This probably sounds pretty dumb, but wouldn't algae blooms produce a lot of oxygen? Although they would die out and decomposers would use up oxygen, is that more than what the algae produced?
Algae produce O2 in the upper layer of water but when they die they stop producing O2. They sink to the seafloor and most get decomposed by bacteria on the seafloor. In this process, bacteria use O2 contained in the bottom layer of water which decreases the dissolved O2 concentration in the bottom water.
These concepts (and much more!) are well described in the excellent open-access paper by Rabalais et al. (2010) Dynamics and distribution of natural and human-caused hypoxia. It also includes case-studies of areas affected by hypoxia and eutrophication around the world. A must read if you're interested in this topic!
As you surmise, the oxygen is consumed by decomposers. From the Wikipedia page on Eutrophication:
Phosphorus is a necessary nutrient for plants to live, and is the limiting factor for plant growth in many freshwater ecosystems. The addition of phosphorus increases algal growth, but not all phosphates actually feed algae. These algae assimilate the other necessary nutrients needed for plants and animals. When algae die they sink to the bottom where they are decomposed and the nutrients contained in organic matter are converted into inorganic form by bacteria. The decomposition process uses oxygen and deprives the deeper waters of oxygen which can kill fish and other organisms.
Eutrophication (from Greek eutrophos, "well-nourished")  is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients. Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. 
Prior to human interference, this was, and continues to be, a very slow natural process in which nutrients, especially phosphorus compounds and organic matter, accumulate in water bodies.  These nutrients derive from degradation and solution of minerals in rocks and by the effect of lichens, mosses and fungi actively scavenging nutrients from rocks.  Anthropogenic or cultural eutrophication is often a much more rapid process in which nutrients are added to a water body from any of a wide variety of polluting inputs including untreated or partially treated sewage, industrial wastewater and fertilizer from farming practices. Nutrient pollution, a form of water pollution, is a primary cause of eutrophication of surface waters, in which excess nutrients, usually nitrogen or phosphorus, stimulate algal and aquatic plant growth.
The visible effect of eutrophication is often nuisance algal blooms that can cause substantial ecological degradation in water bodies and associated streams.  This process may result in oxygen depletion of the water body after the bacterial degradation of the algae. 
Eutrophication in freshwater systems is almost always caused by excess phosphorus.  Humankind has increased the rate of phosphorus cycling on Earth by four times, mainly due to agricultural fertilizer production and application. Between 1950 and 1995, an estimated 600,000,000 tonnes of phosphorus was applied to Earth's surface, primarily on croplands. 
What is eutrophication?
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VIDEO: What is eutrophication? Here's an overview in one minute. Transcript
Eutrophication is a big word that describes a big problem in the nation's estuaries. Harmful algal blooms, dead zones, and fish kills are the results of a process called eutrophication — which occurs when the environment becomes enriched with nutrients, increasing the amount of plant and algae growth to estuaries and coastal waters.
Sixty-five percent of the estuaries and coastal waters in the contiguous U.S. that have been studied by researchers are moderately to severely degraded by excessive nutrient inputs. Excessive nutrients lead to algal blooms and low-oxygen (hypoxic) waters that can kill fish and seagrass and reduce essential fish habitats. Many of these estuaries also support bivalve mollusk populations (e.g., oysters, clams, scallops), which naturally reduce nutrients through their filter-feeding activities.
Eutrophication sets off a chain reaction in the ecosystem, starting with an overabundance of algae and plants. The excess algae and plant matter eventually decompose, producing large amounts of carbon dioxide. This lowers the pH of seawater, a process known as ocean acidification. Acidification slows the growth of fish and shellfish and can prevent shell formation in bivalve mollusks. This leads to a reduced catch for commercial and recreational fisheries, meaning smaller harvests and more expensive seafood.
Did you know?
In September 2017, New York Governor Andrew M. Cuomo announced a $10.4 million effort to improve Long Island's water quality and bolster the economies and resiliency of coastal communities by restoring native shellfish populations to coastal waters. The state plans to establish five new sanctuary sites in Suffolk and Nassau Counties to transplant seeded clams and oysters, and to expand public shellfish hatcheries in the two counties through a dedicated grant program. Eutrophication has had significant economic impacts on Long Island Sound, where commercial shellfisheries have lost millions of dollars annually since 1985. Recent projections indicate that without intervention, the Sound could lose all of its seagrass beds by 2030, and that two-thirds of the Sound could lack enough oxygen for fish to survive.
In recent years, NOAA's National Centers for Coastal Ocean Science (NCCOS), in collaboration with NOAA's Northeast Fisheries Science Center, has enlisted estuaries' indigenous residents, namely, bivalve mollusks, to help slow and, in some cases, reverse the process of eutrophication, since they efficiently remove nutrients from the water as they feed on phytoplankton and detritus.
A groundbreaking modeling project in Long Island Sound showed that the oyster aquaculture industry in Connecticut provides $8.5 – $23 million annually in nutrient reduction benefits. The project also showed that reasonable expansion of oyster aquaculture could provide as much nutrient reduction as the comparable investment of $470 million in traditional nutrient-reduction measures, such as wastewater treatment improvements and agricultural best management practices.
