Nitrogen plays a central role in the control of aquatic ecosystems; it participates to the eutrophication process in lakes, streams, and estuaries and near coastal oceans. As nitrogen is a growth-limiting nutrient in most marine systems, internal recycling processes drive the community dynamics. Phototropic N2 assimilation, essentially realised by plank tonic Cyan bacteria, thus contributes a non-negligible part of total N demand by phytoplankton in the upper mixed layer. Blooming diastrophic Cyan bacteria can relax N-limitations, making the system P-limited. N-fixing Cyan bacteria are liable to greatly influence the community production processes and can impact the coupling of C-N-P cycles. The potential toxicity of some species of Cyan bacteria also motivates a growing concern for the conditions promoting their development. We propose a functional approach of the question to explicitly represent the biological nitrogen fixation process and assess its ecological control by environmental factors. We plan to implement and validate an ID vertical model of nitrogen fixation by Cyan bacteria, taking into account the interaction between nitrogen metabolism and carbon metabolism at a daily simulation scale. This analysis and reflection tool will promote an ecologically based understanding of N2-fixation. Such an accurate description is of the utmost importance to compare the ecological significance of the N-fixation process in various regions. An essential contribution of the present proposal to the project field will be the use of these common controls of biological N fixation to provide a foundation for the development of a regional model that incorporates ecological controls of biological N fixation. Once the model is coupled with an ID vertical ecosystem model, this knowledge will permit (1) to analyse the gradual changes in the nitrogen availability for the development of the whole phytoplankton and (2) to guide the decision-making process of loading managements in order to improve water quality.
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