Efeitos do enriquecimento de nutrientes sobre os processos ecossistêmicos (produtividade primária e secundária) : uma abordagem experimental.

Abstract

Nutrient enrichment on aquatic ecosystem is one of the most widespread human impacts on Earth. The aimed to evaluate experimentally the effects of nutrient enrichment on ecosystem processes of phytoplankton primary productivity (PPL) and zooplankton secondary productivity (PS), and on richness, dominance and composition of zooplankton community. We tested whether ecosystem processes are positively affected with nutrient enrichment, and whether the structure of zooplankton community responds distinctively to enrichment and over time. The experiment ran for 30 days in mesocosms filled with water from the Paraná River. Inocula of phytoplankton and zooplankton from environments of the Upper Paraná River floodplain were added in the mesocosms. The experimental design was randomized and replicated three times, with three treatments: lower nutrient concentration (N1); middle nutrient concentration (N2); and high nutrient concentration (N3). The phytoplankton?s PPL was estimated by the method of light and dark bottles in each mesocosm at the beginning and every five days until the end of the experiment, with a total of seven days of sampling. Zooplankton was collected at the same PPL sampling days by filtering 10 L of water from each mesocosm. Rotifers? PS was estimated by the method of recruitment, and cladocerans and copepods? through the biomass growth method. Richness was the species number in the sample, and the dominance was estimated from community evenness. Nutrient enrichment positively affected primary and secondary productivity, and increased fluctuations over time. Community attributes were only affected by time; the interactions between populations changed in favor of large bodied animals and better competitors, such as large cladocerans and copepods. These effects could reflect negatively in environmental services provided by aquatic ecosystems.