The NOAA scientists used aquaculture modeling tools to demonstrate that shellfish aquaculture compares favorably to existing nutrient management strategies in terms of efficiency of nutrient removal and implementation cost. Documenting the water quality benefits provided by shellfish aquaculture has increased both communities' and regulators' acceptance of shellfish farming, not only in Connecticut but across the nation. In Chesapeake Bay, for example, nutrient removal policies include the harvesting of oyster tissue as an approved method, and in Mashpee Bay, Massachusetts, cultivation and harvest of oysters and clams are part of the official nutrient management plan.
Journal of Introductory Biology Investigations
When a body of water increases its nutrient levels, especially in consideration of phosphorus and nitrogen, which promote the growth and later death of algae, this is called Eutrophication (Olatunji et al., 2015). Eutrophication is a major problem being faced by the agricultural community. Nitrogen is usually the limiting factor in the growth of plants (including algae) because of this farmers add fertilizer to their fields (Chapin et al., 1987). Over applications of fertilizers linked with rain causes the fertilizers to run off into water sources, which leads to eutrophication (Diaz, 2008). The experiment is comparing different nutrient levels of water effect on fertilizer and fertilizers effect on the life cycle of algae. By putting fertilizer in water the dissolved oxygen will decrease over time because decomposers will breakdown the algal bloom caused by the fertilizer and use up all of the dissolved oxygen in the water regardless of the nutrient levels in the water. Though others have attempted to prove the effects of fertilizers in water and found adequate data, we took a different direction by trying to see the effects of different nutrient levels of fertilizer in water. This information is important because knowing how fertilizer reacts in different nutrient levels of water will help predict the impact of fertilizer on the ecosystem in the water.
Chapin, F. S., III, Bloom, A. J., Field, C. B., Waring, R. H. (1987). Plant Response to Multiple Environmental Factors. Bioscience. 37(1), 49-57.
Diaz, R. J. Rosenberg, R. (2008). Spreading Dead Zones and Consequences of Marine Ecosystems. Science. 321(5891), 1-6. doi: 10.1126/science.1156401.
Eutrophication is a process of increasing biomass generation in a water body caused by increasing concentrations of plant nutrients, most commonly phosphate and nitrate.  Increasing nutrient concentrations lead to increasing fecundity of aquatic plants, both macrophytes and phytoplankton.  As more plant material becomes available as a food resource, there are associated increases in invertebrates and fish species. As the process continues, the biomass of the water body increases but biological diversity decreases.  With more severe eutrophication, bacterial degradation of the excess biomass results in oxygen consumption, which can create a state of hypoxia at least in the bottom sediment and deeper layers of water. Hypoxic zones are commonly found in deep water lakes in the summer season due to stratification into the cold oxygen-poor hypolimnion and the warm oxygen-rich epilimnion. Strongly eutrophic freshwaters can become hypoxic throughout their depth following severe algal blooms or macrophyte overgrowths.
According to Ullmann's Encyclopedia,  "the primary limiting factor for eutrophication is phosphate." The availability of phosphorus generally promotes excessive plant growth and decay causing a severe reduction in water quality. Phosphorus is a necessary nutrient for plants to live, and is the limiting factor for plant growth in most freshwater ecosystems.  Phosphate adheres tightly to soil particles, so it is mainly transported by erosion and runoff. Once translocated to lakes, the extraction of phosphate into water is slow, hence the difficulty of reversing the effects of eutrophication.  In marine systems nitrogen and iron are the primary limiting nutrients for the accumulation of algal biomass. 
The sources of excess phosphate are phosphates in detergent, industrial/domestic run-offs, and fertilizers. With the phasing out of phosphate-containing detergents in the 1970s, industrial/domestic run-off and agriculture have emerged as the dominant contributors to eutrophication. 
Natural eutrophication Edit
Although eutrophication is commonly caused by human activities, it can also be a natural process, particularly in lakes. Paleolimnologists now recognise that climate change, geology, and other external influences are also critical in regulating the natural productivity of lakes. A few lakes also demonstrate the reverse process (meiotrophication), becoming less nutrient rich with time as nutrient poor inputs slowly elute the nutrient richer water mass of the lake.   This process may be seen in artificial lakes and reservoirs which tend to be highly eutrophic on first filling but may become more oligotrophic with time. The main difference between natural and anthropogenic eutrophication is that the natural process is very slow, occurring on geological time scales. 
