8.5 Eutrophication

Eutrophication is the result of excess nutrients in the ecosystem coming from fertilizer in farmland running off into water streams during rain, or other such causes, leading to adverse effects within waterways 🐟 The excess nutrients can enter from point sources or non point sources. Point sources are sources of pollution that come from only one place, and are therefore easier to address. Some examples are power plants, wastewater treatment plants, and other industrial discharges. Nonpoint sources of pollution are more difficult to both identify and address because they come from many places at once. Some examples are stormwater runoff from residential areas, cities, and rural areas. Some of those adverse effects are algal blooms as well as low oxygen waters, aka, hypoxic waters, which can kill organisms within coastal waters and estuaries. When the excess algae or plant matter decomposes, large amounts of carbon dioxide is produced, lowering the pH of seawater, aka, ocean acidification. Ocean acidification slows fish and shellfish growth. It also prevents the formation of shells in bivalve mollusks. Because of these issues, there is less for fishermen to catch, which leads to skyrocketing prices 🎣

Eutrophication can result in algal blooms, dead zones, and mass fish kills. 

When waterways receive too many nutrients, specifically nitrogen and phosphorus, algae can start to overgrow rapidly on the surface of the water, leading to less sunlight entering the water. Algal Blooms are a symptom of imbalances within the body of water’s ecology. This effect occurs especially when the water in question is warm and the weather around it is calm. 

Dead zones are caused by eutrophication events. These are aquatic environments that have become hypoxic or have low oxygen due to excessive 🪴 plant matter decaying. Very few organisms are able to survive in these low oxygen conditions and the result is a dead zone void of natural plant and animal life. 

Algae, while natural to the aquatic food chain, of certain types and at high levels can produce mass concentrations of toxins that can affect other local organisms, sometimes leading to fish kills. Algae can also clog fish gills, which again, lead to fish death. Due to the depletion of oxygen within algal bloom filled waterways, fish with low tolerance for little dissolved oxygen levels can die. 

Eutrophic lakes have high levels of nutrients within the waterways. The lakes are characterized as containing large populations of aquatic organisms, but plant and algae grow to levels that can become a nuisance. Oligotrophic lakes have low levels of nutrients. They can be characterized as supporting a not so very high level of aquatic organisms. Many can be split into two layers during summer (stratification). The lowest layer is the hypolimnion, which supports cold water organisms since it is cold. Mesotrophic lakes have a moderate amount of nutrients. They have occasional algae blooms and if they are deep enough, they can also have hypolimnion which becomes low oxygen near summer's end, possibly resulting in phosphorus release from the sediments. 

Hold on.. What does anthropogenic mean? Anthropogenic means originating from human 👫 activity.  Due to intensive practices of agriculture, the activities of industries, and the growth of population, all of which are anthropogenic factors, eutrophic events have increased. All three of the mentioned processes emit copious amounts of nitrogen and phosphorus. Additionally, anthropogenic activities release two times the nitrogen and three times the phosphorus as naturally occurring emissions.

Depending on the region, different levels of nitrogen and phosphorus are emitted. In developed countries such as the US and EU nations, the heavy reliance on animal manure 🐄 and commercial fertilizers within the agricultural field has led to much eutrophication. As rain falls, runoff from these fields goes into waterways, raising the levels of nutrients within them. Within developing countries, untreated wastewater from sewage and industrial activities are the main contributors to eutrophication. Additionally, since sewage facilities and factories are less regulated, sometimes the end wastewater is dumped straight into waterways. 

In estuaries, animals such as bivalve mollusks reduce nutrients within the water naturally by filtering the water and subsequently feeding on them. Therefore, increasing or re-introducing these organisms into a system can help to stop eutrophication. Stormwater facilities that are designed to capture polluted runoff can catch excess nutrients and/or sediments before they reach more fragile waterways ♼ On a more individual level, the public can learn to use fertilizer more wisely by reading and following the manufacturer’s directions, not fertilizing before the ⛈ rain, and only fertilizing when the lawn has nutrient deficiencies. 

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