Variação espaço-temporal das respostas ecofisiológicas da comunidade fitoplanctônica em uma lagoa costeira tropical com múltiplos usos: Lagoa Juara (Serra, ES)

Abstract

This study aimed evaluates the structure, dynamic and physiological responses of phytoplankton community and limnological variables in Juara lagoon, in a spatial and temporal scale. Six samples were taken monthly (september/2009 to march/2010), separated in two groups: dry season (september, october and january) and period influenced by rainfall (november, december and march), in five sampling sites located in a longitudinal gradient in the lagoon, representing regions near the stream, the sea and in the region of deployment of net cages. Samples were collected in sub-surface and bottom water column, always in the morning. Samples were collected using plankton net to qualitative analysis (20 µm) and van Dorn bottle for quantitative analysis. The structure of the phytoplankton community was evaluated on the basis of its main attributes: richness, total density and per Class, abundance, dominance, diversity and equitability. Phytoplankton biomass was evaluated through biovolume and chlorophyll a concentration. The studied environmental variables were: temperature (air/water), transparency, euphotic zone, depth, pH, dissolved oxygen (OD), turbidity, electrical conductivity (CE), total suspended solids (STS), salinity, main nutrients (silicate, nitrate, nitrite, ammoniacal nitrogen, total nitrogen, orthophosphate, total phosphorus), and reason NT/PT water. A total of 214 taxa were found, and the Class Chlorophyceae was the most representative qualitatively and the Class Cyanophyceae quantitatively during the study, mainly in the site 2, where the net cages were deployed. The phytoplankton density was high, presenting hightest values in samples influenced by rainfall and in sites 2, 4 and 5. Data on biovolume and chlorophyll a concentration were highest in dry season and site 4. Generally, the Class Bacillariophyceae, Chlorophyceae and Euglenophyceae showed a greater contribution in terms of biovolume. The Juara lagoon was characterized as a shallow freshwater environment, oligohaline, neutral waters, with high values of temperature, CE, euphotic zone and transparency, low values of OD (except in sites 4 and 5). The OD, pH, CE, salinity and STS values showed significant temporal and spatial variation. The concentration of orthophosfate, total phosphorus, silicates, total nitrogen, reason NT:PT and ammoniacal nitrogen, were high, mainly in sites 4 and 5. The Carlson’s Index of Trophic State ranked the Juara lagoon as mesotrophic (sites 1, 2 and 3) to eutrophic (site 4) and meso-eutrophic in site 5. There isn’t significant vertical spatial variation; in relation of horizontal variation the sites 1, 2 and 3 showed limnological and biological features different from sites 4 and 5, influenced by the system net cage and Laranjeiras stream, respectively, beyond the both sites were near the connection of the sea. Despite the regional importance, the pond Juara showed sign of changes in it environmental conditions committing their uses, particularly intensive culture of fish for domestic consumption, leisure and recreation, evidenced by ecophysiological responses of phytoplankton and the limnological variables analyzed