Phytoplankton biomass is mainly controlled by hydrology and phosphorus concentrations in tropical hydroelectric reservoirs

Abstract

Phytoplankton is widely recognized as being regulated mainly by resources (nutrients and light) and predation by higher trophic levels. In reservoirs, these controls also can be modulated by hydrology, for example through the influence of flow pulses generated by the operation of the dam. In this study, we tested the influence of light, nutrients, and zooplankton grazing pressure, and also hydrology (as water residence time) on the phytoplankton biomass in eight tropical hydroelectric reservoirs, which differ in size, morphometry, location, trophic state, and water residence time. Our hypothesis was that, as these reservoirs are used for hydroelectric purposes, the control that would otherwise be exerted on phytoplankton biomass primarily by resource availability and grazing will also be modulated by hydrology. Low phytoplankton biomass (range of system medians = 12-299 lg C l -1 ) occurred in most systems, except for one highly eutrophic reservoir (median = 1331 lg C l -1 ). Our data showed that phosphorus was more often likely to be the limiting nutrient in these systems, as assessed through nutrient limitation indexes (nitrogen and phosphorus), based on concentrations and ratios. For most reservoirs, excluding the eutrophic system with high cyanobacteria biomass, seasonal water residence time was the variable that best explained phytoplankton variation among the several environmental variables analyzed in this study (P \ 0.0001; adjusted r 2 = 0.38). Hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability.

<b>Preliminary analysis on the use of Trophic State Indexes in a brazilian semiarid reservoir

“…Nonetheless, frequent instabilities in the water column may be verified naturally in reservoirs due to external factors (rain and wind) (DANTAS et al, 2008;FREIRE et al, 2009;CHAVES et al, 2013;MOLISANI et al, 2013), operational (as water release regime) (RANGEL et al, 2012) DANTAS et al, 2008;. These factors may act together by restricting or triggering primary productivity of the systems.…”

“…When nutrient concentrations are above limiting levels, other factors beyond light limitation may limit or co-limit primary production in reservoirs, such as zooplankton grazing, decrease in residence time (RANGEL et al, 2012), exporting organisms and nutrients downstream during periods of high discharge . In this direction, new surveys are being conducted to examine possible influences of these factors on algal production in the Pereira de Miranda Reservoir.…”

The effects of river inflow and retention time on the spatial heterogeneity of chlorophyll and water–air CO&lt;sub&gt;2&lt;/sub&gt; fluxes in a tropical hydropower reservoir

“…The river inflow is affected by the water supply and the operation of upstream dams. In general, the Funil Reservoir is a turbid, eutrophic system, with high levels of phytoplankton (cyanobacteria) biomass (Soares et al, 2012;Rangel et al, 2012).…”

Spatial heterogeneity and seasonal succession of phytoplankton functional groups along the vertical gradient in a mesotrophic reservoir

“…These ecosystems therefore represent important elements of Earth's freshwater network although they often lie in the upper part of the trophic spectrum and their water exploitation can be seriously compromised by the occurrence of toxic cyanobacteria blooms . Conversely to natural lakes, reservoirs are actually subjected to operational procedure, which may interfere with their physical, chemical and biological characteristics (Naselli-Flores, 2000;Rangel et al, 2012). In particular, Mediterranean reservoirs show wide water-level fluctuations, which may influence the ratio between mixing depth and euphotic depth and ultimately influence phytoplankton structure (Naselli-Flores, 2014).…”