Bird populations collapse at Lake Kukkiajärvi as Finland’s waterways darken

A once-crystal-clear lake in Pirkanmaa has lost three-quarters of its visibility in 40 years, and its waterbird populations have plummeted, reports Yle.

At Lake Kukkiajärvi, water clarity has dropped from 8–10 metres in the 1960s to 2–2.5 metres today. Over the same period, nearly all waterbird species have declined sharply, with the common goldeneye and black-throated loon among the hardest hit.

In the early 2000s, 186 goldeneye pairs nested at Kukkiajärvi; by 2024–2025, only 30 remained. Black-throated loon numbers have fallen from 71 pairs to five, and the red-necked grebe has disappeared entirely. Only the common merganser and great crested grebe have held steady, while the whooper swan and Canada goose have thrived.

Raimo Virkkala, a retired senior researcher at the Finnish Environment Institute, links the collapse to large-scale clear-cutting in surrounding forests. The practice releases carbon into the soil, which leaches into the lake, darkening the water.

“Species like the goldeneye and black-throated loon rely on sight to forage. As the water darkens, they struggle to find food,” says Markus Piha, a senior researcher at the Natural Resources Institute Finland.

One in five Finnish lakes is now officially in poor condition. The same pattern—nutrient enrichment, increased algae, reduced light penetration, and shrinking underwater vegetation—is playing out nationwide. Invertebrate populations, a key food source for birds, have dwindled as a result.

Other factors, such as invasive predators like mink and raccoon dogs, climate change, and hunting, may also contribute, though their impact is less documented than that of water quality.