BackgroundRe-openings of estuary barrages for ecosystem restoration are rare globally. The Nakdong River Estuary (NRE), South Korea, provides a unique opportunity to assess the effects of temporary barrage re-opening on fish population structure.MethodsWe conducted monthly fish population surveys both upstream and downstream of the estuary barrage from 2021 to 2023, categorizing results into closing period (CP) and opening period (OP).ResultsDuring the OP, we observed shifts in fish distribution, including decreased proportions of non-indigenous species and altered habitat distributions. Additionally, we observed an increase in brackish water species populations and their upstream movement. Monthly frequency analysis revealed that brackish and non-indigenous species exhibited high frequencies mainly during OP (July to October), while freshwater (demersal) species showed low frequencies during the same period.DiscussionOur study highlights the significant impact of temporary estuarine ecosystem restoration on fish distribution and emphasizes the need for careful consideration of both frequency and duration of estuary barrage re-openings.

Estuarine ecosystems globally are being restored through re-naturalization efforts, including the Nakdong River Estuary (NRE) in South Korea, which partially re-opened in 2019 after decades of disconnection. We investigated how this restoration affected the feeding ecology and fitness of Erythroculter erythropterus (Skygager), a non-native cyprinid that became dominant in the upper NRE following barrage construction. We surveyed fish populations, analyzed stomach contents using Next Generation Sequencing, and assessed morphological changes from May 2018 to June 2022. Following NRE opening, zooplankton and fish diversity indices increased, while phytoplankton and benthic invertebrate diversity decreased. E. erythropterus showed altered prey selection patterns, shifting from diverse prey items to predominantly zooplankton after NRE opening, coinciding with altered vertical migration patterns of plankton communities. While length distribution did not change significantly, the condition factor K decreased significantly, indicating reduced fitness. Body shape parameters also shifted toward more elongated forms, suggesting morphological responses to changed hydraulic conditions and feeding ecology. Our findings demonstrate that controlled estuarine reconnection can affect non-native species through cascading trophic effects, potentially serving both restoration and non-native species management objectives. This study highlights the complex ecological consequences of partial estuarine restoration and the importance of considering multiple trophic levels when evaluating restoration outcomes.

Understanding the ecological characteristics of endangered species is essential for their management and conservation. The dark sleeper (Odontobutis obscurus) is a carnivorous freshwater fish that has been designated an endangered species in South Korea because of its highly restricted habitat range and small population size. However, ecological research on prey items for this species remains severely limited globally, posing challenges for establishing conservation measures such as translocation management. In this study, the food sources and feeding strategies of O. obscurus were identified through non-invasive faecal DNA metabarcoding. A total of 24 O. obscurus specimens were collected from Sanyang Stream on Geoje Island, and faecal DNA was amplified using mitochondrial cytochrome c oxidase I (COI) primer. As a result, 13 prey items were identified, comprising fish and benthic invertebrates. Dietary metrics revealed the overall food composition and major food source of O. obscurus, and the Costello method enabled graphical diet analysis, suggesting that O. obscurus is a generalist, feeding on a wide variety of food sources. The food selectivity index suggested that O. obscurus prefers specific prey items, such as genus Misgurnus, Zacco (fish), Radix (gastropod), and Ephemera (mayfly). This molecular method facilitated the diet analysis of an endangered fish species that cannot undergo dissection, providing crucial information for their management, particularly regarding translocation efforts.

Estuary reopening is a means of restoring estuarine habitats, which has recently been implemented in a few developed countries. The regeneration of a brackish zone in the Nakdong River Estuary (NRE), South Korea, were tested through a series of barrage reopening. During the same period, we conducted extensive fish surveys in the upper part of the NRE barrage on a monthly basis from 2017 to 2021, and subsequently determined whether fish populations and communities were affected by the reopening. The results showed that the reopening of the NRE hardly affected the fish community structure, as non-native species such as Erythroculter erythropterus and Lepomis macrochirus maintained their dominance. Still, we discovered that certain euryhaline species are positively affected by estuary reopening, as total 46 Japanese eels (Anguilla japonica) were captured after the reopening, which had not been detected before the reopening. By comparing the size structure of various fish species, we discovered that size distribution of native and migratory species presented more positively skewed pattern after the reopening, while size structure in non-native species remained relatively unchanged normally distributed pattern. Piecewise structural equation modelling revealed that the NRE had become more complex ecosystem, as migratory fish species abundance and biomass started to show a positive correlation with hydraulic factors such as discharge and negative correlation with seasonality after the reopening. We concluded that estuary reopening created some changes in migratory and native freshwater species but such changes were not notably detected in non-native species. Therefore, appropriate sluice operation methodologies, such as considering the migration seasons of migratory species, should be developed. Additionally, human-involved management policies are required to regulate non-native species populations.

