Spatial-temporal variability of total and size-fractionated phytoplankton biomass in the Yangtze River Estuary and adjacent East China Sea coastal waters, China

Abstract

The spatial and seasonal dynamics of total and size-fractionated phytoplankton biomass (chlorophyll- a) as well as physical and chemical factors in the Yangtze River Estuary and adjacent East China Sea coastal waters were investigated from April 2002 to February 2003. Average surface total and water column integrated chlorophyll a biomass showed a clear seasonal variation in response to the Yangtze River discharge, with the highest in summer (∼4 mg m−3 and >60 mg m−2), intermediate in spring and autumn (∼1 mg m−3 and 26–28 mg m−2), and the lowest in winter (0.5 mg m−3 and <20 mg m−2). Summer maximum chlorophyll a concentrations (>10 mg m−3) occurred at intermediate salinities (∼20–30) region beyond the front zone between 112.5°E and 123°E with sufficient nutrients replenishment for phytoplankton growth. Generally, spatial distribution of size-fractionated phytoplankton showed that phytoplankton biomass was dominated by the large size fraction (>20 μm) in the turbid eutrophic estuarine and near-shore waters, while the small-sized phytoplankton (<5 μm) were dominant in the offshore stations. Phosphate was the main limiting nutrient of phytoplankton biomass in river diluted water and most near-shore stations, while dissolved inorganic nitrogen became the potential limiting nutrient in some offshore stations, except for summer when phosphate limited almost all in the whole investigation region. Controlling the inputs of phosphate loading from the Yangtze River is one of the most effective strategies for reducing the increasing eutrophication and occurrences of harmful algal blooms in Yangtze River Estuary and adjacent East China Sea coastal waters.