The development of an assimilative capacity model for the sustainable management of nutrients within the Ria Formosa in southern Portugal

Mathematical modelling approaches have been widely used to evaluate the capacity of an ecosystem to assimilate anthropogenic wastes. This is essential to develop sustainable management strategies and for the prevention of eutrophication. This project aimed to assess the importance of the benthic-pelagic interactions in Ria Formosa and to develop a simple biogeochemical model for the management of nutrient inputs. This was done by adapting the simple version of the CSTT model, for pelagic eutrophication, to the system and by adding a benthic primary producer, the microphytobenthos (MPB), which was previously indicated as one of the main components of the system.This research project has three main parts: 1) field work that provided context and data for model testing; 2) experimental work, which aimed at evaluating key parameters of the system to be used in the modelling approach; 3) model development work that was used to test hypotheses and provided a synthesis of ecological achievements.An initial assessment of the optimal methodology for MPB chlorophyll extraction was carried out to implement a strategy for an accurate chlorophyll measurement. The MPB temporal, spatial and vertical variability was investigated. The complex temporal pattern revealed a small influence of seasonality. However, phytoplankton was found to have a much more important seasonal component. The most important component of the MPB variability was found to be the small and large scale spatial variability, which explains around 61% of the total variance. MPB was also found to be the most important source of chlorophyll to the lagoon system, contributing around 99% of the total chlorophyll. The experimental approach carried out to investigate the yield of MPB chlorophyll from nutrients, which was previously considered to be one of the most important parameters of the CSTT model for phytoplankton, revealed interesting results. Estimates were found to be larger than the values used for phytoplankton.The development process of the biogeochemical dCSTT-MPB model allowed the investigation of the importance of several processes. Pore water nutrients were found to be essential to support the large MPB community. Moreover, MPB cells were also found to have a large impact on the pelagic chlorophyll concentrations by re-suspension. The model was able to predict concentrations in the appropriate range of values observed in the lagoon. However, the temporal pattern is still weakly simulated and improvements are still required