Nitrogen threat to Chesapeake Bay

The gas nitrogen comprises 78% of the Earth's atmosphere and the element has played a crucial role in building modern civilisation.

The practice of fixing nitrogen from the air - cracking the nitrogen molecules open to make the chemical ammonia - has vastly increased agricultural output across the world.

But when ammonia fertiliser makes its way into rivers and oceans it can be hugely damaging to marine life.

The Chesapeake Bay on the Atlantic coast of the US is one area in which this is happening, as Dr Beth McGee tells the BBC's Rajini Vaidyanathan.