Comment on gmd-2023-33

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Many current biogeochemical models rely on an autotrophic versus heterotrophic food web representation. However, in recent years, an increasing number of studies have begun to challenge this approach. Several authors have highlighted the importance of protists capable of combining photoautotrophic and heterotrophic nutrition in a single cell. These mixotrophic protists are known to play an important role in the carbon cycle. Here, we present a new biogeochemical model that represents the food web using variable stoichiometry. It contains the classic compartments such as zooplankton, phytoplankton and heterotrophic bacteria, and a newly added compartment to represent two types of mixotrophic protists: non constitutive mixotrophs (NCM) and constitutive mixotrophs (CM). We demonstrate that the model correctly reproduces the characteristics of NCM and CM and proceed to study the impact of light and nutrient limitation on planktonic ecosystem structure in a highly dynamic Mediterranean coastal area: the Bay of Marseille (BoM, France), paying special attention to the dynamics of mixotrophic protists in these limiting conditions. In addition, we investigate the carbon, nitrogen and phosphorus fluxes associated with mixotrophic protists and showed that: (i) the portion of the ecosystem occupied by NCM decreases when resources (nutrient and prey concentrations) decrease, although their mixotrophy allows them to maintain a relatively high carbon biomass as photosynthesis increase as food source; (ii) the portion of the ecosystem occupied by CM increases when nutrient concentrations decrease, due to their capability to ingest prey to supplement their N and P needs.