Nutrient levels, trophic status and land-use influences on streams, rivers and lakes in a protected floodplain of Uruguay

Land-use intensification impacts freshwater ecosystems in many agricultural landscapes, including the Rio de la Plata grasslands in Southeastern South America. We evaluated water chemistry of rivers, streams and lakes associated with a protected river-floodplain in the Queguay River basin in Uruguay. From 2019–2021, we measured phosphorus (P) and nitrogen (N) forms, dissolved oxygen (DO), turbidity and other chemical parameters; basin-scale land-use via Sentinel satellite imagery; and trophic status based on nutrient concentrations and Principal Components Analysis (PCA). Results highlighted low nutrient concentrations in the Queguay rivers, indicating this watershed as a national reference for evaluating eutrophication in Uruguay. Levels of total nitrogen (TN) in rivers averaged 430–510 µg/L and total phosphorus (TP) averaged 44–51 µg/L, classifying them as oligotrophic-mesotrophic systems. Streams ranged in TP from 51 to 97 µg/L and in TN from 545 to 701 µg/L, suggesting meso- to eutrophic states according to TP levels, and potential anthropogenic eutrophication in smaller basins with crop cover > 25%. Floodplain lakes are a conservation target within the protected area and had TP averaging 74–109 µg/L, suggesting eutrophic-hypereutrophic states based on TP and potential external nutrient inputs. The positive correlation between basin-scale crop cover and mean TP and soluble reactive phosphorus (SRP) concentrations suggested that land-cover plays a role in nutrient levels of fluvial and lotic systems. This study establishes a baseline for water chemistry in lakes and fluvial ecosystems in the Queguay River basin within an extensive agricultural region of the La Plata River Grasslands. By providing the first time series dataset of water chemistry for this area, we fill a major geographical gap regarding the limnological attributes of freshwater ecosystems in northwestern Uruguay. Results highlight the value of the Queguay basin for freshwater habitat conservation, and the vulnerability of these systems to land cover changes and nutrient loading from external sources.