Fertilizer Runoff, Algal Bloom and Fish Deaths at Dawn

At 2 p.m.: in bright light the algae photosynthesise, releasing oxygen faster than the algae, bacteria and animals respire it, so DO rises above the original level - up to 15 mg/L by week 5. Nitrate is being used up as the algae grow.

At 5 a.m.: there has been no light for roughly ten hours. Photosynthesis has stopped, but respiration continues all night in the algae, the bacteria, the plants and the fish, so oxygen is removed from the water and not replaced. The lowest point comes just before sunrise.

Why the gap widens: the size of the swing depends on how much living and dead plant material there is. At week 0 the pond had little algae, so day and night values were both 8 mg/L. By week 5 the thick surface scum contains a huge mass of algae, so daytime oxygen production is large (15 mg/L) and night-time consumption is equally large (down to 1 mg/L). The difference between 8 and 8, then 15 and 1, is a direct measure of how much biological material has built up.

Extra factor: algae at the surface also block light from submerged plants, which die and are decomposed by bacteria - a process that consumes oxygen around the clock.

Sequence:

1. Rain washes nitrate fertilizer into the pond; nitrate rises from 2 to 22 mg/L.
2. Nitrate is a limiting nutrient, so algae grow rapidly - an algal bloom, visible as green scum by week 4.
3. The bloom produces oxygen by day, which is why the afternoon reading is high; but at night the algae respire and oxygen falls.
4. Algae die, sink, and are broken down by aerobic bacteria, which consume large amounts of dissolved oxygen.
5. By week 5 the pre-dawn DO is 1 mg/L - far below the roughly 4–5 mg/L that most fish need - so the fish suffocate.

Why dawn, not afternoon: at 5 a.m. the pond has been without photosynthesis for about ten hours while respiration has continued, so oxygen is at its lowest. At 2 p.m. the reading is the highest of the whole week (15 mg/L) - the pond looks healthy at the very moment it is most dangerous at night, which is why single daytime measurements mislead.

Verdict on the farmers' claim: they confuse the oxygen made by algae with oxygen available to fish. The same algae that release oxygen by day consume it at night, and their decomposition consumes even more.

Prediction with reasoning:

• Nitrate: stops rising and then falls as it is taken up by algae and plants, diluted by rain and outflow, and slowly buried in sediment. Expect a lag of one to three weeks, because nitrate already in the water and in the sediment is not removed instantly.
• Algae: without the added nutrient, the bloom declines over about two to four weeks; the surface scum breaks up and water clarity improves.
• Pre-dawn DO: rises as the algal mass falls - less night-time respiration and less decomposition. However, recovery is delayed because dead algae already on the bottom keep consuming oxygen while they decompose, so the pond may remain at risk for several weeks even after the bloom has visibly gone.
• Fish: the surviving population must be allowed to recover naturally or be restocked only after pre-dawn DO is consistently above about 5 mg/L.

Physical measure: plant a strip of vegetation (a buffer zone or reed bed) between the field and the pond. Runoff slows, soil particles settle, and plants take up nitrate before it reaches the water - reducing the input at the source rather than treating the symptom.