All over the world lakes are in trouble. An excess of nutrients – from fertilisers, detergents and sewage – is upsetting the balance of life, leading to algal blooms and bottom-water dead-zones. Many places are now trying to clean up their act, but how long does it take for a lake to recover?

Sediment cores drilled from a Swiss lake reveal how long it took for the lake to bounce back after the Romans departed, and indicate we might have to wait centuries for today’s polluted lakes to become properly fresh again.

The Roman city of Aventicum, on the shores of Lake Murten in Switzerland, grew rapidly from around AD30, supporting around 20,000 people during its heyday in the first and second centuries. But now sediments from the lake bed reveal that the land clearance and intensive agriculture associated with the growth of Aventicum resulted in huge nutrient runoff into the lake, wiping out ecosystems and plunging the lake into crisis.

During the third century the Romans abandoned Aventicum and the sediments show that life returned to the lake after about 50 years. But the findings, which are published in Earth and Planetary Science Letters, reveal that it took around 300 years for the lake to fully recover.