The limnological trace of contemporaneous anthropogenic activities in the Pampa Region

Shallow lakes are the most productive freshwater ecosystems in the Argentinian Pampa Region. Only in Buenos Aires province, about 14000 shallow lakes larger than 10 ha exist, which provide important ecosystem services. Since the middle XX century profound changes are occurring in the region as a consequence of the anthropogenic activities, mainly associated to the expansion of agriculture, but also to livestock and urbanization. Different studies strongly suggest that changes in land use, as well as increased eutrophication of lakes, accelerated since 1990’s. In this article, we provide a general overview of the main features of the Pampean shallow lakes and a review of published evidences based on paleolimnological and satellite images studies that indicate changes in the state of the lakes related with eutrophication during the last two centuries. On the other hand, we analyzed the historical changes that are occurring in these systems since the 1980’s until the present by means of the principal variables associated with trophic status. This analysis, based on a data set of 740 samples from 87 shallow lakes, showed that nutrients (TP and TN) and algal biomass (as chlorophyll a) increased over the time, whereas transparency (Secchi depth) decreased; differences between XX and XXI centuries were significant. We also evaluated the change in the cyanobacteria assemblages (abundance and composition) between both centuries. We found that the biomass of this group increased in the XXI century and the composition of the assemblages changed, which nowadays are frequently represented by potentially toxic, bloom forming species. The shallow lakes of the region play an important role as sentinels of changes and all pieces of evidence are showing the impact of the anthropogenic activities.