In the lakes lacking higher aquatic plants, phytoplankton is the most important primary producer, and the basis of material circulation and energy flow in the aquatic ecosystems. Knowledge of phytoplankton primary productivity and its influencing factors is fundamental to the understanding of lake ecosystem dynamics and the assessment of lake ecosystem service functions. Although phytoplankton primary productivity in lakes had frequently been monitored and estimated using the black-and-white bottles, carbon isotope, remote sensing and optical data, there has a limited knowledge for phytoplankton primary productivity high-frequency variation and influencing factors. In our study, we analyzed phytoplankton primary productivity characteristics and influencing factors, using the high frequency monitoring data and synchronous anniversary in situ observation data in littoral zone of Meiliang Bay, Lake Taihu in 2010.
Because the high frequency automatic monitoring data was missing from May to July, we used screening high frequency monitoring dissolved oxygen data to estimated phytoplankton daily primary productivity. Weekly results showed that the lowest phytoplankton primary productivity was recorded during the period from February to April with the mean daily gross primary productivity (GPP) less than 0.20 mgO₂·L^-1·h^-1. During the period from August to October, phytoplankton primary productivity raised, and it reached the highest throughout the year in the second week of September with the maximal mean daily GPP of 1.34 mgO₂·L^-1·h^-1, net ecosystem productivity (NEP) of 0.44 mgO₂·L^-1·h^-1. During the period from November to December, phytoplankton primary productivity reduced with decreasing water temperature.
Correlation analysis showed that the significant relationships were found between main components of phytoplankton (Cyanophyta, Chlorophyta, and Bacillariophyta) and GPP, NEP, and the trend of phytoplankton primary productivity was similar with the change of phytoplankton biomass. Thus the diurnal and seasonal variations of phytoplankton primary productivity can be reflected by the high frequency monitoring dissolved oxygen, making it is feasible to calculate phytoplankton primary productivity using high frequency algal bloom data. Statistical analysis also showed that water temperature is an important factor influencing the change of phytoplankton primary productivity in Lake Taihu. The supply of nitrogen and the selectivity prey of zooplankton were also important for the variations of phytoplankton primary productivity.
The special topographic of lakeshore played an essential role in progress of algal piled, then impacted primary productivity; and this change was exacerbated by meteorological and hydrological processes, which reflected spatial and temporal heterogeneity of phytoplankton primary productivity of eutrophic lake. Based on the meteorological factors data during two algae accumulating periods in August and December, primary productivity changes are significantly influenced by meteorological factors. If there were large amounts of algae with Cyanobacteria as the dominant species, adequate sunlight to promote phytoplankton proliferating quickly, and steady wind to drive algae accumulating, phytoplankton primary productivity significantly increased in the lakeshore region. The high frequency dissolved oxygen monitoring can capture those algae accumulating processes. Thus the high frequency dissolved oxygen monitoring can be used in the monitoring of phytoplankton primary productivity and the warning of algal bloom disaster.