Abstract
The present study reports the phytoplankton abundance, dominance and co-existence relationships in the eutrophic Carpina reservoir, Pernambuco, Brazil. Sampling was carried out at six different depths bimonthly at a single reservoir spanning two climatic periods: dry season (January, September, and November 2006) and rainy season (March, May, and July 2006). Density, abundance, dominance, specific diversity and equitability of the community were determined, along with chlorophyll a, and physical and chemical variables of the environment. Eight species were considered abundant, and their densities corresponded to more than 90% of the total phytoplankton community quantified. Cyanobacteria represented more than 80% of this density. Cylindrospermopsis raciborskii was the only dominant taxon in the dry season, and was co-dominant in the rainy season. C. raciborskii, Planktothrix agardhii and Geitlerinema amphibium had the greatest densities and lowest vertical variation coefficients. The statistical analysis indicated relationships with vertical and seasonal variations in the phytoplankton community and the following variables: total dissolved solids, water temperature, electrical conductivity and pH. The changes in the environmental variables were discrete and regulated by the establishment of precipitation however, they were able to promote vertical and seasonal instability in the structure of the phytoplankton community.
Highlights
Most reservoirs are formed by the damming of rivers and are considered hybrid systems between rivers and lakes (Thornton et al 1990)
Water temperature (x = 27.5 in the dry season; x = 28.1 in the rainy season), dissolved oxygen (x = 4.38 in the dry season; x = 5.18 in the rainy season), electrical conductivity (x = 1914 in the dry season; x = 2060 in the rainy season), total dissolved solids (x = 1246 in the dry season; x = 1699 in the rainy season) and turbidity (x = 23.1 in the dry season; x = 22.0 in the rainy season) values were higher in the rainy season, whereas pH (x = 8.6 in the dry season; and x = 8.5 in the rainy season) was slightly higher in the dry season (Fig. 1)
Cyanobacteria was dominant throughout the study, accounting for an average of 93% of total density in the dry season, and 84% in the rainy season, with an average variation among depths of 1063 × 104 (4.0 m) org.L−1 to 1331 × 104 org.L−1 in the dry season, and from 889 × 104 org.L−1 (10.0 m) to 1440 × 104 org.L−1 in the rainy season (Fig. 3)
Summary
Most reservoirs are formed by the damming of rivers and are considered hybrid systems between rivers and lakes (Thornton et al 1990) The importance of these artificial aquatic systems is huge, especially in the northeastern Brazil, which is largely dominated by a semiarid climate, with long periods of drought (Chellapa and Costa 2003). Problems regarding artificial eutrophication in reservoirs are common and are caused by anthropogenic actions, which lead to innumerous changes in the structure and dynamics of the phytoplankton community. This community is one of the main groups of primary producers and promptly responds to nutrient changes in reservoir aquatic ecosystems. Environmental changes frequently stimulates results in high densities of phytoplankton, known as algal blooms, which could compromise wa-
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