Effect of Rotifer Community to Microhabitat Structure Changes Caused by Summer- Concentrated Rainfall in a Shallow Reservoir, South Korea

Empirical studies suggest that the structural heterogeneity of aquatic ecosystem microhabitat is
determined by the diversity and abundance of macrophytes. However, excessive accumulation of freefloating
macrophytes on the water surface can reduce the biomass of submerged macrophytes,
resulting in a relatively simplified habitat structure. We hypothesized that heavy summer rainfall disrupts
the growth of free-floating macrophytes covering much of the Jangcheok Reservoir’s water surface,
thereby resulting in a more complex habitat structure by allowing development of a more diverse of
macrophytic community. We divided long-term (2008–2017) monitoring data (rainfall, macrophytes, and
rotifers) into two groups: Rainy and Dry years, corresponding to years with annual rainfall higher and
lower than the total annual average, respectively. We found that summer densities of rotifers fell sharply
in Rainy years, but increased continuously in Dry years. This trend resulted in greater autumn densities
in Rainy relative to Dry years, which we attributed to changes in habitat related to differential
macrophyte development. Moderate disturbance of the water surface caused by high summer rainfall
can promote growth of submerged macrophytes by creating large areas of open water and therefore a
more complex autumnal microhabitat structure, resulting in seasonal variations in rotifer community
structures and populations. Moreover, a highly complex microhabitat structure restricts foraging activity
of fish (i.e., Lepomis macrochirus) that prey on rotifers. Based on these findings, we suggest that
summer-concentrated rainfall plays an important role in supporting the density and species diversity of
rotifers. Based on the results of this study, we conclude that summer rainfall plays an important role in
supporting rotifer diversity. Although summer rainfall negatively affects rotifer density, we found
positive effects in terms of species diversity. We suggest that the high autumn rotifer species diversity
of Rainy years can be attributed to the effects of submerged macrophytes and therefore reduced
predation; however, the differences in species composition are largely influenced by summer rainfall.
Thus, a stable environment free of disturbances such as rainfall has a negative effect on the species
diversity of rotifer communities.