Spatial and Temporal Variation of Physicochemical Parameters in a Shallow Reservoir Seasonally Covered by Pistia stratiotes L. in Uruguay

ABSTRACT

Spatial and temporal variation of physicochemical parameters of Cisne reservoir were studied. A clear spatial pattern was observed between the tributaries and the reservoir, with the highest values of conductivity, alkalinity, pH, silicate and phosphate (SRP) in the tributaries. The incorporation of bicarbonate, silicate, SRP and other nutrients by littoral vegetation and the decomposition process of macrophytes could explain the spatial pattern and the temporal variation of the nutrients. The freshwater marsh situated between the tributaries and reservoir acts as sink for silicate and SRP. The temporal and spatial pattern of total phosphorus (TP) was correlated with suspended matter values. The low nitrogen concentration and the ratio of the inorganic nitrogen to SRP suggest that nitrogen is a limiting factor for primary production.

Some seasonal vertical gradients of SRP, TP, and silicate were observed, with the highest concentrations at the surface or at 0.5 m of depth; these could be due to water exchange between littoral and pelagial zones.

According to principal component analysis, three main features characterize the spatial and temporal patterns: a) water mineralization changes, b) the supply of suspended matter and nutrients from the tributaries and littoral zone, and c) the seasonal variations of temperature and oxygen.