The composition and structure of phytoplankton communities are important indicators for evaluating water quality and water ecology. However, characteristics of riverine phytoplankton communities and their relationship with water ecology are not fully understood in remote areas at high altitudes with low ambient temperature and light anthropogenic activities. In this study, phytoplankton communities were investigated in Nujiang River and the upper section of the Lancang River in July-August 2019, with a goal of evaluation of water quality and ecology in the rivers using the functional group classification method in combination with algal diversity index and functional group based Qr index. Furthermore, the key factors influencing phytoplankton community structure were evaluated. The results showed that a total of 44 phytoplankton species from 4 phyla and 36 genera were recorded in the Nujiang River, with an average biomass of 1.74 mg·L^-1 (0.013-13.299 mg·L^-1). The dominant functional group was MP. In comparison, 38 species from 4 phyla and 31 genera were detected in the upper section of the Lancang River, with an average biomass of 1.21 mg·L^-1 (0.079-5.840 mg·L^-1). Besides the major functional group of MP, a large number of moderately eutrophic functional groups such as J, K, W1, and W2 were found in the upper section of the Lancang River, indicating stronger eutrophication than in the Nujiang River. The values of indices indicated that the ecological status in two rivers was at a good condition, with the water quality of the Nujiang River being better than that of the upper section of the Lancang River. The structure of functional groups in the Nujiang River was mainly affected by water temperature, dissolved oxygen, and nitrogen and phosphorus concentrations. In contrast, the structure of phytoplankton communities in the upper section of the Lancang River was mainly regulated by nitrate nitrogen and total nitrogen concentrations. The analysis by coupling with the land-use data of the watersheds showed that algal communities in the Nujiang River and the upper section of Lancang River basins were significantly correlated with bare land and construction land (P<0.05), indicating that soil erosion and human activities have significant impacts on the water ecology of rivers in ecologically sensitive alpine areas, and are the primary controlling factors for the protection of water environment in alpine rivers.

Nujiang River, Lancang River, phytoplankton, hydrogeological evaluation, environmental driver