Paleolimnological reconstruction of eutrophication processes in a sub-tropical South American reservoir over the 20th -21st centuries

HALAC SILVANA; MENGO LUCIANA; GUERRA LUCIA; ANDREA LAMI; MUSAZZI S; LOIZEAU JL; ARIZTEGUI D; EDUARDO L PIOVANO

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2020

0895-9811

Reservoirs can provide a detailed record of the changes in input of sediments and nutrients over a period of decades to centuries. Paleolimnological multi-proxy analysis makes it possible to reconstruct basal environmental conditions of aquatic systems and to identify early evidence of change. Our study aims to reconstruct past environmental changes in the San Roque reservoir (Córdoba, Argentina) related to the temporal dynamic of sedimentary and eutrophication processes covering a period of ca. 100 years..Multi-proxy analysis of the sedimentary record, dated by 210Pb and 137Cs, made it possible to identify two stages during the environmental evolution of the San Roque reservoir. Physical processes, such as fluvial influences and water level variation, dominantly governed stage 1 (Unit C) during the initial infilling of the reservoir. Nutrient load and eutrophication processes controlled stage 2 (Units B and A). Stage 1 (77-55 cm; AD 1921 ±15 to 1965 ±7) occurred before and after the second dam was built and while the level of water increased by ≈ + 8 m; it displays a large variation of mean grain size and maximum values of magnetic susceptibility. Stage 2 (55-0 cm; AD 1965 ±7 to 2017) records a new reservoir base level and the maintenance of high water levels and comparatively more stable conditions. Regarding the eutrophication process throughout stage 2, three sub-stages were defined: a) Sub-stage I (55 to 34 cm; AD 1965 ± 7 to 1985 ± 4) is a period of incipient eutrophication; b) Sub-stage II (34 to 14 cm; AD 1985 ± 4 to 2005 ± 1) is an interval of intensification of eutrophication as shown by the increase of several organic proxies related to the abundance of phytoplankton. Echinenone, zeaxanthin and myxoxanthophyll indicate that cyanobacteria increase their concentration three- to fourfold in comparison with the previous sub-stage. c) Sub-stage III (14 to 0 cm; AD 2005 ± 1 to 2017) shows the transition to a hypereutrophic state. Diatomea (fuco and diato), dinophyceae (diadino), chlorophyta (lut) and cryptophyta (allo) groups show a comparatively higher contribution. Our results mostly highlight that during the last century the main drivers of changes in the environmental state of the San Roque reservoir were trophic, fluvial and hydrometeorological. These results might provide tools to predict future scenarios that are extremely necessary for water management plans under increasing anthropic pressure.