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

We reconstructed and interpreted the historical trends of cyanobacteria of a large South American Reservoir -Salto Grande, built in 1979- using a multiproxy approach. This water body has been suffering recurrent cyanobacteria blooms for at least 15 years, yet when blooms increased remains unexplained, as continuous monitoring started two decades after its construction. We also aimed to identify the possible drivers of cyanobacteria blooms, and to characterize the current inocula of akinetes (dormant cells) for assessing potential future bloom magnitude and composition. For this we obtained duplicate cores at the geographical center of the reservoir (17 m depth) and ran several paleolimnological proxies (geochemical, biological, and physical proxies) and germination experiments. The paleolimnological records obtained allowed reconstructing the history of the dam since its construction (40 years) filling the two decades gap in information. Cyanobacteria were present since the onset of the dam at low numbers but abruptly increased (more than two fold) after year 2003. Cyanobacteria trends tracked the shifts in the phytoplankton as a whole, without triggering major community reorganizations. Increased pigments coincided with a signal of lower oxygen in the sediments (presence of obligate anoxic bacteria, and drop of nutrients and in magnetic susceptibility), increased grazing and sedimentation rate. Regarding environmental historical variables, air temperature significantly increased since 1967, while precipitation, water level and water discharge showed non-significant trends. The akinete pool was low, yet a total of 5 cyanobacteria species germinated in laboratory conditions. Three species -all within Dolichospermum- are also found in contemporary blooms. The relative abundance of hatchings reflected well the composition of Nostocalesblooms. The major changes observed in this ecosystem seem to be related to the interplay between limnological, environmental and human (marked changes in land cover and increased population) causes. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large reservoirs.