Year 11 Edexcel Biology: Case Study Practice – River Pollution and Bioindicators | Year 11 Edexcel 生物：案例分析实战演练

📚 Year 11 Edexcel Biology: Case Study Practice – River Pollution and Bioindicators | Year 11 Edexcel 生物：案例分析实战演练 – 河流污染与生物指示物种

This article guides you through a realistic case study on river pollution, showing how to analyse data, use bioindicators, and evaluate human impacts – exactly as required in Edexcel Year 11 Biology.

本文将通过一个真实的河流污染案例，带你一步步分析数据、利用生物指示物种并评估人类活动的影响，完全针对 Edexcel 11年级生物考试要求。

1. Understanding Bioindicators | 理解生物指示物种

Bioindicator species are organisms whose presence, absence, or abundance reflects the environmental condition of a habitat. In freshwater ecosystems, certain macroinvertebrates are sensitive to pollution, while others are highly tolerant.

生物指示物种是指其存在、缺失或数量能够反映栖息地环境状况的生物。在淡水生态系统中，某些大型无脊椎动物对污染十分敏感，而另一些则具有很强的耐受力。

For example, stonefly nymphs and mayfly nymphs require high dissolved oxygen and quickly disappear when water quality declines. In contrast, sludge worms (Tubifex) and bloodworms (chironomid larvae) can survive in very low oxygen conditions, often thriving below sewage outlets.

例如，石蝇稚虫和蜉蝣稚虫需要较高的溶解氧，一旦水质下降它们会迅速消失；而污泥蠕虫（颤蚓）和红虫（摇蚊幼虫）则能在极低氧条件下生存，甚至在污水排放口下方大量繁殖。

By sampling macroinvertebrates at different points along a river and identifying the species present, biologists can infer the level and type of pollution without expensive chemical testing.

通过沿河流不同地点采集大型无脊椎动物并鉴定物种，生物学家可以在无需昂贵化学检测的情况下推断污染程度和类型。

2. Case Background: River Alder Investigation | 案例背景：奥尔德河调查

The River Alder flows through mixed farmland, a small town, and finally past a wastewater treatment plant before joining a larger river. Local anglers have reported fewer fish and an unpleasant smell downstream. You are tasked with investigating the cause of the decline in water quality.

奥尔德河流经混合农田、一个小镇，最后经过一座污水处理厂后汇入大河。当地垂钓者反映下游鱼类减少且有难闻气味。你需要调查水质下降的原因。

Three sampling sites were chosen: Site A – upstream of all human activity (reference site); Site B – 2 km downstream of intensive agriculture; Site C – 500 m downstream of the town’s sewage treatment works. At each site, water samples were taken for chemical analysis, and kick-sampling was used to collect macroinvertebrates.

选择了三个采样点：地点A – 所有人类活动上游（参照点）；地点B – 集约农业下游2公里处；地点C – 城镇污水处理厂下游500米处。在每个地点采集水样进行化学分析，并使用踢网法收集大型无脊椎动物。

3. Interpreting Chemical Data | 解读化学数据

Before looking at the invertebrates, we analyse the water chemistry. The table below shows key measurements from the three sites.

在看无脊椎动物之前，我们先分析水化学数据。下表显示了三个地点的关键测量值。

Parameter	Site A (Upstream)	Site B (Farmland)	Site C (Sewage outfall)
Dissolved oxygen (mg/L)	9.2	7.5	3.1
Biochemical oxygen demand (BOD, mg/L)	1.8	4.6	12.4
Nitrate (NO₃⁻, mg/L)	0.9	5.2	6.8
Phosphate (PO₄³⁻, mg/L)	0.05	0.42	2.30
Water temperature (°C)	13.2	14.1	15.0

Dissolved oxygen (DO) is a direct measure of how much oxygen is available for aerobic organisms. A high DO, as at Site A, indicates a healthy ecosystem. Site C shows hypoxic conditions (DO below 5.0 mg/L), which can cause fish kills.

溶解氧 (DO) 是衡量可用于需氧生物氧气量的直接指标。像地点A那样的高DO表示生态系统健康。地点C出现缺氧状况（DO低于5.0 mg/L），可能造成鱼类死亡。

Biochemical oxygen demand (BOD) measures the oxygen consumed by microorganisms to decompose organic matter. A high BOD, especially at Site C (12.4 mg/L), strongly suggests organic pollution, most likely from sewage. The slight rise in BOD at Site B also points to organic input, perhaps from livestock waste or uneaten fertiliser.

