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WEEK TWO: HOW DOES WATER SUPPORT LIFE? | ESSAY THREE

How Has Human Activity Affected Earth's Freshwater Ecosystems?

by Eleanor Sterling, Nora Bynum, and Erin Vintinner

While the concept of ecosystem health, like human health, can be difficult to define, healthy ecosystems are those that are able to maintain

structure and function over time in the face of external stress. Earth's water is such a vital biological, physical, and chemical resource that

almost every human activity has the potential to affect its quantity or quality in some way. The major threats typically operate in concert, and

can vary in scale and from place to place. For example, agricultural sediment and pollution are the main threats to freshwater ecosystems in

eastern North America, whereas in western North America, the threats are invasive species, dams, and diversion schemes. Because freshwater

ecosystems are complex and interdependent networks, chemical or physical changes in one area can affect the health of the whole.

Freshwater Ecoregions

Some of our colleagues at the World Wildlife Fund and the Nature Conservancy are working on the cutting edge of evaluating these threats and

their impacts. In their Freshwater Ecoregions of the World (FEOW) project, they have classified a series of freshwater ecoregions that include

the major freshwater habitat types, such as upland rivers, river deltas, large lakes, and floodplain rivers/wetlands. They have mapped many of

the primary threats to these areas so it is possible to view their severity and geographic extent, and build better-targeted conservation strategies.

For example, below we discuss the impact of water withdrawals on freshwater ecosystems. The FEOW project shows us that the areas with

high stress to rivers due to water withdrawals are concentrated in the temperate and tropical regions of North America, Africa, and Asia. Both

species richness (number of different species) and endemism (species found nowhere else in the world) tend to increase toward the equator for

fishes and amphibians (as with many other groups), so these declining water levels in rivers pose a particularly acute threat to global

biodiversity.

What Are the Threats?

Altering Water FlowWe withdraw, transport, and move water to farms and factories, cities and homes. The resulting infrastructure—dams, canals, wells, and

pipelines—fulfills important needs such as generating power, controlling floods and erosion, and facilitating navigation. Yet these feats of

engineering alter a river's flow, often to the detriment of the natural systems that contain much of Earth's freshwater biodiversity. They reduce

or fragment fish habitat like sandbars and slow-flow pools, threaten riverbank vegetation that provides habitat and shade, change water quality,

and disrupt the transportation of organic matter. Downstream, when water is diverted, less flows into deltas and estuaries. This changes salinity

gradients, which in turn affects the vegetation and fish stocks that depend on the outflow of fresh water.

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Channelizing the Kissimmee

In an effort to control flooding from hurricanes, the Kissimmee River in central Florida was transformed from a winding 103-mile river flowinginto Lake Okeechobee (1961) to a 56-mile canal that caused major environmental problems to the region (1983). Today the process is beingreversed by backfilling the canal (2009, left) and restoring the natural state of the river (right). ©Florida Department of EnvironmentalProjection

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Globally, approximately one-fourth of the sediment carried by flowing water now gets trapped behind reservoirs instead of nourishingfloodplains, deltas, and estuaries downstream. And when floodplain communities can no longer gradually absorb sediment loads, the sedimentand associated pollutants are concentrated and carried downstream. In her research, Dr. Melanie Stiassny, curator of the AMNH Department ofIchthyology, has noted that although the era of major dam construction has passed in many parts of the world, a large number of new dams arenonetheless planned for a number of developing countries—despite the fact that their payoffs often fall far short of projections. Dr. Stiassnystudies Africa's lower Congo River, an area of remarkable wild aquatic biodiversity. This part of the river has tremendous hydropowerpotential, but a large dam would completely transform the lower part of the Congo and could threaten the entire ecosystem.

Overharvesting

Overharvesting of plants or animals also threatens ecosystems. Fish are a major source of protein, and around the world, in both fresh and saltwater, they've been harvested faster than they can reproduce. The story of Australia's native Murray cod (Maccullochella peelii peelii),Australia's largest and once-abundant freshwater fish, is typical. Sport fishermen overharvested this slow-growing fish, and dams throughout itsrange further reduced populations, to the point where the Murray cod is critically endangered. Overharvesting top predators can alter foodwebs, changing relationships between fishes, invertebrates, and algae lower on the food chain. Fishing with poisons or explosives takes a toll,killing many species other than the target fish, and can damage aquatic habitats.

