Abstract
The goal of this study was to analyse the vertical structure of the phytoplankton community at the Mundaú reservoir, located in the semi-arid region of northeastern Brazil, and to correlate it to environmental conditions over two distinct seasons, dry and rainy. Samples were collected bimonthly at eight depths in the dry and rainy season for analyses of the physical and chemical variables of the water, as well as density, abundance, dominance, species diversity index and equitability of the community. Analysis of variance (ANOVA-two way) was used to analyse the vertical and seasonal differences, and Canonical Correspondence Analysis (CCA) was used to assess associations between phytoplankton and environmental variables Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju was the only dominant species and Geitlerinema amphibium (C. Agardh) Anagnostidis, Merismopedia punctata Meyen and Synedra rumpens Kützing. Others six taxa were abundant in at least one of the samples. Distinct vertical distribution patterns were observed for the abundant taxa between depths and seasons. The cyanobacteria, with the exception of C. raciborskii, showed similar seasonal patterns, with higher densities in the dry season. The CCA showed a strong correlation between the density of the phytoplanktonic species and abiotic variables. The vertical changes in abundant taxa revealed distinct patterns regulated by the variation in the environmental factors that were directly linked to seasonality, with the success of one or more species being dependent on their life strategies and ecological needs. The present study restates the importance of environmental and seasonal factors for phytoplankton composition and distribution in a freshwater tropical reservoir through a vertical gradient.
Highlights
The phytoplankton community is an important biological component of water bodies, because it is responsible for a large part of the primary production, constituting a food chain basis
Water temperature variations, precipitation, wind direction and speed, environment morphometry, and N:P ratios are reported to be the main factors of an aquatic ecosystem, capable of promoting changes in phytoplankton composition and dynamics and the occurrence of cyanobacterial blooms (Chellappa and Costa, 2003; Jöhnk et al, 2008; Delpla et al, 2009)
In the Mundaú reservoir, abiotic variables and seasonality were observed to change the dynamics of phytoplankton and were responsible for the variation in the vertical distribution of the species
Summary
The phytoplankton community is an important biological component of water bodies, because it is responsible for a large part of the primary production, constituting a food chain basis. Variations of abiotic water conditions occur naturally throughout the day and over the seasons of the year These variations can be either vertical or horizontal and may be closely related to stratification and mixing of the water column, resulting in changes in the availability of light and nutrients for the development of the phytoplankton community (Lopes et al, 2005). Other factors such as herbivory, internal seiches, turbulence and water renewal rate may influence the vertical distribution of the phytoplankton (Litchman et al, 2010; Mellard et al, 2011). The excessive development of the biomass of these organisms (blooms), which often produce toxins, results in an unpleasant taste and odor of the water, affecting the availability of drinking waters, and leads to loss of environmental scenic characteristics compromising thereby its quality for the most varied uses (Heo and Kim, 2004; Salmaso et al, 2012)
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