Environmental filtering of crustacean zooplankton communities in fishless boreal lakes: expectations and exceptions

To test the paradigm that large-bodied zooplankton species dominate in fishless lakes, we evaluated crustacean zooplankton composition of 53 fishless boreal lakes across a broad geographic scale (north-west Saskatchewan, Canada) in relation to contemporary environmental factors. Lake productivity, acid–base status and intensity of invertebrate predation were the three major environmental filters of crustacean zooplankton composition of the survey lakes, while lake morphometry and drainage basin characteristics likely had indirect effects by influencing the variations in above environmental factors. The traditionally expected dominance of large-bodied zooplankton species in fishless conditions only occurred in well-buffered shallow lakes with higher productivity and intense invertebrate predation. Daphnia pulex was dominant in more eutrophic lakes, while higher abundances of Holopedium gibberum were favored by relatively mesotrophic conditions. In exception to the traditional view, small-bodied species dominated in 57% of survey lakes as influenced by interactions of biotic and abiotic factors. Bosmina longirostris and cyclopoid copepods dominated less productive, deeper lakes with low invertebrate predation. Predominance of Leptodiaptomus minutus was supported under unfavorable environmental conditions for other species, i.e. poor buffering capacity and/or low calcium concentration. The present study demonstrates that exceptions to the generally expected species composition in fishless lakes can be caused by interactions among abiotic and biotic factors.