Assessment of 'top-down' and 'bottom-up' forces as determinants of rotifer distribution among lakes in Ontario, Canada

Predation and food supply are generally perceived as important determinants of spatial and temporal variations of populations. The population dynamics of freshwater rotifers have been well researched in this aspect. However, their spatial variations have attracted less attention and have not been studied by simultaneously considering both predation and food supply. We studied spatial variations of rotifer abundance among 34 Canadian boreal lakes. A large part of the variance of rotifer abundance was associated with variables related to trophic status including chlorophyll a and total phosphorus. However, abundances of mesozooplankton such as potential predators and competitors did not correlate with rotifer abundance and did not explain the residual of the regression between rotifer abundance and chlorophyll a. The results of the present study indicated that variation in rotifer abundance among lakes was caused by 'bottom-up' forces related to food supply and not by 'top-down' predatory interactions. This provides a contrast to previous empiric and experimental studies that reported that temporal variations of rotifer abundance were mainly regulated by 'top-down' interactions. This discrepancy suggests that overall differences in rotifer abundance among lakes are mainly determined by 'bottom-up' forces while temporal changes in single lakes are shaped by 'top-down' forces. Meanwhile, the composition of rotifer species was correlated with mesozooplankton abundance as well as trophic status. Rotifer species with long spines or rigid loricae were found in the lakes where mesozooplankton were abundant, which suggests that defensive morphology could have affected the rotifer species distribution among the study lakes.