independent

Sunday 19 January 2020

Increase in number of Dead Zones a cause for concern

Big fish like Tuna need a lot of oxygen to fuel their large bodies.
Big fish like Tuna need a lot of oxygen to fuel their large bodies.

Jim Hurley - Nature Trail

Just before Christmas, media reports from the COP25 climate conference in Madrid informed us that marine life is being left literally gasping for breath as oxygen levels in the world's oceans and seas are being depleted and dead zones are becoming more common. So, what is a dead zone?

Here's the science bit. Oxygen gas in the air dissolves in water much in the same way that sugar dissolves in tea. There is about 8-11 milligramme of dissolved oxygen gas in each litre of seawater (mg/l). All marine life depends on the life-giving gas so if the level of oxygen drops life forms are left literally gasping for breath.

Fish become stressed but can tolerate oxygen levels as low as 3-5mg/l. If the concentration of oxygen drops below 2mg/l, the area is said to be hypoxic, popularly known as a 'dead zone'. Fish can move so they swim away from the area. Life forms like shellfish attached to the seafloor don't have the option to move away so they die throwing the delicate balance of life into disarray.

At least 700 dead zones are now known worldwide with examples of them arguably present in some estuaries around the coasts of Ireland. The number of dead zones has quadrupled in the past half-century and continues to grow at an accelerating rate. The two main factors causing dead zones to form are climate change and nutrient run-off both from fertilisers used by farmers on land and from all of us contributing to wastewater treatment plants and septic tanks.

The problem of oxygen depletion is compounded by a litany of other woes suffered by the world's oceans and seas: they are being over-fished, are becoming warmer and more acid, are losing their ability to both hold oxygen and to capture carbon dioxide gas, are being polluted by plastic rubbish and are being poisoned by an array of pollutants as never before experienced.

The bottom line is that we are on the slippery slope of knowingly destroying the planet's life-support systems while lacking the sense of urgency needed to take action to try to reverse the trend.

Dead zones range from ones with relatively mild issues to ones with acute problems. The EU Water Framework Directive sets out targets to be achieved by 2027. The targets are doable if the political will is there at both central government and local authority level to address the issues in a meaningful way.

The Argus

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