Abstract
Salinity is restricting habitatability for many biota in prairie lakes due to limited physiological abilities to cope with increasing osmotic stress. Yet, it remains unclear how salinity effects vary among major taxonomic groups and what role other environmental parameters play in shaping food-web composition. To answer these questions, we sampled fish, zooplankton and littoral macroinvertebrates in 20 prairie lakes (Saskatchewan, Canada) characterized by large gradients in water chemistry and lake morphometry. We showed that salinity thresholds differed among major taxonomic groups, as most fishes were absent above salinities of 2 g L-1, while littoral macroinvertebrates were ubiquitous. Zooplankton occurred over the whole salinity range, but changed taxonomic composition as salinity increased. Subsequently, the complexity of fish community (diversity) was associated with large changes in invertebrate communities. The directional changes in invertebrate communities to smaller taxa indicated that complex fish assemblages resulted in higher predation pressure. Most likely, as the complexity of fish community decreased, controls of invertebrate assemblages shifted from predation to competition and ultimately to productivity in hypersaline lakes. Surprisingly, invertebrate predators did not thrive in the absence of fishes in these systems. Furthermore, the here identified salinity threshold for fishes was too low to be a result of osmotic stress. Hence, winterkill was likely an important factor eliminating fishes in low salinity lakes that had high productivity and shallow water depth. Ultimately, while salinity was crucial, intricate combinations of chemical and biological mechanisms also played a major role in controlling the assemblages of major taxonomic groups in prairie lakes.
Highlights
The formation of saline lakes is favored in endorheic drainage basins that are located in semi-arid and sub-humid climates where evaporation exceeds precipitation [1]
Our analyses showed that diversity and food-web composition in prairie lakes was predominantly correlated with salinity as the strongest predictor for richness and taxonomic composition of fishes, zooplankton and littoral macroinvertebrates
Fishes disappeared quickly with increasing salinity while littoral macroinvertebrates were ubiquitous among lakes
Summary
The formation of saline lakes is favored in endorheic (interior) drainage basins that are located in semi-arid and sub-humid climates where evaporation exceeds precipitation [1]. Across the Canadian prairies, saline lakes are of Previous studies of species assemblages in saline systems illustrated that food-web composition can be considerably different among lakes. Mesosaline lakes, e.g., altiplano lakes of the Andes [8] and prairie lakes in Saskatchewan [1,9], can have extensive food webs with diverse pelagic and littoral invertebrate communities and fish assemblages that include multiple trophic levels. Very simplified food webs are found in many hypersaline lakes (e.g. Lake Nakuru, Kenya; Great Salt Lake, Utah) where algal communities are often restricted to a small number of species, and higher trophic levels are limited to few large-bodied crustaceans, such as Artemia sp., and birds [10]. Richness and abundance are impoverished above salinities of 5–8 g L-1, with most meso- and hypersaline lakes being fishless [1,3,4,11]
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