Oxygen depletion in the ocean, called hypoxia ( low oxygen ) or anoxia ( no oxygen ) can occur due to natural or anthropogenic ( man made ) processes. In coastal and surface areas, the causes are usually various forms of pollution. Nutrients from farm runoff, sewage system overflow and soil erosion can flow into a river and then into a coastal or estuarine zone and create a condition called eutrophication. ( eu – well , trophic – eat or nourish ). Eutrophication has been documented to exist at various times in the Gulf of Mexico.
When excessive amounts of nutrients flow into an aquatic system, the first affect is a bloom of algae. This resulting bloom of algae then consumes the oxygen in the water do to a process known as respiration. Although there is some transfer of oxygen into a water surface from the atmosphere, the bulk of the oxygen comes from the action of photosynthesis. If the bloom of algae becomes so dense, that the sun is blocked from the deeper depths, then the ability of photosynthesis to keep up with the demands of respiration is diminished. The ecosystem is overwhelmed, and organisms that need oxygen, such as fish die off. While finfish have a good chance of moving on when faced with an algae bloom and resulting eutrophication and anoxia, the more sedentary inhabitants of the coastal reason, clams, mussels and oysters, have fewer options when presented with an anoxic environment.
Complicating the situation is southern latitudes such as the Gulf of Mexico is that in recent years the Gulf water temperature has peaked at record temperatures. While one of the most apparent effects of record high oceanic temperatures is record numbers and size of hurricanes, there could be another less obvious affect of climatic change. Warm water holds oxygen less effectively than cool water, so record Gulf temperatures could be a contributing factor to anoxia now or in the future.
There are natural systems that can reduce the amount of algea in the water column. For example, even in a purely natural state, a huge amount of nutrients flow into the Gulf of Mexico from the Mississippi River. As nutrients bloom, the algea may be rapidly consumed by filter feeding decapods, otherwise known as shrimp. Shrimp are not the only filter feeders that might feast on an algea due to nutrients from a river outflow. Clams and oysters are also prolific filterers.
So with some of the causes of the problem explained, we must get to the point of the article. There are not one ways but two ways that we can attempt to control anoxic coastal areas. First, in the case where there are abnormally large amounts of nutrients coming into the water body, we can make efforts to control them. Efforts might involve changing land use patterns around important watersheds from farming to park and recreation lands, or changing the practices of farmers so as to cause less nutrient runoff into the watershed.
On the other front, we can improve efforts to protect important filter feeders in the aquatic system, whether they be crustaceans, like shrimp, or bivalves like oysters and mussels. As a note, oysters and mussels are also builders and will help control erosion which has been one of the devastation hallmarks of our recent flurry of hurricanes in the Gulf region and elsewhere.
