Distribution of Epilithic Diatom Communities in Relation to Land Use with Descriptions of Three New Species in the Hangang River Watershed, South Korea

Abstract

The Hangang River is the most economically important basin of Korea; it provides drinking water for more than 20 million people living around the river, and is also an important water source for irrigation and navigation. However, with the continuous inflow of wastewater from the land around rivers such as cities and farmland, the water quality has deteriorated. Diatoms are organisms that respond quickly to changes in water quality. Therefore, it is important to understand the relationship between diatom communities and land use in the Hangang River. In the present study, epilithic diatoms collected from December 2017 to April 2018 at 19 sites in the basin were classified and ecologically studied. A total of 118 diatom taxa belonging to 46 genera were identified at the species level with an additional 4 unknown species, occurring in 19 sampling sites. The dominant species were Achnanthidium minutissimum (31.04%), Hannaea arcus var. recta (9.91%), and Achnanthes convergens (9.56%). Based on a cluster analysis with land use, the sampling sites were divided into three groups (G1–G3): 1) G1 included mainly urban rivers, with circumneutral pH, high electric conductivity, and limited canopy. The dominant and subdominant species were Pseudostaurosira elliptica (29.52%) and A. minutissimum (15.06%); 2) G2 included mainly mountain streams, with high forest cover, low water temperature, high pH, and low electric conductivity. The dominant species was Achnanthidium minutissimum (33.66%) and the subdominant species were Hannaea arcus var. recta (15.23%) and Achnanthes convergens (13.67%); 3) G3 included different types of rivers. The water quality was between G1 and G2. The dominant species was Achnanthidium minutissimum (36.46%) and the subdominant species were Encyonema minutum (10.73%), A. alteragracillimum (8.09%) and E. silesiacum (7.07%). Epilithic diatoms were isolated during aquatic ecological health investigations of the Hangang River, South Korea, and they were then cultured. Three species belonging to the genera Fragilaria, Nitzschia and Encyonema were newly proposed based on morphological and molecular data. Their morphological characteristics were analysed using light microscopy (LM) and scanning electron microscopy (SEM). The nuclear small subunit (SSU) rRNA and ribulose-1, 5-bisphosphate carboxylase/oxygenase (rbcL) genes were used to study these species’ molecular characteristics. Herein three new species were proposed: 1) Fragilaria thornata, which has spatulate spines tapering towards their tips; 2) Nitzschia hantanii, which has a small-sized valve with convex margins, besides parallel striae in the central part of the valve that become strongly radial towards the poles; 3) Encyonema equalis, which has a space between stubs, located on the upper siliceous struts, that is consistent whether near the margin or near the raphe. According to the molecular analysis of three new species, the similarities based on the phylogenetic tree of SSU rRNA and rbcL gene were 97.8–99.0% and 96.3%–99.3%, respectively. As the result of all analyses herein presented indicate that Fragilaria thornata, Nitzschia hantanii and Encyonema equalis are indeed new species.