Ischnura asiatica (Brauer, 1865) is a freshwater damselfly belonging to the family Coenagrionidae that is distributed across most of Korea, primarily in areas with low water flow, such as ponds and wetlands. The complete mitochondrial genome of I. asiatica was sequenced by next-generation sequencing. The circular mitochondrial genome was found to be 15,769 bp long, with of 13 protein-coding, two ribosomal RNA, and 22 transfer RNA genes (GenBank accession no. OM310774). Maximum likelihood, phylogenetic analysis showed that this species clustered with other species belonging to the family Coenagrionidae. This study contributes to the phylogeny of damselflies and other members of the family Coenagrionidae.

Environmental DNA (eDNA) extracted from the gut contents of filter feeders can be used to identify biodiversity in aquatic ecosystems. In this study, we used eDNA from the gut contents of the Asian clam Corbicula fluminea to examine biodiversity within estuarine ecosystem. Field sampling was conducted at three points in the Nakdong River Estuary, which is characterised by closed estuarine features resulting from the presence of an estuarine barrage. The collected C. fluminea samples were dissected to separate the gut contents, and the extracted eDNA was amplified using 18S V9 primer targeting all eukaryote-derived DNA. The amplified DNA was sequenced using a next-generation sequencing (NGS) technique, and a BLASTn search was performed based on the National Centre for Biotechnology Information (NCBI) database for taxa identification. We obtained 23 unique operational taxonomic units (OTUs), including fish (approximately 8.70%), copepods (approximately 17.39%), and green algae (approximately 21.74%), representing a wide range of habitats. Furthermore, 8 out of the 20 families were identified through comparisons with reference data from conventional field surveys, and the OTUs of elusive migratory fish were detected. The results support the application of C. fluminea as an eDNA sampler for supplementary biodiversity monitoring.

Invasion by non-native species can cause major changes in the biodiversity of fishes. Despite the urgency of this issue, exotic species management (EM) to reduce the abundance of non-native species combined with follow-up assessments of effects based on comprehensive ecological data are limited. We conducted intensive field surveys of fishes and analysed long-term ecological data spanning 15 years (2007–2022) in Upo Wetland in which EM was conducted targeting two of the non-native species bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides). We found that EMs affected the native fish communities, while no significant changes occurred for the non-native fish communities in terms of catch per effort. At population level, however, EM caused instability in the population structure of the remaining non-native fishes but increased the stability of the native fish populations. In addition, enhanced connectivity between rivers and wetland enhanced the species diversity. In conclusion, we found cascading effects on α-diversity of fish communities and structural changes in populations following non-native fish removal efforts in the Upo wetland.

This study describes the prey DNA fragments found in the feces of a migratory bird species, the little tern, Sternula albifrons (Pallas, 1764), based on a DNA barcoding approach. This species is found in Nakdong Estuary, South Korea, and is a species designated as ‘Least Concern’ (IUCN Red List). Prey identification is a central issue of population conservation, and we applied DNA barcoding (using cytochrome oxidase I; COI) to fecal materials from little tern individuals. We successfully identified prey consumed by little tern individuals. All prey items comprised one phylum including three classes, six orders, and eight families based on a robust dual certification scheme (combined analysis of BLASTn searches and phylogenetic tree construction). Even though the success of identification was largely dependent on the degree of completion of the database in the genebank or BOLD systems, an increased resolution of prey identification to species level is important in predator–prey research. The current study used a small number of fecal samples to evaluate the applicability of the COI barcoding region to avifaunal feces, and more fecal samples are expected to convey increased information that can be used to infer the range of the prey species of little terns.

