Possible consequences of eutrophication

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Ecological impacts

Increased biomass of phytoplankton resulting in algal blooms Phytoplankton are free-floating photosynthesizing microscopic plants that are mostly unicellular and create organic compounds from carbon dioxide dissolved in the water (primary production). They obtain energy through the process of photosynthesis and must therefore live in the euphotic zone of an ocean, sea, lake, or other body of water. They are responsible for much of the oxygen present in the Earth's atmosphere. The term phytoplankton encompasses all photoautotrophic microorganisms in aquatic food webs. Phytoplankton serve as the base of the aquatic food web, providing an essential ecological function for all aquatic life.

Phytoplankton are classified as microalgae and include species from the following divisions: Cyanobacteria (blue-green algae), Chlorophyta (green algae), Prochlorophyta, Euglenophyta, Pyrrhophyta (dinoflagellates ), Cryptophyta (cryptomonads), Chrysophyta, and Bacillariophyta (includes diatoms).

When conditions are right (i.e.nutrients or sunlight or temperature or a combination of these), phytoplankton populations can grow explosively, a phenomenon known as an algal bloom. These conditions can be provided on a local basis by natural run-off from the land or by human inputs (e.g., treated or untreated sewage, farming or urban gardening practices). Blooms in the ocean may cover hundreds of square kilometers and are easily visible in satellite images. A bloom may last several weeks, but the life span of any individual phytoplankton is rarely more than a few days. Blooms can appear as a green discoloration of the water due to the presence of chlorophyll within their cells.

Toxic or inedible phytoplankton species (harmful algal bloom)

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A red tide: Noctiluca scintillans bloom in New Zealand (Photo credit: M. Godfrey)

Harmful algal blooms (HAB) are algal bloom events involving toxic or otherwise harmful phytoplankton such as dinoflagellates of the genus Alexandrium or diatoms of the genus Pseudo-nitzschia. To the human eye,algal blooms can appear greenish, brown, and even reddish- orange (red tides) depending upon the algal species, the aquatic ecosystem, and the concentration of the organisms. Harmful algal blooms may cause harm through the production of toxins or by their accumulated biomass, which can affect co-occurring organisms and alter food-web dynamics. Impacts include human illness (see human health impacts) and mortality following consumption of or indirect exposure to HAB toxins, substantial economic losses to coastal communities and commercial fisheries, and HAB-associated fish, bird and mammal mortalities.

Increases in blooms of gelatinous zooplankton

Zooplankton are heterotrophic plankton. They are primarily transported by ambient water currents,but many have locomotion and are primarily found in surface waters where food resources (phytoplankton or other zooplankton) are abundant. Through their consumption and processing of phytoplankton and other food sources, zooplankton play a role in aquatic food webs, as a resource for consumers on higher trophic levels (including fish).They represent a range of organism sizes including small protozoans and large metazoans. Zooplankton can be divided in two important groups: crustacean and gelatinous zooplankton. Crustacean zooplankton (copepods and krill) are arthropods with a chitinous exoskeleton. These are the most abundant zooplankton. Gelatinous zooplankton have relatively fragile,plastic gelatinous bodies that are at least 95% water and which lack rigid skeletal parts. The most well-known are the "jellyfish" (hydromedusae and scyphomedusae).

Eutrophication is believed to cause an increase in the relative importance of gelatinous zooplankton vs. crustacean zooplankton.In many areas of the world where the natural species diversity has been affected by pollution, over-fishing and, now, climate change, gelatinous plankton organisms may be becoming the dominant predator species.

