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In this paper, the authors use remote-sensing images to monitor the water quality of reservoirs located in the semiarid region of Northeast Brazil. Sentinel-2 MSI TOA Level 1C reflectance images were used to remotely estimate the concentration of chlorophyll-a (chl-a), the main indicator of the trophic state of aquatic environments, in five reservoirs in the state of Ceará, Brazil. A three-spectral band retrieval model was calibrated using 171 water samples, collected from November 2015 through July 2018 in 5 reservoirs. For validation, 71 additional samples, collected from August 2018 through December 2019, were used to ensure a robust accuracy assessment. The TOA Level 1C products performed very well, achieving a relative RMSE of 28% and r2 = 0.80. Data on wind direction and speed, solar radiation and reservoir volume were used to generate a conceptual model to analyze the behavior of chl-a in the surface waters of the Castanhão reservoir. During 2019, the reservoir water quality showed strong variation, with concentration fluctuating from 30 to 95 µg/L We showed that the end of the dry season is marked by strong eutrophic conditions corresponding to very low water inflows into the reservoir. During the rainy season there is a large decrease in the chl-a concentration following the increase of the lake water storage. During the following dry season, satellite data show a progressive improvement of the trophic state controlled by wind intensity that promotes a better mixing of the reservoir waters and inhibiting the development of most phytoplankton.

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Artificial lakes play an important role in water availability in tropical semi-arid Northeastern Brazil. However, in addition to the negative effects that dams have on the water quality of rivers, other practices such as discharge of untreated wastewater, intensive crop fertilization and fish farming have been contributing to water quality deterioration. Climate change may be another driver of water quality decrease. Increases in precipitation and drought intensities as well as temperature increase may redefine the dynamics of planktonic communities and favour species more adapted to the new environment. Water samples were collected in two reservoirs selected based on the recurrence of positive results for saxitoxins, Acarape do Meio (AM) and Sitios Novos (SN). In general, diversity in SN (1.08) was higher than in the AM (0.60). Phytoplankton dynamics in these reservoirs differ from other reservoirs in the same region of Brazil, in the sense that filamentous cyanobacteria are dominant. Furthermore, lower diversity, concentration and richness of chlorophyceae and bacillariophyceae were found in both reservoirs. Increase in diversity was observed during the intense rainy season of 2011, suggesting a slight improvement of water quality. A close inverse correlation between Cylindrospermopsis sp and other cyanobacteria was found in SN (R 2 = -0.862) and in AM (R 2 = -0.997) and the data collected demonstrates that a season with intensive rainfall may lead to lower cyanobacteria concentrations probably due to water dilution and higher flushing rates.Keywords: Artificial reservoirs; Freshwater cyanobacteria; Semi-arid regions; Temporal phytoplankton dynamics; Eutrophication. RESUMOOs lagos artificiais tem um papel primordial na disponibilidade hídrica no semi-árido Brasileiro. Entretanto, além dos efeitos negativos que os barramentos dos rios causam sobre a qualidade da água, a descarga de esgotos, a irrigação e a piscicultura intensivas vêm acentuando a deterioração da qualidade da água em um ritmo sem precedentes nesta região. A mudança climática é outro fator de impacto negativo na qualidade da água. O aumento da intensidade das precipitações pode redefinir a dinâmica das comunidades de plâncton e espécies mais adaptadas a ambientes extremamente variáveis. Os reservatórios estudados, Acarape do Meio (AM) e Sítios Novos (SN), foram selecionados com base na recorrência de resultados positivos para saxitoxinas. A dinâmicas do fitoplâncton encontrada, difere de outras áreas da mesma região, no sentido em que as cianobactérias filamentosas são dominantes. Baixas diversidade, concentração média e riqueza de Chlorophyceae e Bacillariophyceae foram encontrados em ambos os reservatórios, embora tenha sido observado um aumento durante o intenso período chuvoso de 2011, sugerindo uma ligeira melhoria da qualidade da água. Em geral, a diversidade no reservatório SN (1,08) foi maior que no AM (0,60). Uma correlação inversa entre a concentração de Cylindrospermopsis sp e outras cianobactérias foi encontrada nos reservatór...

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Aim: The present study aimed to investigate the composition and toxicity of the phytoplankton in Sítios Novos reservoir, used mainly as drinking water supply for approximately 30,000 people. Methods: Samples were collected between January 2010 and June 2011. Results: During this period 19 taxa of cyanobacteria and 22 of algae were identified. Out of 45 samples collected, algae accounted for no more than 10% of the quantified organisms in 44 samples. Cyanobacteria accounted for 100% of the organisms quantified in three samples and for 99% in other 29 samples. Among the cyanobacteria group, Planktothrix agardhii (Gomont) Anagnostidis & Komárek and Cylindrospermopsis raciborskii (Woloszynska) Seenaya & Subbaraju prevailed and both strains were isolated and identified as paralytic shellfish poison (PSP) producers. C. raciborskii strain has shown to produce SXT and dcSXT while P. agardhii strain has shown to produce dcGTX2 or 3. Conclusions: To the author's knowledge, this is the first report of PSP-producer cyanobacteria species isolated in Northeastern Brazil and the first reported of a P. agardhii synthesizing dcGTX2/3.

ResumoTeor lipídico da microalga marinha Chaetoceros muelleri Lemmermann (Bacillariophyceae) cultivada em diferentes salinidades. A produção e armazenamento de lipídios por microalgas em resposta a variações nos fatores ambientais são específicos para cada espécie de microalga. O presente trabalho teve como objetivo avaliar o crescimento e a concentração de lipídios totais da microalga Chaetoceros muelleri cultivada em três diferentes salinidades (15, 25 e 35). O experimento foi realizado em triplicatas, utilizando frascos de nove litros com volumes úteis de 7 L, sendo cultivo do tipo estacionário ou "batch". A temperatura da sala de cultivo e a intensidade luminosa foram mantidas em 28±1ºC e 20 μ.mol.m -2 .s -1 , respectivamente. Os resultados mostraram que no presente trabalho, o rendimento lipídico (10,41±1,89, 10,87±2,10, 12,33±1,81% nas salinidades 15, 25 e 35, respectivamente) não foi influenciado pela concentração salina (p>0,05). No entanto, a concentração celular máxima sofreu esta influência e foi significativamente maior (p<0,05) nas salinidades de 15 e 25 as quais obtiveram 16,3±1,50 e 17,6±2,90 x 10 5 cel.mL -1 , respectivamente. Key words: Cultivo; Lipídeos; Microalgas AbstractThe production and storage of lipids by microalgae in response to variations in environmental factors are specific for each microalgal species. The present study aimed to evaluate the growth and concentration of total lipids of the microalga C. muelleri Lemmermann cultivated at three different salinities (15, 25 and 35). The experiment was conducted in triplicate, using 9-L containers (7-L working volume), with the culture being stationary or batch type. The temperature and light intensity of the cultivation room was kept at 28±1ºC and 20 μ.mol.