长江口及邻近海域浮游植物现存量的上下行控制分析

利用2010—2011年度3个季节的调查资料以及广义相加模型(GAM)分析,研究了长江口及邻近海域浮游植物现存量(以叶绿素a浓度表征)的上下行控制作用。调查结果显示,在叶绿素高值区,三航次营养盐含量均比整个海区平均值偏低,且春季呈现三季节最低的磷酸盐(PO4-P)和硅酸盐(SiO3-Si)浓度(平均值分别为0.48和8.96μmol/L)以及最高的氮磷比(DIN/P为43.3),为该海域春季甲藻赤潮频发提供了有利条件。夏季叶绿素高值区的硅氮比(Si/DIN)相对整个调查区较高,而春季和秋季却相近,这与夏季藻华种类主要是硅藻相一致。春、夏两季叶绿素高值区的悬浮物浓度(TSS)在时空比较上均显著低值。浮游动物高值区分布与叶绿素高值区分布虽不完全重合,但有交叉或两者相邻。GAM模型分析结果显示,各环境因子变化对长江口及邻近海域叶绿素变化的贡献可达70%以上,且主要影响因子为盐度和营养盐,而与TSS、浮游动物生物量和温度三因子的直接相关性不显著(p0.05)。受长江冲淡水的影响,盐度与DIN、PO4-P、SiO3-Si、TSS等因子间存在显著的相关关系(p0.001),说明盐度对叶绿素变化的影响可能体现了营养盐和光照条件等因子的作用。上述研究结果表明,在长江口及邻近海域,营养盐的上行效应是浮游植物现存量的主要控制作用,而光照条件和浮游动物生物量与浮游植物现存量虽然在时空分布上存在一定的联系,但非决定性控制因素。

Bottom-up and top-down controls of the phytoplankton standing stock off the Changjiang Estuary

Based on the field survey data in August 2010, November 2010 and May 2011, and the following analysis from generalized additive model (GAM), the bottom-up and top-down effects on the phytoplankton standing stock (represented as chlorophyll a concentrations) off the Changjiang Estuary were studied. The results showed that the average concentrations of nutrients in the high chlorophyll zone (HCZ) were lower than those in the entire region. The concentrations of phosphate (0.48 μmol/L) and silicate (8.96 μmol/L) in spring in HCZ were the lowest among the three seasons, and the DIN/P ratio (43.3) was the highest, which help to the frequent dinoflagellate bloom in spring. The Si/DIN ratio in the HCZ was higher than that in the entire region in summer, but very close in spring and autumn, which help to the diatom bloom in summer. The concentrations of total suspended solids (TSS) in the HCZ in spring and summer were significantly low in both spatial and seasonal scale. The spatial distribution of high zooplankton biomass zone was not consistent with the HCZ, but they were crossed or neighboring. The further analysis from GAM showed that the environmental factors can explain more than 70% variability of chlorophyll (log-transformed) off the Changjiang Estuary. Salinity and nutrients were the main factors, while the TSS, zooplankton and temperature had no significant direct correlation with the chlorophyll variability (p>0.05). Moreover, due to the impact of Changjiang Diluted Water, the salinity was significantly correlated to the DIN, phosphate, silicate and TSS (p<0.001), which suggested that the effect of salinity on explaining the chlorophyll variability involved those of both the nutrient and the light. Our results suggested that the bottom-up effects from nutrients were the main control of the phytoplankton standing stock off the Changjiang Estuary. Light condition and zooplankton biomass had a certain relation with the phytoplankton standing stock on the spatial and seasonal distribution, but they were not the key controlling factors.