copepods subject to carnivorous predation, with remineralization of carbon and nitrogen by bacteria attached to detritus and faecal pellets. The copepods become extinct after failing to reproduce in years of low food supply. We show that the risk of extinction can be reduced by allowing cannibalism or by reducing carnivorous predation; we discuss other possibilities: enhancing the food supply by adding new guilds of phytoplankton, and relaxing oligotrophy by allowing other sources of nitrogen injection into the euphotic zone