生化需氧量 (BOD) 测量微生物分解有机物所消耗的氧量。高BOD值，尤其是地点C的12.4 mg/L，强烈表明存在有机物污染，极有可能来自污水。地点B的BOD略有上升也暗示有机物的输入，可能来自牲畜粪便或未利用的肥料。

Elevated nitrates and phosphates at Site B are typical of fertiliser runoff. These nutrients cause eutrophication, which initially boosts plant growth but later depletes oxygen when plants and algae die and are decomposed. At Site C, further increases in these nutrients, along with the huge BOD, confirm a mixed pollution source.

地点B升高的硝酸盐和磷酸盐是肥料径流的典型特征。这些营养物引起富营养化，初期促进植物生长，但当植物和藻类死亡并被分解时，会耗尽氧气。地点C这些营养物进一步增加，加上极高的BOD，证实了混合污染源。

4. Investigating Macroinvertebrate Data | 调查大型无脊椎动物数据

The kick-samples produced the following abundance data for key pollution-sensitive and pollution-tolerant groups (number of individuals per 3-minute sample).

踢网采样得到了以下关键污染敏感与耐污染类群的丰度数据（每3分钟样本的个体数）。

Indicator group	Tolerance level	Site A	Site B	Site C
Stonefly nymphs	Very sensitive	12	1	0
Mayfly nymphs	Sensitive	18	3	0
Caddisfly larvae (cased)	Fairly sensitive	8	4	0
Freshwater shrimp (Gammarus)	Moderate	21	10	2
Leeches	Tolerant	0	4	15
Bloodworms (chironomidae)	Very tolerant	0	2	27
Sludge worms (Tubifex)	Extremely tolerant	0	1	22

Site A is dominated by pollution-sensitive groups, indicating excellent water quality. The almost total absence of tolerant species confirms a pristine environment.

地点A以污染敏感类群为主，表明水质极佳。耐污物种几乎完全缺失，证实了原始环境。

At Site B, sensitive species have declined drastically, while moderately tolerant species like Gammarus persist. The appearance of a few leeches and bloodworms signals some organic enrichment, aligning with the slightly elevated BOD and nutrients.

在地点B，敏感物种数量急剧下降，而像钩虾这样的中等耐受物种仍然存在。少量水蛭和红虫的出现标志着一定程度的有机物富集，这与略微升高的BOD和营养物相符。

Site C is overwhelmed by tolerant and extremely tolerant species, particularly bloodworms and Tubifex. The complete absence of sensitive and fairly sensitive groups makes this a text‑book example of severe organic pollution. These macroinvertebrates can survive because haemoglobin in bloodworms and the high surface‑area‑to‑volume ratio in Tubifex allow respiration in hypoxic water.

地点C被耐污和极耐污物种占据，尤其是红虫和颤蚓。敏感和较敏感类群完全消失，这是严重有机物污染的典型例子。这些大型无脊椎动物能够存活，是因为红虫体内的血红蛋白以及颤蚓的高体表面积‑体积比使其能在缺氧水中呼吸。

5. Applying a Simple Biotic Index | 应用简单生物指数

To quantify the pollution level, we can use a simplified Trent Biotic Index approach. Assign each group a sensitivity score: stonefly = 10, mayfly = 9, caddisfly = 8, Gammarus = 6, leech = 3, bloodworm = 1, Tubifex = 1. Multiply each score by the number of individuals at a site, sum the products, and divide by the total number of individuals to get a weighted average index.

为量化污染水平，我们可以使用简化的特伦特生物指数法。给每个类群分配敏感度分数：石蝇=10，蜉蝣=9，石蛾=8，钩虾=6，水蛭=3，红虫=1，颤蚓=1。将每个分数乘以该地点的个体数，求和后除以总个体数，得到加权平均指数。

Index = Σ(sensitivity score × count) / Σ count

指数 = Σ(敏感分数 × 个体数) / Σ 个体数

Site A: (10×12 + 9×18 + 8×8 + 6×21 + 3×0 + 1×0 + 1×0) / (12+18+8+21) = (120+162+64+126)/59 = 472/59 ≈ 8.0. This high index reflects an unpolluted environment.

地点A：(10×12 + 9×18 + 8×8 + 6×21 + 3×0 + 1×0 + 1×0) / (12+18+8+21) = (120+162+64+126)/59 = 472/59 ≈ 8.0。该高指数反映出未受污染的环境。

Site B: (10×1 + 9×3 + 8×4 + 6×10 + 3×4 + 1×2 + 1×1) / (1+3+4+10+4+2+1) = (10+27+32+60+12+2+1)/25 = 144/25 = 5.76. The index indicates moderate pollution.

