Vanessa Becker

Is the future blue-green or brown? The effects of extreme events on phytoplankton dynamics in a semi-arid man-made lake

Abstract 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 semi-arid 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.

Effects of water level reduction on the dynamics of phytoplankton functional groups in tropical semi-arid shallow lakes

Abstract Droughts are large-scale perturbations that affect freshwater ecosystems worldwide. A water level reduction caused by drought is an important driving factor of phytoplankton dynamics. It has been suggested that a water level reduction alters the light and mixing regime and increases nutrient concentrations and phytoplankton biomass favoring cyanobacterial blooms. We took advantage of two exceptionally dry years in the Brazilian semi-arid region to investigate the effects of the water level reduction on the water quality and phytoplankton communities of two shallow man-made lakes. In both lakes, the water level was reduced by half, while the water turbidity, conductivity, and nutrient concentrations increased. In the deeper lake, the phytoplankton biomass increased and was dominated by a cyanobacteria group as expected, but it decreased in the shallower lake and was dominated by mixotrophic flagellate groups. This was because of sediment resuspension by wind and fish facilitated by a water level reduction and increased the water turbidity more strongly in the shallower than in the deeper lake. Therefore, a water level reduction caused by a drought may either increase or decrease the phytoplankton biomass and cyanobacteria dominance in tropical shallow lakes depending on the lake depth and the concentration of inorganic suspended sediments.

High Primary Production Contrasts with Intense Carbon Emission in a Eutrophic Tropical Reservoir

Recent studies from temperate lakes indicate that eutrophic systems tend to emit less carbon dioxide (CO2) and bury more organic carbon (OC) than oligotrophic ones, rendering them CO2 sinks in some cases. However, the scarcity of data from tropical systems is critical for a complete understanding of the interplay between eutrophication and aquatic carbon (C) fluxes in warm waters. We test the hypothesis that a warm eutrophic system is a source of both CO2 and CH4 to the atmosphere, and that atmospheric emissions are larger than the burial of OC in sediments. This hypothesis was based on the following assumptions: (i) OC mineralization rates are high in warm water systems, so that water column CO2 production overrides the high C uptake by primary producers, and (ii) increasing trophic status creates favorable conditions for CH4 production. We measured water-air and sediment-water CO2 fluxes, CH4 diffusion, ebullition and oxidation, net ecosystem production (NEP) and sediment OC burial during the dry season in a eutrophic reservoir in the semiarid northeastern Brazil. The reservoir was stratified during daytime and mixed during nighttime. In spite of the high rates of primary production (4858 ± 934 mg C m-2 d-1), net heterotrophy was prevalent due to high ecosystem respiration (5209 ± 992 mg C m-2 d-1). Consequently, the reservoir was a source of atmospheric CO2 (518 ± 182 mg C m-2 d-1). In addition, the reservoir was a source of ebullitive (17 ± 10 mg C m-2 d-1) and diffusive CH4 (11 ± 6 mg C m-2 d-1). OC sedimentation was high (1162 mg C m-2 d-1), but our results suggest that the majority of it is mineralized to CO2 (722 ± 182 mg C m-2 d-1) rather than buried as OC (440 mg C m-2 d-1). Although temporally resolved data would render our findings more conclusive, our results suggest that despite being a primary production and OC burial hotspot, the tropical eutrophic system studied here was a stronger CO2 and CH4 source than a C sink, mainly because of high rates of OC mineralization in the water column and sediments.

Influence of extended drought on water quality in tropical reservoirs in a semiarid region