Cultural eutrophication Edit
Cultural or anthropogenic eutrophication is the process that speeds up natural eutrophication because of human activity.  Due to clearing of land and building of towns and cities, land runoff is accelerated and more nutrients such as phosphates and nitrate are supplied to lakes and rivers, and then to coastal estuaries and bays. Cultural eutrophication results when excessive nutrients from human activities end up in water bodies creating nutrient pollution and also accelerating the natural process of eutrophication.  The problem became more apparent following the introduction of chemical fertilizers in agriculture (green revolution of the mid-1900s).  Phosphorus and nitrogen are the two main nutrients that cause cultural eutrophication as they enrich the water, allowing for some aquatic plants, especially algae to grow rapidly. Algae are prone to bloom in high densities and when they die off, their degradation by bacteria removes oxygen, generating anoxic conditions. This anoxic environment kills off aerobic organisms (e.g. fish and invertebrates) in the water body. This also affects terrestrial animals, restricting their access to affected water (e.g. as drinking sources). Selection for algal and aquatic plant species that can thrive in nutrient-rich conditions can cause structural and functional disruption to entire aquatic ecosystems and their food webs, resulting in loss of habitat and species biodiversity. 
There are several sources of excessive nutrients from human activity including run-off from fertilized fields, lawns and golf courses, untreated sewage and wastewater and internal combustion of fuels.  Cultural eutrophication can occur in fresh water and salt water bodies, shallow waters being the most susceptible. In shore lines and shallow lakes, sediments are frequently resuspended by wind and waves which can result in nutrient release into the overlying water, enhancing eutrophication.  The deterioration of water quality caused by cultural eutrophication can therefore negatively impact human uses including potable supply for consumption, industrial uses and recreation. 
Effects in freshwater systems Edit
One response to added amounts of nutrients in aquatic ecosystems is the rapid growth of microscopic algae, creating an algal bloom. In freshwater systems, the formation of floating algal blooms are commonly nitrogen-fixing cyanobacteria (blue-green algae). This outcome is favored when soluble nitrogen becomes limiting and phosphorus inputs remain significant.  Nutrient pollution is a major cause of algal blooms and excess growth of other aquatic plants leading to overcrowding competition for sunlight, space, and oxygen. Increased competition for the added nutrients can cause potential disruption to entire ecosystems and food webs, as well as a loss of habitat, and biodiversity of species. 
When macrophytes and algae die in over-productive eutrophic lakes, rivers and streams, they decompose and the nutrients contained in that organic matter are converted into inorganic form by microorganisms. This decomposition process consumes oxygen, which reduces the concentration of dissolved oxygen. The depleted oxygen levels in turn may lead to fish kills and a range of other effects reducing biodiversity. Nutrients may become concentrated in an anoxic zone and may only be made available again during autumn turn-over or in conditions of turbulent flow. The dead algae and organic load carried by the water inflows into a lake settle to the bottom and undergo anaerobic digestion releasing greenhouse gases such as methane and CO2. Some of the methane gas may be oxidised by anaerobic methane oxidation bacteria such as Methylococcus capsulatus which in turn may provide a food source for zooplankton.  Thus a self-sustaining biological process can take place to generate primary food source for the phytoplankton and zooplankton depending on the availability of adequate dissolved oxygen in the water body. 
Enhanced growth of aquatic vegetation, phytoplankton and algal blooms disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of oxygen needed for fish and shellfish to survive. Eutrophication also decreases the value of rivers, lakes and aesthetic enjoyment. Health problems can occur where eutrophic conditions interfere with drinking water treatment. 
Human activities can accelerate the rate at which nutrients enter ecosystems. Runoff from agriculture and development, pollution from septic systems and sewers, sewage sludge spreading, and other human-related activities increase the flow of both inorganic nutrients and organic substances into ecosystems. Elevated levels of atmospheric compounds of nitrogen can increase nitrogen availability. Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to "point source" pollution from sewage pipes. The concentration of algae and the trophic state of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication. Later stages of eutrophication lead to blooms of nitrogen-fixing cyanobacteria limited solely by the phosphorus concentration. 
Effects in coastal waters Edit
Eutrophication is a common phenomenon in coastal waters. In contrast to freshwater systems where phosphorus is often the limiting nutrient, nitrogen is more commonly the key limiting nutrient of marine waters thus, nitrogen levels have greater importance to understanding eutrophication problems in salt water.  Estuaries, as the interface between freshwater and saltwater, can be both phosphorus and nitrogen limited and commonly exhibit symptoms of eutrophication. Eutrophication in estuaries often results in bottom water hypoxia/anoxia, leading to fish kills and habitat degradation.  Upwelling in coastal systems also promotes increased productivity by conveying deep, nutrient-rich waters to the surface, where the nutrients can be assimilated by algae. Examples of anthropogenic sources of nitrogen-rich pollution to coastal waters include seacage fish farming and discharges of ammonia from the production of coke from coal. 
The World Resources Institute has identified 375 hypoxic coastal zones in the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and East Asia, particularly Japan. 
In addition to runoff from land, fish farming wastes and industrial ammonia discharges, atmospheric fixed nitrogen can be an important nutrient source in the open ocean. A study in 2008 found that this could account for around one third of the ocean's external (non-recycled) nitrogen supply, and up to 3% of the annual new marine biological production.  It has been suggested that accumulating reactive nitrogen in the environment may prove as serious as putting carbon dioxide into the atmosphere. 
