Oceanic Dead Zones: Is agriculture destroying marine habitats?

Continuing on the theme of anthropogenic nitrogen’s impact on the ocean, it is time I did a post dedicated to a major nitrogen-related environmental problem: Oceanic dead zones.   ‘Dead Zones’ are areas of the Ocean where the bottom water has become anoxic (i.e. has low or zero dissolved oxygen concentration), very few organisms are able to survive in such low oxygen conditions.  Dead zones occur along large sections of the coastline of major continents and are continuing to spread over the sea floor, destroying the habitat of many organisms.

Fig. 1. ‘Global distribution of 400-plus systems that have scientifically reported accounts of being eutrophication-associated dead zones.’ (Diaz and Rosenberg, 2008)

These dead zones are created when the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone), where it is broken down by the action of bacteria, a process known as bacterial respiration.  This is problematic because while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The bacteria use up the oxygen dissolved in the water which is essential to all of the other oxygen-respiring organisms on the bottom of the ocean, such as crabs, clams and shrimp, and also those swimming in the water, such as fish and zooplankton.  The overall impact is to make large parts of the ocean uninhabitable for the majority of organisms. For further explanation there is an excellent review of dead zones in the ‘Science Focus’ section of the NASA website, which provided much of the core information for this post.

An influential paper on dead zones is from Diaz and Rosenberg, published in Science in 2008, they state that oceanic dead zones have spread exponentially since the 1960s and that this formation of dead zones is exacerbated by anthropogenic influences on nitrogen entering the ocean due to riverine runoff of fertilizers and the burning of fossil fuels.  This extra nutrient input fuels coastal eutrophication and the accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. The resulting lack of oxygen causes fish to migrate away from affected waters and the death of large numbers of less mobile organisms.

The issue is also discussed in The Scientific American magazine, the article includes this quote from Robert Diaz (co-author of the paper discussed above) highlighting the main cause of the problem  

"The primary culprit in marine environments is nitrogen and, nowadays, the biggest contributor of nitrogen to marine systems is agriculture. It's the same scenario all over the world.." (R. Diaz, Marine Biologist, The College of William and Mary)

The article also investigates various possible measures that could be used to prevent coastal eutrophication, but preventing damage caused by agriculture is highly problematic.  Suggested solutions range from engineering crops to overexpress a gene causing roots to absorb more nitrogen, to large-scale geoengineering projects, such as attempting to artificially aerate the ocean. The latter solution sounds unlikely in my opinion, as trying to directly influence an open system on such a massive scale is sure to be very challenging, even though experiments on a much smaller scale have shown success.  It seems that dead zones are another anthropogenic nitrogen related environmental problem that may require more drastic change to slove!