Strategic conservation can protect viable populations, as in the case of Vietnam's overharvested freshwater turtles. The Center for Biodiversityand Conservation-affiliated scientist Le Duc Minh, at the Centre for Natural Resources and Environmental Studies in Vietnam, has identifiedseveral viable populations in Cat Tien National Park, and has recommended measures such as establishing additional guard stations around akey wetland to protect the wild turtles from increasing demand.

Deforestation

Freshwater ecosystems are also affected by resource extraction on land. For example, deforestation increases runoff, which carries sedimentand pollutants into nearby bodies of water. Experimental research has demonstrated the importance of vegetation for stream flow in watersheds.A classic example of this is from whole watershed experiments done in the late 1960s at the Hubbard Brook Experimental Forest in NewHampshire. After scientists clear-cut a deciduous forest (and suppressed regenerating vegetation with herbicides for three years), the annualvolume of stream flow increased by 40 percent, totally changing the watershed. The increased stream flow represents, in part, the volume ofevapotranspiration (the transport of water from plants into the atmosphere) previously provided by vegetation.

Human Impacts on Native Wildlife

Wild stocks of Australia's Murray cod have been drastically reduced to less than 10 percent of the population present at the time of Europeansettlement of the continent. Today, this species is being stocked into many water reservoirs, but it will take appropriate management of the riversystems in order to restore the population to its original size and distribution. ©Flickr / Guo Chai Lim

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Pollution

A pollutant is a substance that in certain amounts is known to be harmful to living organisms. As both a powerful physical force and a universalsolvent, water is uniquely suited to carry both undissolved and dissolved pollutants.

Pollution that regularly enters water at an easily identifiable location through a direct route, such a pipe from an industrial site or commercialbusiness, is called point-source pollution. These sources are generally easy to identify and can be regulated. Non-point-source pollution, on theother hand, is less distinctly identifiable, occurs irregularly, and is influenced by land use, climate, typography, and hydrology. Runoff from

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agricultural and urban areas, which can be very difficult to monitor and control, typically falls into this category. So does industrial runoff frommines, factories, and refineries. Chemical and radioactive spills; leaks from waste disposal, buried tanks, and pipes; and seepage fromagricultural activities, industrial sites, septic systems, mines and mine waste piles produce intermittent non-point-source pollution that canpollute both surface and groundwater.

Main Sources of Water Pollution:

Pollutant Sources

Organic matter Sewage, agricultural waste, garbage

Pathogens or disease-causingagents Human and animal wastes

Organic chemicals Agricultural use of fertilizers, pesticides and herbicides, industrial processes

Nutrients Agricultural and urban land use practices releasing phosphorus and nitrogen, wastewater from sewagetreatment

Inorganic chemical: heavy metals Agricultural, urban, industrial processes

Inorganic chemical: acids Coal and metal mines, industrial processes, improper disposal

Sediment Runoff from agriculture and construction sites, natural erosion

Heat (thermal pollution) Power plants and industrial facilities

Radioactivity Nuclear power industry, military, natural sources

Pollution is particularly problematic in slower-moving and still water ecosystems, which are natural pollutant reservoirs. Many pollutants—nutrients, suspended solids, and toxic chemicals—ultimately end up in oceans.

Water pollution weakens or kills organisms and disrupts food chains. Although the U.S. has made progress in regulating and reducing point-source pollution, non-point-source pollution is harder to address. While similar laws are on the books in countries around the world, thechallenges are multiple and severe. An estimated 80 percent of wastewater in developing countries is discharged directly to rivers and streamswithout any processing.

The Link Between Pollution and EutrophicationThe level of nutrients in standing-water systems changes over time—a natural process called eutrophication. When first formed, most lakes arenutrient-poor, or oligotrophic, and can support only small numbers of fish, plants, algae, plankton, and bacteria. These aquatic organismsconsume oxygen through respiration. As nutrients accumulate over time, lakes become eutrophic: higher nutrient levels support more plant andalgae life, which consume more oxygen, so oxygen levels drop as the lake becomes highly productive. Agricultural runoff or urban sewage canaccelerate this process, causing human-induced eutrophication. Fed by nutrient influx, algae, plants, and bacteria grow faster, especially at thewater's surface. This reduces photosynthesis because less light penetrates the water, at which point the plants and algae die off. Bacteriadecomposing this matter consume oxygen, which further reduces the amount available for fish and shellfish, resulting in another die-off.Eutrophication can also cause toxic algal blooms that can harm aquatic species.