ObjetivoPeriodos de estiagem sao frequentes no semiarido brasileiro e parecem induzir a degradacao da qualidade da agua, alterando as propriedades fisicas, quimicas e biologicas dos ecossistemas aquaticos. Os reservatorios dessa regiao estao sujeitos a amplas flutuacoes em seus volumes devido a escassez de chuvas e as altas taxas de evaporacao em periodos de estiagem. O objetivo do estudo foi identificar padroes na qualidade da agua de reservatorios durante um periodo de estiagem prolongada. Espera-se que condicoes mais secas e rasas favorecam o crescimento algal divido a maior disponibilidade de nutrientes, causando degradacao da qualidade da agua.MetodosForam realizadas amostragens mensais ao longo de vinte meses (maio de 2011-dezembro de 2012) em dois reservatorios tropicais do semiarido brasileiro. Dados de precipitacao e volume foram obtidos por orgaos ambientais. A transparencia foi medida em campo com disco de Secchi, enquanto que a condutividade, os nutrientes, os solidos suspensos e a clorofila foram analisados em laboratorio. As mudancas temporais nas variaveis ambientais foram analisadas em cada reservatorio, utilizando uma analise de agrupamento two-way e uma analise de componentes principais (ACP).ResultadosO volume de ambos reservatorios reduziu consideravelmente durante o estudo devido ao periodo prolongado de precipitacoes baixas ou ausentes. Foi possivel identificar padroes opostos nas concentracoes de clorofila-a dos reservatorios, a medida que a estiagem se prolongava: no primeiro o crescimento algal foi favorecido e, no segundo, a concentracao de colorofila-a diminuiu devido a alta turbidez inorgânica. Ambos reservatorios apresentaram uma aumento em sua turbidez e condutividade durante o periodo de estiagem prolongada.ConclusoesA reducao do volume durante a estiagem prolongada contribuiu para a degradacao da qualidade da agua devido a alta biomassa algal e a alta turbidez. Fatores locais, como a natureza dos solidos em suspensao, podem desempenhar um papel importante na determinacao da qualidade da agua desses mananciais.

Towards a comprehension of Ceratium (Dinophyceae) invasion in Brazilian freshwaters: autecology of C. furcoides in subtropical reservoirs

Ceratium species are not a common component of freshwater phytoplankton in South America. However, these dinoflagellates have often been observed in many water bodies over the past two and a half decades. We investigated Ceratiumfurcoides’ abundance and morphological variation during its initial phase of colonization (2012–2013) in two subtropical reservoirs in southern Brazil in order to explore which environmental factors were related to the occurrence, persistence and bloom formation of this dinoflagellate in those environments. Biomass of C. furcoides showed a strong seasonal pattern, in which warm seasons led to an increase in population density, resulting in cell-size reduction, while in the cold seasons cells increased in volume. Maximum densities over 2,500 cells ml−1 were observed in spring–summer periods in both reservoirs. C. furcoides’ abundance in the studied reservoirs was associated, primarily, with a combination of optimal conditions of temperature, organic matter, and pH, and secondarily, with nutrient availability. The possible factors for the successful colonization performed by C.furcoides across distinct Brazilian waterbodies include good swimming performance, low herbivory pressure, and ability to form dense blooms, as strategies that allow maintenance of populations and effective dispersal.

Dynamics of plankton and fish in a subtropical temporary wetland: Rice fields

Rice fields are temporary wetlands that harbor many of the same species that breed in natural temporary ponds. These systems have a complex limnology, characterized by rapid physical, chemical, and biological changes. The goal of this study was to evaluate the role of nutrients in the plankton and fish dynamics during a production cycle, based on the auto-ecology of the species related to their adaptations to environmental temporal changes in an irrigated rice field in Southern Brazil. The principal components analysis (PCA) indicated a temporal gradient driven by nutrient availability, grouping sampling periods according to the production cycle. ANOVA indicated temporal differences in the limnological variables during the development of the rice field production cycle. Linear regression showed a positive relationship between chlorophyll a, nutrients and biomass of small and medium filter-feeders. In contrast, planktivorous fish biomass was inversely related to chlorophyll a. This study showed both top down and bottom up processes simultaneously regulating the primary production in the rice field wetland along a temporal gradient the rice production cycle.