Increased biomass of benthic and epiphytic algae

Decreases in water transparency (increased turbidity)

Dissolved oxygen depletion (hypoxia)

Oxygen is required for all life forms on this planet (with the exception of some bacteria). Oxygen depletion, or hypoxia, is a common effect of eutrophication in bottom waters. This effect may be episodic, occurring annually (most common in summer/autumn), persistent, or periodic in the coastal zone. The direct effects of hypoxia include fish kills, which not only deplete valuable fish stocks and damage the ecosystem, but are unpleasant for local residents and can harm local tourism. Oxygenated water is necessary for aquatic animals to breathe. Mobile animals, such as adult fish, can often survive hypoxia by moving into oxygenated waters. When they cannot, such as when young fish need to spend time in the habitat that has become hypoxic area, the result is a fish kill. Non-mobile animals, such as clams, cannot move into healthier waters and are often killed by hypoxic episodes. This causes a severe reduction of the amount, or in extreme cases the complete loss, of animal life in hypoxic zones. Hypoxia develops when a series of conditions occur together. One of the most important conditions is that there must be a large amount of algae. These algae have to sink or otherwise end up on the bottom. This happens when the algae die without being eaten, or when they are eaten by small animals (zooplankton) whose fecal pellets sink to the bottom. On the bottom, the algae or fecal pellets decompose. This process of decomposition consumes oxygen. If the water is not well-mixed, there is no way to replace the oxygen consumed by decomposition, and hypoxia is likely to occur.

Increased incidences of fish kills and dead benthic animals

Species biodiversity decreases and the dominant biota changes

Human health impacts

Harmful algal blooms have the capacity to produce toxins dangerous to humans. Algal toxins are observed in marine ecosystems where they can accumulate in shellfish and more generally in seafood, reaching dangerous levels for human and animal health. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisioning. Around 40 algal species able of producing toxins harmful to human or marine life have been identifie in european coastal waters. Among these Dinophysis, Gymnodium, Pseudo-nitzschia are frequently observed and represent a risk for seafood consumers. The various effects are summarizes in the table.

Disease	Symptoms	Species	Carriers
Amnesic shellfish poisoning (ASP)	Mental confusion and loss of memory, disorientation and sometimes coma	Diatoms from the genus Nitzschia	Shellfish (mussels)
Neurotoxic shellfish poisoning (NSP)	Muscular paralysis, state of shock and sometimes death	Genus Gymnodinium	Oysters, clams and crustaceans
Venerupin shellfish poisoning (VSP)	Gastrointestinal, nervous, haemorrhagic, hepatic and in extreme cases delirium and hepatic coma	Genus Prorocentrum	Oysters and clams
Diarrhoeic shellfish poisoning (DSP)	Gastrointestinal (diarrhoea, vomiting and abdominal pain)	Genus Dinophysis and Prorocentrum	Filtering shellfish (oysters, mussels, cockles and clams)
Paralytic shellfish poisoning (PSP)	Muscular paralysis, difficulty in breathing, shock and in extreme cases death by respiratory arrest	Genus Alexandrium	Oysters, mussels, crustacean and fish

Other marine animals can be vectors for toxins, as in the case of ciguetera, where it is typically a predator fish that accumulates the toxin and then poisons humans.

Recreational and aesthetic impacts

The enrichment of nutrients can result in a massive growth of green algae. The existence of large areas (mats) can inhibit or prevent access to waterways. This decreases the fitness for use of the water for water sports such as skiing, yachting and fishing. The presence of unsightly and smelling scums also makes any recreational use of the water body unpleasant. Many beaches on the North Sea coast are ruined by Phaecystis blooms. When these so-called foam algae die, large flakes of yellowish foam arise at the beach. In extreme cases beaches can be closed by the presence of toxic algal blooms (HAB). Those may be a health hazard to both human and animals (see higher). If the water is used for water treatment purposes, various taste and odeur problems can occur. These lower the perceived quality of the treated water, although do not cause human health problems.

Economic impacts

Nearly all of the above mentioned impacts have direct or indirect economic impacts. In some specific cases, local authorities must rely on eutrophic waters for producing drinking water. Infected water increases the costs of water treatment in order to avoid taste, odeur and toxineproblems in the treated water. Due to the toxins produced byharmful algal blooms commercial fish and shellfish may come unsuitable for consumption. Other fish may die due to oxygen limitation. An example of the scale of the potential economic impact arising from the occurrence of harmful algal blooms in estuaries, is the estimated cost to the US economy of US$100 million per year. This estimated cost includes lost fishery production and the related costs of human illness, stock losses, lost tourism and recreational value.