Historical sediment records from a reservoir with recurrent Cyanobacteria blooms (Salto Grande, Argentina)

GANGI, DANIELA; PLASTANI, SOFÍA; LAPRIDA, CECILIA; DE TEZANOS PINTO, PAULA

Reno, Nevada

Congreso; International Limnogeoglogy Congress ILIC6; 2015

Many reservoirs around theworld suffer the proliferation of cyanobacterial blooms, which pose potentialrisks to both human health and sustainability of ecosystems. Salto Grande Reservoir(Argentina-Uruguay border, 29°43´ to 31°12´ S and 57°06´ to 57°55´ W) filled in1979, is an aquatic system (Uruguay River) affected by recurrent cyanobacterialblooms, mostly during the last decade. Sediments integrate the aquatic systemand catchment environmental conditions into a continuous, high-resolutionarchive of local and regional, natural and human-induced changes, and hence can give information on algalpopulation modifications.  The aim of this work is to analyzeorganic matter, nutrient and fossil pigments from short sediment cores to determinewhen (and why) Cyanobacteria species became dominant. We plan to relate theseresults with abiotic and biotic variables registered in the reservoir (fromstudies undergone just after the reservoir was filled and in subsequent yearsup to the present) in order to identify the forcing factors behind thosechanges. We also plan to study the impact of anthropogenic (agriculture,industries and urbanization) and natural (mainly climatic parameters such astemperature, precipitation) changes that took place in the Uruguay River basin.On February, 2015, we sampled four sites in Salto Grande reservoir, threelocated in arms of the reservoir (3-9m) and one in the main river channel (17m).We will use the latter for paleolimnological purposes and use all sites for assessingthe diversity of dormant phytoplankton and other bioproxies such as ostracods(Arthropoda, Crustacea) in the sediments. In the main river channel (CerroPaloma, 17 mdepth), we obtained a sediment core of 49 cm length. The core was open andmacroscopically described considering grain size, sedimentary structures andcolor by using the Munsell table. The sediments display centimeter-scale laminationand had a predominantly silty-clay composition. We subsampled the core in slices of 1 cm, for running different analysis(organic matter, pigment, nutrients and quantification ofdormant Cyanobacteria cells and ostracods). Model age will be performed by 210Pbdating. Preliminary results indicate that the sediments had high water content(61-66%) and were mostly (mean 88%) of clastic origin (silty clay).Interestingly, alternating dark-gray and light-brown levels have similarcontent of organic matter, and probably reflect in situ redox conditions. Xray images from cores takenfrom the arms of the reservoir display massive bedding as beds appearhomogenous and lacking internal structure, probably as a consequence of intensebioturbation.  This study is supported bythe CARU (Comisión Administradora del Río Uruguay) commission. Within theframe of this ongoing work, we are also planning to run germination experimentsand to quantify akinetes (dormant stages of Nostocales, Cyanobacteria that play a key role inbloom dynamics) in the top core of each of the 4 cores. The data to be obtained will allow estimatingthe annual sedimentation rate in the reservoir, to link Cyanobacteria abundancewith environmental data, and to assess the diversity of dormant cells (whichcan trigger blooms) in Salto Grande reservoir. None of these aspects have been investigateduntil today and will provide valuable information for the reservoir management.