Phytoplankton limitation in Patagonian and Pampean shallow lakes: effect of phosphorus and light

Abstract

In order to evaluate patterns of phytoplankton community structure and primary production limitation, we analyzed the phytoplankton communities (chlorophyll a, biovolume, species richness, and primary production) of a set of 20 shallow lakes encompassing a wide range of phosphorus and light levels. The degree of limitation or co-limitation by phosphorus and/or light was estimated using a combined index. We found a direct relationship between irradiance in the water column (Imean) and the light saturation parameter (Ik) and, a negative relationship between Imean and total phosphorus (TP) concentration. Lakes at the lowest and highest ends of TP tended to display the simplest phytoplankton communities (i.e., only one or two algal classes accounted for the total phytoplankton biovolume). In contrast, lakes with intermediate TP exhibited a more complex phytoplankton composition. Low values of phytoplankton abundance proxies (chlorophyll a and biovolume), species richness, and primary production could be observed at any combination of phosphorus and light availability. However, the highest values of the previous variables were observed only when the relative availability of both resources was similar. The combined index developed provides a single metric for describing light versus nutrient limitation. The importance of its use is that can be applied to the analysis of clear lakes lacking vegetation and inorganic turbid lakes, besides organic turbid and clear lakes with vegetation.