No abstract

Salto Grande is a large South American reservoir on the border between Uruguay and Argentina that was impounded in 1979 and experiences recurrent, massive cyanobacteria blooms. A water-monitoring program was initiated 20 years after the dam was built, hence the causes and onset of cyanobacteria blooms remain poorly known. We collected two sediment cores from the old river channel in the reservoir (z = 17 m) and used physical, chemical and biological variables in the sediments, along with existing limnological data, to explore the history of cyanobacteria in the sub-tropical water body. Cyanobacteria fossil pigments were present at low concentrations during the first 24 years after impoundment, but more than doubled thereafter. Phytoplankton abundance tracked shifts in cyanobacteria pigment concentration, indicating an overall increase in all primary producers. Several sediment variables indicate a decline in water quality after 2003, such as increases in the number of photosynthetic sulfur bacteria and a reduction in sediment magnetic susceptibility. Akinetes (dormant cyanobacteria cells, Daniela Gangi and Marı ´a Sofı ´a Plastani share first authorship.