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 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.

Assessing temporal and spatial variability of phytoplankton composition in a large reservoir in the Brazilian northeastern region under intense drought conditions

“…This finding agrees with other studies performed in semi-arid reservoirs (Medeiros et al, 2015;Costa et al, 2016), that have shown, in contrast with the expectations, that a reduction in water level would led to an increase in cyanobacterial dominance (Naselli-Flores et al, 2007;Yang et al, 2016), other phytoplankton groups (such as cryptomonads and diatoms) could also improve their performance under scenarios of extreme drought.…”

“…Overall, our results suggest that in accordance with other studies, shifts in phytoplankton community were associated to changes in nutrient concentrations and water transparency (Medeiros et al, 2015;Costa et al, 2016). Changes in these environmental conditions were closely associated with hydrological changes in the reservoir, namely the reduction in water level due to years of severe drought.…”

Phosphorus dynamics in the water of tropical semiarid reservoirs in a prolonged drought period

“…The inorganic turbidity can be caused by resuspension of the sediment, through the action of the winds, since the reservoir presented low depths (<2.5 m) during the period of the study. A lower availability of light in the water column may lead to inhibition of phytoplankton growth as reported in other studies in the Brazilian semiarid region (Medeiros et al, 2015;Braga et al, 2015). As a result, the Cruzeta reservoir had lower values of chlorophyll, despite the high SRP concentration, compared to the Gargalheiras reservoir.…”

“…The Cruzeta reservoir had a unique behaviour for semiarid reservoirs as predominance of inorganic solids in this reservoir provided a high turbidity (Braga et al, 2015;Medeiros et al, 2015). In Cruzeta, Chl-a decreased while water volume reduced; this behaviour is contrary to many semiarid reservoirs.…”

“…They are often the main external source of nutrients arriving in the reservoirs during the rainy seasons through leaching of the soil (Oliveira, 2012;Medeiros et al, 2015). This is because, in semiarid regions, diffuse pollution of water sources may be high due to peculiar natural conditions of the region, notably shallow soils with little coverage due to characteristic vegetation (Oyama & Nobre, 2004).…”