Structural and functional phytoplankton responses to nutrient impoverishment in mesocosms placed in a shallow eutrophic reservoir (Garças Pond), São Paulo, Brazil

Abstract

Experiments were performed in mesocosms placed in a shallow eutrophic reservoir for the purpose of testing a nutrient removal technique as an eutrophication reduction method. Garcas Pond is in the ParqueEstadualdasFontesdoIpiranga Biological Reserve, located in the southeastern part of the Municipality of Sao Paulo (Sao Paulo State). Three different treatments were designed and each was conducted within two enclosures (closed systems) containing 360 l of water. Mesocosms built with polyethylene bags and PVC pipes were attached to the reservoir bottom. Treatment dilutions were made by using different proportions of pond and oligotrophic tributary water, following Carlson’s trophic state index modified by Toledo and collaborators. Dilutions constituted the oligo-, meso-, and eutrophic treatments. Ten abiotic and nine biological variables were simultaneously studied over 31 days. Species richness, diversity, evenness, and dominance are discussed. Dissolved oxygen concentration decrease was observed in all treatments during the entire study period, whereas ammonium and free CO2 values increased, indicating decomposition process presence in all treatments. Nutrient impoverishment induced P limitation in all treatments during most of the study period, a fact that, considered together with the free CO2 concentration increase, low alkalinity values, and pH reduction throughout the period clearly indicated a photosynthetic activity decrease. Confinement and nutrient dilution led to changes in floristic composition as well as in phytoplankton biomass in all treatments. The initial community mainly represented by R- and S-strategists (Planktothrix, Cylindrospermopsis and Microcystis) was gradually replaced by C-strategists (Cryptomonas spp., Chloroccoccales in general). Characteristics of the initial succession phases were observed in all treatments. Thus, the community was first inhabited by fast growing species but no important biomass contribution of size fractions was observed. Space liberated by blue-green species that did not adapt to the new nutrient-impoverished conditions was gradually occupied by other algal species, which together contributed to most of the total biomass registered in all treatments.