Terrestrial ecosystems Edit
Terrestrial ecosystems are subject to similarly adverse impacts from eutrophication.  Increased nitrates in soil are frequently undesirable for plants. Many terrestrial plant species are endangered as a result of soil eutrophication, such as the majority of orchid species in Europe.  Meadows, forests, and bogs are characterized by low nutrient content and slowly growing species adapted to those levels, so they can be overgrown by faster growing and more competitive species. In meadows, tall grasses that can take advantage of higher nitrogen levels may change the area so that natural species may be lost. Species-rich fens can be overtaken by reed or reedgrass species. Forest undergrowth affected by run-off from a nearby fertilized field can be turned into a nettle and bramble thicket. [ citation needed ]
Chemical forms of nitrogen are most often of concern with regard to eutrophication, because plants have high nitrogen requirements so that additions of nitrogen compounds will stimulate plant growth. Nitrogen is not readily available in soil because N2, a gaseous form of nitrogen, is very stable and unavailable directly to higher plants. Terrestrial ecosystems rely on microbial nitrogen fixation to convert N2 into other forms such as nitrates. However, there is a limit to how much nitrogen can be utilized. Ecosystems receiving more nitrogen than the plants require are called nitrogen-saturated. Saturated terrestrial ecosystems then can contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient.  This is also the case with increased levels of phosphorus. However, because phosphorus is generally much less soluble than nitrogen, it is leached from the soil at a much slower rate than nitrogen. Consequently, phosphorus is much more important as a limiting nutrient in aquatic systems. 
Raw sewage Edit
Raw sewage is a large contributor to cultural eutrophication since sewage matter is very rich in nutrients. Releasing raw sewage into a large water body is referred to as sewage dumping, which is a large problem in today's society even in developed countries. There are multiple different ways to fix cultural eutrophication with raw sewage being a point source of pollution. Waste collection, incineration, and waste treatment have become common practices in industrialized parts of the world.  A wastewater treatment plant is where the water will be filtered to regular water before discharging back into a large water body. In some areas incineration is used where the solid waste is exposed to high temperatures turning the waste into ash. Convectional sludge incineration systems mostly charge dewatered sludge directly into the incinerator.  Generating an anaerobic environment is also another method where microorganisms degrade the waste without the use of oxygen. An anaerobic system can be used for pretreatment prior to discharging to a municipal wastewater treatment plant.  The incineration method and the anaerobic methods are the most environmentally friendly compared to others.  Anaerobic treatment use substantially less energy, require less chemicals, and incur lower sludge handling costs compared to aerobic treatments as well the biogas produced is a source of renewable energy to generate electricity.  Similarly, incinerating a ton of waste produces electric energy equivalent to 52.1 kWh/ton of combustible waste in the combines heat and power generation this amount displaces electricity that would be provided by an electric utility power plant that uses fossil fuels in Korea. 
Since the agricultural boom in the 1910s and again in the 1940s to match the increase in food demand, agricultural production relies heavily on the use of fertilizers.  Fertilizer is a natural or chemically modified substance that helps soil become more fertile. These fertilizers contain high amounts of phosphorus and nitrogen, which results in excess amounts of nutrients entering the soil. Nitrogen, phosphorus and potassium are the "Big 3" primary nutrients in commercial fertilizers, each of these fundamental nutrients play a key role in plant nutrition.  When nitrogen and phosphorus are not fully utilized by the growing plants, they can be lost from the farm fields and negatively impact air and downstream water quality.  These nutrients can eventually end up in aquatic ecosystems and are a contributor to increased eutrophication.  When farmers spread their fertilizer whether it is organic or synthetically made most of the fertilizer will turn into runoff that collects downstream generating cultural eutrophication.
There are many ways to help fix cultural eutrophication caused by agriculture. Safe farming practices is the number one way to fix the problem. Some safety precautions are: 
- Nutrient Management Techniques - Anyone using fertilizers should apply fertilizer in the correct amount, at the right time of year, with the right method and placement.
- Year - Round Ground Cover - a cover crop will prevent periods of bare ground thus eliminating erosion and runoff of nutrients even after the growing season has occurred.
- Planting Field Buffers - By planting trees, shrubs and grasses along the edges of fields to help catch the runoff and absorb some nutrients before the water makes it to a nearby water body.
- Conservation Tillage - By reducing frequency and intensity of tilling the land will enhance the chance of nutrients absorbing into the ground.
Eutrophication was recognized as a water pollution problem in European and North American lakes and reservoirs in the mid-20th century.  Since then, it has become more widespread. Surveys showed that 54% of lakes in Asia are eutrophic in Europe, 53% in North America, 48% in South America, 41% and in Africa, 28%.  In South Africa, a study by the CSIR using remote sensing has shown more than 60% of the reservoirs surveyed were eutrophic.  Some South African scientists believe that this figure might be higher  with the main source being dysfunctional sewage works that produce more than 4 billion litres a day of untreated, or at best partially treated, sewage effluent that discharges into rivers and reservoirs.  Even with good secondary treatment, most final effluents from sewage treatment works contain substantial concentrations of nitrogen as nitrate, nitrite or ammonia. Removal of these nutrients is an expensive and often difficult process.
