Estrutura fitoplanctônica em reservatórios cearenses - associação hidroclimática e estado trófico

Abstract

are strongly influenced by climatic conditions. Phytoplankton react intensely and quickly to constant and strong seasonal fluctuations, reflecting on the physicochemical characteristics of the water and the biological communities. This can cause changes in the composition, abundance and diversity of species, favoring the occurrence of cyanobacterial blooms that are possibly regulated by hydroclimatic and biotic factors. The scope of this study was to analyze the effects of the variation of the fraction of the available volume on the structure of the phytoplankton community, in the wet and dry seasons, and to associate with biotic indices, degrees of trophy and hydroclimatic elements. For this purpose, a set of four reservoirs (Acarape do Meio, Banabuiú, Jaburú I and Jaburú II) were selected, whose analyzed period comprised from January / 2014 to October / 2018, with samples collected at a depth of 0.3 m in the lake area of each reservoir. A database composed of physico-chemical and biological variables (microalgae and cyanobacteria) was examined. Phytoplankton populations were quantified using the Utermöhl method and identified using inverted microscopy. For taxonomic analysis of the groups and systematization of phytoplankton in the reservoirs, specialized bibliographies were used for each division. Important biotic indexes were calculated in the study of the structure of the phytoplankton community. Descriptive analysis was applied with groupings of semiannual averages, characterizing seasonality and correlations between hydroclimatic, physical-chemical, biotic variables and fraction of the available volume through Pearson's correlation coefficient with a significance level of 5% (α = 0,05). Principal component analysis (PCA) was applied to the matrix of hydroclimatic data, nutrients and biotic indexes, with emphasis on the variation of the operational level of the reservoirs. The phytoplankton community was represented by 81 taxa, distributed among the divisions Cyanophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae, Zygnematophyceae, Cryptophyceae, Dinophycea. The most representative divisions in the Jaburú I and II and Banabuiú reservoirs were Cyanophyceae (98% average) and Bacillariophyceae (57.8%) in Acarape do Meio. The species Microcystis sp. was highlighted with 77% classified as the most frequent taxon, followed by Monoraphidium contortum with 49% and Raphidiopsis sp. with 46% frequency. The Shannon-Wiener index ranged between 2.10 and 0.36 bits.cel-1, classifying all reservoirs as having low diversity. Margalef's wealth was also considered low. The trophic status index for the semiarid classified all reservoirs as eutrophic or hypereutrophic for most of the evaluated period. The analysis of the main components of hydroclimatic elements, nutrients and biotic indexes explained the variability of water quality in relation to the degree of trophy. There is a hydroclimatic effect that favors the predominance of cyanobacteria in the reservoirs of the semiarid region of Ceará, compromising the water quality of these water systems, mainly due to the depletion of the available volume and the intake of water for supply.