Enormous clusters of dead fish are surfacing all across the planet

[The title was written by my editor. Not accurate.]

Aquatic oxygen-depletion zones, or dead zones, are linked in a recent review to climate change and exist as functions of variables such as agricultural water pollution and rising temperatures.

by John Tyburski

Copyright © Daily Digest News, KPR Media, LLC. All rights reserved.

First published at Daily Digest News on November 11, 2014

Eutrophication is a process in which a body of water accumulates an abundance of nutrients (such as fertilizer brought in from runoff) that result in excessive plant and algae growth. During eutrophication, water can become hypoxic, or oxygen-depleted. Hypoxic conditions are worsened by higher temperatures, as warmer water in general holds less dissolved oxygen than does cooler water. Without adequate oxygen, aquatic animals perish. Sites where this die-off occurs are called dead zones.

Researchers Andrew Altieri and Keryn Gedan recently reviewed eutrophication research and found that 94 percent of existing dead zones are located in regions that will, according to their climate change predictions, undergo temperature increases of at least two degrees Celsius by the end of the century. Among the variables that will influence dead zones in the future and change as the climate changes are temperature, ocean acidification, rising sea level, precipitation, and wind and storm patterns. Each of these variables may contribute to worsening dead zones through multiple causal pathways ending in hypoxia and the ecological responses to hypoxia.

“We’ve underestimated the effect of climate change on dead zones,” said study lead author Andrew Altieri, a researcher at the Smithsonian’s tropical center in Panama.

Altieri and Gedan considered 476 dead zones across the globe, including 264 in the U.S. Standard computer climate models show future temperature increases for these dead zones. They note that based on current trends in climate change and how the aforementioned variables currently operate in dead zones, they predict that future climate changes will result in these variables interacting with one another in ways that multiply the effects any one variable may have by itself.

Lake and ocean waters absorb oxygen from the atmosphere on their surfaces, and these surface waters slowly mix with deep, colder water and deposit oxygen below the surface. However, as surface temperatures increase, less oxygen is absorbed. What is more, greater temperature differentials between surface and deep waters results in less mixing.

“It’s like Italian dressing that you haven’t shaken, where you have the oil and water separate,” Altieri said.

The researchers call for an integrated, multidisciplinary approach to managing dead zones in our changing climate in their review published on Monday in the journal Global Change Biology.