Efeito da heterogeneidade de habitats sobre o fitoplâncton no reservatório de Moxotó, Rio São Francisco,Brasil

Abstract

Given the multiplicity of uses to which reservoirs are generally subjected, it is expected to find different conditions of composition, density and biomass of phytoplankton, due to possible spatial and seasonal variations of limnological conditions and land uses in the basin. Aiming to know the structure of phytoplankton and its variation in function of spatial heterogeneity in Moxotó Reservoir, located in the semiarid region of Northeastern Brazil, samples were analyzed in two periods, rainy (June 2009) and dry (December 2009). Samples were collected at a station in body (dam upstream) and at five in arms representing several human activities (aquaculture, agriculture, urban settlement, landfill and without punctual activity). Limnological variables, richness, density and biomass of phytoplankton were analyzed. Abundance/biomass comparison curves (ABC) were used to analyze phytoplankton structure. Algae diversity between periods and samples was estimated by cluster analysis, using species presence/absence data. Samples were ordinate (NMDS) by abundance and biomass of phytoplankton descriptor species. The relation between phytoplankton and limnological data was accessed by Pearson’s univariate correlation and BIOENV multivariate analysis. The reservoir presented regulated flow, short theoretical retention time (<7 days) and high water transparency (median zEUF: zMAX = 1.0). Phytoplankton biomass was consistent with low-productive systems (median = 0.9 mg L-1), limited mainly by phosphorus in rainy period (median SRP = 8.4 μg L-1), and nitrogen in dry period (median DIN = 36.5 μg L-1). Phytoplankton composition showed marked seasonality, especially for diatoms, with main contributions of planktonic in rainy period and typically periphytic in dry period. Under limitation by SRP and DIN, organisms adapted to mixotrophy (dinoflagellates and euglenoids) reached higher biomass. In contrast, nanoplanktonic organisms (chlorophytes and cryptophytes) were dominant in conditions of high nutrient availability. The studied sites showed three distinct environments: body, arms with submerged macrophytes and arms under influence of tributaries. Among human activities observed, agriculture was considered the most striking in relation to eutrophication. Moreover, proliferation of submerged macrophytes in arms can cause impairment for multiple uses, despite apparent benefits on water quality (increased transparency). Human activities, presence of submerged macrophytes and retention time influenced the availability of resources, being considered the driving forces of phytoplankton structure.