Linking primary productivity and benthic community shifts in Salto Grande Reservoir

Abstract

The Salto Grande Hydroelectric Complex was built in 1979 in the course of the Uruguay River, upriver from the cities of Concordia (Argentina) andSalto Grande (Uruguay). Since year 2000, the Reservoir has been suffering recurrent blooms of cyanobacteria. In this study we analyze for the firsttime sedimentary pigments from the sediments of its reservoir in order to reconstruct phytoplankton communities prior to regular monitoring effortsinitiated in year 2000. Based on this paleolimnological archive and a well- established 210Pb chronology, we aimed to analyze if the phytoplanktoncomposition has changed significantly during the last decades, and which could have been the main environmental drivers that lead cyanobacteriablooms. During the first 20 years after inundation, diatoms, cryptophytes, chlorophytes and cyanobacterias dominate the phytoplankton communities.An abrupt change in productivity occurred after ca. 2003, but with no major shift in phytoplankton composition. This change is reflected in benthiccommunities: the increase in pigments belonging to sulphurphotosynthetic anoxygenic bacteria (i.e. green sulphur and purple sulfur bacteria) suggest amarked lowering in the oxygen concentrations. The causes of dysoxia are still elusive, but the interplay between climatic tendencies promoting waterstratification especially during summers, and enhanced carbon flux related to higher phytoplankton productivity, could have increased the oxygendemand: the utilization of metabolizable organic matter could have exceeded the rate of oxygen supply due to stratification, favoring dysoxia. Thiswork highlights the potential of the paleolimnological approach for disentangling modern ecological trajectories in absence of monitoring programs.