Dead zones are exactly that, deadly. They are oxygen-deprived or hypoxic bodies of water. Not much survives in these oceanic deserts as, not surprisingly, marine life needs a constant intake of oxygen to live. Large swaths of ocean habitats that would otherwise be teeming with life become biological wastelands. Hypoxic zones may occur naturally, however many more are caused by unsustainable agricultural practices around the world. This is big problem for wildlife and for us all.
Dead fish in the Gulf of Oman dead zone – one of over 400 dead zones on the planet
There are a number physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those dead zones created by humans. In this context nutrients are fertilizers, mainly containing nitrogen and phosphorus, used in agriculture. It is interesting to note that 77% of all agricultural land globally is used to farm or feed the seventy billion farm animals reared annually to feed nearly eight billion humans.
Excess nutrients carried by water as runoff from agricultural lands or piped as wastewater into the Earth’s marine environment can stimulate an overgrowth of algae. Phytoplankton are microscopic algae that contain chlorophyll and require sunlight to grow which is why they float near the surface of the ocean. The volume of phytoplankton in coastal locations increases exponentially where nutrient is being discharged directly or carried by rivers to their estuaries. The reason for this is that nutrients used in agriculture are food for phytoplankton. Zooplankton are small marine animals that spend most of their lives drifting in water eating phytoplankton. Fish consume the zooplankton.
Aerial view of where the Mississippi River meets the Gulf of Mexico – this creates the Gulf of Mexico dead zone which is one of over 400 dead zones on Earth
Now, the resulting zooplankton and fish faeces as well as the plankton corpses are consumed by bacteria as it all sinks to the ocean floor. The bacterial process of decomposition absorbs the oxygen from the water, dramatically lowering the amount that remains available to fish, sea mammals and other marine life.
Everything in nature is interdependent and follows complex patterns of interactions which is why we should be mindfully aware of the effect that our actions have on the Earth’s ecosystems.
Two parts per million or less of oxygen in seawater is considered a low-oxygen level. This low concentration of oxygen would cause mobile marine life to leave the contaminated area thereby seriously disturbing the predator prey relationships and further disrupting the ecosystem. As the oxygen content in the water deteriorates some animals may still survive however their biological and reproductive development would be altered. This would eventually make the population of these species in this area extinct.
Satellite view of the Baltic Sea dead zone – which is one of over 400 dead zones on globally
Since the 1970s, dead zones became more common and widespread, almost doubling each decade since the 1960s when there were 45 dead zones. A 2008 study found more than 400 hypoxic zones existing worldwide covering a total area of 245,000 km2.
The largest dead zone in the world lies in the Arabian Sea, covering almost the entire 165,000 km2 Gulf of Oman. The third largest dead zone sits in the Gulf of Mexico in the United States, averaging almost 22,700 km2 in size. The Baltic Sea hypoxic zone covered 70,000 km2 in 2018.
There are more than one hundred locations where nutrient is being released into the Mediterranean Sea and Malta appears to be one of them.
The global warming crisis has made the occurrence of marine dead zones more likely. A warming climate could lead to more rainfall in areas prone to nutrient runoff, making a bad situation worse. The Oceans are also becoming warmer and warm water naturally carries less oxygen than cooler water, making it easier for dead zones to form.
The Oceans are running out of oxygen and this creates dead zones
Oceanic dead zones can be brought back to life. This would be achieved by introducing preventive measures at the point of origin, on land, to reduce nutrient runoff through better farm management practices, such as using less fertilizer, using crop covers to help anchor the soil in place and the recapture and purification of the irrigation water.
Dead zones are yet another catastrophic collapse of more marine ecosystems, caused entirely by humans, that is sucking the life out of our rivers, lakes, seas and oceans. We need to encourage and incentivise organic agricultural practices and substantially reduce our dependence on meat as a food source. I would have thought this much was obvious.
This article was published in the Senior Times of the Times of Malta on the 18th September 2020
Source: National Geographic
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