Responses of phytoplankton functional groups to simulated winter warming

We investigated the seasonal dynamics of phytoplankton functional groups and the relevant environmental factors (water temperature, total nitrogen content, total phosphorous content and water transparency) in Dianchi Lake, China. We also examined the growth rates and physiological characteristics of representative species in laboratory cultures. In the field experiment, five dominant functional groups, including M (mainly consisted of Microcystis aeruginosa; Microcystis wesenbergii and Microcystis flos-aquae), H1(mainly consisted of Aphanizomenon flos-aquae), J (mainly consisted of Scenedesmus; Pediastrum and Coelastrum), F (mainly consisted of Oocystis and Kirchneriella) and P (mainly consisted of Melosira), were determined. Groups M and J were prevalent throughout the year and comprised more than 90% of the total biomass. Group M was prevalent at a relatively high temperature in summer and autumn; by contrast, group J was dominant at low temperature in winter and early spring. Co-cultivation laboratory experiments revealed that the biomass and the density of Microcystis sp., which is the representative species of the functional group M, were higher at 18 degrees C than at 13 degrees C. Conversely, the density and the biomass of the representative species of the functional group J (consisted of Pediastrum duplex, Coelastrum microporum and Scenedesmus obliquus) were higher at 13 degrees C than at 18 degrees C. At low temperatures, group M (Microcystis spp.) cannot successfully survive and grow at low temperatures, exhibiting various stress responses, such as inhibited photosynthetic activities and reduced phosphorous utilization. However, low temperature increased soluble carbohydrate contents of Microcystis, which favored the fast development of Microcystis, once the temperature warmed. Currently, climate warming is occurring in the Dianchi Lake basin; thus, future climate warming in winter (+/- 5 degrees C) may compromise the advantages of group J and promote the abundance of group M. The water transparency, dissolved oxygen and biodiversity in the lake would be further reduced. Furthermore, the increased microcystin and odor produced by Microcystis would considerably threaten the food web structure and lake ecosystem functions