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Background This study aimed to identify NO3−-N sources using the stable isotope δ15N in Chydorus sphaericus (OF Müller), to investigate hydrological characteristics and nutrient states in artificial wetlands near the Nakdong River. Chydorus sphaericus is dominant in wetlands where aquatic plants are abundant, occurring in high density, and is sensitive to wetland water pollution, making it suitable for identification of NO3−-N sources. Results NO3−-N sources for each wetland were strongly dependent on hydrological characteristics. Wetlands with sewage or rainfall/groundwater as their main sources had high levels of NO3−-N, whereas wetlands with surface water as their main input had comparatively lower levels. Since wetlands with sewage and rainfall/groundwater as their main water sources were mostly detention ponds, their inputs from tributaries or the main river stream were limited and nutrients such as NO3−-N easily become concentrated. Changes in NO3−-N levels at each wetland were closely associated with δ15N of C. sphaericus. Interestingly, regression analysis also showed positive correlation between δ15N of C. sphaericus and NO3−-N level. Conclusions We conclude that the nitrate stable isotope (δ15N) of C. sphaericus can be used to elucidate the hydrological characteristics of riverine wetlands. This information is important for maintenance and conservation of artificial wetlands at the Nakdong River.

The objective of our study was to investigate sexual reproduction of Daphnia magna associated with mating behaviors and hatching rates, according to different algal food sources. Since a diatom is known to contain more abundant long‐chain poly unsaturated fatty acids (PUFAs), we hypothesized that the diatom‐consuming D. magna would exhibit more successful reproduction rates. Upon the hypothesis, we designed three experiments using two algal species, a green alga (Chlorella vulgaris) and a diatom (Stephanodiscus hantzschii). From the results, we found that the mating frequency and copulation duration increased in the treatment with S. hantzschii, resulting in a significant increase of hatching rates of resting eggs. In the other two repetitive mating strategies (e.g., one female vs. multiple males, and one male vs. multiple females), we found that the hatching rates of resting eggs were greater in the S. hantzschii treatment. In addition to the mating strategy, male body size significantly increased in the diatom treatment, hence average diameter of penis was also statistically different among the treatments (greater diameter in the S. hantzschii treatment). To examine the effect of algal food quality, we estimated quantity of fatty acids in the two algal species. Our result showed that S. hantzschii had a higher proportion of long‐chain PUFAs than C. vulgaris. Furthermore, a stable isotope analysis revealed that carbon and nitrogen originated from S. hantzschii were more assimilated to D. magna. In summary, our study manifested that diatom consumption of D. magna leads to more successful sexual reproduction. We then discussed how the diatom consumption of zooplankton influences food web dynamics in a freshwater ecosystem.

Zooplankton community distribution depends largely on the microhabitat characteristics of the water body. It has been reported that macrophytes provide microhabitats for zooplankton (e.g., space and food resources). To date, studies have focused on the overall influence of macrophytes on zooplankton (e.g., positive relationships with zooplankton diversity); however, the morphological characteristics of macrophytes have not been intensively studied. To fill this gap in knowledge, we investigated zooplankton abundance and diversity, macrophyte characteristics (types, dry weight, and species number), and physicochemical parameters (water temperature, dissolved oxygen, pH, conductivity, and chlorophyll a) by using the 1 × 1 m quadrat method. We surveyed 164 wetlands in South Korea during spring (May to June), prior to the summer monsoon. Patterning zooplankton distribution was accomplished using a Self-organizing map (SOM). We used 34 input variables (zooplankton genera) to train the model. The distribution of five plant habit parameters (no plant, emergent, free-floating, floating-leaved, and submerged) was investigated with a trained SOM plane, by environment data masking. Based on a U-matrix, three clusters were identified from the model. Zooplankton assemblages were positively related to macrophyte characteristics (i.e., dry weight, species number, and plant type). In particular, free-floating plants supported rotifers, such as Testudinella, and cladocerans, such as Alona, Chydorus, Diaphanosoma, and Ilyocryptus (mostly epiphytic). Submerged plants were associated with planktonic rotifers, such as Filinia, Ploesoma, Synchaeta, cladocerans, such as Daphnia, and copepods, such as Eucyclops and Macrocyclops. On the basis of these results, we suggest that the microhabitat structure, created by macrophytes, is an important factor in determining the diversity and abundance of zooplankton communities, because the different species compositions of macrophytes support diverse zooplankton genera in these habitats. The results indicate that macrophytes are the key components of lentic freshwater ecosystem heterogeneity, and the inclusion of diverse plant species in wetland construction or restoration schemes will result in ecologically healthy food webs.

In the present study, we investigated morphological changes of hatchlings from asexually and sexually produced eggs of Daphnia galeata and D. similis against fish kairomones. In both species, hatchlings from asexual eggs showed induced traits, such as short body length, long tail spine and high relative tail spine length when influenced by fish kairomones. However, hatchlings from sexual eggs expressed high relative tail spine length regardless of kairomones presence. The results showed that the inducibility of morphological defences depends on the egg type, with inducible defences predominant in asexual eggs while fixed defences are employed by the offspring of sexual eggs in Daphnia.

