Global sensitivity analysis of a trophodynamic model of the Gulf of Trieste

Abstract A global sensitivity analysis is performed on a trophodynamic model of the Gulf of Trieste by using the Morris’ method. This screening method allows to highlight the most important processes in the system and to single out and rank the most relevant parameters of the model. The analysis is applied to a trophodynamic model specifically developed for the Gulf of Trieste, the northernmost part of the Adriatic Sea. In agreement with the specific ecological literature on the area of interest, the model considers two groups of phytoplankton (diatoms and nano-pico phytoplankton) and two groups of zooplankton (meso and microzooplankton). Heterotrophic bacteria are explicitly included in the model in order to simulate their role in DOC degradation and in phosphorus cycle, that is considered the limiting nutrient in the area of interest. The nutrient and carbon content in particulate organic matter and dissolved organic matter is also included in order to realistically reproduce the uncoupling of the nutrient and carbon cycles in the marine ecosystem. Model results reproduce fairly well the observed alternation of classical food chain and microbial food web as a result of the nutrient availability and the climatological factors used to force the system. The results of the global sensitivity analysis indicates that the most relevant parameters are those related to the growth formulations of the two phytoplankton groups, the total phosphorus in the system, the decay rate of particulate organic phosphorus and the mortality rate of bacteria. The analysis outlines that in spring and summer different processes within the two trophic pathways are dominant, and that while primary producers are mainly bottom–up controlled, bacteria experience both top–down and bottom–up controls.

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