Horizontal distribution of water quality between early and dense water plant seasons in Lake Shirarutoro, Kushiro Wetland

The dense growth of aquatic macrophytes can affect lake water quality. We studied the water chemistry at 40 sites on Lake Shirarutoro when dense beds of water chestnut (Trapa japonica) expanded in 2007–8. The distribution patterns between the early (June) and middle (August) vegetation water plant seasons were compared. In this lake, the dense beds of water chestnut did not necessarily correspond to the lack of dissolved oxygen at the lake bottom. Principal component analyses (PCA) of 20 variables showed that the lake could be divided into two areas: one was a central area from the main inflowing river to the outflow (‘area A’) and the other was an area adjacent to the southern wetland (‘area B’). The factor loadings of the first principal component (PC) showed high positive value for K, Ca, Na, Cl, Mg, and D-Si, and high negative for dissolved organic phosphorus (DOP), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved iron (D-Fe). ‘Area A’ was influenced by the high concentration loading of K, Ca, Na, and Cl from a northern tributary; ‘area B’ was influenced by the adjacent wetlands. The factor loadings of the second PC showed high negative values for the total phosphorus (TP), total nitrogen (TN), volatile solids (VS), and chlorophyll a, all of which were indicators of eutrophication. The variables whose factor loading in the first PC changed between June and August were only the soluble reactive phosphorus (SRP) and SO4 . Therefore, we concluded that the distribution of the lake water chemistry was more influenced by watershed condition rather than the dense water plant beds.