Dissimilarities in the recent histories of two lakes in Portugal explained by local-scale environmental processes

The palaeolimnology of two shallow lakes, Vela and Braças, in coastal Portugal, were investigated. The lakes are situated in a transitional area between a Holocene dune field and agricultural fields. This study focused on the local palaeoecology and palaeolimnology inferred from the stratigraphic records of pollen of hygrophilous taxa and non-pollen palynomorphs, with particular emphasis on algal remains, e.g. Pediastrum. The record of local-scale vegetation changes was utilized to reconstruct the evolution of the local environment in and around the lakes and to identify and explain dissimilarities between the two water bodies. Initially, the sites were wetlands similar to the dune flats and seasonal pools found in the region today. Lake genesis in both basins occurred relatively synchronously, about 2–3 centuries ago, in response to regional geo-hydrological and climatic forcing. Biotic and abiotic dissimilarities between the two lakes have existed since the time of lake formation. Knowledge of the modern ecology of local vascular plants and algae was used to elucidate differences in the recent histories of the two basins. Lake Vela has been more alkaline and displays higher trophic status than Lake Braças. Dissimilarities between the two lakes are probably explained by differences in the flow paths and amounts of input waters, with Lake Vela receiving more nutrient-rich waters from fertilized agricultural fields. Lake Braças receives greater inflow of water that passes through relatively nutrient-poor, acid soils.