地点B：(10×1 + 9×3 + 8×4 + 6×10 + 3×4 + 1×2 + 1×1) / (1+3+4+10+4+2+1) = (10+27+32+60+12+2+1)/25 = 144/25 = 5.76。该指数指示了中度污染。

Site C: (10×0 + 9×0 + 8×0 + 6×2 + 3×15 + 1×27 + 1×22) / (0+0+0+2+15+27+22) = (0+0+0+12+45+27+22)/66 = 106/66 ≈ 1.61. A value below 2 is typical of heavily polluted water.

地点C：(10×0 + 9×0 + 8×0 + 6×2 + 3×15 + 1×27 + 1×22) / (0+0+0+2+15+27+22) = (0+0+0+12+45+27+22)/66 = 106/66 ≈ 1.61。低于2的数值是重度污染水体的典型特征。

6. Linking Chemical and Biological Evidence | 关联化学与生物学证据

The chemical and biological datasets reinforce each other. At Site B, nitrate and phosphate levels are far above background, while DO is slightly depressed. The macroinvertebrate community shifts from sensitive to moderately tolerant organisms. This is classic diffuse agricultural pollution – nutrient enrichment that initially increases productivity but harms sensitive species.

化学和生物学数据集相互印证。在地点B，硝酸盐和磷酸盐水平远超背景值，而DO略有下降。大型无脊椎动物群落从敏感型转向中等耐受型。这是典型的农业面源污染 – 营养物富集起初提高了生产力，却伤害了敏感物种。

At Site C, the extreme values of BOD, phosphate, and low DO coincide with a community dominated by organisms that thrive in low‑oxygen, high‑organic‑matter environments. The data point unequivocally to a point source of organic pollution: the sewage treatment works. The raised water temperature (15.0 °C) further reduces the solubility of oxygen, compounding the stress on aquatic life.

在地点C，BOD、磷酸盐的极端值和低DO与一个由低氧、高有机物环境生物主导的群落相吻合。数据明确指向一个有机物污染的点源：污水处理厂。升高的水温（15.0 °C）进一步降低了氧的溶解度，加剧了对水生生物的压力。

In an exam answer, always cross‑reference the numbers: e.g., ‘The BOD of 12.4 mg/L explains the absence of mayfly nymphs, as they cannot tolerate oxygen demand exceeding 5 mg/L.’

在考试答案中，一定要交叉引用数据：例如，“BOD为12.4 mg/L解释了为何没有蜉蝣稚虫，因为它们无法耐受超过5 mg/L的需氧量。”

7. Evaluating Pollution Sources | 评估污染源

At Site B, the source is non‑point (diffuse). Rainwater washes fertilisers and animal waste from fields into the river. Control measures would need to address farming practices over a wide area, not a single pipe.

在地点B，污染源是面源（扩散型）。雨水将肥料和动物粪便从田间冲入河流。控制措施需针对大面积农业操作，而非某一根排污管。

At Site C, the discharge from the treatment plant is a point source. This is easier to regulate, but the data suggest the plant’s secondary treatment may be inadequate – high BOD means organic matter is not being fully digested before release.

在地点C，污水处理厂排放是一个点源。这更容易监管，但数据表明该厂的二级处理可能不够充分 – 高BOD意味着有机物在排放前未被完全分解。

It is also worth considering the combined effect: the nutrients arriving from Site B could be fuelling algal blooms downstream, which the sewage discharge worsens. When algae die and sink, they add to the organic load, creating a positive feedback loop of deoxygenation.

还值得考虑联合效应：从地点B来的营养物可能在下游引发了藻华，而污水排放使其恶化。当藻类死亡下沉时，增加了有机负荷，形成了缺氧的正反馈循环。

8. Proposing Remedial Actions | 提出补救措施

For agricultural runoff, buffer strips of vegetation along riverbanks can trap sediments and absorb nutrients. Reducing fertiliser application rates and timing applications to avoid rainy periods can lower nitrate and phosphate inputs.

针对农业径流，沿河岸种植植被缓冲带可以截留沉积物并吸收营养物。降低肥料施用率并避开雨季施肥，可减少硝酸盐和磷酸盐的输入。

For the sewage works, upgrading to tertiary treatment (e.g., phosphate stripping) would reduce eutrophication risks. Aerating the effluent before release could bring BOD down, while maintaining a strict monitoring programme for effluent quality is essential.

针对污水处理厂，升级到三级处理（如去磷工艺）将降低富营养化风险。在排放前对出水进行曝气可降低BOD，同时必须坚持严格的出水质量监测计划。

From an ecological perspective, reintroducing pollution‑sensitive species could be a long‑term restoration goal once water quality consistently improves. However, biological recovery often lags behind chemical recovery.

