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Hypoxic or so-called dead zones are areas of the ocean (and rarely in lakes or even rivers) which lack oxygen to provide normal marine life (Simon, n.d.). This condition is usually caused by a complex of reasons which includes chemical, biological, and physical aspects. I consider that the emergence of hypoxic zones mainly depends on the area and human activity there. For example, in agricultural areas, the major reason for hypoxic zones will be chemicals since the water is rich in “nitrogen and phosphorous from agricultural runoff” (Simon, n.d.). The growth of concentration of chemical nutrients in the water causes the active algae blooming which drains underwater oxygen thus conditioning the lack of oxygen (Simon, n.d.). However, other factors such as “sewage, vehicular and industrial emissions and even natural factors” have an impact on the development of hypoxic zones (Simon, n.d.).
It is considered that hypoxic zones can have natural roots. Nevertheless, their prevalence in the seas and oceans near the locations of big industrial or agricultural facilities proves the human impact (Simon, n.d.). As of 2008, there were over 400 hypoxic zones detected around the world (Simon, n.d.).
One of the biggest hypoxic zones in the US (area of 8,500 square miles) is in the Gulf of Mexico (Simon, n.d.). The Mississippi River runs into the Gulf filled with nutrients from farms. The condition of water in the area caused the decline of the shrimp industry. Moreover, lack of oxygen conditioned the reproductive problems for fish. Thus, a chemical reason caused biological one. However, hypoxic zones can be reversible in case the factors conditioning them are reduced or removed.
At present, biological invasions are a burden for marine ecosystems. These invasions are an extensive global change that is a threat to “the conservation of biodiversity and natural resources” (Simberloff et al., 2013, p.58). Invasion science which emerged at an intersection of ecologic studies, social sciences, resource management, and economics investigates biological invasions.
The influences of invasions by alien species are varied and hard to evaluate since their effect is often delayed. Different researchers define these impacts as “good” or “bad” (Simberloff et al., 2013). However, the impacts of marine invasive species are mainly detrimental. For example, the migration of rabbitfish (Siganus spp) reduced the habitat complexity and species richness. Moreover, it changed food webs in the area (Simberloff et al., 2013). Another example is the introduction of Nile perch (Lates niloticus) which led to the extinction of more than 150 native fish species, increase of algal blooms, and rapid growth of prawn populations (Simberloff et al., 2013). Sometimes the effect of biological invasion can touch ecosystems other than marine. Thus, an introduced predator such as Rainbow trout (Oncorhynchus mykiss) usurped terrestrial insects which made local char to migrate for food. In its turn, it caused the growth of algal biomass and decreased insect and spider populations (Simberloff et al., 2013).
Research by Preston, Henderson, and Johnson (2012) investigated the effects of alien fish predators and alien bullfrogs on marine communities. They revealed the facts that mosquitofish, for example, reduced the survival rate of tree-frog from 74% to 7% (Preston et al., 2012, p.1258). Also, alien fish species had an impact on nutrients and plankton. However, despite a substantial influence on the local species, invaders did not have a significant impact at one another.
References
Preston, D.L., Henderson, J.S., & Johnson, P. (2012). Community ecology of invasions: Direct and indirect effects of multiple invasive species on aquatic communities. Ecology, 93(6), p.1254-1261.
Simberloff, D., Martin, J.-L., Genovesi, P., Maris, V., Wardle, D.A., Aronson, J., … Vila, M. (2013). Impacts of biological invasions: What’s what and the way forward. Trends in Ecology & Evolution, 28, 58-66.
Simon, R. (n.d.). What causes ocean “Dead zones?”Scientific American. Web.
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