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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.

In aquatic ecosystem, the food web structure based on composition and abundance of prey species influences the feeding characteristics of predators, resulting in changes to the structure and function of the entire food web. Zooplankton mainly feed on phytoplankton, but the method of analyzing the feed source through visual and microscopic identification of gut contents has limitations in applicable species and the low resolution of the analysis results. In this study, potential vs. eaten food sources of Sinocalanus tenellus, a small and medium-sized copepod species dominant in brackish waters, were analyzed based on phytoplankton operational taxonomic units detected from the habitat’s raw water and treated whole bodies of S. tenellus. The alpha diversity and dominant/subdominant species of each potential and eaten food source were identified, and they were compared across seasons. At the same time, the feeding selectivity index (Ei) was calculated by identifying overlapping species in the potential vs. eaten food source pool. As a result, it was confirmed that not all surrounding potential food sources are fed on by S. tenellus, and that its diet can vary depending on the presence/absence of preferred prey. Metabarcoding techniques is considered to be an effective way to identify fluctuations in major food sources of zooplankton at the genus or species level. If uncertainties such as incomplete species identification as identified from the results of this study are improved through improving of analysis methods such as application of species-specific primers in the future, it will be useful for securing information on the feeding characteristics of small to medium-sized zooplankton.

Climate warming and eutrophication caused by anthropogenic activities strongly affect aquatic ecosystems. Submerged macrophytes usually play a key role in shallow lakes and can maintain a stable clear state. It is extremely important to study the effects of climate warming and eutrophication on the growth of submerged macrophytes in shallow lakes. However, the responses of submerged macrophytes to climate warming and eutrophication are still controversial. Additionally, the understanding of the main pathways impacting submerged macrophytes remains to be clarified. In addition, the influence of seasonality on the growth responses of submerged macrophytes to climate warming and eutrophication requires further elucidation. In this study, we conducted a series of mesocosm experiments with four replicates across four seasons to study the effects of rising temperature and nutrient enrichment on the biomass of two submerged macrophytes, Potamogeton crispus and Elodea canadensis. Our results demonstrated the seasonality and species specificity of plant biomass under the influence of climate warming and eutrophication, as well as the main explanatory factors in each season. Consistent with the seasonal results, the overall results showed that E. canadensis biomass was directly increased by rising temperature rather than by nutrient enrichment. Conversely, the overall results showed that P. crispus biomass was indirectly reduced by phosphorus enrichment via the strengthening of competition among primary producers. Distinct physiological and morphological traits may induce species-specific responses of submerged macrophytes to climate warming and eutrophication, indicating that further research should take interspecies differences into account.

Periphyton plays an important role in lake ecosystems processes, especially at low and intermediate nutrient levels where periphyton contribution to primary production can be similar to or exceed that of phytoplankton. Knowledge of how periphyton responds to key drivers such as climate change and nutrient enrichment is, therefore, crucial. We conducted a series of mesocosm experiments over four seasons to elucidate the responses of periphyton communities to nutrient (low and high, TN-0.33 mg L–1 TP-7.1 μg L–1 and TN-2.40 mg L–1 TP-165 μg L–1, respectively), temperature (ambient, IPCC A2 scenario and A2 + 50%) and plant type (two submerged macrophytes with different morphological structural complexity: Potamogeton crispus and Elodea canadensis, and their corresponding plastic imitations with similar size and structure). We found a noticeable seasonality in the abundance and composition of periphyton. In spring and summer, periphyton abundances were significantly higher in the turbid-high-nutrient state than in the clear-low-nutrient state, and in summer they were notably higher at ambient temperature than in climate scenario A2 and A2 + 50%. In contrast, periphyton abundances in autumn and winter were not influenced by nutrient and temperature, but they were notably higher on plants with a more complex morphological structure than simple ones. The genus composition of periphyton was significantly affected by nutrient–temperature interactions in all seasons and by plant type in winter. Moreover, periphyton functional composition exhibited noticeable seasonal change and responded strongly to nutrient enrichment and temperature rise in spring, summer, and autumn. Our results suggest that the effect of warming on periphyton abundance and composition in the different seasons varied with nutrient state and host plant type in these mesocosms, and similar results may likely be found under field conditions.