Effects of polyaluminum chloride and lanthanum-modified bentonite on the growth rates of three Cylindrospermopsis raciborskii strains

In tropical and subtropical lakes, eutrophication often leads to nuisance blooms of Cylindrospermopsis raciborskii. In laboratory experiments, we tested the combined effects of flocculant polyaluminum chloride (PAC) and lanthanum-modified bentonite (LMB) on the sinking and growth rates of three C. raciborskii strains. We tested the hypothesis that the combination of PAC and LMB would (1) effectively sink C. raciborskii in a test tube experiment and (2) impair C. raciborskii growth, irrespective of the biomass of the inoculum (bloom) and the strain in the growth experiment. We tested the recommended (LMB1) and a three-times higher dose of LMB (LMB3). The combined addition of PAC and LMB enhanced the sedimentation of all C. raciborskii strains. Moreover, both the PAC and LMB doses decreased the phosphate concentration. PAC and LMB1 decreased the growth rate of all strains, but the efficacy depended on the biomass and strain. The combined addition of PAC and LMB3 inhibited the growth of all strains independently of the biomass and strain. We conclude that a low dose of PAC in combination with the recommended dose of LMB decreases C. raciborskii blooms and that the efficiency of the technique depends on the biomass of the bloom. A higher dose of LMB is needed to obtain a more efficient control of C. raciborskii blooms.

Extreme drought favors potential mixotrophic organisms in tropical semi-arid reservoirs

Climate change is affecting the global hydrological cycle and is causing drastic changes in the freshwater hydrological regime. Water level (WL) reduction caused by drought tends to increase the concentration of nutrients favoring the dominance of cyanobacteria. We hypothesized that the WL reduction favors the dominance of cyanobacteria at regular dry conditions, but at extremely dry events mixotrophic algae would thrive because of light limitation due to increased resuspension of sediments on the water column. To test our hypothesis, we compared phytoplankton traits and water quality variables between two sets of reservoirs located in two watersheds with contrasting precipitation regimes within the Brazilian semi-arid. The reservoirs were compared in a dry period and in an extremely dry period to evaluate the response of the variables to an extreme drought. Drought intensification decreased the reservoirs’ WL and water transparency and increased the total phosphorous. Cyanobacteria dominated in the dry period, and the contribution of mixotrophic algae increased in the extremely dry period. Thus, phytoplankton with mixotrophic potential was favored by the extreme drought. This result suggests that this can be one possible scenario for phytoplankton communities in reservoirs of semi-arid regions if extreme droughts become more frequent because of climate change.

Water volume reduction increases eutrophication risk in tropical semi-arid reservoirs

Aim: Global patterns of temperature and precipitation have significantly changed over the last century and nearly all predictions point to even greater changes by the end of 2100. Long periods of drought in semi-arid regions generally reduce reservoirs and lakes water level, increasing the nutrients concentrations in the water. Our principal hypothesis is that water volume reduction, driven by prolonged droughts, will increase reservoirs susceptibility to eutrophication and accordingly an increase in trophic state. To test this hypothesis, we used a comparative analysis of ecosystems in a space-for-time substitution approach, in a Brazilian semi-arid region, to predict the consequences of reservoirs water volume reduction on key limnological variables. Methods: We sampled 16 reservoirs located in two sub-basins with contrasting rainfall regimes, inserted on Piranhas-Açu watershed. The Seridó River basin (SB) is dry and the Piancó River basin (SB) is humid, with annual mean precipitation of 500 and 700 mm, respectively. Linear regressions analyzes were performed to assess whether the percentage of maximum volume stored (%MVS) is a good predictor for total phosphorus (TP), total nitrogen (TN) and chlorophyll-a (CHLA). In addition, a two factorial analysis of variance (two-way ANOVA) was performed to test for period (dry, very dry and extremely dry), basin (SB and PB) and their interactions effects on TP, TN, CHLA, conductivity, turbidity, and Secchi depth. Results: The results showed a reduction in the reservoirs %MVS both for PB and SB regions. At the extremely dry period, all reservoirs were classified as eutrophic, but TP concentrations reached much higher values in SB than in PB. The linear regressions analyses showed that the TP and TN were negatively related to %MVS during all periods sampled. The two-way ANOVA showed that there were significant basin and period effects on TP, TN, Secchi depth and turbidity, whereas for CHLA and conductivity only basin effects were observed. In addition, we found significant interaction effects between period and basin on TP, TN and turbidity. Conclusions: We conclude that the contrasting levels of rainfall observed between the two basins affect the water quality and trophic state of the reservoirs and these effects are magnified by water volume reduction. Therefore, our findings might help to predict the consequences of rainfall reductions on freshwater ecosystems of Brazilian drylands.