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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 μg/L/d and 9.6 μg/L,respectively,whilst the rate of increase in fluorescence and maximum fluorescence of the 25% dilution treatment were 0.54 μg/L/d and 2.0 μg/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-3,NO-2,PO3-4 and SiO2-3 were apparently consumed but not NH+4.The extent and rate of consumption of NO-3 and SiO2-3 were very similar during the first 48 h;the extent of consumption was lower with lower initial nutrient concentration.PO3-4 levels in the 100%,75% and 50% dilution treatments were depleted within 48 h.This suggested that PO3-4 was the likely limiting factor for phytoplankton growth at salinities below 26.Meanwhile,the initial concentrations of NO-3,SiO2-3 and PO3-4 in different treatments had significant positive correlations with their uptake rates during the exponential growth period.For NO-3 the uptake rates for the 100%,75%,50%,25% and 0% dilution treatments were 25.39,19.24,12.84,6.04 and 0.21 μmol/d,respectively.For SiO2-3,the uptake rate decreased from 14.34 μmol/d for the 100% dilution treatment to 3.73 μmol/d for the 25% dilution treatment.For PO3-4,uptake rates decreased from 0.46 μmol/d for the 100% dilution treatment to 0.02 μmol/d for the 0% dilution treatment.For the same dilution treatment uptake rates of the nutrients could be sequenced as follows: NO-3SiO2-3PO3-4.(3) The DIN/P(DIN:Dissolved Inorganic Nitrogen,DIN=NO-3+NO-2+NH+4) ratio for all treatments,except for the 0% treatment,increased during the phytoplankton exponential growth period.For the 100% dilution treatment the ratio doubled as PO3-4 was consumed very rapidly,while DIN decreased slowly.Similarly,the ratio DIN to PO3-4 consumed in 100%,75% and 50% dilution treatments was higher between 48 h and 96 h compared to consumption within the first 48 h.This indicated that under sufficient nutrient conditions,phytoplankton absorb N and P with an increasingly greater ratio during the exponential growth period.The DIN/Si ratio decreased to about 0.7 times the original level during the first 48 h of incubation.This reflected the low initial DIN/Si value compared to the diatom uptake ratio(ΔDIN/ΔSi) during the incubation period.The results demonstrated variation in the extent and rate of phytoplankton growth for different freshwater-saline water mixtures,and the resultant nutrient gradient.Such mixing processes may cause local blooms which change nutrient structure,and could result in phytoplankton regime shifts.

An increase in N2 fixation by diazotrophic organism due to increased stratification driven by climate changes,may decrease phosphate concentrations and result in P limitation in the oligotrophic upper ocean,which challenges the traditional view that nitrogen is generally the primary nutrient limiting phytoplankton productivity in oceanic waters.Which nutrient,N or P,is the most limiting nutrient for phytoplankton growth in the oligotrophic Pacific Ocean has been on debate over recent years.More studies on nutrient limitation are apparently needed to resolve this debate in the Pacific Ocean.In August and September of 2009,nutrient enrichment bioassays were conducted at two representative stations,N1(160.58°E,21.61°N) in the western Pacific Ocean with extremely low nutrient(below detect limit) and Chl a concentrations,and N2(154.12°W,10.12°N) in the eastern Pacific Ocean with shallower nutricline due to the influence of equatorial currents,in order to examine the spatial variability in the potential limiting nutrient for phytoplankton growth in the Pacific Ocean.Nutrients were added in 5 combinations in bioassays: control(no addition),NO3+PO4(N+P);NO3+SiO4(N+Si),PO4+SiO4(P+Si),and NO3+PO4+SiO4(N+P+Si).The limiting nutrient was judged based on the response of algal biomass and nutrient depletion among treatments.Phytoplankton exhibited different response to nutrient additions at two study sites.Phytoplankton biomass increased dramatically in response to both N and P additions at station N1 where the concentrations of Chl a increased from 0.03 μg/L at the beginning to 2.12 μg/L and 1.83 μg/L in N+P+Si and N+P treatments at the end of incubation,respectively.However,the maximum Chl a concentration achieved in N+Si treatment was slight higher than that in and P+Si treatment.Furthermore,P was depleted before N and Si in the N+P and N+P+Si treatments at station N1.In contrast,algal biomass was significantly stimulated only when both N and P were added at station N2,where the concentrations of Chl a increased profoundly from 0.10 μg/L to 0.34 μg/L and 0.40 μg/L at the end in N+P+Si and N+P treatments,respectively,while they was not stimulated in the N+Si and P+Si treatments Furthermore,N always disappeared before P and Si in N+P,N+Si and N+P+Si treatments.These results showed that the limiting nutrient varied spatially during summer in the Pacific Ocean.N and P co-limitation occurred at both stations,with P being the primary limiting nutrient at N1 and N at N2.In addition,changes in the N ∶ P ratios during the incubation demonstrated distinct patterns,likely due to difference in the phytoplankton composition.N ∶ P ratios rose from 15.7 at the beginning to 59.2 at the end of the incubation at N1,while N ∶ P ratios decreased from 15.3 to 0.06 at N2.This implied that the uptake ratio of N ∶ P was lower than the Redfield ratio(16N ∶ 1P) at N1,but higher than the Redfield ratio at N2.This might explain why P was the primary limiting nutrient in the western Pacific Ocean but N in the central Pacific Ocean.It is speculated that P limitation possibly is associated with N2 fixation in the oligotrophic western Pacific Ocean.