Conserving and enhancing freshwater biodiversity are global issues to ensure ecosystem integrity and sustainability. To meet this, it is critical to understand how the biological assemblages are determined by environmental gradients in different spatial scales. Nevertheless, information on their large-scale environmental relationships remains scarce in Korea. We aimed to understand nationwide spatial distribution patterns of benthic macroinvertebrates and important environmental factors affecting their distribution in 388 streams and rivers across Korea. A total of 340 taxa, belonging to 113 families in 23 orders of five phyla, were identified. Assemblage composition in most Korean streams included a few predominant colonizers and a majority of rare taxa. Cluster analysis based on benthic macroinvertebrates classified a total of 720 sampling sites into five clusters according to the pollution levels from fast-flowing less polluted streams with low electrical conductivity to moderately or severely polluted streams with high electrical conductivity and slow water velocity. Canonical correspondence analysis revealed that altitude, water velocity and streambed composition were the most important determinants, rather than watershed and water chemistry variables, for explaining the variation in macroinvertebrate assemblage patterns. The results provide basic information for establishing the conservation and restoration strategies of macroinvertebrate biodiversity against anthropogenic disturbances and developing more confident bio-assessment tools for diagnosing stream ecosystem integrity.

Anthropogenic land use within watersheds has substantial effects on aquatic habitats and biological communities. From September 2006 to December 2008, we investigated the effects of land use on benthic macroinvertebrate communities by comparing Song Stream and Odae Stream, two adjacent mountain streams in Korea whose watersheds have different land use patterns. Song Stream is significantly disturbed by agricultural activities in the watershed, whereas Odae Stream is relatively undisturbed and is surrounded by a well-conserved forest area. Song Stream had significantly higher levels of all nutrients and sediment-related factors due to the adjacent agricultural area. As a result, Song Stream had markedly lower species community indices, such as taxa richness and abundance. In Song Stream, macroinvertebrate scrapers and predators were most adversely affected, whereas collector-gatherers became a dominant group. Based on correlation and multivariate analyses, total dissolved solids had the strongest negative relationship with macroinvertebrate assemblages, followed by electrical conductivity, total nitrogen, and pH. The proportion of cobble in stream substrate was positively related to the richness and abundance of macroinvertebrates. Our results indicate that disturbances caused by agricultural land use, particularly sand deposition, had significant adverse effects on macroinvertebrate habitats and on the biotic integrity of benthic macroinvertebrate communities.

The extent of anthropogenic land use in watersheds determines the amount of pollutants discharged to streams. This indirectly and directly affects stream water quality and biological health. Most studies have therefore focused on ways to reduce non-point pollution sources to streams from the surrounding land use in watersheds. However, the mechanistic pathways between land use and the deterioration of stream water quality and biological assemblages remain unclear. This study estimated a structural equation model (SEM) representing the impact of agricultural and urban land use on water quality and the benthic macroinvertebrate index (BMI) using IBM AMOS in the Nam-Han river systems, South Korea. The estimated SEM showed that the percent of urban and agricultural land in the watersheds significantly affected both the water quality and the BMI of the streams. Specifically, a higher percent of urban land use had directly increased the biochemical oxygen demand (BOD) and total phosphorus (TP), and deteriorated the BMI of streams. Similarly, higher proportions of agricultural land use had also directly increased the BOD, total nitrogen (TN), and total phosphorus (TP) concentrations, and lowered the BMI of streams. In addition, it was observed that the percent of urban and agricultural land use had indirectly deteriorated the BMI through increased BOD. However, we were not able to observe any significant indirect effect of the percent of urban and agricultural land use through increased nutrients including TN and TP. These results indicate that increased urban and agricultural land use in the watersheds had directly and indirectly affected the physicochemical characteristics and benthic macroinvertebrate communities in streams. Our findings emphasize the need to develop more elaborate environmental management and restoration strategies to improve the water quality and biological status of streams.

