Salinity as a Major Driver for Submerged Aquatic Vegetation in Coastal Lagoons: a Multi-Year Analysis in the Subtropical Laguna de Rocha

This paper analyses 6 years of temporal and spatial dynamics of submerged aquatic vegetation (SAV) in a subtropical choked coastal lagoon in Uruguay. In this lagoon, macrophyte proliferation is frequently observed. Nutrient inputs and a highly dynamic salinity regime may be promoting the observed changes in SAV. We explored the main potential factors causing plants proliferation. We took seasonal samples of SAV biomass and physicochemical variables in vegetated areas of the entire lagoon and analysed the spatial distribution with correspondence analysis. Generalised linear models were used to quantify the contribution of physicochemical variables to SAV species biomass, total biomass and richness. Salinity was the most important factor determining SAV structure, causing a decrease of richness and biomass at salinity values higher than ca. 10. The SAV temporal and spatial dynamics were also determined by the species life strategies. Freshwater and slightly brackish conditions allowed competitive species to develop high biomass and cover, while marine water declined biomass and richness promoting stunted species dominance. The wide variation of salinity resulted in a highly dynamic SAV community, and the artificial opening of this coastal lagoon sand bar could therefore have strong effects on SAV. Further eutrophication due to land use intensification might enhance SAV variability and the alternating dominance of submerged angiosperms, macroalgae and phytoplankton.