The limnology and plankton ecology of prarie saline lakes of Saskatchewan

Abstract

Prairie saline lakes are unusual ecosystems with major ecological, economic and recreational importance. Despite their ecological value, anthropogenic activities are conducted with little knowledge of their impacts. Early studies conducted between the 1940s and 1980s were semi-quantitative and limited by available technology. Historical quantitative data sets collected between 1989 to 1995 for nine Saskatchewan prairie lakes whose salinity ranges from 0.2-170.3 g TDS·L-1 were analyzed to determine if there was any relationship between salinity, nutrient concentrations, plankton biomass and community composition. Prairie saline lakes are characterized by high N and P concentrations which increase along a specific conductivity gradient. Specific conductivity is significantly related to total phosphorus, soluble reactive phosphorus, sum nitrogen, ammonia and dissolved oxygen concentrations. Chlorophyll α concentrations were significantly correlated with total phosphorus, sum nitrogen, silica, temperature, surface area and Secchi depth. Sestonic ratios suggest N- and P-limitation for phytoplankton were present in all of the study lakes, but were less severe in the higher salinity lakes. In eutrophic fresh water lakes, elevated nutrients are correlated with high phytoplankton biomass. In prairie saline lakes, phytoplankton biomass is low relative to N and P concentrations. In addition, crustacean zooplankton biomass is high and phytoplankton biomass alone does not appear sufficient to support this zooplankton biomass. The phytoplankton community composition is most strongly affected by salinity, with a greater proportion of flagellate phytoplankton taxa occurring as salinity increases. There is also an unexpected decrease in the proportion of cyanobacteria in the phytoplankton community. As salinity increases, the crustacean zooplankton community shifts from a mixed community to one exclusively dominated by Artemia. Regression models of phyto- and zooplankton biomass developed in fresh water systems were applied to the study lakes to investigate their applicability. Total phytoplankton biomass and inedible phytoplankton biomass were overestimated by TP, while edible phytoplankton biomass was underestimated TP. Zooplankton biomass was not accurately predicted by ch1 α concentrations or total phytoplankton biomass, but predictions based on TP were accurate. The saline lakes tend to have a greater proportion of small, flagellate phytoplankton taxa which are a prime food source for crustacean zooplankton. In these lakes the total phytoplankton biomass was lower than would be expected based on TP concentrations. It is possible that these phytoplankton taxa are capable of withstanding the zooplankton grazing pressure while still managing to outcompete larger, slower growing phytoplankton taxa for nutrients. It is also possible that there is an alternate food source available to the zooplankton, such as benthic algae and/or bacteria.