Nitrogen is the basic building block of plant and animal proteins, and is essential to all forms of life. But studies in recent times have shown that human activity, including agriculture, energy production and transport, have overwhelmed the natural nitrogen cycle. This has resulted in excess nitrogen in the ecosystem, causing a range of ill effects from reduced soil fertility to toxic algal blooms. The main culprit is the excessive use of fertilisers in agriculture, which increased by about four times between 1960 and 1995. Most of this fertiliser runs off into the waterbodies, where it stimulates the growth of algae and other aquatic plants. This process, known as eutrophication, cuts out the oxygen supply to fish and other marine animals, asphyxiating them.