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Fish Kills

The Problem

Sick lakes are characterized by two key physical changes that occur.

1. Stratification or layering of the water column; the upper levels are warmer, the lower levels are colder
2. Deoxygenation of the water column; the upper levels near the surface have more oxygen, the lower levels have virtually none. Over time the deoxygenated layers rise from the bottom to take up more and more of the water column and the oxygenated layers take up less and less.

This means that layers that fish can live and breathe in steadily decrease so their nutrient uptake and that of the whole oxygen breathing food chain is constrained. As muck accumulates on the bottom it covers clean sandy spots where fish would normally spawn. This again means less fish. So various feedback loops kick in that all lead to less fish and less competition for invasive weeds, algae and cyanobacteria.

How To Make It Worse

Virtually every common lake treatment or intervention exacerbates the problem by promoting nutrient recycling that helps invasive weeds, algae and toxic cyanobacteria compete more effectively for available nutrients so that they bloom and dominate.

» Algaecides

Algaecides quickly kill algae so that the dead algae fall to the bottom to decompose and perpetuate the nutrient recycling.

» Chemical Treatments

Such as alum that clear the water and precipitate phosphorus down into the sediment to perpetuate nutrient recycling.

» Conventional Aeration

Conventional Aeration raises dissolved oxygen levels in the upper layers of the water column. Because phosphorus precipitates out of the water at higher dissolved oxygen levels acts in much the same way as chemical precipitants by recycling phosphorus back into the sediment.

The difference is that chemical precipitation tends to retain phosphorus in the sediment a little longer, whereas when conventional aeration fails to oxygenate the sediment phosphorus rapidly becomes dissolved in the water again, fueling weed and algae growth.

How To Make It Better

All the organisms that make up a sustainable productive food chain from zooplankton to insects, mollusks and fish need oxygen to respire. So any solution to the problem has to start with destratifying and reoxygenating the water column and oxygenating the sediment.

Once the environment is capable of supporting a productive food chain, interventions must be made to ensure that the organisms at the base of the food chain can compete and out-compete weeds, algae, and cyanobacteria for the available nutrients.