We compared two metabolome profiles of a small centric diatom species, Stephanodiscus hantzschii Grun., grown under conditions with enriched nutrients but different temperatures. This species proliferates in eutrophic rivers during winter. We investigated the population dynamics and internal metabolite changes of Stephanodiscus by performing a simple culture experiment at different temperatures (5 and 15 degrees C). We applied the H-1 nuclear magnetic resonance (NMR) technique to fully grown cells to obtain the metabolite profiles of S. hantzschii. Growth rates were significantly different at different temperature conditions (0.99 +/- 0.11 day(-1) at 15 degrees C and 0.21 +/- 0.12 day(-1) at 5 degrees C, n=10). Characterized metabolites included saturated and unsaturated fatty acids, AXP (including AMP, ADP and ATP), and UDP-glucose and UDP-galactose, all of which are important for energy metabolism. These metabolites were abundant within S. hantzschii cells grown at 15 degrees C but were not prolific in those grown at 5 degrees C. Furthermore, other H-1 NMR spectrum uncovered very little amounts of metabolites. Based on these observations of cell growth rate, although required nutrients were supplied, colder temperatures suppressed population growth through the deactivation of various internal metabolisms. Thus, winter proliferation of this species is opportunistic, implying that survival success led to dominance in freshwater ecosystems with neither resource competition nor grazing pressure.

The Nakdong River has suffered from hydrological alterations in the river channel and riverine area during the Four Major Rivers Restoration Project (FMRRP). As these anthropogenic modifications have induced intensive algal blooms, the prediction of algal abundances has become an important issue for securing a source of drinking water and ecosystem stability. This study aimed to assess the changed river system in terms of chlorophyll a concentrations using artificial neural network (ANN) models trained for the pre-FMRRP period and tested for the post-FMRRP period in the middle reaches of such a river-reservoir system, and identify the descriptors that consistently affect algal dynamics. A total of 19 variables representing biweekly water-quality and meteo-hydrological data over 10 years were used to develop models based on different ANN algorithms. To identify the major descriptor to the algal dynamics, sensitivity analyses were performed. The best and most feasible model incorporating five parameters (wind velocity, conductivity, alkalinity, total nitrogen, and dam discharge) based on the topology of a probabilistic neural network with a smoothing parameter of 0.028 showed satisfactory results (R = 0.752, p < 0.01). Some mismatches were found in the post-FMRRP period, which may be due to a discrete event with a newly adapted over-wintering species and different causes of the summer growth of cyanobacteria owing to the river alteration. Based on the lowest sensitivity of dam discharge and the combination results of environmental management with total nitrogen, ANN modelling indicated that short-term water quality variables are persistent factors shaping algal dynamics.

Three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy was used to investigate the characteristics of dissolved organic matter (DOM) in five typical eutrophic agricultural reservoirs. Based on catchment ecosystem, the five reservoirs were divided into three pollution sources of livestock, living, and farmland sources. The quantities and qualities of DOM in the reservoirs were analyzed. Our results showed that DOM characteristics were different in eutrophic reservoirs based on source. More protein-like components were observed in the reservoirs with the living sources, while more humic-like components were seen in the reservoir with farmland sources. Additionally, correlation analysis showed different sources for protein-like and humic-like components. Protein-like components originated mainly from phytoplankton (endogenous sources), and humic-like components were from terrestrial sources. Furthermore, the high values of specific fluorescence parameters were consistent with a dominant role of endogenous DOM in eutrophic water bodies, with FI values (fluorescence index) of approximately 1.9, and β:α values (freshness index) greater than 0.7. This result indicated that mixed features dominated endogenous sources in the reservoirs, regardless of terrestrial pollution sources. By comparing our fluorescence characteristics and historical references, we confirmed that catchment ecosystems related to human activities are important factors in determination of the characteristics of DOM in aquatic environments. However, complex and extensive eutrophication requires endogenous control of water bodies, which will play a central role in improving water environments and sustainable use of reservoirs. Therefore, this study provides an effective basis for water quality assessment of eutrophic agricultural reservoirs.

This review summarizes the history of freshwater zooplankton research in Korean Society of Limnology and necessary future topics that are remain poorly investigated in South Korea based on main research topics of published articles focusing on lakes, reservoirs, rivers and wetland ecosystems. In Korea, a total 450 freshwater zooplankton species have been reported (85 species of cladocera, ca. 230 species of rotifera since 1939, and 138 species of copepoda), and they cover 10% of total zooplankton species list. In the present paper, we provide recent species list of zooplankton found in Korea and their taxonomic keys. Over periods of 45 years, there are 25 published papers for zooplankton biota in lentic ecosystems in Korean Journal of Ecology and Environment (KJEE). The ecology of zooplankton communities in rivers has focused on the mechanisms involved in regulating their abundance, diversity and spatio-temporal patterns of genus Bosmina and rotifers (genus Brachionus and Keratella) that are most frequently found from Korea. On the other hand, the studies on zooplankton in wetland has focused on Alona, Chydorus and Ceriodaphnia with special emphasis on their relationships with aquatic macrophytes. Even though studies on the freshwater ecosystem in Korea have a long history, a few of studies on zooplankton biota were conducted at rice paddy, Dumbeong and wetlands. During the last two decades, experimental advances in freshwater zooplankton ecology and understanding of structure and function of this biota were made from a series of articles mainly in journal of KJEE. For future studies, quantitative, experimental and interdisciplinary approaches would be key words to understand zooplankton ecology and their roles in aquatic ecosystems under changing environments, and we have suggested necessary zooplankton research fields and future directions.

