📚 Biological Consequences of Water Pollution by Sewage: Eutrophication | 污水污染水体的生物学后果：富营养化
Sewage and agricultural runoff introduce excess nutrients into aquatic ecosystems, triggering a chain of biological events known as eutrophication. This process dramatically alters water quality, depletes dissolved oxygen, and often leads to the collapse of local food webs. For Edexcel IGCSE Science students, understanding eutrophication is essential because it links ecology, human impact, and practical environmental management. In this article, we will explore the step-by-step mechanism of eutrophication, its devastating effects on aquatic life, and the methods used to prevent it.
生活污水和农业径流将过量的营养物质带入水生生态系统，从而引发一连串被称为富营养化的生物事件。这一过程会剧烈改变水质，耗尽溶解氧，并常常导致当地食物网的崩溃。对于 Edexcel IGCSE 科学课程的学生来说，理解富营养化至关重要，因为它将生态学、人类影响和实际环境管理紧密联系在一起。本文将逐步探讨富营养化的机制、其对水生生物的破坏性影响以及预防富营养化的方法。
1. Introduction to Water Pollution and Sewage | 水污染与污水简介
Water pollution occurs when harmful substances enter bodies of water, making them unsafe for organisms and human use. One major source of pollution is untreated or partially treated sewage, which contains organic matter, pathogens, and high concentrations of nutrients such as nitrates and phosphates. These nutrients, while essential for plant growth in small amounts, can cause severe ecological imbalances when present in excess.
当有害物质进入水体，导致其对生物和人类使用变得不安全时，就发生了水污染。一个主要的污染源是未经处理或仅部分处理的污水，其中含有有机物、病原体以及高浓度的硝酸盐和磷酸盐等营养物质。这些营养物质虽然在少量时对植物生长至关重要，但一旦过量就会引发严重的生态失衡。
2. What is Eutrophication? | 什么是富营养化？
Eutrophication is the process by which a body of water becomes overly enriched with nutrients, leading to excessive growth of algae and other aquatic plants. The term originates from the Greek 'eu' (well) and 'trophe' (nourishment). While some lakes are naturally eutrophic, human activities have greatly accelerated this process through sewage discharge and the use of fertilisers. The sudden increase in plant growth sets off a deteriorating sequence that reduces biodiversity and water quality.
富营养化是指水体因营养物质过度富集，导致藻类和其他水生植物过度生长的过程。该术语源自古希腊语中的 'eu'（良好）和 'trophe'（营养）。虽然有些湖泊是天然的富营养湖，但人类活动通过排放污水和使用化肥大大加速了这一过程。植物突然大量繁殖引发一系列退化性变化，降低了生物多样性和水质。
3. Nutrient Enrichment: Nitrates and Phosphates | 营养物质富集：硝酸盐与磷酸盐
The primary culprits in cultural eutrophication are nitrate (NO₃⁻) and phosphate (PO₄³⁻) ions. Phosphates are often the limiting factor for plant growth in freshwater; even a small addition can spark massive algal blooms. Sources include phosphate-rich detergents, human and animal waste, and fertilisers washed from farmland. Nitrates, highly soluble in water, easily leach from agricultural soil into rivers and lakes, adding to the nutrient load.
人为富营养化的主要元凶是硝酸根离子（NO₃⁻）和磷酸根离子（PO₄³⁻）。磷酸盐通常是淡水中植物生长的限制因子；哪怕只增加少量磷酸盐，也能引发大规模的藻华。来源包括富含磷酸盐的洗涤剂、人畜粪便以及从农田冲刷出的化肥。硝酸盐则极易溶于水，能从农业土壤渗入河流和湖泊，进一步增加营养负荷。
4. The Process of Eutrophication: Step-by-Step | 富营养化的过程：逐步解析
The sequence of eutrophication can be broken down into clear stages. First, nutrients enter the water. Then, algae and aquatic plants grow rapidly, forming dense green mats on the surface. As the algae die, they sink and are decomposed by aerobic bacteria. This decomposition consumes dissolved oxygen. Oxygen levels drop sharply, causing the death of fish and other aerobic organisms. Finally, anaerobic bacteria take over, releasing toxic gases and further degrading the habitat.
