Abstract
Abstract. The biogeochemistry of carbon and nutrients (N,P) in the surface layer of the ocean strongly depends on the complex interactions between primary producers (phytoplankton) and remineralizers (heterotrophic bacteria). To understand how these interactions impact the overall DOC dynamics in the surface layer of the Mediterranean Sea, we implemented, using Eco3M (Ecological Mechanistic Modular Modelling tool), a multi-element model with a mechanistic description of primary production. We studied the model steady state results under various nutrient conditions and fixed cell abundances. By doing so, we show how the bottom up control of osmotrophs growth can impact the overall DOC dynamics in the system. Based on our set of parameters, the biogeochemical characteristics displayed by the model appear realistic when compared to literature data for the Mediterranean basin. Differences in DOC dynamics between N and P limited systems in the model, lead to the conclusion that the unusually high N:P ratio of the Mediterranean Sea may favour the uncoupling between growth and carbon production leading to higher DOC accumulation compared to systems with lower N:P ratio.
Highlights
During summer, the surface layer of the Mediterranean Sea is a stratified Low Phosphate Low Chlorophyll (LPLC) system (Moutin et al, 2008)
The carbon budget in the model is inferred from the balance between carbon production through primary production, grazing or mortality and carbon respiration which depends on the cellular C-content of phytoplankton and on the efficiency at which heterotrophic bacteria assimilate DOC
Within the range of total nitrogen (TN) and total phosphate (TP) used in this study, inorganic nutrient concentrations obtained at steady state range from 0 to 15 nM and from 0 to 370 nM for phosphate and nitrogen respectively (Fig. 3b)
Summary
The surface layer of the Mediterranean Sea is a stratified Low Phosphate Low Chlorophyll (LPLC) system (Moutin et al, 2008). Inorganic nutrients concentrations are often close to or below the detection limit in this layer and elemental content of osmotrophs is rarely quantified directly on board This makes the understanding of the coupling between carbon, nutrients and growth at the cell level difficult to assess. The observed response in bioassay experiments is likely to be a relatively complex phenomenon involving cell physiology for uptake, as well as, storage of limiting and near limiting resources. This is regarded as a key issue when trying to understand the coupling between nutrient and carbon in the eupohtic zone. We chose to look at the steady state results assuming constant cell abundance and a conservative amount
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