富营养化水体的营养盐限制性研究综述

Review of studies on the limitation of nutrients in the eutrophic water

水体富营养化是全球目前以及今后相当一段时期内的重大水环境问题。氮、磷作为浮游植物生长所必需的营养盐,是水体富营养化的主要限制因子。通过对近年来水体富营养化受氮、磷营养盐限制的研究进行综述,研究了营养盐来源及其对浮游植物生长的影响,总结了营养盐限制性差异的影响因素,分析了缓解富营养化的营养盐主控因子。分析表明:营养盐的来源主要分为外源和内源,受外界环境及人类活动影响显著;浮游植物的生长受N/P比值及其绝对浓度的共同作用;在不同的水文、气候和人类活动强度下,水体富营养化主要营养盐限制因子存在差异,并且存在限制性转换和共同限制的情况;缓解水体富营养化措施的提出应在综合考虑多种因素的前提下进行。

Background, aim, and scope As a global environmental issue, eutrophication has been recognized as the most common and severe problem, which originated from the excessive input of nutrients from agricultural run-off, untreated industrial and urban discharges. The primary symptom of eutrophication is excessive production of phytoplankton and other microorganisms. Two elements, nitrogen（N） and phosphorus（P）, explain most of the experimentally diagnosed nutrient limitation of algal growth in inland waters under natural or human-modifi ed conditions. Therefore, in recent years, a variety of studies focused on the interactions between cyanobacterial proliferation and nutrient limitation has been investigated and many valuable results were obtained. However, there is a strong controversy now to the alternative possibilities for controlling one nutrient preferentially（P） or two nutrients with equal emphasis（N, P）. And the short of systematic summaries about them exists. This paper sums up the experience centring on nutrient limitation problems in order to provide a reference of eutrophication control. Materials and methods A lot of literatures were retrieved around nutrient limitation of eutrophication; based on this, we made a systematic analysis from some aspects, including the sources of nutrients in water bodies, the growth restriction of phytoplankton by nutrients, the influence of nutrient limitation, and the nutrients control measures for eutrophication relieving. Results The results show that exogenous nutrients of contaminated lakes and rivers mainly come from agricultural activities, industrial production, and domestic wastewater; endogenous pollutions were basically due to the release of sediment nutrients. The limiting nutrient concept holds that nutrient defi ciency at any given time in a photosynthetic organism can be traced to a single element, which is the element available in the least amount relative to the needs of the organism. Therefore, in order to control the excessive algal growth, it is important to know which element limits the expansion of algal populations when their growth stops because of nutrient depletion. The growth restriction of phytoplankton infl uenced by nutrients refl ected in two aspects：（1） the absolute concentration of nutrients;（2） the relative ratio of nutrients（N, P）, the concentrations of which jointly affected the growth of algae. However, there exists difference of nutrient limiting factor of eutrophication, which was affected by hydrology, weather and the intensity of human activities. The limiting nutrient concept is more complex for an entire community or ecosystem than it is for a single organism. For example, species may differ, even among organisms of similar type（e.g., algae）, in their optimal internal N： P ratios, and their ability to store critical nutrients or to take up a nutrient at low concentrations. Thus, it is possible in a mixed community of algae for some species to be limited by phosphorus and others to be limited by nitrogen. In addition, it is possible for an environment to be very near the nutrient limitation thresholds for N and P simultaneously. Thus, a slight enrichment with one element could cause the other element to become limiting. A third possibility is that seasonal or spatially heterogeneous changes may occur in the relative availability of potentially limiting nutrients. All of these circumstances have been documented experimentally and in nature. Finally, we suggest that the controlling of N and P loadings could be considered to relieve the eutrophication, and, P inputs should be primary controlled in comparison with N. Much more attention has been given to P limitation than to N limitation in inland waters for three reasons：（1） control of total phosphorus in effl uents is feasible and is increasingly being required by regulations;（2） N2 fi xation by cyanobacteria（also known as blue-green algae） has been assumed to make N control ineffective, but could be made more feasible by recognition that a substantial portion of dissolved organic nitrogen is not bioavailable;（3） the correlation between chlorophyll（an index of algal abundance） and total P among lakes is stronger than the correlation between chlorophyll and total N. Discussion The results also show that the sources of nutrients include exogenous and endogenous pollutions. The nutrient limitation factor was unlike under different nutrient conditions. The concentrations of N and P as well as their ratio could be an alternative reason that infl uences the proliferation of algae. In addition, external conditions such as climate, environment, hydrology as well as the intensity of human activities would affect the concentrations and their ratios. In order to control eutrophication, the concentration difference should be considered to relieve the degree of pollution. Moreover, the nutrient limitation factor of eutrophication should be determined for taking appropriate control methods. Conclusions Comprehensive studies show that the dogenous and exogenous sources of nutrients which are remarkable influenced by human activities and external environment. The growth of plankton is influenced both by the ratio of nitrogen and phosphorus and their absolute concentrations. The main limiting factors of nutrients that lead to eutrophication are different with different climate, hydrology and human activity intensity of water area. Recommendations and perspectives In the future study, mechanism discussions should be emphasized to do in-depth analysis on nutrient restriction of eutrophication. At the same time, the response relationships between nutrients（N, P） and algae proliferation should be clarifi ed in the future, in order to complete the forecasting and forewarning model of eutrophication. Additionally, the joint of a variety of biotechnologies need to be improved to explore the interactions among different species and ecological security.