The length-weight relationships (LWRs) for three freshwater fish species in catchment area of Hoiya River, Republic of Korea, were analyzed. Fish samples were collected quarterly using scoop-nets (5mm mesh size), casting nets (7mm mesh size) from 2017 to 2018. For LWRs using the general equation from Froese (2006), the parameter b ranges from 3.14 (Oryzias latipes) to 3.37 (Iksookimia longicorpa). LWRs data of these two species have not yet been listed in FishBase.

In this study, we examined the ontogenetic changes in prey selection pattern of largemouth bass Micropterus salmoides (Lacepede, 1802) in accordance with habitat type. The stomach contents of largemouth bass were analysed with respect to predator size classes for two different habitats (Nakdong River main channel and the Upo Wetlands tributary) in South Korea, that differed in the diversity of potential prey items. The diversity (H′) of invertebrate diet items in stomach of largemouth bass was found significantly different between the study sites. Small large mouth bass (<100 mm) preferred to consume decapods and mayflies in the river and wet land systems respectively. Fishes >100 mm in the river and >200 mm in the wet land system preferred fish as their prey. Results demonstrated variations in composition of prey items as influenced by habitat characteristics as well as habitat use type, contributed to the delayed ontogenetic diet shift in the largemouth bass population.

Macrophytes are commonly utilised as habitat by epiphytic species; thus, complex macrophyte structures can support high diversities and abundances of epiphytic species. We tested the hypothesis that the presence of aquatic macrophytes is an important factor determining Sida crystallina (O.F. Müller, 1776) distribution.

The proliferation of freshwater cyanobacteria is a serious environmental problem that often reduces water quality. In this study, we observed the possible influence of the Indian Ocean Dipole (IOD), represented by the Dipole Mode Index (DMI), on the dynamics of cyanobacterial blooms in a river system (the lower Nakdong River). Korean monsoon rainfall (KMR) was preceded by 5-month moving average DMI of similar to 1.5 years, and the KMR and DMI were negatively associated (r(2) = 0.53, n = 15, p < 0.01). In turn, a decrease in KMR was responsible for a decrease in total dam discharge (r(2) = 0.64, n = 15, p < 0.01), resulting in an abrupt increase in cyanobacterial cell density at the study site (r(2) = 0.94, n = 15, p < 0.01). We found a strong and significant positive correlation between cyanobacterial density of the Nakdong River and a 5-month moving average DMI (r(2) = 0.95, n = 15, p < 0.01). These correlations suggest that a positive DMI decreased KMR and therefore water discharge, resulting in an excessive proliferation of the cyanobacterial density during the monsoon period. Despite a variety of uncertainties, the presence of the IOD is believed to play a role in triggering freshwater cyanobacterial proliferation through a signal propagation pathway during summer drought occurrence in the monsoon period.

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.

Macrophytes play a major role in the structuring of aquatic environments, and create diverse microhabitats. Therefore, these plants represent an important factor regulating the zooplankton biomass, taxonomic composition, and distribution in freshwater ecosystems. In the current study, we examined the effects of the structural heterogeneity provided by various macrophytes. We identified four habitat types in this study: (1) open water (without macrophytes), (2) the helophyte zone, (3) the pleustophyte zone, and (4) the mixed vegetation zone (containing pleustophytes, nymphaeids, and elodeids). We tested the hypothesis that complex habitat structures support large zooplankton assemblages. Specifically, we collected zooplankton samples from a total of 119 sampling points in the Upo Wetlands, South Korea, during the spring and autumn of 2009. The largest zooplankton assemblage was found in the mixed macrophyte zone, followed by the helophyte and pleustophyte zones. The pleustophyte zone supported larger zooplankton assemblages during autumn compared to spring. Differences in zooplankton assemblages were considered to be strongly related to seasonal variation in the development and growth of pleustophytes. However, two-way ANOVA revealed that seasons had no significant influence on the zooplankton density and diversity. Instead, different habitat types substantially determined zooplankton characteristics. In conclusion, we demonstrated that wetland areas with high macrophyte species diversity contribute toward higher zooplankton diversity.