从生态角度看，一旦水质持续改善，重新引入污染敏感物种可作为长期恢复目标。然而，生物恢复通常滞后于化学恢复。

9. Typical Exam‑Style Questions | 典型考题演练

Question 1: Explain why bloodworms can survive at Site C while stonefly nymphs cannot. (4 marks)

问题1：解释为什么红虫能在地点C存活而石蝇稚虫不能。(4分)

Model answer: Bloodworms have haemoglobin‑like molecules that bind oxygen efficiently even at low concentrations. They can also tolerate high levels of organic matter and low pH often accompanying sewage. Stonefly nymphs require consistently high dissolved oxygen (≥8 mg/L) and are highly sensitive to any drop; their gills are easily clogged by suspended solids, so they cannot survive the hypoxic, turbid conditions at Site C.

标准答案：红虫含有类血红蛋白分子，即使在低浓度下也能有效结合氧气。它们还能耐受高浓度有机物和污水伴随的低pH。石蝇稚虫需要持续高溶解氧（≥8 mg/L）并对任何下降高度敏感；它们的鳃容易被悬浮固体堵塞，因此无法在地点C的低氧、浑浊条件下存活。

Question 2: Using data from the tables, calculate the percentage decrease in dissolved oxygen from Site A to Site C. (2 marks)

问题2：利用表格数据，计算从地点A到地点C溶解氧的百分比减少量。(2分)

Answer: (9.2 – 3.1)/9.2 × 100 = (6.1/9.2)×100 ≈ 66.3% decrease.

答案：(9.2 – 3.1)/9.2 × 100 = (6.1/9.2)×100 ≈ 66.3% 的减少。

Question 3: Evaluate whether the pollution at Site C is entirely from the sewage works, using both chemical and biological evidence. (6 marks)

问题3：利用化学和生物学证据，评价地点C的污染是否完全来自污水处理厂。(6分)

Evaluation: The chemical evidence (BOD 12.4 mg/L, high PO₄³⁻, low DO) strongly indicates a point source consistent with untreated or partially treated sewage. However, nitrate levels are also elevated (6.8 mg/L), implying that agricultural runoff from upstream enriches the water. Biologically, the community is dominated by extremely tolerant taxa, but their abundance may be boosted by nutrients from both sources. Thus, while the sewage works is the primary cause of the hypoxia, agricultural diffuse pollution likely contributes, and the two sources interact. No single source explains the full picture.

评价：化学证据（BOD 12.4 mg/L，高PO₄³⁻，低DO）强烈表明点源与未处理或部分处理污水一致。但硝酸盐水平也升高（6.8 mg/L），意味着上游农业径流也在富集水体。生物学上，群落由极耐污类群主导，但其数量可能受两种来源的营养物共同促进。因此，虽然污水厂是缺氧的主因，农业面源污染很可能也扮演了角色，两者相互影响。没有单一来源能解释全部现象。

10. Key Skills and Tips for the Exam | 考试关键技能与技巧

Always quote specific data when supporting a conclusion. For instance, ‘Site C had zero stonefly and mayfly nymphs, compared with 12 and 18 at Site A, showing a severe decline in water quality.’ This demonstrates analytical thinking, not just description.

在支持结论时一定要引用具体数据。例如，“地点C的石蝇和蜉蝣稚虫为零，而地点A分别为12和18，表明水质严重下降。”这体现了分析性思维，而非简单描述。

Use scientific terminology accurately: biochemical oxygen demand, eutrophication, hypoxia, point/non‑point source, bioindicator, tolerance. Pair each term with a clear explanation. Do not just label a graph ‘it goes down’ – state ‘there is a negative correlation between distance downstream and dissolved oxygen, likely due to organic loading.’

准确使用科学术语：生化需氧量、富营养化、缺氧、点源/面源、生物指示物种、耐受性。每个术语都要配以清晰解释。标注图表时不要只说“它下降了”，而要说“下游距离与溶解氧之间存在负相关，很可能是由有机负荷引起的”。

Practice linking tables of data with descriptions of community structure. When you see a drop in sensitive groups and a rise in tolerant ones, always connect it to a measurable chemical change, such as decreased DO or increased BOD. This forms a complete ’cause‑and‑effect’ chain that gains high marks.

练习将数据表与群落结构描述联系起来。当你看到敏感类群减少而耐污类群增加时，务必将其与可测量的化学变化（如DO下降或BOD上升）建立联系。这就形成了完整的“因果”链，能获得高分。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Year 11 Edexcel Biology: Case Study Practice – River Pollution and Bioindicators | Year 11 Edexcel 生物：案例分析实战演练 – 河流污染与生物指示物种