Characterization and causes analysis for algae blooms in large river system

With the effects of human activities and climate change, river algal blooms have frequently occurred worldwide. Compared with eutrophication issues in lakes and reservoirs, the characteristic identification and cause analysis of river algal blooms have been rarely studied, and the cause of river algal bloom remains an academic difficulty. This study aimed to analyze the characteristics and causes of algal blooms in a large river system in China, the Hanjiang River, from 2004 to 2014 by applying various statistical approaches. Statistical analyses included seasonal trend decomposition procedure based on Loess, cross wavelet analysis, Spearman analysis, and redundancy analysis. Results showed the following. (1) The high risk period of algal bloom in Hanjiang River occurred from mid-February to early-March, and the peak value tended to move forward from early-February to mid-February after 2009. (2) The water level of Yangtze River, total P, and water temperature were main contribution factors for river algal blooms downstream of Hanjiang River in China, contributing 44.5%, 39.6%, and 11.4%, respectively. (3) Hydrologic factors played important roles for the occurrences of river algal blooms when a suitable hydrodynamic condition existed between water flow of Hanjiang River and water level of Yangtze River. Finally, the key factors leading to river algal blooms are discussed, and further studies are suggested to ascertain the influence of complex hydrodynamic conditions in order to reveal the cause of river algal blooms.