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Wildfires are an increasing problem in Europe and elsewhere. While their immediate effect on air quality is obvious, they can also trigger less visible but significant environmental effects, including algal blooms.

An algal bloom is the rapid growth of algae and other plant-like microorganisms. These blooms can become so dense that they change the colour of the lake and seas. Although most are harmless, some produce dangerous toxins. When their blooms die, their decomposition depletes oxygen in the water, creating so-called dead zones.

Algal blooms are often fuelled by excess nutrients from human activities, such as fertiliser runoff and sewage dumping. But smoke and ash from wildfires settling on the water can also trigger them. This material contains iron, which is scarce at sea, and even small amounts can stimulate algal growth.

This phenomenon was dramatically illustrated during Australia’s 2019-20 “black summer” bushfires, when vast plumes of iron-rich ash spread across the Southern Ocean. The resulting algal bloom, visible from space, covered an area larger than Australia.

Smoke can also contain toxins that inhibit algal growth, while dense smoke clouds may reduce the sunlight needed for photosynthesis. The overall effects are therefore complex. Even so, it is clear that wildfires can have environmental consequences that extend far beyond the destruction of local vegetation in terms of time and distance.