Eutrophication

Natural eutrophication is the process by which lakes gradually age and become more productive. It normally takes thousands of years to progress. However, humans, through their various cultural activities, have greatly accelerated this process in thousands of lakes around the globe. Cultural or anthropogenic "eutrophication" is water pollution caused by excessive plant nutrients. During the 1960's, Lake Erie was undergoing rapid cultural eutrophication and was the subject of much concern. The ELA was established in 1

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968 to experimen

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tally investigate this problem. Between June, 1969 and May, 1976, it was virtually the sole focus of whole-ecosystem experimental studies at the ELA.

Humans add excessive amounts of plant nutrients (primarily phosphorus, nitrogen, and carbon) to streams and lakes in various ways. Runoff from agricultural fields, field lots, urban lawns, and golf courses is one source of these nutrients. Untreated, or partially-treated, domestic sewage is another major source. Sewage was a particular source of phosphorus to lakes when detergents contained large amounts of phosphates. The phosphates acted as water softeners to improve the cleaning action, but they also proved to be powerful stimulants to algal growth when they were washed or flushed into lakes.

The excessive growth, or"blooms", of algae prom

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oted by these phosphates changed water quality in Lake Erie and many other lakes. These algal blooms led to oxygen depletion and resultant fish kills. Many native fish species disappeared, to be

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replaced by species more resistant to the new conditions. Beaches and shorelines were fouled by masses of rotting, stinking algae. A means to control this problem became a paramount need.

Using small, natural lakes as experimental systems, scientists at the ELA were able to add various combinations of nutrients and determine which of the major plant nutrients (carbon, nitrogen, phosphorus) was the key to controlling cultural eutrophication in lakes. Over a number of years, seven different ELA lakes (227, 304, 302, 261, 226, 303, 230) were experimentally fertilized in various ways. Two of these lakes (227 and 226) were particularly important in demonstrating that phosphorus was the key nutrient for the control of eutrophication.

By the mid-1970's, North American interest in eutrophica

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tion had waned. However, this "nutrient pollution" problem remains the number one water quality problem worldwide. Eutrophication research at the ELA has continued in Lake 227, albeit on a much reduced scale. After more than three decades, ELA scientists continue to unravel the details of algal responses to nutrients and the food chain effects that accompany them.

