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

Abstract

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.

Influence of extreme hydrological events in the quality of water reservoirs in the semi-arid tropical region

“…Recent studies confirm that most of the semiarid reservoirs have been showing high concentrations of nutrients, solids and algal biomass in severe drought. (GOMES et al, 2015;MEDEIROS et al, 2015;BRAGA;BECKER, 2016;ROCHA JÚNIOR et al, 2018). Therefore, factors such as the climate of the region, size, depth, in addition to the…”

“…The climatic and hydrological factors play important roles in the input, output, water level and water residence time of the reservoirs, directly affecting the limnological patterns BARONE, 2005;SOARES et al, 2012). Seasonal fluctuations in reservoir water levels are associated with changes in the physical, chemical and biological characteristics of these systems and may result in a decrease in water quality in aquatic ecosystems during periods characterized by low rainfall and reduced water volumes (ARFI, 2003;BARONE, 2003;ROCHA JÚNIOR et al, 2018). In addition, global or local climatic changes, when affect the hydrological regime, may compromise the water quality of the water sources, especially as regards the eutrophication process (BRAGA; BECKER; .…”

Long-Term Interannual and Seasonal Links between the Nutrient Regime, Sestonic Chlorophyll and Dominant Bluegreen Algae under the Varying Intensity of Monsoon Precipitation in a Drinking Water Reservoir

“…An elevated nutrient level (N, P) as a result of nutrient loading, for instance, is mostly considered as a significantly important issue that could pose a hazard to the water resource ability in serving the desired ecological functioning [ 18 ]. The variations in nutrient cycling impact the reservoir water quality in numerous ways, such as increased chances of enrichment, intensive organic matter degradation triggering abrupt oxygen depletion, reduced water clarity due to surge in phytoplankton biomass, and sudden inflow or washing out phenomena of nutrients [ 19 , 20 ].…”

Avaliação de nutriente limitante da produção algal em reservatórios do semiárido brasileiro

“…Dessa forma, nos reservatórios eutrofizados do NEB, onde vivem cerca de 50 milhões de pessoas, a identificação do nutriente limitante do crescimento algal é fundamental para o seu gerenciamento. Apesar do conhecimento acumulado sobre a limnologia e o manejo desses reservatórios LIMA NETO, 2017;LACERDA et al, 2018;MOURA et al, 2019), que envolvem as fontes de cargas de nutrientes (FREITAS; RIGHETTO; ATTAYDE, 2011; VIDAL; CAPELO NETO, 2014; ARAÚJO; LIMA NETO; BECKER, 2019) e a influência da seca na qualidade da água (BRASIL et al, 2016;ROCHA JÚNIOR et al, 2018;LIRA;LIMA NETO, 2020), as pesquisas que avaliaram o nutriente limitante da produtividade primária e seus fatores associados são raras e locais. Assim, o presente estudo teve por objetivo testar a hipótese de que o fósforo total seria o principal nutriente limitante do crescimento fitoplanctônico em reservatórios do NEB.…”

“…Segundo a razão de Redfield, os açudes cearenses aqui avaliados, em sua maioria, apresentaram o PT como nu-triente limitante do crescimento algal. Esse resultado apresenta-se em concordância com os estudos desenvolvidos porRocha Júnior et al (2018), nos quais o PT foi considerado um importante fator relacionado com a estimativa da biomassa fitoplanctônica e com a qualidade da água em reservatórios de regiões tropicais e subtropicais. Também está de acordo com Paulino, Oliveira e Avelino (2013), que avaliaram diversos açudes monitorados pela COGERH no período de 2008 a 2011, Nutriente limitante da produção algal para grandes reservatórios do Ceará de acordo com os períodos úmido P1 (2008-2012) e seco P2 (2013-2017) e o modelo aplicado.…”