Analysis of coastal lagoon metabolism as a basis for management

This work was carried out in a shallow eutrophic coastal lagoon (St. André lagoon, SW Portugal) which is artificially opened to the sea each year in early spring. Macrophytes, mainly Ruppia cirrhosa, are keystone species in this ecosystem covering up to 60% of its total area with peak biomasses over 500 g DW m^–2. The main objectives were to study ecosystem metabolism, to evaluate the metabolic contribution to the community of the macrophyte stands and their influence in the development of thermal stratification and bottom oxygen depletion.The work combined an experimental and a modelling methodology. The experimental approach included open water, mesocosm and microcosm seasonal experiments. During these experiments several physical, chemical and biological parameters were monitored in the lagoon and in plastic enclosures (mesocosms) for periods of 24 hours. The microcosm experiments followed the light-dark bottle technique. The simultaneous use of these different methodologies allowed the analysis of the contribution of the planktonic and benthic compartments to the ecosystem's oxygen budget.The modelling work was based on the mathematical simulation of heat and gas exchanges in a vertically resolved water column, under different macrophyte densities. Several simulations were carried out, in order to investigate the importance of the macrophytes in the development of water column stratification and anoxia.The simulation results suggest that macrophytes may greatly influence thermocline and oxycline development. This influence is proportional to their biomass and canopy height. It is suggested that controlled macrophyte biomass removal of up to 25% of available biomass in summer, may be useful in preventing bottom anoxia without compromising benthic net primary production.