Empirical studies suggest that the structural heterogeneity of aquatic ecosystem microhabitat is determined by the diversity and abundance of macrophytes. However, excessive accumulation of free-floating 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.

As cladocerans are a primary food source for fish, predator avoidance is important to sustain cladoceran populations. We hypothesized that Bosmina longirostris and Daphniaobtusa would show different vertical distributions that depend on environmental variables and their life cycle phase. Quarterly monitoring was implemented in three water column layers (upper, middle, and bottom) in the Nakdong River Estuary. Cladocerans were mostly observed during summer; B. longirostris and D.obtusa were most abundant and exhibited different vertical distributions. Large (>600 μm) D.obtusa individuals were mainly distributed in the bottom layer (9–11 m) during the daytime and in the upper layer (1–3 m) at night. Utilization of the bottom layer by large D. obtusa was possibly a defense strategy to avoid fish predation. Although the bottom layer was not supported by lower water temperatures and dissolved oxygen than the upper or middle layers, as suggested in the previous study, we assumed that high turbidity replaced this role as a place shunned by fish (and thus a refuge for Daphnia obtusa). In contrast, smaller individuals remained in the upper layer at all times because of the low predation risk. The consumption of B. longirostris by fish was low, as the largest B. longirostris (411 μm) was smaller than the small-sized Daphnia. From this finding, we suggest that the vertical distribution of cladocerans likely depends on selectivity feeding based on fish size rather than the presence/absence of fish. We considered that these results are an important advance in understanding distribution patterns of cladocerans related to environmental features, as well as their key predators.