In several locations around the globes, scientists have found dead zones, regions of water that are partially or totally lacking in dissolved oxygen. These zones are typically found near the ocean, and result from a variety of natural and human-induced causes.
A recent study charted over 400 of these dead zones, ranging in size from 1 square kilometer to over 70,000 square kilometers. All of these locations thus far discovered are in coastal regions in close proximity to human habitation. This alone suggests a possible cause of their low-oxygen state.
In some cases, this human-centric diagnosis would be correct. Anoxic (low or no oxygen) zones at the ocean floor are very often a result of pollution.
The cause of anoxic bottom waters is fairly simple: the organic matter produced by phytoplankton at the surface of the ocean sinks to the bottom, where it is subject to breakdown by the action of bacteria, a process known as bacterial respiration. The problem is, while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, the sea floor bacteria use oxygen and give off carbon dioxide during respiration. The oxygen used by bacteria is the oxygen dissolved in the water, which is the same oxygen that all of the other oxygen-respiring animals on the bottom, like crabs, clams, and a host of mud-loving creatures, and those that are swimming in the water, like fish and octopi, require for life to continue.
The main issue is not “how,” as that is fairly well-known. The issue is why these dead zones are on the increase, and what role human activity has to play in this problem.
These dead zones appear to be areas where surface phytoplankton activity has been enhanced, leading to the production of more organic matter. This matter sinks to the bottom and is broken down by sea floor bacteria, which use even more oxygen due to the increased infall.
Most of these dead zones are near river outlets, where silt-laden waters join the sea. More often than not, these waters are also full of nitrogen compounds, fertilizer and animal waste from agricultural and industrial operations along their length. This material, then, is the most likely culprit in the enhancement of phytoplankton activity. Phytoplankton are simply plants, after all. Even the silt is laden with nutrients from increased soil runoff from cultivated fields.
Another factor in sea floor anoxia is restricted water flow. This prevents oxygenated water from flowing freely in to replace the depleted deep waters. When this is combined with nitrate-rich runoff from polluted rivers, the results can be severe. Even small systems, with relatively low agricultural runoff, can see the effects magnified by geological features. Saanich Inlet, on Vancouver Island, is one such location, where a bench across the mouth of the inlet comes within 70 meters of the surface, while the inlet itself averages more than twice that depth. The surface waters are fine, and teem with life, while 60 meters down there is nothing. As it sits close to the University of Victoria, the anoxic zone in Saanich Inlet can be easily studied.
While the problem is understood, the next steps are unclear. With modern agriculture, fertilization is required, and it is very difficult to keep fertilizer and other agricultural byproducts from finding their way into rivers and streams, and thus to the ocean. Industrial runoff, however, can be more tightly controlled, and that is a first step.