C, N and P stoichiometric mismatch between resources and consumers influence the dynamics of a marine microbial food web model and its response to atmospheric N and P inputs

Results from the DUNE experiments reported in this issue have shown that nutrient input from dust deposition in large mesocosms deployed in the western Mediterranean induced a response of the microbial food web, with an increase of primary production rates (PP), bacterial respiration rates (BR), as well as autotrophic and heterotrophic biomasses. Additionally, it was found that nutrient inputs strengthened the net heterotrophy of the system, with NPP : BR ratios < 1. In this study we used a simple microbial food web model, inspired from previous modelling studies, to explore how C, N and P stoichiometric mismatch between producers and consumers along the food chain can influence the dynamics and the trophic status of the ecosystem. Attention was paid to the mechanisms involved in the balance between net autotrophy vs. net heterotrophy. Although the model was kept simple, predicted changes in biomass and PP were qualitatively consistent with observations from DUNE experiments. Additionally, the model shed light on how ecological stoichiometric mismatch between producers and consumers can control food web dynamics and drive the system toward net heterotrophy. In the model, net heterotrophy was notably driven by the parameterisation of the production and excretion of extra DOC from phytoplankton under nutrient-limited conditions. This mechanism yielded to high C : P and C : N ratios of the DOM pool, and subsequent postabsorptive respiration of C by bacteria. The model also predicted that nutrient inputs from dust strengthened the net heterotrophy of the system; a pattern also observed during two of the three DUNE experiments (P and Q). However, the model was not able to account for the low NPP : BR ratios (down to 0.1) recorded during the DUNE experiments. Possible mechanisms involved in this discrepancy were discussed.