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

Abstract

<p><strong>Abstract.</strong> 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.</p>

Introduction to project DUNE, a DUst experiment in a low Nutrient, low chlorophyll Ecosystem

“…This contribution was triggered by the strength of the stoichiometric mismatch between the resource (dissolved organic matter) and bacteria. Although the model was kept simple, it highlights how stoichiometric mismatch between producers and consumers can influence the response of a planktonic food web to nutrient addition (Pondaven et al, 2014).…”

“…In parallel, we used a stoichiometric microbial food web model to investigate how ecological stoichiometric mismatches within the food web -that is, resources and consumers have distinct elemental composition -result in differential consumer-driven nutrient recycling (CNR; Sterner and Elser, 2002) and how, in turn, CNR feedbacks on the magnitude of the ecosystem respond to the addition of nutrients from dust (Pondaven et al, 2014). This model was used as a theoretical tool; that is, it was not optimized to fit observations from DUNE experiments.…”

Introduction to the project DUNE, a DUst experiment in a low Nutrient, low chlorophyll Ecosystem

“…We used a stoichiometric microbial food web model to investigate how the stoichiometry of consumer driven nutrient recycling (Sterner, 1990) may influence the ecosystem response to N and P dust addition. This model considers simultaneously the differential recycling of C, N and P in a food chain which includes 7 compartments (bacteria, autotrophic pico and nanoplankton, heterotrophic flagellates, ciliates and mesozooplankton) (Pondaven et al, 2013). Growth rates of bacteria and phytoplankton were controlled by the availability of dissolved inorganic and/or organic N, P and C. The model results showed that bacteria and zooplankton had substantial effects on the supply rate and the stoichiometry of N and P in the dissolved pools, thereby exerting a control Figures on the predicted primary production.…”