Magnitude of Blue-Green Algal Blooms in a Saline Desert Lake Evaluated by Remote Sensing: Evidence for Nitrogen Control

Seventy-six percent of the 1972–86 variability in remotely sensed maximum surface water biomass of Nodularia spumigena blooms in Pyramid Lake, Nevada, was explained by concentrations of surface water total nitrogen (TN) during the preceding winter circulation. Very large summer–autumn Nodularia blooms were recorded by Landsat subsequent to low early winter surface water TN concentrations, very low fluvial discharge, and fluvial TN loads as predicted, but contrary to predictions, also when fluvial discharge and TN loads were very high. The observed cause of large Nodularia blooms following high fluvial nitrogen influx was temporary meromixis produced by the large volume of freshwater transporting this nitrogen. Incomplete winter lake circulation reduced internal transport of hypolimnetic nitrogen to surface waters. Meromixis resulted in early stable summer thermal stratification and early depletion of epilimnetic inorganic nitrogen, followed by early and large Nodularia blooms. Variations in magnitude of historic Nodularia blooms in a saline desert lake support the generality of blue-green algal dominance in lakes with low nitrogen and high phosphorus concentrations (i.e. low N:P ratios). However, climatic forcing of fluvial discharge and lake mixing patterns were also shown to be important determinants of seasonal succession of phytoplankton to blue-greens.