Temporal variation in cyanobacteria species composition and photosynthetic activity in experimentally induced blooms

Temporal variation in the photosynthetic activity of bloom-forming cyanobacteria was evaluated by the maximum quantum yield of photosystem II (F _v/F _m) using a 3′-(3,4-dichlorophenyl)-1′, 1′-dimethyl urea (DCMU) methodology. To evaluate the F _v/F _m of cyanobacteria in phytoplankton communities containing several algal groups, phycocyanin was directly excited at 590 nm, and in vivo fluorescence at 680 nm emission was measured before and after DCMU addition. Using a cultured Microcystis strain, we first tested the relevance of the method and found that F _v/F _m values measured under phycocyanin excitation approximately corresponded to those under chlorophyll a (chl a) excitation (440 nm). Second, we monitored temporal variations in F _v/F _m for induced cyanobacterial blooms in experimental ponds. Cyanobacterial F _v/F _m was usually lower than overall phytoplankton F _v/F _m measured under chl a excitation, suggesting that potential photosynthetic activity of bloom-forming cyanobacteria was lower than in other algal groups, such as green algae and diatoms. We also found that temporal variations in F _v/F _m explained the shift of dominant species from Microcystis aeruginosa to Aphanizomenon issatschenkoi. Our results consistently demonstrated that this method is simple and useful for evaluation of cyanobacterial potential photosynthetic activity in natural phytoplankton communities.