Seasonal variation of phytoplankton nutrient limitation in Lake Taihu, China: A monthly study from Year 2011 to 2012

• Water quality, phytoplankton biomass and community changed seasonally.
• Cyanophyta dominated but other phyla had remarkable contributions in cold seasons.
• Microcystin content increased exponentially with cyanobacterial proportion.
• Phytoplankton could be limited by N, P, and no nutrients in different seasons.
• Nutrient limitation is more dynamic than what we thought before.

Lake Taihu is the third largest freshwater lake in China with severe eutrophication issues. However, it remains ambiguous how its phytoplankton growth is limited by various nutrients in different seasons. A series of bottle-enrichment assays in Meiliang Bay was thus performed once a month from July, 2011 to June, 2012 in the present study. The initial chlorophyll a concentration and phytoplankton cell density ranged from 4.70 to 34.6 μg/l and from 1.25×106 to 6.72×108 cells/l with three peaks in July, November, and March. Although Cyanophyta was dominant (30.9–99.2 percent) in most cases, other phyla like Chlorophyta, Bacillariophyta, and Cryptophyta could account for as much as 69.1 percent of total phytoplankton in cold seasons. The microcystin-LR content in the particulate phase followed a similar seasonal pattern as Cyanophyta. It further went up exponentially with the proportion of cyanobacteria in phytoplankton suggesting more toxigenic species and (or) upregulated microcystin synthesis when the contribution of Cyanophyta was enhanced. On the other hand, the dissolved concentrations of various nitrogen and phosphorus species reached their maxima in late spring and autumn, respectively. According to its growth response to nutrient addition, phytoplankton in Meiliang Bay was restricted by nitrogen in August, October, and November. No nutrient limitation occurred in July, September, and April, whereas phosphorus deficiency prevailed in the other months. Overall, nutrient limitation in Lake Taihu and possibly other aquatic ecosystems worldwide may be more dynamic than what we thought before, which should be considered to eliminate eutrophication.

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