Formation of cyanobacterial blooms in Lake Chaohu and the photosynthesis of dominant species hypothesis

Elucidation of the causes of cyanobacterial harmful algal blooms (CHABs) may be a precondition for their control. We have investigated Lake Chaohu since 2007 ; identifying phytoplankton species, observing seasonal variation in dominant species, measuring primary productivity, detecting changes in limnological characteristics, identifying 'leading factors', and then assaying the ecophysiology of photosynthesis in the dominant cyanobacteria. We also analyzed the historical events relating to CHABs in this lake. Our studies showed that phytoplankton diversity varied seasonally, and dominant cyanobacteria represented more than 74% of the total phytoplankton cells. Dominant species in 2008 to 2009 included Microcystis viridis ( in April, May, June, October, November and December); Microcystis wesenbergii ( in July and August); Microcystis aeruginosa (in September); and Anabaenaflos-aquae (in January, February and March). Blooms were recorded over 100 years ago in this lake, and no appropriate explanations have been advanced for their causes. Since the1930s, researchers have presented the following ten hypotheses on bloom formation: (A) the TN/TP hypothesis; (B) the inorganic nitrogen hypothesis; (C) the buoyancy hypothesis; (D) the storage strategy hypothesis; (E) the low light hypothesis; (F) the high pI-L/low CO2 hypothesis; (G) the elevated water temperature hypothesis; (H) the trace element hypothesis; (I) the zooplankton grazing hypothesis; and (J) the evolutionary adaptation hypothesis. Although these hypotheses explain why cyanobacteria successfully compete over eukaryotic algae in most lakes and reservoirs, they cannot clarify why different dominant cyanobaeterial species appear in seasonal succession in Lake Chaohu. A new hypothesis is needed. Based on our understanding, we have constructed 'the photosynthesis of dominant species hypothesis', as follows: (1) Blooms include various species of eyanobacteria and algae. Bloom initiation is related to cell density, and also to primary productivity. We collected and measured phytoplankton monthly in different water depths at six points in the western part of Lake Chaohu. In 2008 to 2009, collected phytoplankton consisted of 85 species ( in 5 phyla). Both cell density and primary productivity were highest during the summer, and lowest during winter. (2) During blooms, dominant species grew more quickly and had the greatest biomass of the phytoplankton. There were four dominant species and these constituted ov