From toxic blooms to dead zones, how wildfires transform oceans and lakes

Wildfires, once extinguished, can leave a lasting imprint on aquatic ecosystems, transforming oceans and lakes in profound ways. While their immediate impact on air quality is evident, wildfires also unleash less apparent but substantial environmental repercussions.

One such consequence is the onset of algal blooms, which are rapid proliferations of algae and other microorganisms. These blooms can manifest in such dense concentrations that they alter the color of the affected water bodies. While most algal blooms are benign, some produce hazardous toxins.

The decomposition of these blooming algae, however, saps oxygen levels in the water, resulting in so-called dead zones. Algal blooms are frequently exacerbated by excess nutrients from human activities, such as fertilizer runoff and sewage disposal. Yet, smoke and ash from wildfires can also precipitate these blooms. When this material settles on the water, it introduces iron, a nutrient usually scarce in marine environments. Even minute quantities of iron can ignite algal growth.

This phenomenon was vividly depicted during Australia's 2019-20 "black summer" bushfires, when enormous clouds of iron-rich ash wafted across the Southern Ocean. The ensuing algal bloom, discernible from space, sprawled across a surface larger than Australia itself.

Smoke from wildfires can also harbor toxins that stifle algal growth, while thick smoke blankets can diminish the sunlight vital for photosynthesis. Consequently, the effects of wildfires on aquatic ecosystems are multifaceted and far-reaching, extending not only in time but also across vast distances.