Is The World Slowly Dying?

CNN has an article which reports on a peer reviewed manuscript published in Science which discusses the fact that the world's oceans contain over 400 dead zones. That's not a good thing.

These dead zones are the result of eutrophication, when massive amounts of nutrients are released into an ecosystem. It is particularly bad when that ecosystem happens to be a coastal watershed, stream, river, pond or lake. These nutrients are typically runoff from agricultural lands, into adjacent waterways, which are then flushed into the oceans. Eutrophication is not a "Good Thing" as it typically leads to excessive plant, algae, and/or microbial growth, which in turn effectively chokes off the existing biodiversity in the area. That's bad.

As Diaz and Rosenberg write:

The worldwide distribution of coastal oxygen depletion is associated with major population centers and watersheds that deliver large quantities of nutrients (Fig. 1 and table S1). Most of these systems were not hypoxic when first studied, but it appears that from the middle of the past century, the DO concentrations of many coastal ecosystems have been adversely affected by eutrophication. The observed declines in DO have lagged about 10 years behind the increased use of industrially produced nitrogen fertilizer that began in the late 1940s, with explosive growth in the 1960s to 1970s (4). For marine systems with data from the first half of the 20th century, declines in oxygen concentrations were first observed in the 1950s in the northern Adriatic Sea (5), between the 1940s and 1960s in the northwestern continental shelf of the Black Sea (6), and in the 1980s in the Kattegat (7). Localized declines of DO levels were noted in the Baltic Sea as early as the 1930s, but it wasn’t until the 1960s that hypoxia became widespread (7). Localized hypoxia had also been observed since the 1930s in the Chesapeake Bay (8) and since the 1970s in the northern Gulf of Mexico (9) and many Scandinavian coastal systems (7). Paleoindicators (foraminifera ratios and organic and inorganic compounds) show that hypoxia had not been a naturally recurring event in these ecosystems (10, 8). The number of dead zones has approximately doubled each decade since the 1960s (fig. S1 and table S1).

DO stands for dissolved oxygen which is the measure of oxygen which is a measure of the level of oxygen saturation in a system. Since a sizeable portion of life relies on oxygen, a poor DO level is an ominous sign. There are several "levels of hypoxia", with about half of the dead zones being annual phenomenon, typically occurring once a year. An additional 25% have multiple hypoxic events each year. Some systems do not experience hypoxic episodes every year (~17% of the dead zones) but still are cause for concern. In all cases, these hypoxic events result in mortality for a portion of the existing organisms within the ecosystem.

Light at the end of the tunnel?

There is hope however. With effective nutrient management, several systems have reduced the dead zones in the surrounding ecosystems. As the authors relate:

The management of nutrients and carbon inputs has virtually eliminated dead zones from several systems, including the Hudson and East Rivers in the United States and the Mersey and Thames Estuaries in England (31, 32). However, in other systems, such as the Chesapeake Bay, the management of nutrient input has not improved DO. Nevertheless, the management of sewage and pulp mill effluents has led to many small-scale reversals in hypoxia (table S1).

There are several ways to help reduce eutrophication, such as building riparian zone buffers around farmland, organic farming, better fertilization practices, as well as sound and effective environmental policy. These steps, and many more, are going to be necessary in order to help the worlds oceans survive and continue to provide us with food and life.

R. J. Diaz, R. Rosenberg (2008). Spreading Dead Zones and Consequences for Marine Ecosystems Science, 321 (5891), 926-929 DOI: 10.1126/science.1156401