富营养化的过程可分为几个清晰的阶段。首先，营养物质进入水体。接着，藻类和水生植物快速生长，在水面形成厚厚的绿色覆盖层。藻类死亡后沉入水底，由需氧细菌进行分解。这一分解过程消耗溶解氧。氧含量急剧下降，导致鱼类和其他需氧生物死亡。最后，厌氧细菌占据主导，释放有毒气体并使栖息地进一步恶化。
5. Algal Blooms and Light Penetration | 藻华与光照穿透
An algal bloom is a visible sign of eutrophication. The dense layer of algae blocks sunlight from reaching submerged plants. Without sufficient light, these plants cannot photosynthesise, so they die. This not only destroys the primary producers that form the base of the food chain but also adds more dead organic matter for bacteria to decompose, worsening the oxygen crisis.
藻华是富营养化的可见标志。浓密的藻层阻挡阳光照射到沉水植物。由于缺乏足够的光照，这些植物无法进行光合作用，从而死亡。这不仅摧毁了构成食物链基础的初级生产者，而且增加了更多的死亡有机物供细菌分解，使氧气危机雪上加霜。
6. Death and Decomposition of Algae | 藻类的死亡与分解
aerobic respiration during decomposition can be represented simply as: organic matter + O₂ → CO₂ + H₂O + energy. The huge quantity of dead algae places a massive demand on the dissolved oxygen in the water.
藻华的生命周期很短；当营养供应减少或环境条件改变时，藻类会大量死亡。死去的细胞沉入水底，腐生细菌和真菌随即开始分解它们。这一分解过程是需氧的，也就是说需要氧气。分解过程中的有氧呼吸可简单表示为：有机物 + O₂ → CO₂ + H₂O + 能量。大量死亡的藻类对水中的溶解氧产生了巨大需求。
7. Oxygen Depletion: Biochemical Oxygen Demand (BOD) | 溶解氧耗竭：生化需氧量（BOD）
Biochemical Oxygen Demand (BOD) is a key measure in water quality studies. It quantifies the amount of dissolved oxygen needed by aerobic microorganisms to break down the organic material in a water sample over five days at 20 °C. A high BOD value indicates a high level of organic pollution. In eutrophic waters, BOD soars as bacteria work to decompose the algal remains, dropping dissolved oxygen to near zero in extreme cases. Low oxygen conditions are termed hypoxia; a complete lack of oxygen is anoxia.
生化需氧量（BOD）是水质研究中的一项关键指标。它量化了在 20°C 条件下、五天内，需氧微生物分解水样中有机物所需的溶解氧量。BOD 值高意味着有机物污染严重。在富营养化的水体中，随着细菌努力分解藻类残骸，BOD 飙升，极端情况下溶解氧可降至接近零。低氧状态被称为低氧；完全缺氧则称为无氧。
8. Consequences for Aquatic Life | 对水生生物的后果
Most aquatic animals, such as fish, amphibians, and many invertebrates, rely on dissolved oxygen for respiration. When oxygen levels fall below 2–3 mg/L, many species become stressed, lose their ability to reproduce, or suffocate. Mobile organisms may flee the area if they can, leading to a local extinction of fish populations. Bottom-dwelling organisms, which cannot escape quickly, are often the first to die. The result is a lifeless zone dominated by anaerobic bacteria that produce hydrogen sulfide, giving the water a rotten-egg smell.
大多数水生动物，如鱼类、两栖动物和许多无脊椎动物，依赖溶解氧进行呼吸。当氧含量降至 2–3 mg/L 以下时，许多物种会受到胁迫，丧失繁殖能力，或者窒息而死。能够移动的生物如果来得及就会逃离该区域，导致鱼类种群在局部地区灭绝。无法迅速逃离的底栖生物往往最先死亡。结果就形成一片由厌氧细菌主导的无生命区，这些细菌会产生硫化氢，使水体散发出臭鸡蛋般的气味。
9. Impact on Food Chains and Ecosystems | 对食物链与生态系统的影响
Eutrophication disrupts every trophic level. The loss of submerged plants removes a vital food source and shelter for herbivores and young fish. Zooplankton, which normally feed on algae, may initially increase but then crash as the algae die and oxygen disappears. Predators that rely on small fish will starve or migrate. Over time, the ecosystem may shift from a clear-water state with high biodiversity to a turbid, algae-dominated state with very few species, a change that is often difficult to reverse.
