Ocean dead zones are large bodies of water-typically in the ocean, but can also occasionally occur in lakes and even rivers-that do not have enough oxygen to support marine life. Algal blooms and hypoxia are the leading causes of dead zones and impact the oxygen levels in the ocean(2012, Scientific American).
Algal blooms are created by phosphorus, nitrogen, and other nutrients. They increase the productivity and fertility of marine ecosystems(1996, National Geographic). Organisms such as phytoplankton, algae, and seaweed grows quickly and excessively on the water’s surface, which is an algal bloom. Algal blooms prevent light and oxygen from being absorbed by organisms beneath the surface because there are too many organisms covering the surface(1996, National Geographic), thus creating dead zones. Sunlight is necessary for organisms that need light to function and live off of and oxygen is necessary for almost all aquatic life. Every species living beneath the surface is greatly reduced because there is no longer sunlight nor oxygen to live off of. Algal blooms also create larger-scaled problems, such as human illness. Shellfish like oysters, are filter feeders. They filter water and absorb microbes that are associated with algal blooms. The microbes are high in nutrients that they are toxic to people, which can cause humans to bome sick or even die from shellfish poisoning(1996, National Geographic). Algal blooms also lead to the death of many marine mammals and shore birds that rely on fish and other aquatic organisms to feed off of. Herons and sea lions depend on fish to survive and with fewer fish beneath the algal blooms, the sea lions and herons die because of insufficient food. This causes the rest of the food web to die which then wipes out entire ecosystems.
The cause of such “hypoxic” (lacking oxygen) conditions is usually eutrophication, which is an increase in chemical nutrients in the water, such as phosphorus and nitrogen(1996, National Geographic), leading to excessive blooms of algae/bacteria that deplete underwater oxygen levels. At normal levels, these nutrients feed the growth of an organism called cyanobacteria, or blue green algae, but when there are too many nutrients, cyanobacteria grows out of control and is harmful to the marine ecosystem. When the excess cyanobacteria, algae, and phytoplankton sink to the seafloor, they are decomposed by bacteria. Although oxygen is now able to move freely throughout the water, the process of decompostion uses up almost all the oxygen(1996, National Geographic). Thus, preventing other organisms receive the oxygen they need. The lack of oxygen creates dead zones in which most aquatic species cannot survive in.
Algal blooms and Hypoxia are the causes of dead zones, but the excess in nutrients doesn’t come naturally. Humans are negatively impacting the oceans and are the MAIN CAUSE of ocean dead zones.