Phytoplankton composition changes during water level fluctuations in a high-altitude, tropical reservoir

Abstract

To understand the responses of phytoplankton to water level fluctuations (WLFs) in tropical conditions at high altitude, limnological monitoring was performed monthly in Valle de Bravo (VB) reservoir, a waterbody affected by relatively strong WLFs due to water scarcity in central Mexico during 2008–2009. The thermal behaviour of the reservoir was warm, monomictic (Nov–Feb), and water levels fell to 12 m below capacity. Observed hypolimnetic temperature increases in VB are considered indirect evidence of boundary mixing events, previously attributed to internal waves driven by strong diurnal winds along a 7 km fetch. Unusually low biomasses were observed during circulation, probably due to low light availability (Zeu/Zmix < 1), grazing, and relatively low temperatures (<20 °C). In contrast, enhanced phytoplankton biomass was observed throughout stratification, which could be explained by nutrient inputs to the epilimnion through boundary mixing, as previously proposed for VB, and by higher (>20 °C) temperatures. Dominant species were Cyclotella ocellata, Fragilaria crotonensis, Woronichinia naegeliana, and Microcystis wesenbergii. Nitrogen-fixing cyanobacteria (Nostocales), a recurrent problem in this reservoir, were markedly less abundant than during high-water periods (2002–2005), probably due to dissolved inorganic nitrogen (DIN) inputs from the hypolimnion. A profuse formation of heterocysts was observed at the same time that DIN and the DIN to soluble reactive phosphorus ratio (DIN:SRP) were low (<2 μmol L−1 and ~1 respectively), suggesting heterocyst development could be a complementary proxy to assess DIN limitation. Intense water level decreases in this reservoir resulted in increasing biomasses of planktonic diatoms and a significant decrease in noxious algae such as Nostocales, thus ameliorating the water quality of the reservoir.