The Argentinean Pampas is an ecologically disturbed region where patchy wetlands form complex habitats with surrounding agricultural areas. Numerous bird species concentrate in these sites at breeding colonies and communal roosts. For two consecutive years, point censuses were made of birds in the vicinity of a roosting and breeding area at Los Padres Lake in the south-east Pampas's plain. In parallel with these censuses, limnological analyses (e.g. nutrient concentration, conductivity, water transparency and temperature) were conducted to correlate changes in the structure of waterbird assemblages with environmental variability. Seasonal variability in the structure of waterbird assemblages could be due to changes in the birds' habitat use and reproductive phenology through the year (e.g. colonies, foraging and roosting sites) and land use in neighboring areas (e.g. for agricultural practices). Waterbird abundance in July and August correlated with phosphorus and nitrogen concentrations, which were linked to changes of waterbird assemblages and their feeding behavior. The present study shows the potential importance of waterbird communities in contributing biotic nutrients to Pampas wetlands.