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Picture a pristine coastal marine habitat. The water is full of life; invertebrates, birds, larval and juvenile fish, zooplankton, and phytoplankton. However, coastal habitats are often places of dense human populations, and we inevitably produce waste and use various chemicals in our everyday life. As a result, this waste – whether intentionally or unintentionally – ultimately finds its way into the nearby water. This waste is full of nutrients – molecules and elements necessary for biological processes. This nutrient loading benefits phytoplankton and algae, because it includes key nutrients for photosynthesis. The algae and phytoplankton grows at a high rate, overwhelming its predators, and takes over the marine community in dense blooms. Because photosynthesis consumes oxygen, these areas become oxygen-deprived (hypoxic) which leads to loss of overall biodiversity. It is this cascade of events that is known as eutrophication.
Humans have an intimate connection with the oceans, whether they know it or not. A large amount of the things we consider as “waste” find their way into the oceans (see the “How” tab for more on this). Something we must all remember is the connectivity of ecological systems. A change in one factor will inevitably cause change in another factor, which causes a change in a different factor, etc. This notion is at the heart of eutrophication; it is a cascade of events that cause damaged ecosystems and potential risks to human health.
Defining eutrophication briefly is easy, but the definition will likely not be complete. For our purposes, it can be defined as nutrient loading which leads to increased photosynthesis, increased algal/phytoplankton growth, and decreased oxygen levels.
The first question you may have is: how does all our “waste” find its way to the oceans? This is an interesting question, and one important distinction has to be made. In this instance, “waste” is not synonymous with garbage.
Think about things like sewage, fertilizers, detergents, furnaces, and construction sites. All of these use or produce chemicals which contain nutrients. They may get into bodies of water by the acts of people (sewage pipes discharging into nearby water sources) or acts of nature (rainwater carrying fertilizers into the water). Either way, human activity can have a great deal of impact on marine ecosystems, no matter what you are doing.
This picture perfectly displays possible sources of nutrient loading into freshwater and marine ecosystems. When we think about eutrophication we think about phytoplankton and algae growth, along with photosynthesis. The limiting nutrients for photosynthesis are phosphorus and nitrogen, and most of these sources provide the marine environments with these elements. This spike in photosynthetic rate and consumption of oxygen by algae and phytoplankton is what triggers the damaging effects of eutrophication and hypoxia.
Coastal eutrophication has become a more common problem for marine and freshwater ecosystems all over the world, and the United States is no exception. Reports of low dissolved oxygen because of eutrophication occurred in 64% of our estuaries from 2000-2008, while this number was only 37% from 1980-1989. Although the tolerance to hypoxic conditions varies from species to species, there is no doubt that deprived oxygen levels have a negative effect on these ecosystems.
The diagram above gives you an idea of the basic process and effects. You’ll notice oxygen deficiencies, cloudy water, and decreases in biodiversity. All of these effects are no doubt harmful to the balance of an ecosystem. Oxygen shortages can turn into full on oxygen absences; many areas that suffer extreme eutrophication turn into “dead zones”; named so because of the pure and absolute absence of any life.
Nutrient loading and eutrophication lead to the growth of all types of algae. Certain types of algae produce toxic substances which can kill fish and become embedded in the tissues of clams, oysters, scallops, and mussels; they can also cause illness in humans. When these toxin-producing algae bloom, it is called a harmful algal bloom (HAB).
HABs are also known as red or brown tides, because the dense growth of algae stains the water to these colors. Although red and brown are used as general terms, the colors come from specific groups of algae. Brown tides are caused by a genus of microalgae called Aureococcus (mainly A. anophagefferens and A. lagunesis) and red tides are caused by dinoflagellates such as Alexandrium, Cochliodinium, and Karenia brevis.
The direct danger posed by HABs lies in the toxins that they produce. These toxins can paralyze, suffocate, and kill fish on large scales, and cause tissues of marine animals in the area (fish, shellfish) to be laced with these toxins. Unfortunately, these toxins are also harmful to humans, and anyone who eats seafood containing these toxins will likely become ill. Environmental agencies take great care in monitoring water quality where shellfish are harvested in order to prevent people from consuming seafood that has these toxins present, as it can cause neurotoxic, diarrhetic, and paralytic shellfish poisoning (NSP, DSP, PSP). These illnesses can be severe, depending on the quantity of toxin ingested. PSP can be fatal to humans.
Although this is frightening, the normal beachgoer or seafood eater should not worry. Many agencies take great care in monitoring water quality and HABs. Pay attention to announcements or signs regarding shellfishing restrictions, and always stay aware on the latest news regarding the condition of the water.
Despite the fact that many sources of nutrients that promote eutrophication come from industrial and population-wide sources, coastal eutrophication is something that you can help to prevent!
Monitoring how much of a certain chemical you use can go a long way in determining how much nutrients you contribute to the ecosystem. Use fertilizers sparingly – people often use more than is necessary to make your lawn or plants beautifully green. Buy environmentally friendly soaps for things like cars and boats. Often, the labels will indicate if it is safe for discharge into soil or water. Even then, try not to use excessive amounts.
Practicing an environmentally sustainable lifestyle will go a long way to prevent the consumption of fossil fuels, which also release nutrients and carbon dioxide which fuel algal growth. By being an overall steward for the environment, you will help spread awareness and to prevent many environmentally damaging processes, including eutrophication.
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