LINS, R. P. M.; http://lattes.cnpq.br/3327055945085150; LINS, Ruceline Paiva Melo.
Resumo:
This work studied the structure and dynamics of the phytoplankton
community of a eutrophic reservoir of semiarid region, with intensive fish farming in
cages, focusing on the patterns of temporal and spatial distribution of life strategies
and functional groups characteristic. Sampling occurred between August 2007 and
July/2009, with bimonthly frequency in the first year and at monthly intervals
thereafter, in the euphotic and aphotic zone of the points of confluence of tributary
rivers (PC), near the fish net cages (PT) and near the dam itself (PB). The
variables analyzed were: rainfall, water temperature, transparency, pH, electrical
conductivity, alkalinity, dissolved oxygen, soluble reactive phosphorous (SRP),
dissolved inorganic nitrogen (DIN), phytoplankton biomass and the presence of
cyanotoxins (microcystin-LR) in water. The approach of phytoplankton functional
groups and ecological and morphological attributes was used to describe the
trajectory of species between periods. Seasonality promoted the establishment of
distinct periods, drought and rainy. During the drought favorable environmental
conditions in the reservoir such as high temperature (>24°C), alkaline pH (minimum
7.0 and maximum 10), limited availability of underwater light (Zeu:Zmis≤1), eutrophic
conditions (IET=61.2±5.1-PC, 62.5±5.4-PT, 60.7±6.6-PB) and environmental stability
associated with high water residence time (146 days), contributed to the
establishment of high phytoplankton biomass consisting mainly of filamentous
cyanobacteria (R strategists), belonging to the functional groups S1, Sn, H1 and MP,
typical of eutrophic environments with low light availability. Coccoid cyanobacteria of
M and K were also recorded. During this period, the dominant species were:
Planktothix agardhii, Pseudanabaena limnetica, Cylindrospermopsis raciborskii,
Dolicospermum circinalis, Oscillatoria lacustre, Microcystis aeruginosa, Microcystis
sp. and Aphanocapsa incerta. Reduction in biomass and in the dominance of
cyanobacteria occurred in the rains, especially in times of overflow of the reservoir,
when invasive species (C strategists) and stress tolerant (S strategists), not colonial
flagellate (Botryococcus braunii), single-celled flagellate (Chlamydomonas sp. and
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Peridinium umbonatum) and non-flagellated unicellular (Closterium sp.) dominated or
co-existed in the environment, favoring the increase in biomass of the groups: F, P,
Lo and X2, typical of meso-eutrophic environments. These taxa were predominant
especially at the confluence of tributary rivers and near the dam itself, during overflow
events. Next to the cages cyanobacteria dominated and the biomass values were
higher, indicating that the activity of intensive fish farming was an important factor of
anthropogenic disturbance in the reservoir. Of the eight species of cyanobacteria
dominant (≥ 50% of the total biomass), Planktothrix agardhii, Pseudanabaena
limnetica, Cylindrospermopsis raciborskii, Dolicospermum circinalis, Oscillatoria
lacustre, Microcystis aeruginosa, Microcystis sp. are capable of producing toxins.
During the study the formation of toxic blooms and production of microcystin-LR was
recorded, with the highest values observed in the dry season (minimum=0.93μg.L-1-
dezembro/2008 and maximum=27.29μg.L-1-december/2007). The occurrence of
microcystin alert to possible risks to public health since this reservoir is used to
supply human activity and intensive fish farming. The use of functional groups,
ecological and morphological attributes was effective tools for understanding the links
between environment and species, and assessing the ecological health of the
reservoir.