Agricultural Run-off

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Eutrophication Observed at Lough Hyne (Image Credit: Rob McAllen)

Agricultural run-off can occur when rainfall causes any chemicals or fertilisers applied to crops or livestock to be washed off into river systems and be carried downstream to the sea. In recent years the use of toxic biocides, like DDT (the world’s first and best known pesticide), have been reduced, but fertiliser is still causing major issues. Nitrates and phosphates in the fertilisers are major causes of nutrient enrichment, also known as eutrophication. This increase in nutrients can directly cause algal blooms (sometimes known as red tides), which can pose dangers to marine life and humans alike. In recent years, these blooms have become increasingly more frequent (Sellner et al., 2003).

Aerial view of Red Tides at Lough Hyne (Image Credit: Rob McAllen)

What makes the blooms so detrimental to the marine environment, is the resulting oxygen depletion. The nutrients in the water cause the rapid growth of algae, which continue to proliferate until all nutrient supply has been exhausted, the algae then dies off and sinks to the seabed where bacteria help break it down. It is this breakdown that reduces the level of oxygen in the water, creating anoxic “dead zones” where it is impossible for most marine organisms to continue their survival. Many of these blooms/red tides also contain harmful toxins that can become airborne, causing health complications in humans and animals alike (Watkins et al., 2008). These toxins can also bio-accumulate in filter feeding organisms, like mussels, which is what causes neurotoxic shellfish poisoning, a disease caused by the consumption of contaminated shellfish. In 1997, up to two dozen people contracted diarrhetic shellfish poisoning from consuming only 10-12 individual mussels (Twiner et al., 2008). Depending on toxicity levels, species and site, the incidence of these infections can range from 0.2% to 14% (Hinder et al., 2011). This poses particular danger to the shellfish industries based in  Bantry Bay area in West Cork whose industry shifted to processed mussel populations in the early 90’s in part due to the impact of these naturally occurring tides (Bantry Bay charter).


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