Spring phytoplankton blooms in Xiangxi Bay of Three-Gorges Reservoir: spatiotemporal dynamics across sharp nutrient gradients

After the impoundment of Three-Gorges Reservoir, the specific hydrodynamics created sharp nutrient gradients in Xiangxi Bay. To discover how spring blooms in Xiangxi Bay respond to sharp nutrient gradients, we investigated the phytoplankton community composition and dynamics along nutrient gradients and analyzed the strategies of different types of phytoplankton in filling their niches. We observed a total of 148 taxa of 59 genera belonging to seven phyla. Phytoplankton concentration ranged from 0.03 x 10(7) to 3.74 x 10(7) cell L-1 with two pulses during the spring bloom period. Bacillariophyta was the predominant group, which accounted for 75.4% of total abundance. Canonical correlation analysis (CCA) revealed that phytoplankton occurred in optimal niches for their growth across the nutrient gradients. Specifically, species of Bacillariophyta were found in the region with Si:DIN > 1, and species of Cryptophyta, Chrysophyta, Euglenphyta, Pyrrophyta, and some of Chlorophyta were most abundant in the region with DIN:PO4P < 16. High abundances of Cyanophyta and some Chlorophyta were observed in the region where DIN:PO4P > 16. The CCA also indicated a potential nutrient limitation of Si on Bacillariophyta, of DIN on Cyanophyta and some Chlorophyta, and of PO4P on Cryptophyta, Euglenphyta, Pyrrophyta, and some Chlorophyta. Moreover, as a complement for the traditional Redfield ratio of 16N:1P in determining whether N or P limits a system, our study revealed that the stoichiometric ratio of 1Si:1DIN is an important criterion to determine limitation for diatoms