Phytoplankton is typically the most important primary producer in lake ecosystems, strongly influences nutrient concentrations, and supports higher trophic levels such as zooplankton and filter-feeders. Knowledge of primary production, and its spatial and temporal variations, is of special relevance for Meiliang Bay in the northern part of the Lake Taihu, China, which has three important inflowing rivers which carry domestic sewage and industrial waste water from the adjacent cities of Changzhou and Wuxi. Although primary production in oceans had frequently been estimated using the Vertically Generalized Production Model (VGPM), this approach has rarely been used in inland lakes. In our present study, phytoplankton primary production in Meiliang Bay, Lake Taihu, was estimated from February to December in 2009 based on VGPM using the MODIS-derived data for chlorophyll a, total suspended solid concentrations, and water surface temperature.
There were significant spatial variations in the monthly mean daily primary production in 3 of the 4 seasons: spring (March, April, May), summer (June, July, August), and autumn (September, October, November), and primary production gradually decreased from inner Meiliang Bay to outer Meiliang Bay. In inner Meiliang Bay, suitable qualities and quantities of nutrients, photosynthetically available radiation, and water temperature, provided good conditions for algal growth in these three seasons. However, there was no significant spatial variation in the monthly mean daily primary production in winter (February, December) due to the death and degradation of algae in the whole of Meiliang Bay. There was however, significant spatial variation in the annual mean daily primary production in Meiliang Bay, Lake Taihu in 2009. Time series analysis showed that the seasonal variation of primary production was: summer > autumn > spring > winter. The annual mean daily primary production was 1,291.61 mgC·m^-2·d^-1, with the highest primary production in August. The trend in temporal variations of the primary production in Meiliang Bay was consistent with that of the chlorophyll a concentration. Temproal variations in primary production were less closely linked to water temperatures which peaked in June and August, and to photosynthetic available radiation intensity which peaked in July.
To explore the effect caused by the sediment resuspension, the phytoplankton primary production estimated by the VGPM model in Meiliang Bay was normalized to per chlorophyll a concentration, water temperature, and photosynthetically available radiation intensity. A significant negative relationship was found between the total suspended solid concentration and the normalized primary production, and a power function relationship was also found between them with a determination coefficient value of 0.487 by non-linear curve fitting. Thus, it could be demonstrated that the increase of total suspended solid concentration from wind-driven sediment resuspension could cause primary production to decrease markedly. The mean maximum carbon fixation rate was( 6.48±0.17) mgC·mgChla^-2·h^-1 when the water temperature ranged from 19℃ to 21℃. Furthermore, water temperature was significantly and positively correlated to primary production in Meiliang Bay. A positive correlation was found between water temperature and the maximum carbon fixation rate when water temperature was less than 21℃.