We collected zooplankton from May to October, 2011, with the aim of understandingthe zooplankton community distribution and food web interaction between the openwater and littoral (aquatic plants) zones in two small reservoirs with different landcovers(Sobudang, Myeongdong). Small-sized reservoirs are more abundant in SouthKorea, and a total of 51 and 65 species of zooplankton were identified at the twosmall reservoir (Sobudang and Myeongdong), where zooplankton densities weremore abundant in the littoral zone than in the open water zone. Cladocerans andcopepods densities were also higher in the littoral zone, in contrast, rotifers showedhigher densities in the open water zone (t-test

In this study, we investigated what environmental factors mainly influence the pattern of zooplankton communities in the regulated Nakdong River. Fifteen years of data (N = 629, 1994 to 2008) were used to analyze the patterns by means of self-organizing map (SOM), that is capable of extracting information through competitive and adaptive properties. A total of 34 variables (18 groups of zooplankton and 16 physicochemical parameters) were patterned onto the SOM, and then each variable was compared in accordance with three different annual rainfall patterns (i.e., dry, moderate, and wet). Finally, seven clusters were categorized according to Euclidian distance between the clusters on the SOM. Clear seasonal features for the variables were interpreted in five major categories, and two more subcategories were partitioned by a specific feature of zooplankton abundance and appearance. In the result, the patterns of zooplankton assemblage were mainly distinguished by water temperature, Secchi transparency, and river flow rate. In particular, the SOM result depicted a clear pattern of zooplankton assemblage clusters partitioned by annual precipitation, which accentuated the prolonged effect of dam regulation on plankton assemblage in a regulated river system.

It has been hypothesized that the production of diapausing eggs in Daphnia can be induced by fish kairomones. A population of Daphnia could survive severe predation using this predator avoidance strategy. However, in changing environments, diapausing eggs experience various temperature conditions, and hatchlings at emergence may be exposed to the same predation risks as their mothers. Therefore, staying in diapause or an immediate response upon hatching to available environmental information could be important for hatchling survival. For this study, we investigated the impact of water temperature (10, 15, 20, and 25°C) in the presence and absence of fish kairomones (Lepomis macrochirus) on the hatching success of resting eggs (D. galeata). Results show that no diapausing eggs hatched at the lowest temperature (10°C), and the highest hatch percentage occurred at 15°C. Although higher water temperatures reduced hatching success, diapausing eggs hatched more quickly. The number of hatchlings was significantly higher after exposure to fish kairomones, and this was more noticeable at higher temperatures (20 and 25°C). The present results suggest that the diapausing eggs were produced as a predator avoidance strategy in Daphnia; however, the presence of fish works as a positive signal to increase hatchlings when the diapausing stage is terminated.

We tested the hypothesis that the spatial distribution and diet composition of Asplanchna species might be affected by the presence of aquatic macrophytes in 33 wetlands in South Korea. We estimated the densities of Asplanchna and other rotifer and crustacean, together with environmental parameters, in both vegetated and open water zones, from May to June 2011. In the present study, two species of Asplanchna, A. priodonta and A. herrickii, were observed and significantly more abundant in open water zones lacking macrophytes. In particular, the density of A. priodonta was higher than that of A. herrickii, and the density of A. priodonta was strongly positively correlated with the area of open water. In addition, gut content analysis was used to determine their dietary preferences, with the finding that there was apparent differentiation in food source utilisation between the two Asplanchna species; A. priodonta consumed some protozoa, phytoplankton, and exclusively pelagic rotifer, while A. herrickii consumed primarily Euglena. In particular, Keratella and Polyarthra were most commonly consumed by A. priodonta in open water. Macrophytes represent a suitable habitat for epiphytic rotifer but not for pelagic rotifer; this characteristic drives pelagic rotifer such as Asplanchna towards open water and may be responsible for the significant negative correlation that we observed between macrophyte and Asplanchna densities.

Key-words: submerged macrophyte, macrophyte habitat, zooplankton, species diversity, shallow wetland Submerged macrophytes improve the structural heterogeneity of microhabitats in aquatic ecosystems, often providing an important habitat for zooplankton. However, excessive development of free-floating macrophytes on the water surface can reduce the biomass of submerged macrophytes and result in a relatively simple habitat structure. We hypothesized that controlling the development of free-floating macrophytes would result in a more complex habitat structure by promoting the development of submerged macrophytes. After applying three experimental treatments (NR, no removal; IR, intermediate removal; CR, complete removal of free-floating macrophytes), we found that CR of free-floating macrophytes improved the growth and development of submerged macrophytes and supported a large zooplankton assemblage. However, the largest zooplankton assemblage (in terms of abundance and diversity) was recorded after the IR treatment. Although submerged macrophytes were abundant in the CR treatment, the number, abundance, and density of zooplankton species were much lower than those in the IR treatment. Preferential selection of different macrophyte types by zooplankton presumably led to variation in plant utilization of niches, and the simultaneous presence of different macrophyte life forms created a complex microhabitat structure that induced high species diversity and zooplankton density.