Recent cyanobacteria abundance in a large sub-tropical reservoir inferred from analysis of sediment cores

GANGI, DANIELA; PLASTANI, MARÍA SOFÍA; LAPRIDA, CECILIA; LAMI, ANDREA; DUBOIS, NATHALIE; BORDET, FACUNDO; GOGORZA, CLAUDIA; FRAU, DIEGO; DE TEZANOS PINTO, PAULA

JOURNAL OF PALEOLIMNOLOGY

SPRINGER

Lugar: Berlin; Año: 2020

0921-2728

Abstract Salto Grande is a large South American reservoir on the border between Uruguay andArgentina that was impounded in 1979 and experiences recurrent, massive cyanobacteria blooms. Awater-monitoring program was initiated 20 years after the dam was built, hence the causes and onset ofcyanobacteria blooms remain poorly known. We collected two sediment cores from the old riverchannel 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 waterbody. Cyanobacteria fossil pigments were present at low concentrations during the first 24 years afterimpoundment, but more than doubled thereafter. Phytoplankton abundance tracked shifts in cyanobacteria pigment concentration, indicating an overall increase in all primary producers. Several sedimentvariables indicate a decline in water quality after 2003, such as increases in the number of photosyntheticsulfur bacteria and a reduction in sediment magnetic susceptibility. Akinetes (dormant cyanobacteria cells, Order Nostocales) in recent reservoir deposits were abundant and five species germinated under laboratoryconditions, underscoring the ability of akinetes to initiate cyanobacteria blooms. The germinated assemblage reflected closely the composition of cyanobacteria blooms in the reservoir. Recorded increases in airtemperature and decreases in wind speed, together with other variables (e.g. nutrients), can promote thelarge, recurrent cyanobacteria blooms. Invasion of the bivalve Limnoperna fortunei apparently promotedcyanobacteria blooms by preferential feeding on other phytoplankton taxa, and perhaps by altering nutrientconcentrations and ratios. This work highlights the potential for using multiple variables in sedimentcores from large reservoirs to better understand the responses of biota to multiple environmental stressors.