Many ecological effects can arise from stimulating primary production, but there are three particularly troubling ecological impacts: decreased biodiversity, changes in species composition and dominance, and toxicity effects.
- Increased biomass of phytoplankton
- Toxic or inedible phytoplankton species
- Increases in blooms of gelatinous zooplankton
- Increased biomass of benthic and epiphyticalgae
- Changes in macrophyte species composition and biomass
- Decreases in water transparency (increased turbidity)
- Colour, smell, and water treatment problems depletion
- Increased incidences of fish kills
- Loss of desirable fish species
- Reductions in harvestable fish and shellfish
- Decreases in perceived aesthetic value of the water body
Decreased biodiversity Edit
When an ecosystem experiences an increase in nutrients, primary producers reap the benefits first. In aquatic ecosystems, species such as algae experience a population increase (called an algal bloom). Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. Oxygen is required by all aerobically respiring plants and animals and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off.  In extreme cases, anaerobic conditions ensue, promoting growth of bacteria. Zones where this occurs are known as dead zones.
New species invasion Edit
Eutrophication may cause competitive release by making abundant a normally limiting nutrient. This process causes shifts in the species composition of ecosystems. For instance, an increase in nitrogen might allow new, competitive species to invade and out-compete original inhabitant species. This has been shown to occur  in New England salt marshes. In Europe and Asia, the common carp frequently lives in naturally Eutrophic or Hypereutrophic areas, and is adapted to living in such conditions. The eutrophication of areas outside its natural range partially explain the fish's success in colonising these areas after being introduced.
Some algal blooms resulting from eutrophication, otherwise called "harmful algal blooms", are toxic to plants and animals. Toxic compounds can make their way up the food chain, resulting in animal mortality.  Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten, neuro- and hepatotoxins are released which can kill animals and may pose a threat to humans.   An example of algal toxins working their way into humans is the case of shellfish poisoning.  Biotoxins created during algal blooms are taken up by shellfish (mussels, oysters), leading to these human foods acquiring the toxicity and poisoning humans. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Other marine animals can be vectors for such toxins, as in the case of ciguatera, where it is typically a predator fish that accumulates the toxin and then poisons humans.
At the most extreme levels, eutrophication is identifiable by sight and smell.
When conditions become repulsive and drastic steps are required to control obnoxious growths of algae, then there is no longer need to marshall experts or scientific equipment to explain what has happened.
However, as water bodies change their chemical and biological status, identifying the scale and the causes of the problem are prerequisites to identifying a remediation strategy.
Within eutrophic water bodies, nutrients are in constant flux and a determination of concentrations of N and P may not provide good evidence of the current eutrophic state. In early studies on the Great Lakes,total solids, calcium, sodium, potassium, sulfate, and chloride provided good supporting evidence of eutrophication even though they were not themselves implicated. These ions were indicative of general anthropogenic inputs and provided good surrogates for nutrient inputs 
Qualitative assessments of water based on obvious signs of eutrophication such as changes to the species of algae present or their relative abundance will typically be too late to avoid the damage caused by eutrophication to biotic diversity 
Quantitative assessments at regular intervals of key chemical and biological indicators can provide statistically valid data for identifying the earliest onset of eutrophication and monitoring its progress. Typical parameters used include Chlorophyll-a, total nitrogen, total and dissolved phosphorus, biological or chemical oxygen demand and secchi depth level. 
Eutrophication poses a problem not only to ecosystems, but to humans as well. Reducing eutrophication should be a key concern when considering future policy, and a sustainable solution for everyone, including farmers and ranchers, seems feasible. While eutrophication does pose problems, humans should be aware that natural runoff (which causes algal blooms in the wild) is common in ecosystems and should thus not reverse nutrient concentrations beyond normal levels. Cleanup measures have been mostly, but not completely, successful. Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. These efforts have had a 90% removal efficiency.  Still, some targeted point sources did not show a decrease in runoff despite reduction efforts.
Minimizing nonpoint pollution Edit
Nonpoint pollution is the most difficult source of nutrients to manage. The literature suggests, though, that when these sources are controlled, eutrophication decreases. The following steps are recommended to minimize the amount of pollution that can enter aquatic ecosystems from ambiguous sources.
Riparian buffer zones Edit
Studies show that intercepting non-point pollution between the source and the water is a successful means of prevention.  Riparian buffer zones are interfaces between a flowing body of water and land, and have been created near waterways in an attempt to filter pollutants sediment and nutrients are deposited here instead of in water. Creating buffer zones near farms and roads is another possible way to prevent nutrients from traveling too far. Still, studies have shown  that the effects of atmospheric nitrogen pollution can reach far past the buffer zone. This suggests that the most effective means of prevention is from the primary source.
Prevention policy Edit
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems,  but it is generally agreed that a policy regulating agricultural use of fertilizer and animal waste must be imposed. In Japan the amount of nitrogen produced by livestock is adequate to serve the fertilizer needs for the agriculture industry.  Thus, it is not unreasonable to command livestock owners to collect animal waste from the field, which when left stagnant will leach into ground water.
