Possible consequences of eutrophication

Introduction

Enhanced plant production and improved fish yields are sometimes described as positive impacts of eutrophication, especially in countries where fish and other aquatic organisms are a significant source of food. However detrimental ecological impacts can in turn have other negative consequences and impacts which are described below. Essentially the entire aquatic ecosystem changes with eutrophication.

Ecological impacts

Increased biomass of phytoplankton resulting in algal blooms

MODIS satellite image of a phytoplankton bloom in the Barents Sea (Photo credit: NASA/GSFC/Jeff Schmaltz/MODIS Land Rapid Response Team)

Phytoplankton or microalgae are photosynthesizing microscopic organisms. They contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean where sunlight penetrates the water. In a balanced ecosystem they provide food for a wide range of organisms such as whales, shrimp, snails and jellyfish. Among the more important groups are the diatoms, cyanobacteria, dinoflagellates and coccolithophores. Phytoplankton species also require inorganic nutrients such as nitrates, phosphates, and sulfur which they convert into proteins, fats and carbohydrates. When too many of these nutrients (by natural or anthropogenic cause) are available in the water phytoplankton may grow and multiply very fast forming algal blooms. Algal blooms may occur in freshwater as well as marine environments. Only one or a small number of phytoplankton species are involved and some blooms discolor (green, yellow-brown or red) the water due to their high density of pigmented cells. Blooms in the ocean may cover a large area and are easily visible in satellite images.

Toxic or inedible phytoplankton species (harmful algal blooms)

Harmful algal blooms are bloom events involving toxic or harmful phytoplankton. These cause harm through the production of toxins or by their accumulated biomass, which can effect co-occurring organisms and alter food web dynamics. Impacts include:

• Human illness
• Mortality of fish, birds and mammals following consumption or indirect exposure to HAB toxins
• Substantially economic losses to coastal communities and commercial fisheries

Increased in blooms of gelatinous zooplankton

Phytoplankton are the food source for numerous other organisms, especially the zooplankton. Zooplankton are heterotrophic plankton. They are primarily transported by ambient water currents but many have locomotion. Through their consumption and processing of phytoplankton and other food sources they play a role in aquatic food webs as a resource for higher trophic levels including fish. Zooplankton can be divided in two important groups: crustacean (copepods and krill) and gelatinous zooplankton. Gelatinous zooplankton have relatively fragile, plastic gelatinous bodies that contain at least 95% water and which lack rigid skeletal parts. The most well-known are the jellyfish. Eutrophication is believed to cause an increase in the relative importance of gelatinous versus crustacean zooplankton. On many areas of the world where the natural species diversity has been affected by pollution, over-fishing and climate change gelatinous zooplankton organisms may be becoming the dominant species.

Human health impacts

Harmful algal bloom species 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 as well as animal health. Examples include paralytic, neurotoxic and diarrhoeic shellfish poisoning. Several algal species able of producing toxins harmful to human or marine life have been identified in European coastal waters. The table gives an overview of some species that are regularly observed and represent a risk for seafood consumers.

Disease	Symptoms	Species	Carriers
Amnesic shellfish poisoning (ASP)	Mental confusion and memory loss, disorientation and sometimes coma	Diatoms of 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 and hemorrhagic, hepatic symptoms and in extreme causes delirium and hepatic coma	Genus Prorocentrum	Oysters and clams
Diarrhoeic shellfish poisoning (DSP)	Gastrointestinal symptoms (diarrhoea, vomiting and abdominal pain)	Genus Dinophysis and Prorocentrum	Filtering shellfish (oysters, mussels and clams)
Paralytic shellfish poisoning (PSP)	Muscular paralysis, difficulty in breathing, shock and in extreme causes death by respiratory arrest	Genus Alexandrium and Gymnodinium	Oysters, mussels, crustacean and fish

Other marine mammals can be vectors for toxins, as in the case of ciguatera, where it is typically predator fish whose flesh is contaminated with the toxins originally produced by dinoflagellates and then poison humans. Symptoms include gastrointestinal and neurological effects.

Socio-economic impacts

Nearly all of the above described impacts have a direct or indirect socio-economic impact.

Aesthetic impacts

Algal blooms are unsightly and can have unpleasant odors. For example the appearance of a white yellowish foam on the beach in spring on the shores along the North Sea. The foam is formed by the wind that sweeps up the decaying remains of Phaeocystis algal colonies.

Beaches can be closed if high levels of bacteria are found in the water (Photo credit: Elizabeth Halliday, Woods Hole Oceanographic Institution)

Impact on recreation and tourism

The enrichment of nutrients to an ecosystem can result in a massive growth of algae. The existence of such areas of increased vegetation can inhibit or prevent access to waterways. This decreases the fitness for use of the water for water sports (swimming, boating and fishing).

Economical impacts

In some specific cases local authorities must rely on eutrophic waters for production of drinking water. Infected waters increases the costs of water treatment in order to avoid taste, odor and toxin problems in the treated water. Due to the toxins produced by harmful algal blooms commercial fish and shellfish may become unsuitable for consumption (the water becomes unsuitable for drinking even after treatment) resulting in potential economical and financial problems for the fishing industries. In extreme cases beaches are closed due to the presence of toxic algal blooms.

References

1. Eutrophication and health. European Commission (2002). Office for Official Publications of the European Communities: Luxembourg. ISBN 92-894-4413-4.28 pp.
2. The National Eutrophication monitoring Programme Implementation Manual (Murray et al., 2002).