When humans dump nutrients into the oceans, whether it’s fertilisers running off from farms or sewage pouring in from cities, the results are usually predictable. The influx of nitrogen and phosphorus quickly becomes too much of a good thing. It fuels the growth of algae that stop sunlight from reaching underwater plants, introduce toxic substances into the food chain, and deplete the water of oxygen. This process, known as eutrophication, transforms clear, teeming coastal waters into green, slimy, choking wastes.

But Elkhorn Slough is different. This huge Californian estuary, surrounded by farmland, receives 150 times more nitrogen fertiliser than it did a century ago. It should be an algal mess but it’s actually a thriving marine reserve. Flocks of