Policy concerning the prevention and reduction of eutrophication can be broken down into four sectors: Technologies, public participation, economic instruments, and cooperation.  The term technology is used loosely, referring to a more widespread use of existing methods rather than an appropriation of new technologies. As mentioned before, nonpoint sources of pollution are the primary contributors to eutrophication, and their effects can be easily minimized through common agricultural practices. Reducing the amount of pollutants that reach a watershed can be achieved through the protection of its forest cover, reducing the amount of erosion leeching into a watershed. Also, through the efficient, controlled use of land using sustainable agricultural practices to minimize land degradation, the amount of soil runoff and nitrogen-based fertilizers reaching a watershed can be reduced.  Waste disposal technology constitutes another factor in eutrophication prevention. Because a major contributor to the nonpoint source nutrient loading of water bodies is untreated domestic sewage, it is necessary to provide treatment facilities to highly urbanized areas, particularly those in underdeveloped nations, in which treatment of domestic waste water is a scarcity.  The technology to safely and efficiently reuse waste water, both from domestic and industrial sources, should be a primary concern for policy regarding eutrophication.
The role of the public is a major factor for the effective prevention of eutrophication. In order for a policy to have any effect, the public must be aware of their contribution to the problem, and ways in which they can reduce their effects. Programs instituted to promote participation in the recycling and elimination of wastes, as well as education on the issue of rational water use are necessary to protect water quality within urbanized areas and adjacent water bodies.
Economic instruments, "which include, among others, property rights, water markets, fiscal and financial instruments, charge systems and liability systems, are gradually becoming a substantive component of the management tool set used for pollution control and water allocation decisions."  Incentives for those who practice clean, renewable, water management technologies are an effective means of encouraging pollution prevention. By internalizing the costs associated with the negative effects on the environment, governments are able to encourage a cleaner water management.
Because a body of water can have an effect on a range of people reaching far beyond that of the watershed, cooperation between different organizations is necessary to prevent the intrusion of contaminants that can lead to eutrophication. Agencies ranging from state governments to those of water resource management and non-governmental organizations, going as low as the local population, are responsible for preventing eutrophication of water bodies. In the United States, the most well known inter-state effort to prevent eutrophication is the Chesapeake Bay. 
Nitrogen testing and modeling Edit
Soil Nitrogen Testing (N-Testing) is a technique that helps farmers optimize the amount of fertilizer applied to crops. By testing fields with this method, farmers saw a decrease in fertilizer application costs, a decrease in nitrogen lost to surrounding sources, or both.  By testing the soil and modeling the bare minimum amount of fertilizer are needed, farmers reap economic benefits while reducing pollution.
Organic farming Edit
There has been a study that found that organically fertilized fields "significantly reduce harmful nitrate leaching" compared to conventionally fertilized fields.  However, a more recent study found that eutrophication impacts are in some cases higher from organic production than they are from conventional production. 
Shellfish in estuaries Edit
One proposed solution to stop and reverse eutrophication in estuaries is to restore shellfish populations, such as oysters and mussels. Oyster reefs remove nitrogen from the water column and filter out suspended solids, subsequently reducing the likelihood or extent of harmful algal blooms or anoxic conditions.  Filter feeding activity is considered beneficial to water quality  by controlling phytoplankton density and sequestering nutrients, which can be removed from the system through shellfish harvest, buried in the sediments, or lost through denitrification.   Foundational work toward the idea of improving marine water quality through shellfish cultivation was conducted by Odd Lindahl et al., using mussels in Sweden.  In the United States, shellfish restoration projects have been conducted on the East, West and Gulf coasts.  See nutrient pollution for an extended explanation of nutrient remediation using shellfish.
Seaweed farming Edit
Seaweed aquaculture offers an opportunity to mitigate, and adapt to climate change.  Seaweed, such as kelp, also absorbs phosphorus and nitrogen  and is thus useful to remove excessive nutrients from polluted parts of the sea.  Some cultivated seaweeds have a very high productivity and could absorb large quantities of N, P, CO2, producing large amount of O2 have an excellent effect on decreasing eutrophication.  It is believed that seaweed cultivation in large scale should be a good solution to the eutrophication problem in coastal waters.
Geo-engineering in lakes Edit
Geo-engineering is the manipulation of biogeochemical processes, usually the phosphorus cycle, to achieve a desired ecological response in the ecosystem.  Geo-engineering techniques typically uses materials able to chemically inactivate the phosphorus available for organisms (i.e. phosphate) in the water column and also block the phosphate release from the sediment (internal loading).  Phosphate is one of the main contributing factors to algal growth, mainly cyanobacteria, so once phosphate is reduced the algal is not able to overgrow.  Thus, geo-engineering materials is used to speed-up the recovery of eutrophic water bodies and manage algal bloom.  There are several phosphate sorbents in the literature, from metal salts (e.g. alum, aluminium sulfate,  ) minerals, natural clays and local soils, industrial waste products, modified clays (e.g. lanthanum modified bentonite) and others.   The phosphate sorbent is commonly applied in the surface of the water body and it sinks to the bottom of the lake reducing phosphate, such sorbents have been applied worldwide to manage eutrophication and algal bloom.      
