Large balls of diatoms

The large balls of diatoms that once grew in Iceland’s Lake Mývatn have mostly vanished. They have been starved of light by algae blooms fed by pollution. Photo: Arctic Images, Alamy

Cheryl Katz writes in National Geographic that Iceland’s last living lake balls are disappearing. The fluffy green supersize diatoms as large as a head of cabbage are one of the planet’s most unusual plants.

This distinctive form of the freshwater alga Aegagropila linnaei is exceedingly rare. Northern Iceland’s Lake Mývatn, one of Europe’s most important waterbird habitats, used to host the largest lake ball colonies on Earth. This lake had all the right environmental factors: swirling currents to nudge the clusters so they grow outward in perfect, round shapes; consistent winds to stir the currents; a stable bottom for support; and most of all, clean, translucent water.

Vast colonies once teemed in Lake Mývatn, renowned for its dramatic aquascape and abundant wildlife. But like so many lakes worldwide these days, thick blooms of potentially toxic cyanobacteria are increasingly turning Mývatn’s water opaque.

Beneath the blanket of slime, the lake balls are starved for sunlight. The chief culprit behind the algae blooms is pollution – fertilizers and sewage that feed them with nitrogen and phosphorus. Warming waters also may be amping up algae. Scientists predict climate change will make future outbreaks worse.

Healthy, full-grown lake balls haven’t been seen at Lake Mývatn since 2013. Their disappearance signals an ecosystem collapse threatening one of the world’s most important breeding grounds for ducks.

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