Distribution of submerged macrophytes and environmental conditions in the rivers of South Korea

Abstract

The river ecosystems in Korea has been greatly altered from lotic to lentic conditions due to large-scale weir construction. The distribution and abundance of submerged macrophytes have been rapidly expanding along five major rivers. Despite the general importance of submerged macrophytes, fast growing and dense submerged macrophytes can have detrimental effects on river ecosystems. Understanding both the species composition of submerged macrophytes and the major environmental factors related to aquatic plants is important for river management. This study focused on identification of the relationships between the submerged macrophytes species and environmental factors. I performed vegetation survey and measured environmental factors at 197 surveyed sites along the five major rivers from May to September, 2014–2015: Han River (71 sites), Geum River (43 sites), Nakdong River (46 sites), Yeongsan River (27 sites), and Seomjin River (10 sites). Ceratophyllum demersum, Hydrilla verticillata, Myriophyllum spicatum, Najas graminea, N. marina, Potamogeton crispus, P. maackianus, P. malaianus, P. octandrus, P. oxyphyllus, P. pusillus, and Vallisneria natans, were distributed in the river regions. The most abundant species of submerged macrophytes in all rivers were H. verticillata, M. spicatum, and P. crispus. Based on the analysis of dissimilarities, species composition of submerged macrophytes and the environmental conditions in the four major rivers (Han, Geum, Nakdong, Yeongsan Rivers) were similar, whereas the occurrence rates (prevalence) of submerged macrophytes differed. However, ammonium nitrogen, nitrate nitrogen, and total nitrogen concentrations were significantly different between the four rivers. In particular, Han River had higher ammonium nitrogen concentrations and Yeongsan River had lower nitrate nitrogen and total nitrogen concentrations, than the other rivers. Environmental factors associated with the occurrence of submerged macrophytes were related to light availability, such as chlorophyll a, suspended solids, and water temperature during the growing season, whereas nutrient concentration was not an important factor. Najas marina became established rapidly and was an important species over the three year at two monitoring sites, Yangpyeong site in Han and Sangju site in Nakdong Rivers. According to the comparison of submerged macrophyte diversity among Ceratophyllum demersum community, Hydrilla verticillata community, Myriophyllum spicatum community, Potamogeton crispus community, Vallisneria natans community, and others community, species in V. natans community had relatively even coverage and this community exhibited the highest diversity. From generalized linear models, the ammonium nitrogen and nitrate nitrogen concentrations, and water velocity had the most influence on the Shannon diversity index and species richness, which decreased with high nutrient concentrations and rapid water flow. The results indicated that submerged macrophyte diversity was highest under low productivity and low disturbance conditions. I examined the environmental factors that characterize the potential habitats of two most abundant submerged plants, Myriophyllum spicatum and Hydrilla verticillata, using generalized additive models. Potential habitats of M. spicatum were linked with chlorophyll a, nitrate nitrogen, suspended solids, water temperature, water depth, and water velocity. In the case of H. verticillata, electrical conductivity and suspended solids were important in determining habitat factors. Monitoring of vegetation and environmental conditions in river ecosystems is important because dispersal and composition of submerged macrophytes are affected by both water quality factors and water velocity. Preservation of free-flowing rivers with a variety of hydrological features is needed to provide the water quality needed to ensure submerged macrophyte diversity and to control submerged macrophytes abundance to an appropriate level. These results provide information on the initial distribution of submerged macrophytes and on potential submerged macrophytes habitats for the management of river ecosystems after the large-scale weir construction.