富营养河口水体藻华粒级结构的调控机制研究

Researches on regulatory mechanism of algal bloom size structure in eutrophic estuarine water

为探究富营养河口水体藻华粒级结构的调控机制,本研究利用枯水期珠江口上游河水、下游海水及其等比例混合水进行培养实验,跟踪监测水体中叶绿素a和营养盐的浓度变化,并利用稀释实验估算藻类生长速率(μ)和小型浮游动物的摄食速率(m),以阐明上行控制(营养盐刺激)和下行控制(摄食影响)对藻类粒级结构的影响。结果显示:营养加富能增加藻类的生物量,藻类群落的优势粒级由超微型和微型转换为小型;加富河水中μ维持2~3 d高值后下降,速率为(1.13±0.37)d^(-1),加富海水中μ逐步增加,速率为(1.06±0.16)d^(-1),加富混合水中μ轻微波动,速率为(0.58±0.14)d^(-1),总体上小型藻类μ最大。3组加富水体中m总体均先增大后下降,粒级差异不明显。藻类被小型浮游动物摄食率(m/μ)随粒级增大而减小,说明富营养刺激大粒级的生长,大粒级面临的被摄食压力较小。m/μ随藻类每日的比生长速率(μChl a)降低而增加,说明藻华前期由上行控制主导,后期下行控制作用相对加强。本研究表明,富营养化不仅能够改变藻华的生物量,而且能影响其粒级结构,初步阐明了富营养河口水体中藻华粒级结构的调控机制。

In order to examine the regulatory mechanism of size structure of algal blooms in eutrophic estuarine waters, we used river water, sea water and their mixed water in Zhujiang River Estuary during dry season and conducted incubation experiments to examine changes of nutrients and chlorophyll a(Chl a) concentrations. Algal growth rate(μ) and microzooplankton grazing rate(m) were estimated by dilution experiments to examine the effects of bottom-up(nutrients stimulation) and top-down control(microzooplankton grazing) on size structure of algal blooms. We found that nutrient additions increased the peak of Chl a concentration, and phytoplankton community dominance changed from picophytoplankton and nanophytoplankton to microphytoplankton. Generally, μ kept high in the first 2 to 3 days and then declined(1.13±0.37) per day in nutrient added river water;μ kept increasing(1.06±0.16) per day in nutrient added sea water and slightly fluctuated(0.58±0.14) per day in nutrient added mixed water with microphytoplankton having the highest μ. In contrast, m increased in the first 2 days or 3 days and then decreased, and there were no size differences in all treatments. The microzooplankton grazing vs algal growth(m/μ)increased from microphytoplankton, nanophytoplankton, to picophytoplankton, indicating that larger size phytoplankton were under less top-down control. In addition, m/μ increased as daily algal specific growth rate decreased,indicating that bottom-up control played a stimulating role at early stage, and the top-down control played a more important role in the late stage of algal blooms. This study suggests that eutrophication can make a difference in both the magnitudes and size structure of algal blooms in estuarine waters.