The diatoms are an ecologically important group of algae that have been extensively studied by ecologists and taxonomists. However, the large-scale patterns of diatom distribution and the factors underlying this distribution are largely unknown. The aims of this study were to identify the large-scale spatial patterns of benthic diatom assemblages in Korean streams and rivers, and to assess the importance of numerous environmental factors on diatom distribution. We classified 720 study sites based on diatom flora. Benthic diatoms, water chemistry, altitude, and riparian land cover and use were characterized by multivariate analyses, Monte Carlo permutation tests, and indicator species analysis. In total, we identified 531 diatom taxa. Diatom assemblages were mostly dominated by species of the genera Achnanthes, Navicula, Nitzschia, Cocconeis, Fragilaria (Synedra included), Cymbella, Gomphonema, and Melosira. Cluster analysis partitioned all 720 sites into eight groups based on diatom species composition. Canonical correspondence analysis indicated that altitude, land cover and use, current velocity, electrical conductivity, and nutrient levels explained a significant amount of the variation in the composition of assemblages of benthic diatoms. At the national scale, a downstream ecological gradient was apparent, from fast-flowing, mostly oligotrophic highland streams to slow-flowing, mostly eutrophic lowland rivers. Our data suggest that spatial factors explain some of the variation in diatom distribution. The present investigation of the spatial patterns of benthic diatoms, the ecological determinants of diatom occurrence, and the identification of diatom indicator species contributes to development of a program for assessing the biological integrity of lotic ecosystems in Korea.

Satellite observations and modeling data have suggested a significant increase in net primary production in the Arctic Ocean over the last decade due to retreating sea ice and the development of light availability caused by Arctic warming. Subsequently, under‐ice blooms (UIBs) are being recognized as an important phenomenon from the traditional perspective. However, the role of sea‐ice algae in UIBs is still unknown due to the limited availability of continuous observations. We analyzed data on primary producer‐derived lipid biomarkers from sinking particles collected over 1 year using time‐series sediment traps on the East Siberian Sea and Chukchi Sea slopes. Based on the seasonal changes in sympagic organic carbon derived from the data of the ice proxy (IP₂₅) flux and pelagic biomarkers, such as highly branched isoprenoid trienes, epi‐brassicasterol and dinosterol, a UIB was identified in summer 2018 on the East Siberian Sea slope. Compared to the nutrient distribution on the Chukchi Sea slope, the UIB on the East Siberian Sea slope might have been triggered by the nutrient supply. The estimated flux‐weighted mean sympagic organic carbon value measured during the UIB period (May−August) was 1.04 mg m⁻² d⁻¹ on the East Siberian Sea slope, approximately five times greater than recorded that on the Chukchi Sea slope (0.23 mg m⁻² d⁻¹) during the same period. Our findings suggest that the importance of sea‐ice algae as primary producers has increased as the UIB phenomenon has become more important in the Arctic Ocean and that sea‐ice environments face changes due to Arctic warming.

Eutrophic freshwater ecosystems are vulnerable to toxin-producing cyanobacteria growth or harmful algal blooms. Cyanobacteria belonging to the Nostocales order form akinetes that are similar to the seeds of vascular plants, which are resting cells surrounded by a thick membrane. They overwinter in sediment and germinate when conditions become favorable, eventually developing into vegetative cells and causing blooms. This review covers the cyanobacterial akinete of the Nostocales order and summarizes the environmental triggers and cellular responses involved in akinete germination and formation based on data from the literature. It also emphasizes the intimate and dynamic relationship that exists between the germination and formation of akinete in the annual life cycle of cyanobacteria. After comparing many published data, it is found that the tolerance ranges for factors affecting both akinete germination and formation do not differ significantly and are broadly consistent with the tolerance ranges for vegetative cell growth. However, the optimal range varies with different species and strains of cyanobacteria. The life cycle of cyanobacteria, as a result of akinete germination and formation, has a seasonal periodicity and spatial connectivity between the water column and the sediment. However, during the summer growing season, intimate coupling between akinete formation and germination can occur in the water column, and this can contribute to high population densities being maintained in the water column. During this time, shallow sediment could also provide suitable conditions for akinete germination, thereby contributing to the establishment of water column populations. The information summarized in this review is expected to help improve our shared understanding of the life cycle of the Nostocales cyanobacteria while also providing insights into the monitoring and management of harmful algal blooms.