热带水库中铁的供应对浮游植物群落季节演替的驱动机制

Phytoplankton seasonal succession characterized with cyanobacteria dominating in mixing period and chlorophyta dominating in stratification period in tropic reservoirs, Southern China. It is critical for illuminating the succession mechanism to understand the factors restraining competitive advantage of cyanobacteria in stratification period. It was put forward that cyanobacteria growth and competitive ability in phytoplankton community are regulated by internal loading of ferrous ions because of higher iron demand in cyanobacteria growth than its eukaryotic competitors. The internal Fe from surface sediment is hard to arrive at epilimnion because of strong stratification in summer in the tropical reservoirs, and iron concentration is low because of high pH and dissolved oxygen concentration. Meanwhile, the iron demand of cells will increase as the enhanced photosynthesis. We hypothesize that cyanobacteria growth is limited because of decreased Fe loading, and the synthesis of cyanobacteria photosynthesis and photoprotection systems are inhibited, consequently, the phytoplankton community change from cyanobateria dominating with higher iron demand to chlorophyta dominating. To test the hypothesis, we will select three representative tropical reservoirs in different trophic status, and seasonally investigate their phytoplankton community and iron concentration to quantify the relationships among community chemometrics (Fe, P, C), pigmentation ratio, species composition and iron concentration by statistic analysis; we will test the limiting effect of iron on phytoplankton and understand the difference of the effect among species by short-term nutrient addition in-situ bioassays; and we will test the effect of iron on phytoplankton succession by long-term Fe addition and eliminate in-situ enclosure experiments. Then, we will illuminate the seasonal characteristics of iron supply in tropical reservoirs and its role and mechanism in modifying the seasonal succession of phytoplankton communities and dynamics of cyanobacterial blooms.

在我国热带水库，混合期蓝藻占优势及分层期绿藻占优势是浮游植物群落季节演替的主要特征，揭示分层时期蓝藻优势受抑制的机制是阐明季节演替机制的关键。Molot模型基于蓝藻较其它藻类需铁更高，认为湖上层铁的供应调控蓝藻竞争优势。在热带水体中,夏季持续强烈的分层导致底层铁难以进入真光层，高pH值和氧化环境使得湖上层Fe浓度极低，强烈的光合作用也需要消耗更多的铁。我们推测：热带水库分层期铁供应减弱限制蓝藻生长，蓝藻光合与光保护系统合成受到抑制，导致高铁需求的蓝藻型群落向绿藻型转变。本项目对三座不同营养状态的典型水库的浮游植物和铁浓度进行季节观测，分析群落化学计量学（Fe、P、C）、色素比例、种类组成与铁浓度的定量关系；以原位添加实验明确铁限制性作用及种间差异，在围隔中进行添加和去除实验明确铁对群落演替进程的控制作用，阐明热带水库铁供应季节特征及其在浮游植物群落季节演替及蓝藻水华发生中的作用和机制。

浮游植物是水库生态系统最重要的初级生产者，影响生态系统健康和水质。对浮游植物群落季节演替机制的理解，于水库供水保障和水资源的可持续利用有重要意义。本项目提出热带水库分层和氧化环境导致铁元素的季节性变化，使铁在部分时期成为浮游植物生长限制性资源，对浮游植物群落结构有重要调节作用，铁元素是浮游植物群落季节演替的重要驱动因子。为验证该假设本研究通过室内实验，明确了铁对浮游植物种群生长限制作用及种间差异；通过原位和室内群落培养实验证实铁对群落结构和演替过程的调控作用；通过3座水库的生态监测证实了热带水库中Fe的季节动态及其对浮游植物群落演替的重要性。本研究发现，与氮和磷广泛限制性不同，铁的限制作用主要体现在蓝藻季节动态上，湖下层厌氧条件的出现导致底泥中铁的释放是水体铁的重要来源，对水库蓝藻水华的发生有重要作用，持续的铁供应是支持拟柱孢藻水华发生的重要条件。研究建议，今后热带水库浮游植物群落研究应不限于磷限制范式，进一步解析氮、磷和铁共限制模式下浮游植物群落对营养盐变化的响应机制。以上研究结果和结论从理论上丰富了我们对热带水体浮游植物群落演替机制的认识，并为热带水库水质管理和生态系统健康管理提供有价值的理论依据。