Differences in the Vertical Distribution of Two Cladoceran Species in the Nakdong River Estuary, South Korea

Abstract

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.

Behavioral Avoidance Response of Daphnia to Fungal Infection Caused by Metschnikowia Species in a Temperate Reservoir

“…Therefore, some cladoceran species avoid predators by distributing themselves in spaces where predators are reluctant to go (e.g., areas with low DO or phytoplankton) or face difficulties in search for food. The bottom layers of lakes and reservoirs [ 67 , 68 ] and spaces covered by aquatic macrophytes [ 69 , 70 , 71 ] have been suggested as representative refuge spaces for cladocerans to avoid predators. Fish that search for food using vision restrict themselves from foraging activities in spaces complexly created by the leaves and stems of aquatic macrophytes or deep-water levels with low light penetration.…”

“…Kim and Choi [ 68 ] suggested that high turbidity at the bottom layer of an estuary of the Nakdong River, which has a relatively low water depth, induced diel vertical migration of D. obtusa . Inorganic or organic materials transported from the upper and middle streams of the Nakdong River are accumulated in this area, and turbidity of the bottom layer is high owing to the high residence time of those materials.…”

Changes in Antibiotic-Resistance Genes Induced by the Grazing Effect in Three Cladoceran Species

“…Choi and Kim [ 44 ] pointed out that frequent movements of pelagic cladocerans, including D. obtusa , make them easy prey for predators searching for food. As long as there is no refuge, such as aquatic macrophytes in freshwater ecosystems, D. obtusa numbers can be very low or absent due to the continuous foraging activity of fish on cladocerans [ 19 , 45 ]. In particular, it is more difficult for D. obtusa to inhabit areas when fish specialize in the predation of cladocerans, such as Lepomis macrochirus in Korea’s freshwater ecosystems [ 19 ].…”

The Use of Winter Water Temperature and Food Composition by the Copepod Cyclops vicinus (Uljanin, 1875) to Provide a Temporal Refuge from Fish Predation

“…Furthermore, the lack of light and the low dissolved oxygen in the hypolimnion of lakes and reservoirs restrict fish distribution; thus, the deeper layer can be used as shelter by some zooplankton species ( Daphnia spp. ; [ 22 , 23 , 24 ]). Empirical studies explain that the use of refuges by zooplankton is more of an evolutionary mechanism than one with morphological changes, such as increases in length of the head or spin [ 25 , 26 ].…”