Modelling the response of microbial food web to an increase of atmospheric CO 2 partial pressure in a marine Mediterranean coastal ecosystem (Brusc Lagoon, France)

Abstract

This article deals with the influence of an atmospheric pCO 2 increase on the phytoplankton and microbial food web studied with mesocosms experiments and mathematical models. Experiments used three mesocosms: T350 and T700 were enriched in nutrients whereas TA700 was abiotic. The TA700 and T700 mesocosm atmospheres had a double pCO 2 concentration while this one of T350 was normal. In parallel, two ecological dimensionless models with condensed trophic levels have been developed. Models take into account chemical and biological processes of the carbon cycle by considering environmental variables, nutrient concentration (carbon, nitrogen, phosphorus), diatoms, bacteria, nanoprotozoa and microprotozoa biomass, particular and dissolved matter. For diatoms, two types of assimilation have been considered: assimilation of total inorganic carbon (MDIC model) and assimilation of dissolved carbon only (MCO 2 model). These models have been calibrated with experimental data and a sensitivity analysis has been conducted. In both models, the increase of atmospheric pCO 2 leads to a decrease of pH that is countered by the phytoplankton carbon assimilation. The phytoplankton presence limits the decrease of pH due to the increase of atmospheric pCO 2. In the MDIC model, the phytoplankton and the microbial loop were not influenced by atmospheric pCO 2 increase, considering modelled processes. The total inorganic carbon concentration was not limiting for phytoplankton compared to nitrogen or phosphorus. In the MCO 2 model, pCO 2 increase leads to a weak increase of phytoplankton and nano- and microprotozoa biomass. Another hand, more significant changes on carbon fluxes and organism productions could be occurred.