富营养化扰乱每一个营养级。沉水植物的消失令食草动物和幼鱼丧失了重要的食物来源和庇护所。通常以藻类为食的浮游动物最初可能数量大增，但随后会因藻类死亡和氧气消失而锐减。依赖小鱼为生的捕食者则会饿死或迁移。久而久之，生态系统可能从清澈、生物多样性高的状态转变为浑浊、以藻类主导且物种极为稀少的状态，这种变化往往很难逆转。
10. Case Study: Eutrophication in Lakes and Rivers | 案例分析：湖泊与河流中的富营养化
One well-documented example is Lake Erie in North America during the 1960s and 1970s. Excessive phosphate inputs from detergents and agricultural runoff caused severe algal blooms, massive fish kills, and the near total disappearance of commercially important species. Concerted efforts to reduce phosphate levels, including bans on phosphates in household products and improved sewage treatment, have since helped the lake recover partially. Similar eutrophication events are now common in many Chinese lakes, such as Lake Tai, where efforts to control nutrient input are ongoing.
一个记录详尽的例子是 20 世纪 60 至 70 年代的北美伊利湖。来自洗涤剂和农业径流的过量磷酸盐输入导致了严重的藻华、大规模的鱼类死亡以及重要经济鱼种的几近消失。此后，通过协调一致降低磷酸盐含量的努力，包括禁用家用产品中的磷酸盐和改进污水处理，该湖已部分恢复。如今类似的富营养化事件在中国的许多湖泊（如太湖）也很常见，控制营养物质输入的工作正在持续进行中。
11. Prevention and Control of Eutrophication | 富营养化的预防与控制
Prevention is more effective and cheaper than trying to reverse eutrophication once it has occurred. Key strategies include treating sewage to remove nitrates and phosphates before discharge, using phosphate-free detergents, applying fertilisers more precisely on farmland to minimise runoff, and creating buffer strips of vegetation along water bodies to absorb excess nutrients. In severely affected bodies of water, techniques such as aeration — pumping air into the water — can transiently increase dissolved oxygen, while dredging nutrient-rich sediments can reduce the internal nutrient load.
一旦富营养化发生，预防比试图逆转更有效、更经济。关键策略包括在排放前处理污水以去除硝酸盐和磷酸盐、使用无磷洗涤剂、在农田上更精确地施肥以减少径流，以及沿水体建立植被缓冲带以吸收多余的营养。在严重受影响的水体中，可以使用曝气（向水中泵入空气）的方法暂时增加溶解氧，而清挖富含营养的底泥则可降低内部营养负荷。
12. Exam Tips and Common Misconceptions | 考试提示与常见误区
the death of fish. A common mistake is to claim that algae directly consume all the oxygen — ath → more decomposition → oxygen depletion → death of aerobic organisms. Using terms like 'biochemical oxygen demand' and 'anaerobic conditions' can gain additional marks.
Edexcel IGCSE 的考试题目经常要求考生排列富营养化的各个阶段，或解释污水污染如何导致鱼类死亡。一个常见的错误是声称藻类直接消耗了所有的氧气——实际上，氧气的耗竭是由细菌分解死亡的藻类造成的。另一个误区是遗漏光照受阻这一环节。作答时务必将答案结构安排得合乎逻辑：营养物质输入 → 藻华 → 阻挡阳光 → 植物死亡 → 更多分解 → 氧气耗竭 → 需氧生物死亡。使用“生化需氧量”和“厌氧条件”等术语能帮你获得额外分数。
Published by TutorHao | IGCSE Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图