Competition or niche segregation between Holopedium and Daphnia; empirical light on abiotic key parameters

A survey of 346 Norwegian lakes revealed some consistent patterns with regard to the occurrence of Holopedium gibberum versus Daphnia spp. Both biomass and frequency of Holopedium were negatively correlated with pH and Ca-concentration, and this species rarely occurred at pH > 8, or Ca-concentrations > 10 mg l^-1. On the contrary, the various Daphnia species, although showing different susceptibilities to fish predation and different preferences for lake productivity, were all positively correlated with Ca. Statistical analysis as well as laboratory experiments and biochemical considerations, suggest that Holopedium may be a superior competitor at very low Ca-concentrations, but is replaced by Daphnia at higher pH and increasing Ca-content. High pH in itself is possibly disadvantageous for Holopedium by preventing formation of the muco-polysaccaride mantle.