The Pond and the Fertiliser Runoff

Nitrate is a limiting nutrient, so added nitrate allows rapid algal growth, producing an algal bloom. The dense surface layer shades submerged plants, which die back, reducing oxygen production by photosynthesis and removing habitat. When algae die, bacteria decompose them and respire, consuming dissolved oxygen. At night, algae and plants respire without photosynthesising, so oxygen falls further; at dawn, oxygen is at its lowest after a whole night of respiration. Fish need a minimum dissolved oxygen level, and the dawn minimum falls below it, causing deaths. In the afternoon photosynthesis has raised oxygen again, so fish survive. The chain is: nutrient enrichment, then bloom, then shading and death of submerged plants, then decomposition, then oxygen depletion, then fish kills. This is eutrophication.

Herbicide would kill algae quickly, but the dead algae would then be decomposed by bacteria, which would consume even more oxygen and could worsen oxygen depletion and fish deaths. It also does not address the nitrate source, so the bloom would return. Dredging removes nutrient-rich sediment, which helps in the short term, but it is expensive, disturbs the pond bed and its organisms, and again does not stop further runoff. The root cause is fertiliser entering the pond, so the effective long-term measure is to reduce runoff, for example by leaving buffer strips of vegetation between field and pond, reducing fertiliser application, and timing application to avoid rain. A sensible plan combines source control first, with sediment removal only if nutrient stores remain high, and avoids herbicide because of the oxygen risk.