长江口锋面附近咸淡水混合对浮游植物生长影响的现场培养

Abstract

通过2009年6月调查航次,获得了营养盐等参数断面分布,表明咸淡水混合是控制营养盐分布的主要因素。为了解不同盐度梯度下浮游植物生长与营养盐吸收的关系,采集两个站位水样分别代表长江冲淡水(C1站)和外海水(I10站),按C1站水样比例,分100%、75%、50%、25%、0%不同比例混合进行现场模拟咸淡水混合培养,有以下认识:(1)平行结果表明培养过程中活体荧光极大值在100%混合组,且淡水比例越低,指数生长期0-48 h内生长速率越低,100%、75%、50%、25%组分别为1.18/d、1.12/d、1.14/d、0.77/d;(2)低于26盐度的水体中PO₄^3-在48 h内可被迅速消耗而产生限制作用,是控制浮游植物生长的潜在限制因子;(3)除0%组外,各混合组DIN/P(DIN:溶解无机氮,Dissolved Inorganic Nitrogen,DIN=NO₃^-+NO₂^-+NH₄⁺)比值在浮游植物指数生长期有升高趋势,100%组DIN/P比值增加了一倍。各组培养48 h后DIN/Si比值逐渐降低至原来的0.7左右,初始DIN/Si小于一定时间内硅藻吸收的(ΔDIN/ΔSi)比,是造成各组DIN/Si比值减小的原因。以上结果表明咸淡水混合过程中形成的营养盐梯度可造成浮游植物生长程度和速率差异,且可因局部浮游植物旺发而改变海水营养结构。;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 <em>in vivo</em> fluorescence concentration. During the 3 day incubation period, the rate of increase in fluorescence and maximum <em>in vivo</em> 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 <em>in vivo</em> 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) NO₃^-, NO₂^-, PO₄^3- and SiO₃^2- were apparently consumed but not NH₄⁺. The extent and rate of consumption of NO₃^- and SiO₃^2- were very similar during the first 48 h; the extent of consumption was lower with lower initial nutrient concentration. PO₄^3- levels in the 100%, 75% and 50% dilution treatments were depleted within 48 h. This suggested that PO₄^3- was the likely limiting factor for phytoplankton growth at salinities below 26. Meanwhile, the initial concentrations of NO₃^-, SiO₃^2- and PO₄^3- in different treatments had significant positive correlations with their uptake rates during the exponential growth period. For NO₃^- 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 SiO₃^2-, 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 PO₄^3-, 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: NO₃^->SiO₃^2->PO₄^3-. (3) The DIN/P(DIN:Dissolved Inorganic Nitrogen,DIN=NO₃^-+NO₂^-+NH₄⁺) 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 PO₄^3- was consumed very rapidly, while DIN decreased slowly. Similarly, the ratio DIN to PO₄^3- 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.