Marine dead zones are areas of seawater that have become depleted of oxygen. Dead Zones are a global problem mainly caused by algal blooms fed by increased nutrients entering the oceans from agricultural fertilizers and sewage.
The first dead zones appeared in the 1960s with 49 dead zones around the world, then 87 in the 1970s, and 162 in the 1980s. The dead zone global count is 405 today (2008). Twenty of these Dead Zones are in Ireland and include many of the best known estuaries, such as those of the Barrow, Blackwater, Bandon, Slayney, Suir, and Feale Rivers. Bays include Donegal Bay and Harbours include Castlteownbere, Killybegs, and Dungarven.
Even where oxygen levels do not fall far enough to cause a dead zone , reduced levels can damage the growth and reproduction of species and alter the marine ecosystem.
Harmful algal blooms (Habs) can occur which contain toxins that can kill fish and accumulate in shellfish. Other species that consume the shellfish or fish can become sick or even die - including humans. Toxic shellfish are an increasing problem in Irish waters.
What can we do?
It is vital that there are adequate buffer zones around all water bodies on agricultural land, particuarly in headwaters. At the moment the buffer zone for chemical fertilisers is a mere 1.5m and as low as 3 m for organic fertilisers. Almost 100% of nitrate can be removed by buffers 20-30 m wide, while in examples of forested buffers, 10 m achieved over 70% retention of nitrogen.