For the sustainable use of lake ecosystem services—water resources, aquatic habitats for biodiversity conservation, and aesthetic values as waterfront space—ecosystem health assessments using biota are implemented as important national environmental monitoring projects. Zooplankton play a key role as an important linkage in the material circulation as secondary producers in lake ecosystems. At the same time, they influence the composition and biomass of other communities through biological interactions. In this review, we summarize useful zooplankton indices for ecosystem health assessment and suggest considerations for their use. Suggestions are provided for the practical application of indirectly measured zooplankton biomass, as well as the potential and limitations of eDNA application, which has recently been actively utilized in biological monitoring.

Chum salmon (Oncorhyncus keta) is a cold-water species reported to migrate within a wide range of habitats, including Korea, Japan, North America, and Russia, playing important roles in the river–sea nutrient cycle and food web. However, research on this species has not been widely performed in South Korea owing to its geographical location at the southern edge of migration. In this study, we analyzed the spatial distribution and morphological characteristics of chum salmon migrating to South Korea using the length–weight relationship. We also analyzed 3 years of catch, sex ratio, and individual information (total length (cm), weight (kg), n = 4400) from ten rivers (eight in the East coast and two on the South coast) with a total of 17 years of water quality and the distance they traveled (n = 50) using multivariate analysis. As a result, we discovered a trend of less migration in the southern part of South Korea for all individuals migrating to South Korea. Furthermore, the weight ratio of males/females was significantly different (p < 0.05). Based on the length–weight relationship analysis, the a and b values were between 0.0011 and 0.038 and 2.65 and 3.49, respectively. In the correlation analysis, catch is negatively correlated with distance traveled and temperature (p < 0.05), and positively correlated with pH, dissolved oxygen, distance, and female ratio (p < 0.05). This is possibly the result of differences in water quality during early life stages or the presence of an estuarine barrage at the mouth of the Nakdong River. This research may be used as fundamental distribution and morphological variations of chum salmon in South Korea.

Dissolved organic matter (DOM) in rivers are an important factor in pollution management due to the abundance of stored carbon. Using fluorescent spectroscopy, we investigated the temporal and spatial dynamics of DOM compositions, as well as their properties, for two of the major four regulated rivers—Han River (HR) and Geum River (GR) in South Korea. We collected eight sampling sites, four from each of the two rivers (from close to the weirs) in order to observe the integrated effects of different land use (terrestrial input) during the monsoon (July) and post-monsoon periods (September). High integral values of DOM compositions (July: 30.81 ± 9.71 × 10³ vs. September: 1.78 ± 0.66 × 10³) were present in all eight sites after heavy rainfall during the monsoon period, which indicated that Asian monsoon climates occupy a potent role in the DOM compositions of the rivers. Regarding DOM compositions, tryptophan-like and fulvic acid-like components were predominant in HR and GR, especially in GR with high integral values of protein-like and humus components. However, the properties of terrestrial DOM between HR and GR are markedly different. These results considered due to the different land use, where the terrestrial DOM shows a low degree of humification due to a high percentage of agriculture and urban land use in GR. Furthermore, these two rivers are typical regulated rivers, due to their weir constructions. High values of DOM components were present in the downstream of the weirs; however, increasingly high patterns appeared in the HR because of heavy rainfall (511.01 mm in HR; 376.33 mm in GR). In addition, a lower increasing trend of humic-like component was present in the GR due to a low percentage of forest land use/cover. These results suggest that the effect of the weir on rivers can be highlighted by the different percentages of land use/cover under the conditions of the monsoon period. Hence, DOM fluorescence can serve as an effective indicator for providing an early signal for the complex impacts of the different land use and rainfall in the regulated river systems.

Calanoida is a representative oligo-mesotrophic indicator species, frequently used as an index for assessing lake ecosystems’ health. Additionally, they function as trophic intermediates in the food web, connecting primary producers and higher consumers within lake ecosystems’ food chains. However, research cases that present the implications of habitat environment assessments, such as ecosystem structure and water quality, represented by these taxa remain insufficient. In this study, we conducted research across 49 lakes in South Korea, analyzing the occurrence characteristics of Calanoida and examining the correlations between Calanoida abundance and water quality parameters and the morphological-based functional groups of phytoplankton. Calanoida were more frequently observed in lakes characterized by greater size, increased depth, and reduced levels of anthropogenic land use. Furthermore, Calanoida occurrence was more probable in environments characterized by lower electrical conductivity and suspended solids concentrations. Their presence was also associated with conditions where Large mucilaginous phytoplankton (MBFG7), which includes cyanobacteria, and Large filamentous phytoplankton (MBFG3) were prevalent. An analysis of the environmental factors influencing the increase in Calanoida abundance revealed an inverse relationship between their abundance and water quality factors, including nutrient levels. This trend was observed consistently across all genera. Additionally, Calanoida were observed to maintain a high abundance in environments where the presence of the Large mucilaginous phytoplankton group (MBFG7) was relatively high. In contrast, Cyclopoida exhibited varying occurrence characteristics by genus in response to different water quality factors. Based on these results, we suggest that Calanoida, commonly used as an indicator of mesotrophic conditions, can also serve as a valuable indicator for evaluating the functionality of the food web. While Calanoida struggle to inhabit environments characterized by degraded water quality, they demonstrate the ability to adapt and persist in environments containing large, mucilaginous, or filamentous phytoplankton species that are typically challenging for other zooplankton to graze.