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Aerial view of Lake 227 in 1994. Note the bright green colour caused by algae stimulated by the experimental addition of phosphorus for the 26th consecutive year. Lake 305 in the background is unfertilized. Image via Wikipedia Eutrophication is a syndrome of ecosystem responses to human activities that fertilize water bodies with nitrogen (N) and phosphorus (P), often leading to changes in animal and plant populations and degradation of water and habitat quality. Nitrogen and phosphorus are essential components of structural proteins, enzymes, cell membranes, nucleic acids, and molecules that capture and utilize light and chemical energy to support life. The biologically available forms of N and P are present at low concentrations in pristine lakes, rivers, estuaries, and in vast regions of the upper ocean. Pristine aquatic ecosystems function in approximate steady state in which primary production of new plant biomass is sustained by N and P released as byproduc Image via Wikipedia ts of microbial and animal metabolism. This balanced state is disrupted by human activities that artificially enrich water bodies with N and P, resulting in unnaturally high rates of plant production and accumulation of organic matter that can degrade water and habitat quality. These inputs may come from sewage treatment plants or run-off of fertilizer from farm fields or suburban lawns. Algal bloom in Orielton Lagoon, Australia, 1994. (Photo by Geoff Prestedge) Eutrophication was first evident in lakes and rivers as they became choked with excessive growth of rooted plants and floating algal scums, prompting intense study in the 1960's-70's and culminating in the scientific basis for banning phosphate deterge Image via Wikipedia nts (a major source of P, the most frequent culprit in eutrophication of lakes) and upgrading sewage treatment to reduce wastewater N an Image via Wikipedia d P discharges to inland waters. Symptoms of eutrophication in estuaries and other coastal marine ecosystems (where N is the most frequent contributor to eutrophication) were clearly evident by the 1980's, as human activities doubled the transport of N and tripled the transport of P from Earth's land surface to its oceans. Eutrophication has emerged as a key human stressor on the world's coastal ecosystems. Nutrient enrichment of marine waters promotes the growth of algae, either as attached multicellular forms (e.g. sea lettuce) or as suspended microscopic phytoplankton, because algae can grow faster than larger vascular plants. Small increases in algal abundance or biomass have subtle ecological responses that can increase prod Image via Wikipedia uction in food webs sustaining fish and shellfish, even producing higher fish yields. However, over-stimulation of algal growth leads to a complex suite of interconnected biological and chemical responses that can severely degrade water quality and threaten human health and sustainability of living resources in the coastal zone. Fish Kill in the Salton Sea as a result of eutrophication. As algal biomass builds during blooms it forms aggregates that sink and fuel bacterial growth in bottom waters and sediments. Bacterial metabolism consumes oxygen. If the rates of aeration of water by mixing are slower than bacterial metabolism, then bottom waters become hypoxic (low in oxygen) or anoxic (devoid of oxygen), creating conditions stressful or even lethal for marine invertebrates and fish. Seasonal occurrences of dead zones devoid of oxygen and animal life have expanded in the Gulf of Mexico (where the dead zone has approached the size of New Jersey), the Baltic Sea, and Sea of Marmara as a consequence of eutrophication from nutrients delivered by large rivers. Seagrasses are important communities in undisturbed shallow coastal ecosystems, providing essential habitat for many species of m Image via Wikipedia arine animal Image via Wikipedia s. The distribution and abundance of seagrasses have greatly diminished in nutrient-enriched coastal waters, such as Chesapeake Bay and Danish estuaries, where water transparency and light availability to rooted plants have declined as result of phytoplankton growth and fouling of the grass blades by epiphytes and biofilms. These habitat changes propagate through food webs, and the abundance and species diversity of fish and shellfish decrease as seagrasses are eliminated from nutrient-enriched coastal waters. Some phytoplankton species ex Image via Wikipedia crete large quantities of mucilage during blooms that is whipped into foam by wind mixing and washes ashore, making beaches undesirable for holiday visitors. Other phytoplankton species produce toxic chemicals that can impair respiratory, nervous, digestive and reproductive system function, and even cause death of fish, shellfish, seabirds, mammals, and humans. The economic impacts of harmful algal blooms can be severe as tourism is lost and shellfish harvest and fishing are closed across increasingly widespread marine regions. Marine scientists are trying to determine if and how nutrient enrichment selectively promotes the growth of harmful algal species, and if the frequency of harmful algal blooms has increased globally in response to nutrient enrichment. Protection of marine waters from the harmful consequences of nutrient enrichment is a challenge to resource managers because the sources and delivery routes of N and P are diverse. Combustion of fossil fuels produces gaseous nitrogen oxides, and animal production and fertilizer use produce volatile ammonia, two sources of atmospheric N that can be carried by winds and deposited on coastal waters and lakes hundreds of kilometers from their origin. Modern high-yield agriculture and urban gardeners are dependent upon commercial fertilizers that became cheap to produce in the mid 20th century – the era in which N and P concentrations began to increase in surface waters carrying agricultural and urban runoff to the sea. The world's human popu Image via Wikipedia lation is growing disproportionately in the coastal zone, creating an additional challenge of reducing nut Image by Getty Images via Daylife rient inputs from municipal waste, septic systems, and fertilizer runoff from lawns and gardens. Projections indicate that the largest Image by Tasumi1968 via Flickr future increases in N and P delivery to the coastal ocean will occur in eastern and Image via Wikipedia southern Asia where populations and economies are growing most rapidly. The eutrophication problem illustrates how human activities on land can degrade the quality of coastal waters and habitats, with potentially large economic and ecological costs. Solutions to the coastal eutrophication problem require changes in all these activities within the watersheds and airsheds connected to coastal waters. Commitments to these solutions are now beginning – the European Union's Water Framework Directive mandates strategies to reduce N and P delivery to coastal waters, and a 2000 National Research Council report recommended a National Coastal Nutrient Management Strategy for the United States. Proposed solutions to the eutrophication problem are multidimensional and include actions to restore wetlands and riparian buffer zones between farms and surface waters, reduce livestock densities, improve efficiencies of fertilizer applications, treat urban runoff from streets and storm drains, reduce N emissions from vehicles and power plants, and further increase the efficiency of N and P removal from municipal wastewater. As coastal fish and shellfish aquaculture expand, management considerations of this rapidly growing internal source of nutrients will be required as well.

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