Dead Zones

A new image by NASA reveals the extent of the world’s marine dead zones, which a study in 2008 found were doubling every decade. At that time 415 dead zones had been identified worldwide. Dead zones are regions of the ocean where dissolved oxygen has fallen to such low levels that most marine species can no longer survive. Such conditions are often seasonal.

Created by satellite, the red circles on this map show the location and size of many of our planet’s dead zones. Black dots show where dead zones have been observed, but their size is unknown. Darker blues in this image show higher concentrations of particulate organic matter, an indication of the overly fertile waters that can culminate in dead zones. Image courtesy of NASA.

The so-called dead zones are caused by agricultural run-off, especially nitrogen-rich fertilizers, as well as the burning of fossil fuels. Pollutants from these sources cause marine eutrophication, whereby the ecosystem receives too many nutrients, triggering massive algae blooms, which eventually die and are broken down bacteria. In breaking down the algae blooms, the bacteria consume excessive amounts of oxygen, essentially starving the marine system. Therefore the largest and most extreme dead zones occur near high populations and run-off areas for agriculture fertilizers.

See here for the December 2010 interview with Professor Ove Hoegh-Guldberg, one of the scientists from the Australian Research Centre of Excellence for Coral Reef Studies who recently published a review of dead zone research in the journal Science.

Algae accumulations are common in many waterways around the planet, and a bloom doesn’t necessary spell doom. The Alaska blob is now drifting out to sea with no visible harm done, and smaller blooms occasionally float down even small rivers and streams. But depending on the type and the amount of algae involved, a run-of-the-mill plankton party can quickly escalate into what’s known as a “harmful algal bloom,” or HAB.
Only a fraction of the world’s algae species are toxic, but things get ugly when they get together. Probably the most notorious toxic algae are those responsible for red tide — rosy plumes that billow below the surface (see photo), soon followed by the stench of poisoned, rotting fish. The toxin usually irritates the eyes and skin of people who swim during red tides, and can even become airborne, creating a “stinging gas” that hovers over a beach. Other toxic algae may pass their poisons slowly up the food chain by bioaccumulation, causing ailments like ciguatera fish poisoning, which can involve nausea, vomiting and neurologic symptoms.
Nontoxic blooms are no saints either, since the large, slimy mats they generate often interfere with a wide range of coastal business, from the feeding habits of right whales and fishermen to the antics of would-be beach-goers. They can also smother coral reefs and seagrass beds, endangering the diverse animals living there, including some commercially important fish.
Not even the worst algae blooms, however, create hypoxic zones on their own. A true dead zone is a team effort — individual algae within a bloom die and rain into the depths below, where they’re digested by deep-water bacteria, a process that consumes oxygen. Yet even with this sudden oxygen drain, wind-driven ocean churning normally stirs down enough oxygenated surface water to cure any temporary hypoxia. Certain natural conditions, namely warm weather and a layering of fresh and salty surface water, are often needed for a dead zone to form.
More on models for DZs here…