Breakthrough research carried out at the Experimental Lakes Area (ELA) in Ontario, Canada in the 1970s  provided the evidence that freshwater bodies are phosphorus-limited. ELA is a fully equipped, year-round, permanent field station that uses the whole ecosystem approach and long-term, whole-lake investigations of freshwater focusing on cultural eutrophication. ELA was earlier co-sponsored by the Canadian Departments of Environment and Fisheries and Oceans, with a mandate to investigate the aquatic effects of a wide variety of stresses on lakes and their catchments   and is now managed by the International Institute for Sustainable Development.
The United Nations framework for Sustainable Development Goals recognizes the damaging effects of eutrophication upon marine environments and has established a timeline for creating an Index of Coastal Eutrophication and Floating Plastic Debris Density (ICEP).  The Sustainable Development Goal 14 specifically has a target to prevent and significantly reduce pollution of all kinds including nutrient pollution (eutrophication) by 2025. 
What Are Signs of Oxygen Stress?
In the natural environment, it may be difficult to identify oxygen stress in hard clams. Clams have the ability to close their valves in response to hypoxic or anoxic conditions and can keep their valves closed for several days. Long-term responses may include gaping of the valves. Signs of adverse environmental conditions in juvenile or adult hard clams may go unnoticed because they are infaunal, which means that they live buried in the sediment. However, stressed clams may rise to the surface of the sediment or fail to bury. These signs are not necessarily specific indications of oxygen stress they may also be associated with infectious or noninfectious diseases or other adverse environmental conditions such as high temperature and low salinity.
Effects on Life
Besides being ugly, when an algal bloom occurs, it has a devastating effect on aquatic animals. As large populations of algae and other organisms reproduce, many also die off, and their bodies sink to the bottom of the lake or ocean. Over time, a substantial layer of dead and decomposing organisms fills the bottom.
Microbes that decompose these dead organisms use oxygen in the process. The result is the depletion of oxygen in the water, a condition known as hypoxia. Since most fish, crabs, mollusks, and other aquatic animals depend on oxygen as much as land-based animals, the end result of eutrophication and algal blooms is the creation of an area where no aquatic animals can live—a dead zone.
Dead zones resulting from eutrophication are a growing problem worldwide. According to some sources, 54 percent of the lakes in Asia are eutrophic. The numbers are similar for lakes in Europe, while in North America, almost half the lakes suffer from eutrophication.
This loss of aquatic life has a devastating effect on fisheries and the fishing industry. According to researchers at Carlton College who have studied the immense dead zone in the Gulf of Mexico, that body of water is a major source area for the seafood industry.
The impact goes beyond the fishing industry. Recreational fishing, which is a significant driver of the tourism industry, also suffers from a loss of revenue. Algal blooms can have a severe impact on human health. Humans can become seriously ill from eating oysters and other shellfish contaminated with the red tide toxin. The dinoflagellate that causes red tides can cause eye, skin and respiratory irritation, as well as an allergic reaction (coughing, sneezing, tearing, and itching) to swimmers, boaters, and residents of those coastal areas.
The Process of Eutrophication
The Earth’s population grew exponentially in the past century. To meet the food demands of so many people, agricultural practices were intensified. Field size is increased, heavy machinery is used, and pesticides are added in order to produce more food. These new methods, however, led to negative consequences such as deforestation, and pollution. A particular detrimental consequence is the process of eutrophication which is caused by the use of fertilizers in order to increase crop yield.
Many steps lead to eutrophication. First of all, fertilizers are added to crops to increase plant size and growth rate. This is possible because fertilizers have nutrients, such as nitrogen and phosphate, which help plants grow. Whenever it rains, some the fertilizer is washed along with rain water as runoff water into streams and other water bodies. This step is magnified due to deforestation. Trees roots are very large, normally they would be able to absorb some of the runoff water and nutrients. Agricultural areas often have very few trees, in order to maximize crop planting area, so the nutrient-rich water simply continues to flow.
The runoff water will eventually fall into a body of water. At the bottom of the water, there is algae, an aquatic plant, initially benefited by the nutrient rich water. It will absorb the nutrients and that will promote its growth. Due to the excess of phosphates and nitrates, algae can increase in size considerably. It might become so large that it will block other plants and photosynthesizing organisms from the sunlight who then die. Ultimately, those oxygen producing organisms will die which leads to the decrease of the oxygen level in the bottom of the water. The algae that grew very fast will also die. It is important to note that algae and photosynthesizing plants are essential in an ecosystem because they produce the oxygen that so many other animals need. When all of these plants are dead, they will need to be decomposed, and the decomposers are bacteria who require oxygen to perform their ecological role. These bacteria will decompose these plants, but will also use up a lot of the oxygen in water in doing so since the algae was significantly larger.
