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.

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.…”

Extreme droughts drive tropical semi-arid eutrophic reservoirs towards CO2 sub-saturation

“…The current study was performed in four man-made reservoirs located in a tropical semi-arid region during a prolonged drought period from 2012 to 2014 (Mendonça Júnior et al, 2014;Costa et al, 2016). It is already documented that the regular annual dry periods in this area increase the trophic state of the reservoirs and they are usually mostly in eutrophic conditions (Brasil et al, 2016).…”

“…This region is dominated by reservoirs that have been turning eutrophic due to nutrient inputs from land and concentration of the nutrients in the water because of high evaporation rates and long hydraulic retention times (Chellappa et al, 2009;Barbosa et al, 2012;Brasil et al, 2016;Costa et al, 2016).…”

Cyanobacterial blooms in freshwaters bodies in a semiarid region, northeastern Brazil: A review

“…Extreme climatic events are already being observed in Northeast Brazil, causing changes in the phytoplankton community, especially in the dominance of cyanobacteria (e.g., Lira et al, 2014;Câmara et al, 2015;Brasil et al, 2016;Costa et al, 2016). Bouvy et al (2000) made one of the most important reports on cyanoHABs (in almost 40 reservoirs in the Northeast) associated with climate change, when the severe drought of 1998 (a consequence of the El Niño in 1997) made the environment favourable for cyanobacteria proliferation, specially Cylindrospermopsis raciborskii.…”