The Phytoplankton Community Dynamics And Driving Factors During Bloom Forming Period In Taihu Lake, China

Rapidly growing anthropogenic inputs, associating with agriculture, aquaculture, urbanization and industrial expansion is a primary cause of the water quality declining. Within the past several decades, many of our freshwater lakes have changed from productive ecosystems to ones experiencing sudden trophic changes, biogeochemical alterations, and a deterioration in habitat quality. Nuisance and harmful phytoplankton blooms are becoming more common generally. These blooms are increasing worldwide and representing a serious threat to drinking water supplies, and the economic sustainability. Nutrients over richment of waters by urban, agricultural, and industrial development have promoted the growth of cyanobacteria. Phytoplankton utilizes light, carbon dioxide, a range of inorganic and organic nutrients to generate compounds by photosynthesis, which provides fundamental energy and matter for food webs. The ecological characteristics of phytoplankton species composition and abundance may be in response to aquatic environment gradients to a certain extent. Taihu Lake has experienced accelerating eutrophication over the past several decades. It has changed from a mesotrophic, diatom-dominated lake to hyper-eutrophic, cyanobavteria-dominated system, with Microcystis blooms now occurring regularly. In the last few years, already a large literature referring to field observation and numerical modeling of cyanobacterial blooms, and the phosphorus controlling of phytoplankton biomass seems more valuable than the nitrogen and phosphorus controlling. However, more researches have focused on the mechanism of algal bloom and relationships analysis between phytoplankton community structure and the possible controlling factors. There is apparently deficient about the spatial and temporal characteristics of phytoplankton community structure. In addition, studied area mostly confined to the comparative between cyanobacteria bloom zone and central lake zone. This paper studied the processes of spatial-temporal variability in the phytoplankton community structure, biomass, dominance species, and the nutrients concentrations characteristics in Taihu Lake during2009-2010year. We analyzed biological responses to these nutrient alterations including shifts in probable nutrient limitations for phytoplankton growth, shifts in phytoplankton production, and indicators of eutrophication. Considering the occurrence of cyanobacterial bloom, the aims of the present study were also to analyze the linkage between nutrient-eutrophication and dominance species Microcystis, to try exploring the competitive mechanism of dominance species. This paper aimed to provide the information about phytoplankton community composition and succession changes, and to aid understanding of lake ecosystems as well as for effective management of lake.(1) A total of207phytoplankton species were identified from the May2009to August2010, belonged to8phyla,79genera. Over the whole water column the Chlorophyceae dominated total species richness, followed by Bacillariophyceae, and then the Cyanophyceae. However, Cyanophyceae dominated during the study period, with peak abundance occurring in summer and autumn. Based on the studies of the past40years about Taihu Lake, result showed that phytoplankton species composition and cell abundance had changed significantly. The general trend was the species number reduced while cell abundance accelerated sharply, and with relatively little changes about dominant species and common genera. These reflected that the seriousness of water pollution and eutrophication status.(2) Through the comparative study of phytoplankton community spatial distribution in bloom and non bloom period, we found that phytoplankton community structure was with obvious characteristics during bloom period. In May, there was visible bloom strips distribution in lake, the cyanobacterial biomass was about1.01～21.56%of total phytoplankton biomass. While in the bloom period, the block and patchy distribution of cyanobacteria could be observed, the proportion of cyanobacteria biomass was increased significantly, and the ratio often comprised of an excess of90%at some water zone. Microcystis not only was the dominant species of cyanobacteria, also was the dominant species of total phytoplankton. The phytoplankton biomass values and cell abundance could be as indicators of water quality and eutrophication progress of Taihu Lake. (3) In January, due to water temperature was4℃, Microcystis couldn’t find or rarely exist in surface layer, while hibernated in surface layer or mid layer of sediments. Taihu Lake is a larger shallow lake with the temperature difference between surface layer and bottom water in general is between0～1℃. Microcystis could withstand lower water temperatures and would be expected to survive at sediment during winter in Taihu Lake. Although with higher nutrients concentrations and Microcystis species, the algal bloom was not forming due to the lower water temperature. Thus, the lower water temperature is the most significant factor in blooming occurrence.(4) The nitrogen content and phosphorus content in Taihu Lake were high and the nitrogen concentration was higher than phosphorus’s. Nutrients contents, especially increasing nitrogen values could not influence on phytoplankton biomass, but may have impact on phytoplankton community structure and succession needed to be studied further future. Phosphorus could be a key factor to determine the distribution of cyanobacterial bloom in Taihu Lake.(5) The experiment results showed that Microcystis aeruginosa growth had different sensitivity to nitrogen sources; ammonium and nitrate were the major inorganic nitrogen. M. aeruginosa growth preferred ammonium to nitrate, and organic nitrogen (CO(NH2)2) could promote Microcystis aeruginosa growth. Therefore, recent frequent cyanobacterial bloom may relate to urea concentrations and be worth to study further.(6) Among three phosphorus sources, M. aeruginosa standing crop was highest by phosphorus sources of Na5P3O10, while by the Na4P2O7with the lowest values, and the M. aeruginosa standing crop decreased about40%-59%.There was relative importance of picophytoplankton to total phytoplankton biomass. A comprehensive observation of that picophytoplankton, including phytoplankton biomass, growth rates and so on, has contributed to explainations of water bloom in Taihu Lake. Then, the water body turbulence from other factors, such as wind force, wind direction, bottom terrain and light, and the influence from interaction between those factors on phytoplankton community should be studied further.