Algal blooms and eutrophication

Eutrophication is the enrichment of a water body with nutrients, principally nitrogen and phosphorus, and it is the most widespread water quality problem in fresh water globally.

Why the dying matters more than the growing

A healthy algal bloom produces oxygen. The problem comes afterwards. When the bloom exhausts its nutrients and dies, the mass of dead organic material is decomposed by bacteria, and that decomposition consumes oxygen — enormous quantities of it, fast.

Dissolved oxygen crashes. Fish and invertebrates die. Their decomposition consumes more oxygen still, and the collapse reinforces itself. This is why fish kills so often follow a bloom rather than accompanying it.

The limiting nutrient

Algal growth is limited by whichever nutrient runs out first. In most fresh waters that is phosphorus; in most marine waters it is nitrogen. Reducing the limiting nutrient breaks the chain; reducing the other one alone achieves very little, which is why phosphorus stripping at wastewater works has been the priority in fresh water.

Cyanobacteria

Cyanobacteria have advantages that let them dominate in enriched, warm, stratified water: many can fix atmospheric nitrogen, so phosphorus alone can sustain them, and many can regulate their buoyancy to position themselves in the water column.

Why treatment must remove cells before oxidising

Cyanotoxins are largely held inside the cells while a bloom is healthy. Aggressive pre-oxidation ruptures the cells and releases the toxin into solution, where it is much harder to remove. The correct order is physical removal of intact cells by coagulation and filtration first, and oxidation only afterwards for any dissolved fraction.

The same logic applies at home: boiling water containing a cyanobacterial bloom releases intracellular toxin and concentrates the dissolved fraction. It makes the problem worse, not better.

Why recovery is slow

Phosphorus accumulated in lake sediment over decades continues to release back into the water column, particularly under the anoxic conditions the blooms themselves create. This internal loading means a lake can keep blooming for years after external inputs are cut. Lough Neagh is the clearest current UK example.