Possible consequences of eutrophication

Introduction

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Eutrophication is widely seen as a negative trend in lakes and the sea,but on the land it can be even beneficial. Increases in the productivity of plants are more welcomed, particularly where crops and commercially managed forests are concerned. Terrestrial ecosystems are also normally spared from the more harmful side effects of eutrophication. The ecological impacts of eutrophication are summarized in the figure and discussed in the sections below.

Ecological impacts

Increased biomass of phytoplankton resulting in algal blooms

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Algal bloom on the shore of Langstone harbour in Hampshire © Solent News and Photo Agency

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) | Shellfish (mussels) | Canada (1987): 153 cases, 3 deaths |- | Neurotoxic shellfish poisoning (PSP) | Muscular paralysis, state of shock and sometimes death | Genus Gymnodinium | Oysters, clams and crustaceans | Florida (1977?) |- | Venerupin shellfish poisoning (VSP) | Gastrointestinal, nervous, haemorrhagic, hepatic and in extreme cases delirium and hepatic coma | Genus Prorocentrum | Oysters and clams | Japan (1889): 81 cases, 51 deaths, Japan (1941): 6 cases, 5 deaths, Norway (1979): 70 cases |- | Diarrhoeic shellfish poisoning (DSP) | Gastrointestinal (diarrhoea, vomiting and abdominal pain) | Genus Dinophysis and Prorocentrum | Filtering shellfish (oysters, mussels, cockles and clams) | Japan (1976-1982): 1300 cases, France (1984-1986): 4000 cases, Scandinavia (1984): 300-400 cases |- | 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 | Philippine (1983): 300 cases, 21 deaths, United Kingdom (1968): 78 cases, Spain (1976): 63 cases, France (1976): 33 cases, Italy (1976): 38 cases, Swiss (1976): 23 cases, Germany (1976): 19 cases |}

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.

Links

European Commission, Eutrophication and health (2002)(PDF)[1]

Republic of South-Africa, Department Water Affairs,South African National Water Quality Monitoring Programmes Series, National Eutrophication Monitoring Programme - Implementation Manual-Final Draft,2.Eutrophication (PDF) [2]