Structure of plankton and waterbird communities under water level fluctuations: two case studies in shallow lakes of the Patagonian steppe

The Buenos Aires Lake Plateau, a unique semi-arid environment and priority area for waterbird conservation in Argentina, has recently shown reductions in lake sizes due to scarce rainfall. This study assessed the impact of contrasting hydrological conditions on the abiotic variables of lakes Chapu and Cervecero, as well as on the structure of plankton and how this affected the use of the lakes by waterbirds. We show that hydrological conditions shape the abiotic and biological features of these lakes. In 2015, both lakes were vegetated, but by 2016, their water level had dropped, causing decreased macrophyte coverage. In 2017, the lakes dried up completely, but were refilled the following year. The partial drought of 2015-2016 led to reduced concentrations of nutrients and food that support a high density and richness of birds, which are mostly aquatic and terrestrial feeders. After the extreme drought in 2017 and refilling phase in 2018, there was an improvement in the water quality of lake Chapu, as indicated by its increased clarity compared to that in 2015, while Cervecero remained turbid. Increased homogenization in plankton and waterbird species composition became evident in both lakes. There was a cyanobacteria bloom in Cervecero, while in Chapu Gammaproteobacteria were dominant. Both lakes supported few waterbirds, of which aquatic-terrestrial feeders and terrestrial herbivores were dominant. The abundance of the endemic and endangered hooded grebe (Podiceps gallardoi Rumboll 1974) declined with the cover of macrophytes, which these birds use when establishing their colonies. In sum, this study revealed changes in the conditions of lakes Chapu and Cervecero and their plankton communities in response to hydrological shifts, with implications for waterbird assemblages and, thus, biodiversity conservation.