Narration: synthetic voice. Script drafted with AI assistance and approved by co-author Stan Matwin.

Why did Darlings Lake, in Canada, turn green two summers running?

Blue-green algae are really bacteria. Fed too much phosphorus from farms, towns and wastewater, they multiply.

Harmful blooms can use up oxygen, make toxins that hurt wildlife, and harm people's health.

A Dalhousie University team and local groups asked: can satellites track these blooms?

A buoy measures only where it floats. Satellites photograph the whole lake almost daily, in three-metre squares.

Algae soak up red light but bounce back the shade just past red. That gives each square a greenness score.

But marsh plants are green all summer. So seven bloom-free days were averaged and taken away from each picture.

A simple computer method then sorted the squares into six groups, like laundry by shade. The strongest groups showed the bloom.

The raw map lights up plants and bloom alike. Take away the baseline, and the plants fade.

Checked by eye on three bloom days, the maps agreed well every time.

Two buoys measured a pigment the algae make. The score matched closely: zero point eight nine out of one. Another scored zero point one four.

Blooms started in mid-July both years. They spread across the lake in twenty twenty-one, but stayed near shore in twenty twenty-two.

After a dry spring, July twenty twenty-one brought nearly three times its usual rain. That seems to have helped the blooms spread.

This was a first test, on one lake. Haze, smoke and wind can still fool it.

By Catherine Evans and colleagues, in Frontiers in Remote Sensing, twenty twenty-five.