Environmental changes in the southeastern Pampa plain (southeastern South America) during the last millennium based on multiple lacustrine indicators and historical records

Paleoenvironmental analyses have been widely used to infer the fluctuations in past environmental conditions of lakes and reservoirs due to either anthropogenic and/or climatic influences. The Siete Lomas shallow lake (37° 08′ S, 57° 38′ W) is located in an area with a long history of anthropogenic impacts in the Pampa plain, southeastern Argentina. Our aim was to reconstruct the paleoenvironmental history of the Siete Lomas shallow lake and to identify the main forcings of change during the last 1000 years. A multi-proxy analysis was performed including pollen and non-pollen palynomorphs, diatoms, fossil pigments, plant macrofossils and the associated fauna, Total Phosphorus (TP) and organic matter. Two main environmental stages (the latter including two substages) throughout the history of the Siete Lomas lake were recognized as a result of the action of natural variability and anthropogenic drivers. Stage I (1100–1880/1900 AD) recorded a pre-agricultural period when human impacts were negligible, and was considered as reference conditions. In this stage, a temporary, brackish and oligotrophic lake was inferred. Stage II was composed by two substages. Substage II-1 (1900–1945 AD) suggested the establishment of a permanent freshwater lake with higher pelagic primary production and an incipient eutrophication process, associated to increased anthropogenic activity and higher precipitation. Nutrient load and eutrophication processes controlled Substage II-2 (1945–2021), as a productive lake subject to increased eutrophication as a consequence of nutrient loadings from the surrounding catchment was identified. In particular, a highly productive, turbid-hypereutrophic and permanent lake related to the increase in agricultural activities was recognized for the period 2005–2021 AD. Overall, during the early period of the lake history, changes in dominant communities were mainly controlled by climate whereas after 1900 AD anthropogenic and natural forcing synergistically promoted the generalized degradation of the water quality of the Siete Lomas shallow lake.