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Previous issue date: 2010-06-29 === The dams are limnic ecosystems of great importance for its multiple uses, among them,
water supply for the public and to culture of artisanal fish are most relevant. The aim of the
present study is to evaluate the physical-chemical characteristics and the phytoplankton
community in two chosen sites (Point 1 littoral zone of point source; Point 2 pelagic zone of
non-point source) of the Minister Jo?o Alves dam, which is also known as Boqueir?o de
Parelhas/RN. This represents the spatial distribution of the phytoplankton species in order to
understand any possible alterations of the water quality and the phytoplankton composition in
relation to the water quality originating from the impact of the tilapia, Oreochromis niloticus,
culture. The study period also encompasses temporal variations exhibited in two seasons of an
annual cycle, one during the dry season (Oct, Nov and Dec of 2008 and Jan of 2009), and the
other rainy season (Mar, Apr, May and June of 2008) to extend the observation. The physicalchemical
parameters, such as pH, temperature, electrical conductivity, concentration of dissolved
oxygen were measured in situ and the values of the inorganic nutrients (nitrate, ammonium and
orto-phosfato) and chlorophyll in the laboratory. The quali-quantitative analyses of the
phytoplankton had been carried through sedimentation technique and the enumeration of the
random of 400 cells, colonies and filaments counted using Sedgwick-Rafter counting chamber.
The results of pH varied widely from the acidic to alkaline range with the minimum of 5.8 (? 0.8)
and the maximum of 9.2 (? 0.7-0.8), at point 1 and 2. The dissolved oxygen content was higher in
the rainy period than that in the dry period. The maximum electrical conductivity was of 1409
?Scm-1 in point 1 and 431 minim of ?Scm-1, in point 2. There was a considerable alteration in the
levels of inorganic nutrients such as nitrate-nitrogen, ammoniacal nitrogen and orthophosphate
during the two cycles of study period. Phytoplankton assemblages presented a picture of alternate
dominance among species Cyanobacteria, Bacillariophyceae and Chlorophyceae. The trophic
state index diagnosed to the category of mesotrophic, which is based on the values of chlorophyll,
total phosphorus and Secchi-disc measurements. The wind driven turbulence of the water column
and the fresh inflow of water (flushing and dilution) during rainy season acted as constraint and
did-not allow an exaggerated growth of the species of cyanobacteria. On the basis of the present
we conclude that the culture of tilapias in cage-culture fails to produce pollution load that could
compromise the quality of the water of the dam, probably be due to small dimension of the culture
in relation to the size, volume of the water and the reservoir capacity support its own environment === Os a?udes s?o ecossistemas l?mnicos de grande import?ncia por seus usos m?ltiplos,
dentre estes, o abastecimento p?blico e a piscicultura artesanal s?o os mais relevantes. O presente
estudo objetivou comparar as caracter?sticas f?sico-qu?micas e a comunidade fitoplanct?nica em
dois pontos amostrais (Ponto 1 Pontual; Ponto 2 N?o-Pontual) do a?ude Ministro Jo?o Alves,
tamb?m conhecido como Boqueir?o de Parelhas, Parelhas/RN, observando poss?veis altera??es na
qualidade da ?gua em fun??o do cultivo de til?pia do Nilo, Oreochromis niloticus, durante dois
per?odos, englobando um per?odo chuvoso (mar., abr., maio e jun. de 2008) e um per?odo de
estiagem (out., nov. e dez. de 2008 e jan. de 2009). Os par?metros f?sico-qu?micos, tais como pH,
temperatura, condutividade el?trica, concentra??o de oxig?nio dissolvido, foram medidos in
situ e os valores dos nutrientes inorg?nicos (nitrato, am?nio e orto-fosfato) e clorofila-a foram
analisados em laborat?rio. As an?lises quali-quantitativa das microalgas foram realizadas ap?s
sedimenta??o 24 horas e transferindo 1 ml a c?mara de Sedgwick-Rafter e quantificando 400
individuos de esp?cies. Os resultados mostraram que o pH apresentou-se alcalino durante a maior
parte do estudo, com m?nimo de 5,8 (? DP 0,8) no ponto 1 e m?ximo de 9,2 (? DP 0,7;0,8), no
ponto 2, ambos durante o per?odo chuvoso. A estratifica??o do oxig?nio dissolvido foi maior no
per?odo chuvoso que no per?odo seco. A condutividade el?trica m?xima foi de 1409 ?Scm-1 no
ponto 1 e m?nima de 431 ?Scm-1 no ponto 2, tamb?m durante o per?odo chuvoso. Para os
nutrientes inorg?nicos de um modo geral foram observadas oscila??es nas suas concentra??es
durante os dois per?odos de estudo. A composi??o fitoplanct?nica apresentou domin?ncia
alternada entre Cianobacteria, Bacillarioficeas e Clorof?ceas. O ?ndice de estado tr?fico foi
diagnosticado na catergoria mesotr?fico, com base nas concentra??es de clorofila a, f?sforo total
e nos valores de transpar?ncia obtidos pelo disco de Secchi. O vento e o fator turbul?ncia da ?gua
do a?ude, assim como o fluxo de ?gua do a?ude n?o permitiram o crescimento exagerado das
esp?cies de cianobact?rias. Com base no presente estudo, conclu?-se que o cultivo de til?pias em
tanques-rede n?o produziu carga poluidora que pudesse comprometer a qualidade da ?gua do
a?ude em rela??o ?s vari?veis analisadas, provavelmente devido ? pequena dimens?o do cultivo
em rela??o ao tamanho e capacidade suporte do ambiente
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