Analysis of spatio-temporal variation in phytoplankton and its relationship with water quality parameters in the South-to-North Water Diversion Project of China

The Middle Route of the South-to-North Water Diversion Project of China (MRSNWDPC), the longest trans-basin water diversion project in the world, has been in operation for over 6 years. The water quality of this mega hydro-project affects the safety of more than 60 million people and the health of an ecosystem over 160,000 km2. Abnormal algal proliferation can cause water quality deterioration, eutrophication, and hydro-project operation issues. However, few studies have investigated and reported planktonic algae and their relationship with the water quality of this trans-basin water diversion project. Here, spatio-temporal characteristics of algal cell density (ACD) and 11 water quality parameters, including water temperature (WT), pH, dissolved oxygen (DO), permanganate index (CODMn), 5-day biochemical oxygen demand (BOD5), fecal coliforms (F. coli), total phosphorus (TP), total nitrogen (TN), ammonia nitrogen (NH3−N), fluoride (F−), and sulfate (SO42−) in the MRSNWDPC from May 2015 to February 2019 were determined using multivariate statistical approaches. Consistent seasonal variation in ACD was observed each year, which grew in spring and then continuously decreased from summer to winter. Summer and winter are the seasons with the highest and lowest ACDs, with average values of 572.95 × 104 cell/L and 157.09 × 104 cell/L, respectively. The NH3−N was positively correlated with ACD growth in all seasons, with Pearson correlation coefficients ranging from 0.594 to 0.738 (P < 0.01). The results of the principal component analysis show that the sources affecting the water quality variation in this project are complex, and NH3−N was the most critical water quality parameter affecting ACD variation, which was linked to ACD in four seasons with strong positive loadings ranging from 0.754 to 0.882, followed by CODMn. The management department of the MRSNWDPC should focus on key periods of phytoplankton control in spring and summer; in addition, variation in the concentrations of NH3−N and CODMn merits special attention. This study provides a helpful reference for the water quality security and algae control strategy of the MRSNWDPC and similar projects in the world.