Clustering evaluation of water quality for various classes of in-flow rivers in connected brackish lakes.

Brackish waters and estuaries at the lower reaches of rivers accumulate organic matter and nutrients from various sources in the watershed. Sufficient light and shallow water depth stimulate phytoplankton growth, resulting in a more diversified ecosystem with higher trophic levels. For effective watershed management, it is crucial to characterize the water quality of all rivers, including small and medium-sized ones. Our field survey assessed water quality parameters in 26 inflow rivers surrounding Lakes Shinji and Nakaumi, two consolidated brackish lakes in Japan. The parameters included water temperature, salinity, chlorophyll-a, and nutrients. The study used hierarchical clustering. The Silhouette Index was used to assess clustering outcomes and identify any difficulties in dispersion across clusters. The 26 rivers surrounding Lakes Shinji and Nakaumi were classified into six groups based on their water quality characteristics. This classification distinguishes itself from earlier subjective methods that relied on geographical factors. The new approach identifies a need for improved management of river water quality. The results of the cluster analysis provide valuable insights for future management initiatives. It is important to consider these findings alongside established watershed criteria.

DNA metabarcoding focusing on the plankton community: an effective approach to reconstruct the paleo-environment.

DNA metabarcoding (DNA-MB) targeting the whole plankton community is a promising approach in studies of sediment samples from water bodies, but its effectiveness in ancient material is not well demonstrated. We applied DNA-MB of plankton in a sediment core to reconstruct the paleo-environment of Lake Shinji, Japan, through a marine lagoon/freshwater lake transition during the past 2300 years. We interpreted core-sample plankton taxonomy and habitat by reference to the modern plankton community in water samples. OTUs (operational taxonomic units) belonging to Dictyochophyceae were 81.05% of the total reads in sediments. However, Ciliophora, Copepoda and Labyrinthulea formed the majority of plankton taxa in the water samples, suggesting that they are under-represented in sediment. A drastic change in plankton composition correlated with a large decrease in sediment sulfur concentration, implying the change of aquatic environment from marine lagoon to freshwater lake. This event took place ca. 1200 CE in Lake Shinji. A 250 year-long transitional period followed, during which the total DNA sequence reads were very low. This suggests that salinity fluctuations created a hostile environment for both marine and freshwater plankton species. Our results show that DNA-MB of the whole plankton community is effective in reconstructing paleo-environments.