Enormous amounts of nitrogen fertilizer are applied to agricultural land to increase crop productivity. However, the use of such fertilisers can be damaging to the environment, resulting in leaching of nutrients to freshwater and marine habitats. Increasing the amount of nutrients like nitrogen into coastal waters promotes the growth of plants and algae.
When the plants and algae decompose, this strips the water of its oxygen. Known as eutrophication, this process creates “dead zones” where organisms such as fish cannot survive.
For the first time, research has investigated the role of commercial fishing in the removal of nitrogen from the sea. Globally, commercial fisheries are big business and since 1985, around 85 million tonnes of marine fish have been harvested annually.
Although the impact of this on declining fish stocks is well-documented, the role of fishing in transferring nitrogen back to land is less well-studied. Fish accumulate nitrogen in the sea and when they are consumed as food by humans, they play their part in returning nitrogen, via humans, back to the land.
By quantifying the amount of nitrogen harvested in fish biomass and using data for global nitrogenous fertiliser use, obtained from the Food and Agricultural Organization (FAO), Canadian and American researchers have compared nitrogen inputs and outputs over 4 decades and in 58 coastal zones around the world.
In the early 1960s, fisheries were removing 63 per cent of nitrogen that entered the oceans as fertiliser, but by the year 2000 only 23 per cent was transferred back to land.
Therefore, while both inputs and outputs of nitrogen have increased, nitrogen removal via fishing has not kept pace with fertiliser inputs. This is largely due to the collapse in fisheries from over fishing.
The researchers concluded that fishery harvests still play a significant role in nitrogen cycling in many coastal regions. However, they state that removal of nitrogen via fishing is not a solution for nutrient pollution of coastal waters as humans continue to exploit marine resources unsustainably.
The EU Water Framework Directive (WFD2) refers to the 1996 Nitrates Directive as an action programme to regulate nitrogen pollution from agricultural and urban waste-water treatment sources.
Understanding the flow of contaminating nutrients like nitrogen both into and out of water bodies, could contribute to a total ecosystem approach to water quality management.