Summer Stratification and a Fish Kill

The sequence is eutrophication:

1. Nitrate and phosphate enter from both sources and are no longer limiting, so algae and cyanobacteria grow rapidly; chlorophyll rises from 4 to 95 micrograms per litre.
2. A dense surface scum forms, blocking light to submerged plants, which die. The algae themselves also die as nutrients are used up and as the bloom ages.
3. Aerobic bacteria decompose the dead algae and plants, consuming dissolved oxygen. Respiration by the algae at night also removes oxygen without replacement by photosynthesis.
4. Oxygen at 1 m falls to 1.1 mg per litre, far below the requirement of most fish, so fish die or leave. Decomposition in the bottom layer is not replenished because the lake is stratified and the layers do not mix.
5. Additional stresses include toxins from cyanobacteria, raised pH during daytime photosynthesis, and the loss of plant cover used for spawning.

The key judgement is to present this as a causal chain rather than a list, and to place the oxygen depletion downstream of the nutrient increase and the bloom, not as an independent event.

The surface layer is in contact with the atmosphere, so oxygen can diffuse in and wind-driven mixing can replenish it. It is also the photic zone, where photosynthesis by the algae produces oxygen during daylight. Consumption is high, which is why the value still falls slightly from 9.5 to 8.4, but supply roughly balances demand.

The 1 m layer sits below the dense surface scum, so light penetration is reduced and photosynthesis there is low or absent; respiration and bacterial decomposition continue. Because the lake is stratified, the warm surface water is less dense than the cooler water below and the two layers do not mix, so oxygen from the surface cannot be replaced by downward mixing. The 1 m measurement may also be within the developing anoxic zone depending on the depth of the thermocline, so 1.1 mg per litre reflects oxygen consumption without replenishment. The contrast is therefore evidence that physical stratification, not just biological oxygen demand, controls the vertical pattern.

Concentration alone cannot apportion blame; load matters, and each source has different chemistry and flow behaviour.

• Flow and load: measure discharge at the effluent pipe and at the main inflow, then multiply by concentration to compare kilograms of nitrogen and phosphorus per day. A high concentration in a small flow may be a smaller load than a lower concentration in a large river.
• Nitrogen stable isotope ratios: effluent nitrogen often has a distinctive isotopic signature compared with synthetic fertiliser, so the ratio in algae or in the inflow can indicate the dominant source.
• Sewage tracers: caffeine, artificial sweeteners or pharmaceuticals survive treatment and are absent from fertiliser, so their presence points to effluent.
• Timing: sample during dry weather, when flow is mostly effluent, and after heavy rain, when farmland runoff peaks. A concentration spike after rain suggests agricultural input.
• Nutrient ratios: effluent tends to be rich in phosphorus relative to nitrogen, whereas fertiliser runoff often has a higher nitrogen-to-phosphorus ratio; the ratio in the lake water can hint at the mix.

Because both sources contain both nutrients, no single measurement is conclusive; the answer should combine load estimates with at least one tracer.