The influence of freshwater-saline water mixing on phytoplankton growth in Changjiang Estuary

The distribution of nutrients and salinity recorded during the June 2009 cruise in the Changjiang Estuary indicated that dilution by Changjiang River water and seawater mixing were the main factors controlling nutrient behavior. To better understand the implications of this variation, phytoplankton growth and nutrient uptake along a salinity gradient representing dilution by the Changjiang River and seawater respectively were recorded. Two water samples were collected from the sampling stations C1 and I10 (as used in the June 2009 cruise) which were representative of the freshwater and saline water end-member. After diluting portions of the two samples by 100%, 75%, 50%, 25% and 0% with freshwater to simulate different levels of freshwater-saline water mixing, the samples were incubated for 3 days. The results were as follows: (1) the higher the percentage of freshwater, the faster was initial growth and the higher the in vivo fluorescence concentration. During the 3 day incubation period, the rate of increase in fluorescence and maximum in vivo fluorescence of the 100% dilution treatment were 2.9 ( mu g/L/d and 9.6 mu g/L, respectively, whilst the rate of increase in fluorescence and maximum fluorescence of the 25% dilution treatment were 0.54 mu g/L/d and 2.0 mu g/L respectively. The in vivo fluorescence of the 0% dilution treatment was low, probably due to the low nutrient levels in the offshore seawater. The lower the percentage of freshwater, the lower the growth rate of phytoplankton during the exponential growth period, e.g. 1.18/d, 1.12/d, 1.14/d and 0.77/d for the 100%, 75%, 50% and 25% dilution treatments respectively. (2) NO super(-) sub(3), NO super(-) sub(2), P0 super(3) sub(4) super(-) and SiO super(2) sub(3) super(-) were apparently consumed but not NH super(+) sub(4). The extent and rate of consumption of NO super(-) sub(3) and SiO super(2) sub(3) super(-) were very similar during the first 48 h; the extent of consumption was lower with lower initial nutrient concentration. PO super(3) sub(4) super(-) levels in the 100%, 75% and 50% dilution treatments were depleted within 48 h. This suggested that PO super(3) sub(4) super(-) was the likely limiting factor for phytoplankton growth at salinities below 26. Meanwhile, the initial concentrations of NO super(2) sub(3) super(-), SiO super(2) sub(3) super(-) and P0 super(3) sub(4) super(-) in different treatments had significant positive correlations with their uptake rates during the exponential