夏季长江口不同粒级浮游植物碱性磷酸酶活性

碱性磷酸酶活性(alkaline phosphatase activity, APA)是海洋研究中用于反映浮游植物磷限制状态的重要指标。在长江口等“氮过剩”海域,磷是控制海域初级生产力水平的主要因子,然而,磷限制的范围常常难以界定,当前对不同粒级浮游植物的磷限制效应所知甚少。该文根据2020年夏季长江口航次资料,给出了海洋表层各粒级浮游植物(Net:≥20 μm;Nano:2~20 μm;Pico:0.8~2 μm)APA、浮游细菌APA(0.2~0.8 μm)和溶解态APA(<0.2 μm)的空间分布特征,并分析了APA与环境要素间的相关性。结果显示,各粒级浮游植物APA均与无机磷酸盐浓度(dissolved inorganic phosphate, DIP)呈显著负相关,这表明DIP浓度是影响浮游植物APA分布的主要因素。在平面分布上,各粒级浮游植物APA在近口门的光限制区均较低,且呈现自口门向外逐渐升高的趋势,与DIP的分布特征相反。Net和Nano级APA平均值分别为 (40.28±32.35) nmol/(L·h)和(52.38±34.78) nmol/(L·h)]显著高于Pico级APA平均值为(28.43±20.23) nmol/(L·h)],这表明大粒级浮游植物可能更易受DIP下降的影响。该研究中,诱导浮游植物APA快速升高的DIP浓度为0.159 μmol/L,与近岸区磷限制经验阈值相接近。该研究揭示了夏季不同粒级浮游植物对长江口磷分布的响应特征,有助于理解长江口初级生产过程的环境调控机制。

Alkaline phosphatase activity of size-fractionated phytoplankton in the Changjiang Estuary in summer

Alkaline phosphatase activity (APA) is an important indicator of phytoplankton phosphorus limitation status in the marine science study. In the “nitrogen-excess” areas such as the Changjiang Estuary (CJE), phosphorus is a major factor controlling the primary productivity in the sea. However, the extent of phosphorus limitation is often difficult to define and little is known about the effect of phosphorus limitation on phytoplankton at different sizes. In this study, the spatial distribution of phytoplankton (Net: ≥20 μm; Nano: 2~20 μm; Pico: 0.8~2 μm) APA, bacterioplankton APA (0.2~0.8 μm) and dissolved APA (<0.2 μm) in the surface layer of the CJE in the summer of 2020 were given, and the correlation between APA and environmental factors were analyzed. The results showed that size-fractionated phytoplankton APA was negatively correlated with dissolved inorganic phosphate (DIP), indicating that DIP concentration was the main factor affecting the distribution of size-fractionated phytoplankton APA. In the spatial distribution, phytoplankton APA was lower in the light-limitation zone near the mouth, and showed an increasing trend from the mouth to the east, contrary to the distribution of DIP. Net and Nano phytoplankton APA (mean values of (40.28±32.35) nmol/(L·h) and (52.38±34.78) nmol/(L·h), respectively) were significantly higher than Pico phytoplankton APA (mean values of (28.43±20.23) nmol/(L·h)) in the CJE, implying that large size phytoplankton were more susceptible to DIP decline. In this study, the DIP concentration for inducing a rapid increase in phytoplankton APA was 0.159 μmol/L, which was close to the empirical threshold for nearshore phosphorus limitation. This study revealed the characteristics of phytoplankton response to phosphorus distribution in the CJE at different grain levels in summer, and also contributed to the understanding of the environmental regulatory mechanisms of primary production processes in the CJE.