Stream Bioassessment Below an Outfall
A high school ecology class compares two riffles in the same river 3 km apart. Site A is upstream of a wastewater treatment plant outfall; site B is 500 m downstream. In April, each site is kick-net sampled for 3 minutes at one riffle. Site A yields 14 EPT taxa, 22 total taxa, and 310 individuals. Site B yields 5 EPT taxa, 9 total taxa, and 480 individuals, mostly midge larvae. Field measurements are site A: dissolved oxygen 9.2 mg/L, 11°C, cobble substrate; site B: dissolved oxygen 6.8 mg/L, 14°C, sand and silt substrate. The class concludes that the outfall caused a decline in water quality and that Site B is impaired. The teacher asks whether the evidence supports a cause-and-effect claim and what else the class would need to measure.
1. List at least three confounding variables and explain how each weakens the conclusion that the outfall caused the biological difference.
Read the worked answer
Substrate is a major confounder. EPT taxa such as stoneflies and mayflies require cobble and gravel interstitial spaces; silt and sand bury those spaces and reduce habitat. Site A is cobble and Site B is sand/silt, so the lower EPT richness at B could reflect habitat rather than wastewater. Temperature is another confounder: Site B is 3°C warmer, which lowers oxygen solubility and raises metabolic oxygen demand. The lower dissolved oxygen at B could therefore be partly a temperature effect, not only a pollution effect.
Sampling design adds more weakness. Each site was sampled once, at one riffle, for 3 minutes, in one month. There is no replication at the site level, so the class cannot separate site-specific natural variation from an outfall effect. There is also no before-discharge or upstream-reference series, and no measure of discharge, flow, depth, or water velocity. The downstream increase in total individuals with more midges could reflect organic enrichment, but it could also reflect slower flow and finer sediment. Because these variables differ at the same time as the outfall, the comparison is confounded and cannot establish causation.
2. Design a sampling plan that would allow a stronger causal inference about the outfall's effect.
Read the worked answer
The class should sample multiple riffles at multiple upstream reference sites and multiple downstream sites, with replication at each site and repeated sampling across seasons and before and after any change in discharge. Sites should be matched for substrate, depth, velocity, and canopy so that habitat is not confounded with position relative to the outfall. Sampling effort should be standardized by area or time, and the same net mesh and sorting protocol should be used everywhere. Organisms should be identified to a consistent taxonomic level and summarized with a validated index such as EPT richness or a multimetric index.
Physicochemical measurements should accompany the biological sampling: dissolved oxygen measured at dawn and in late afternoon to capture diel swings, temperature, pH, conductivity, turbidity, nutrients, biochemical oxygen demand, discharge, and current velocity. A before-after, control-impact design would be stronger than a single upstream-downstream snapshot. If the school cannot sample before discharge, it can at least establish a gradient with several distances downstream and several independent reference streams of similar size and substrate.
3. The downstream dissolved oxygen is 6.8 mg/L at 14°C. Explain whether this value by itself demonstrates organic pollution, and what measurement would help.
Read the worked answer
At 14°C, air-saturated water holds roughly 10 mg/L of dissolved oxygen, so 6.8 mg/L is about two-thirds of saturation. That is low enough to be a concern for sensitive invertebrates and fish, and it is below many water-quality criteria, but it does not by itself prove organic pollution. Oxygen is affected by temperature, atmospheric pressure, groundwater input, turbulence, photosynthesis, and respiration. Warmer water holds less oxygen, so part of the difference from Site A may be thermal.
A single daytime grab sample is especially weak because photosynthesis by algae can raise oxygen during the day and respiration can depress it before dawn. The class should measure dissolved oxygen over a full diel cycle, especially the pre-dawn minimum, and compare upstream and downstream at the same times. Biochemical oxygen demand, ammonia, nitrate, phosphate, and discharge would help identify whether the outfall is adding oxygen-consuming organic matter or nutrients. If downstream pre-dawn oxygen is consistently lower and BOD is higher while temperature and substrate are accounted for, the case for an outfall effect becomes much stronger.