In arid and semi-arid regions, a hydrological regime characterized by an annual cycle of drought and rainy seasons changes the volume and retention time of reservoirs. Such changes affect the limnological characteristics and lead to changes on phytoplankton community. Phytoplankton seasonal succession was studied in a Brazilian eutrophic semiarid water supply reservoir (Cruzeta). In this study, the changes in the biomass and species composition of phytoplankton during two annual hydrological cycles were analyzed, and the driving factors were evaluated.The composition of phytoplankton alternated between filamentous cyanobacteria, such as Cylindrospermopsis raciborskii, under conditions of mixing, and colonial species, such as Microcystis panniformis and Sphaerocavum brasiliensis, under conditions of high water column stability. The lower water level during a severe drought favored adaptive phytoplankton species with low requirement for resources, such as diatoms and cryptomonads. Extreme events, i.e., torrential rains and severe droughts, governed by the hydrological regime of the semi-arid region led to strong altered availability of resources in the watershed, directing the spatial and temporal dynamics of the phytoplankton in the Cruzeta man-made lake. The results showed an unusual behavior of the phytoplankton community contradicting the expectations about the climatic change scenario. Instead of an expected increase in cyanobacteria, the severe drought led to low biomass and resources, favoring diatoms and cryptomonads.

AimDrought periods often occur in Brazilian semiarid region and are supposed to induce water quality degradation by changes in physical, chemical and biological properties of freshwater ecosystems. Reservoirs in this region are used as drinking-water supplies and are exposed to wide volume fluctuations during drought periods due to lack of precipitation and high evaporation rates. This study aimed to identify patterns on water quality of two reservoirs during a long drought period. It was expected that more arid and shallower conditions would favor algal growth by enhancing nutrient availability, causing a decrease on water quality.MethodsThe study was based on monthly sampling over 20 months (May 2011 to December 2012) at two tropical reservoirs on Brazilian semiarid region. Precipitation and volume data were obtained from environmental agencies. Transparency was measured on field using a Secchi disk and conductivity, nutrients, suspended solids and chlorophyll-a were analyzed on laboratory. Temporal changes in all environmental variables were analyzed in each reservoir using two-way cluster analysis and also principal component analysis (PCA).ResultsThe volume of both reservoirs decreased considerably over the study because of low or shortage of precipitation. It was possible to detect two opposite patterns of chlorophyll-a in each reservoir throughout the drought season: in the first one phytoplankton growth was favored, while in the second one chlorophyll-a decreased by high inorganic turbidity. Both reservoirs tended to increase their turbidity and conductivity during the drought period due to shallow conditions, which probably contributed to sediment resuspension.ConclusionsWater level reduction during the extended drought period, contributed for water quality degradation due to high algal biomass and also high turbidity found during drought period. Local factors, as the nature of suspended solids, play an important role on predicting water quality.

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Cite as: Mendonça Júnior, J.R. et al. Extreme droughts drive tropical semi-arid eutrophic reservoirs towards CO 2 sub-saturation. Acta Limnologica Brasiliensia, 2018, vol. 30, e101. Abstract: Aim:This study aimed to evaluate the carbon dioxide (CO 2 ) dynamics in tropical semi-arid reservoirs during a prolonged drought period as well as to test if the trophic state affects the CO 2 saturation. Methods: This study was performed in four reservoirs located in the tropical semi-arid region in the northeast of Brazil. All samplings were performed between 9 and 12 am using a Van Dorn Bottle. Samples for partial pressure of carbon dioxide (pCO 2 ) measurements were taken in the sub-surface as well as samples for total phosphorus and chlorophyll-a. Correlation analysis and linear regression were used to detect relations among the calculated pCO 2 , water volume and chlorophyll-a. Results: The water level reduction due to atypical droughts caused chlorophyll-a concentrations to increase, which in turn, led to CO 2 reduction in the water. However, CO 2 concentrations were very variable and an alternation between CO 2 sub-saturation and super-saturation conditions was observed. This paper showed that water volume and chlorophyll-a were important regulators of CO 2 in the water, as well as important carbon balance predictors in the tropical semiarid reservoirs. Conclusions: The results of this paper indicate that the eutrophication allied to drastic water level reductions lead to a tendency of autotrophic metabolism of these systems.Keywords: carbon dioxide partial pressure (pCO 2 ); carbon sink; carbon source; eutrophication; reservoirs.Resumo: Objetivo: Avaliar a dinâmica do dióxido de carbono (CO 2 ) em reservatórios tropicais semiáridos durante um período prolongado de seca e testar se o estado trófico afeta a saturação de CO 2 . Métodos: Este estudo foi realizado em quatro reservatórios localizados no semiárido tropical do Nordeste do Brasil. Todas as amostragens foram realizadas entre as 9 e as 12 horas com a garrafa Van Dorn. Amostras para pressão parcial de dióxido de carbono (pCO 2 ) foram tomadas na sub-superfície, assim como para fósforo total e clorofila-a. As análises de correlação e regressão linear foram usadas 2 Mendonça Júnior, J.R. et al.