The Earth Story

Sea lilly

Largest Gulf of Mexico Dead Zone in more than a decade

Every year, farmers throughout the central United States deploy large amounts of nitrogen- and phosphorus-based fertilizers to help grow food. Every year, rains come. That water picks up a portion of those fertilizers and carries it downstream, where it eventually causes what is known as the Gulf of Mexico Dead Zone.

Dead zones occur in the ocean in areas where there is too much biologic productivity. When ocean-living algae dies, it sinks and begins to decay, using up oxygen in the process. If there isn’t enough oxygen to use up all the algae, the ocean can run out of oxygen, creating areas that are “hypoxic” or “anoxic”, free of the oxygen that other animal species need to live. For example, if fish swim into these zones, they will literally die of suffocation.

In June, areas in Mississippi and the Ohio River Valley received above-average rainfall (http://tmblr.co/Zyv2Js1r39Jl4). That rainfall washed the year’s fertilizers down the Mississippi River system and into the Gulf of Mexico, where it fed the growth of algae plumes that have since died, using up the oxygen. This cycle happens every year, but because of the timing and volume of the flow, this year’s dead zone is the second largest on record and the largest since 2002.

These dead zones push fish and other economically important species out of their normal habitats and can also put populations under stress by reducing their numbers, so they’re not only an environmental issue, they’re an economic issue. The US Environmental Protection Agency and the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force have set a goal of reducing the areal extent of the dead zones to below 5000 square kilometers, but the 16,700 square kilometer dead zone this year shows that goal is far from being met. The area defined as the dead zone is marked in black on this figure and the degree of oxygen depletion is colored in red – the darkest red areas are extremely low oxygen, far below where animal life can find enough oxygen to live.

The Gulf of Mexico Dead Zone is likely the world’s best-monitored dead zone and it is the 2nd largest human-caused dead zone, next to the one that appears in the Baltic Sea.

-JBB

Image credit: NOAA http://1.usa.gov/1N8hgdn

Read more: http://bit.ly/1K5FFNK

Global warming may cause major marine oxygen depletion.

Scientists investigating the changing conditions as the globe warmed 5 degrees Celsius during the several thousand year transition into the current interglacial epoch at the end of the last ice age (a rise similar to the outer limits predicted for the end of only this century) have come up with unsettling data suggesting that a period of extreme oxygen depletion in the sea accompanied the transition.

Reversing the geological maxim of the present being the key to the past when investigating geological processes, in climatology the past is seen as the key to the future. They seek analogous events in the past to what might be expected from man-made global warming, and using what they can glean make predictions for the future.

The event encompassed the period between 17,000 and 10,000 years ago, and was revealed by a detailed analysis of sediment cores taken across the Pacific Ocean. The cores showed the sea bottom footprint of oceanic dissolved oxygen loss, which was presumably accompanied by large dead zones in the sea and a redistribution of marine life due to changes in habitats. The transition was abrupt and basin wide (From the sub arctic to Chile), and penetrated the water column from the surface to a depth of three kilometres. It accompanied other signs of a changing climate such as increased temperature and atmospheric carbon or sea level rise.

One common factor in all warm periods, from interglacials to the Mesozoic, it a tendency for the system to steer towards stratified oceans, with a thin oxygenated rind stirred by winds and currents and a mostly anoxic zone filling the depths. The longer the warm period, the more established and stable this Earth system becomes.

If temperatures rise by the same amount in a short time, the effects are unpredictable, since the event would be unprecedented in geological history. Major changes are already occurring to marine oxygen levels and distribution, some due to agricultural runoff causing plankton blooms that suck the oxygen out of the surface layers. Oxygen levels are falling in every ocean basin, and the size of the risk that we seem to be taking just increased by another large notch.

Most of the ocean ecosystem depends on oxygenated water, as do the fisheries that we depend on for protein. They are already affected by fisheries migrating as their habitat zones change, overfishing, pollution and dead zones due to excess agricultural runoff. Recent research from Tasmania is exploring the southerly migration of many commercial species and its implications for the future.

Add in the increased acidification of the oceans due to the dissolved CO2 forming carbonic acid which is already affecting some shellfish populations (see our past coverage at http://on.fb.me/1C3moNZ), the fisheries and ecosystems of the world ocean are probably heading for a battering over the course of this century.

Loz

Image credit: NASA

Original paper, free access: http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0115246 http://www.theguardian.com/environment/climate-consensus-97-per-cent/2015/jan/29/new-research-reveals-extreme-oxygen-loss-in-oceans-during-past-climate-change http://www.theguardian.com/environment/2015/jan/29/australian-fish-moving-south-as-climate-changes-say-researchers