Geo-climatic factors weaken the effectiveness of phytoplankton diversity as a water quality indicator in a large sediment-laden river.

Abstract The study aims to determine whether phytoplankton diversity can be used as an indicator of water quality in large-scale sediment-laden rivers with heterogeneous environmental conditions. We hypothesized that environmental factors, such as sediment load, water surface slope, and precipitation, may impact the effectiveness of using phytoplankton diversity as a water quality indicator. To test this hypothesis, the Yellow River was selected for phytoplankton diversity and water quality assessments. We measured water quality parameters, calculated phytoplankton diversity indices, and collected data on geo-climatic variables at 130 sampling points in the Yellow River mainstream over two seasons (spring and autumn) in 2019. The results of the water quality assessment based on phytoplankton diversity indices were compared with those based on water quality index (WQI). Correlation analysis, multiple stepwise regression, distance-based redundancy analysis, and regression modeling were used to explore the biogeographical patterns and drivers of phytoplankton diversity. According to the WQI, the water quality gradually deteriorated from the source to the estuary of the river. Three biodiversity indices (Margalef, Pielou, and Shannon-Wiener) indicated that the water quality varied dynamically in the middle reaches of the river. The actual relationships between the biodiversity indices and WQI did not fit well with the standard curves of water quality classification based on the respective biodiversity indices and WQI. Variation partitioning analysis indicated that water quality parameters, such as sediment and nutrient load, were the dominant drivers of variation in phytoplankton diversity in most cases, with the contribution ranging from 11.0% to 25.7%. Geo-climatic factors, such as water surface slope and annual mean precipitation, also affected phytoplankton diversity, with the contribution reaching 27.8%. Therefore, in sediment-laden rivers with a large geographical span and complex environment, phytoplankton diversity cannot be used as a suitable water quality indicator, albeit it can reflect habitat changes to a certain extent.