Spatial and temporal dynamic of trophic relevant parameters in a subtropical coastal lagoon in Brazil.

Coastal lagoons are ecologically and economically important environments but a relative low number of studies were carried out in subtropical and permanently closed coastal lagoons. The present study aimed at assessing the temporal and spatial dynamic of trophic relevant water quality parameters in the small, deep and freshwater Peri coastal lagoon, South Brazil. During the 19 sampled months (March/2008-September/2009) spatial homogeneity (horizontal and vertical) was registered in all seasons for all variables, a condition related to the strong wind influence and low human occupation in the lagoon watershed. Seasonal variations of the water quality could be observed and they can be explained mainly by variation on temperature, wind forces and direction and rainfall, characteristic from the subtropical weather. Comparing this study with two others conducted in Peri lagoon in 1996 and 1998, no critical differences that evidence alteration in the water quality were found, but climate differences may have influenced in some small variations observed. The use of four trophic state indices indicated that indices designed for temperate lakes are inappropriate for the subtropical Peri lagoon. The lagoon was classified as oligotrophic for nutrients concentrations and meso-eutrophic for transparency and chlorophyll-a, which can be explained by the high densities and monodominance of the cyanobacterium Cylindrospermopsis raciborskii and the high recycling rates observed in warmer water bodies, when compared to the temperate ones.