When the deep water is removed of oxygen, much of the fauna and flora at the bottom of the water will start dying. When the oxygen content is less than 2 milligrams per liter, the water is called hypoxic, and the depth becomes a dead zone. Fortunately, if waste and nutrient runoff is prevented from entering the water, the dead zone can eventually decrease and become viable again.
DE BELLIS, T. Environmental Biology. Lecture 7: Pollution. Dawson College. Fall 2014.
Scientists Say: Eutrophication
This canal may look lovely, but all that green algae blooming on the surface is a sign of eutrophication, which could eventually suffocate any animals living below.
Sahehco/Wikimedia Commons (CC BY-SA 3.0)
Eutrophication (noun, “Yu-TRO-fih-CAY-shun”)
This is a process in which a body of water receives a large bounty of nutrients, especially phosphates. These chemicals can enter the ecosystem naturally or through pollution such as fertilizer runoff. Algae and plants in the water respond to the extra phosphates by growing rapidly. But when the algae and plants die, bacteria break them down. As the bacteria go to work, they use up the oxygen in the water. Without dissolved oxygen in the water, many fish and other animals may suffocate.
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In a sentence
When fertilizer runoff causes eutrophication, the lack of oxygen can kill other animals.
(for more about Power Words, click here)
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
bacterium (plural bacteria) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.
decomposition The process by which compounds in once-living things are broken down and returned to the environment the process by which something decays or rots.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
eutrophication The process by which a body of water becomes full of nutrients, which stimulate the growth of plants and algae. When these organisms die, bacteria decompose them and use up the water’s dissolved oxygen in the process. Without oxygen, animals cannot live in the water and the ecosystem may collapse.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
phosphate A chemical containing one atom of phosphorus and four atoms of oxygen. It is a component of bones, hard white tooth enamel, and some minerals such as apatite. It is a primary ingredient in most plant fertilizers.
nutrients Vitamins, minerals, fats, carbohydrates and proteins needed by organisms to live, and which are extracted through the diet.
runoff The water that runs off of land into rivers, lakes and the seas. As that water travels over land, it picks up bits of soil and chemicals that it will later deposit as pollutants in the water.
sediment Material (such as stones and sand) deposited by water, wind or glaciers.
About Bethany Brookshire
Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.
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Why is oxygen used up in eutrophication? - Biology
I'l bet you are wondering that because you already know that plants can make oxygen. You probably already know that in photosynthesis, plants take CO2 from the air, water (H2O) from their roots, and energy from the sun, and make sugar (C6H12O6).
What a lot of people don't realize is that when there's little or no light, plants do the same thing we do. The break down the sugar to release CO2, water, and energy. This requires oxygen. The reason is pretty complex, but basically, electrons get passed around, and oxygen has to pick them up at the end of the process.
If you measured the amount of oxygen and CO2 dissolved in a lake, how do you think the daytime levels would compare to the nighttime levels? Would a plant need oxygen if it were under lights 24 hours a day?
Plants respire, just like we do. When a plant doesn't have access to light, it burns sugar to make energy, consuming energy. It's just that plants use sugars to build their bodies as well as an energy storage, so over the course of a plant's life, as it grows, it makes more sugar than it burns, and so releases more oxygen than it consumes.
Plants need oxygen for the same reason you and Ido -- without oxygen we can't convert the carbohydrates, fats, and proteins we eat into energy. We call this process respiration, and the formula for this sort of reaction is like this:
sugar + oxygen --> carbon dioxide + water + energy
So we breathe in oxygen and eat food, and we exhale carbon dioxide and excrete water.
This exact same reaction goes on in every living cell, including all plant cells. But of course plants don't have to eat food, because they make their own food using photosynthesis.
The formula for photosynthesis is basically this:
carbon dioxide + water + sunlight --> sugar + oxygen
You can see that this is basically the reverse of respiration, but plants convert the energy in sunlight into the chemical bonds of the sugar. When cells respire, they break those bonds and get the energy out of them. Anyway, you can see that photosynthesis produces oxygen as a waste product, so for the most part plants don't have to breathe in extra oxygen -- they can just use the oxygen that they produce during photosynthesis. However, plants only perform photosynthesis in the green parts, like leaves and stems, but all plant cells need oxygen to respire. Cells in the leaves get plenty of oxygen from photosynthesis, but cells in the roots often need to get oxygen from the environment to stay alive. Even though roots are buried, they can absorb oxygen from the small air spaces in soil. This is why it's possible to 'drown' plants by watering them too much.
If the soil is way too wet, the roots are smothered, the roots can't get any oxygen from the air, and the cells in the roots die. Without those root cells, the rest of the plant dies. Some plants have evolved adaptations to deal with extremely wet soil.
Mangroves are trees that live in swampy environments along the coast in the tropics. The roots of mangroves are often entirely under saltwater, so they have special structures called pneumatophores (Greek for "air carrier") that act like snorkels, sticking up out of the water to get a oxygen for the roots.