Spatiotemporal Responses Of Phytoplankton In Lake Taihu To Nutrient Limitation

Taihu, the third largest freshwater lake in China, is suffering severe harmful algae blooms (HABs) as a result of eutrophication. After the breakout of Wuxi drinking water crisis in2007, the government began to make more efforts in Taihu comprehensive renovation. Phytoplankton limitation by various physicochemical factors is critical to control HABs. However, such researches had been rarely performed in Taihu. Therefore, investigating nutrient limitation pattern for the phytoplankton of this lake would have great significance in understanding the underlying mechanisms of HABs and in the establishment of effective management strategies.The objectives of the present study are to study the spatiotemporal variation of nutrient limitation, the relations between nutrient limitation and phytoplankton community, as well as the effects of nutrient limitation on microcystin production. For these purposes, different types of nutrient species in both the dissolved and particulate phases, phytoplankton growth responses to nutrient enrichment, their community composition, and particulate microcystin content were measured, based on which the spatiotemporal variation of phytoplankton nutrient limitation together with its underlying mechanisms was revealed.According to the survey of Lake Center, Xukou Bay and Meiliang Bay, we found that Phytoplankton biomass decreased spatially from Meiliang Bay, Lake Center to Xukou Bay and phytoplankton cell density in Meiliang Bay showed three peaks in March, July, and December. Cyanophyta was dominant in different lake areas and was dominant (30.9-99.2%) in most cases in Meiliang Bay while other phyla like Chlorophyta, Bacillariophyta and Cryptophyta can account for as much as69.1%of total phytoplankton in cold seasons. Cryptophyta biomass was less in Xukou Bay than that in Meiliang Bay and Lake Center because Xukou Bay was a grass type lake and the biodiversity was better. The microcystin-LR content in the particulate phase ranged from0.94-43.0μg/g and was highest in July, August and November (31.3-43.0μg/g) in Meiliang Bay. Phosphorus concentration in water column decreased spatially following the same pattern as phytoplankton biomass, but nitrogen didn’t exhibit any spatial pattern. DIN and TDP concentration reached the highest values in late spring and late autumn, respectively.The nutrient-enriched experiment told us that enriched nutrient changed the phytoplankton community in Xukou Bay:the percentage of Chlorophyta and Bacillariophyta increased but Cyanophyta was still dominant. The content of microcystin-LR content was promoted when nutrient was enriched although the distribution of microcystin-LR content showed no spatial pattern, while the content was promoted when nutrient was enriched. Also we found that the microcystin-LR content in the particulate phase increased exponentially with the ratio of Cyanophyta. Phosphorus induced phytoplankton growth together with a distinct drop in Lake Center and Xukou Bay while the biomass in control and+N treatments was the same. We infer that phytoplankton in those two areas was limited by phosphorus. Phytoplankton suffered complex nutrient limitation in Meiliang Bay. According to the nutrient-enriched experiment, nitrogen limitation occurred in August, October and November and no nutrient limitation occurred in July, September and April while phytoplankton was limited by phosphorus during the other months.