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

GANGI, D.; PLASTANI, M.S.; LAPRIDA, C.; DE TEZANOS PINTO, P.

Reno

Congreso; International Limnogeological Congress; 2015

Many reservoirs around the world suffer the proliferation of cyanobacterial blooms, which pose potential risks 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 in 1979, is an aquatic system (Uruguay River) affected by recurrent cyanobacterial blooms, mostly during the last decade. Sediments integrate the aquatic system and catchment environmental conditions into a continuous, high-resolution archive of local and regional, natural and human-induced changes, and hence can give information on algal population modifications. The aim of this work is to analyze organic matter, nutrient and fossil pigments from short sediment cores to determine when (and why) Cyanobacteria species became dominant. We plan to relate these results with abiotic and biotic variables registered in the reservoir (from studies undergone just after the reservoir was filled and in subsequent years up to the present) in order to identify the forcing factors behind those changes. We also plan to study the impact of anthropogenic (agriculture, industries and urbanization) and natural (mainly climatic parameters such as temperature, precipitation) changes that took place in the Uruguay River basin. On February, 2015, we sampled four sites in Salto Grande reservoir, three located 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 assessing the diversity of dormant phytoplankton and other bioproxies such as ostracods (Arthropoda, Crustacea) in the sediments. In the main river channel (Cerro Paloma, 17 m depth), we obtained a sediment core of 49 cm length. The core was open and macroscopically described considering grain size, sedimentary structures and color by using the Munsell table. The sediments display centimeter-scale lamination and 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 of dormant Cyanobacteria cells and ostracods). Model age will be performed by 210Pb dating. 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 similar content of organic matter, and probably reflect in situ redox conditions. Xray images from cores taken from the arms of the reservoir display massive bedding as beds appear homogenous and lacking internal structure, probably as a consequence of intense bioturbation. This study is supported by the CARU (Comisión Administradora del Río Uruguay) commission. Within the frame of this ongoing work, we are also planning to run germination experiments and to quantify akinetes (dormant stages of Nostocales, Cyanobacteria that play a key role in bloom dynamics) in the top core of each of the 4 cores. The data to be obtained will allow estimating the annual sedimentation rate in the reservoir, to link Cyanobacteria abundance with environmental data, and to assess the diversity of dormant cells (which can trigger blooms) in Salto Grande reservoir. None of these aspects have been investigated until today and will provide valuable information for the reservoir management.