Human-induced eutrophication can also occur in coastal areas and coral reefs. In these marine systems, point- and non-point-source pollutioncan cause nutrient overenrichment, triggering the same pattern of algal and bacterial overgrowth, a drop in oxygen levels, and die-offs.

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Eutrophication

Human-induced eutrophication results from an accelerated influx of agricultural runoff or urban sewage. Fed by this nutrient influx, algae,plants, and bacteria grow faster. Notice the absence of life in the coral covered with macroalgae compared to the healthy coral. ©NASA /GSFC

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Invasive SpeciesFree of the controls that were present in their native habitat, such as predators and competition, some plants and animals spread withoutrestraint, especially through interconnected freshwater ecosystems, becoming invasive species. Invasive species have emerged as a primarycause of biodiversity loss worldwide, with major economic and environmental consequences.

Humans have dispersed thousands of freshwater, estuarine, and marine species across the globe in myriad ways: in the ballast or on the hulls ofships, when fishermen stock lakes with sport fish or use non-native bait that escapes, when new channels connect bodies of water, and when petowners empty aquariums into waterways, to name a few.

Not all introduced species succeed in their new environment. But once established, they can eliminate or hybridize with native species, causingcommunities to lose genetic diversity. Worldwide, two-thirds of the freshwater species introduced into the tropics and more than half of thoseintroduced to temperate regions have established self-sustaining populations. Many have become invasive, competing directly with nativespecies for food or space, or indirectly by altering the food web or physical environment. For example, the water hyacinth plant (Eichorniacrassipes), which is native to South America and forms vast free-floating impenetrable mats, is now widespread in all of the places CBCscientists work, and across the tropics and subtropics in general. Growing at an explosive rate, the water hyacinth rapidly clogs waterways,blocks sunlight, and alters oxygen levels, causing submerged vegetation and fish to die off. Billions of dollars a year are spent on mechanicaland chemical efforts to combat water hyacinth invasions. Since water hyacinth growth is exacerbated by nutrient influx, reducingeutrophication is key.

Invasive species have emerged as a primary cause of freshwater biodiversity loss worldwide, with major economic and environmentalconsequences. Mechanical, chemical, or biological attempts to control their spread are expensive and usually ineffective. The key is to preventinvasions from happening through education campaigns and by targeting the pathways of invasion, such as ballast water.

The Toll on Freshwater SpeciesOver one-third of the world's known freshwater-dependent species are threatened. Animals under threat include:

Water birdsDecades of information on population status and trends are available for many species. Twenty-one percent of those that are known to bepredominantly wetland-dependent are extinct or globally threatened. Studied by the CBC's Dr. Felicity Arengo, the Andean flamingo(Phoenicoparrus andinu) and the JamesÕ flamingo (Phoenicopterus jamesi) inhabit shallow wetlands in the Andean regions ofArgentina, Bolivia, Chile, and Peru. Increased agriculture, mining, and industry, along with unregulated tourism, have caused severepopulation declines in the last few decades.Freshwater fishOver the last few decades, at least 2,000 species (20 percent of the world's described freshwater fish) have been listed as extinct, ordesignated as endangered or threatened. For example, drastically declining sturgeon populations in Eurasia and North America resultfrom a combination of fishing pressure (several species are prized for their roe, which is made into caviar), loss of migratory routes, andhabitat degradation.

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AmphibiansNearly one-third of the world's amphibian species are threatened with extinction. Highly permeable skins make them particularly

vulnerable to air and water degradation.

MammalsMany aquatic or semi-aquatic species, including freshwater otters, seals, manatees, river dolphins, and porpoises, are among the most

threatened mammals in the world. Among them is Russia's endemic Lake Baikal seal (Phoca sibrica), which is harmed by pesticides

from the agricultural runoff and industrial wastes in the lake.

Freshwater Habitats are Under Pressure

Around the world, river channels have been fragmented, watersheds and deltas degraded, and floodplains (which connect wetlands and riparian

ecosystems) have been lost. By 1985, humans had drained vast areas of inland and coastal marshes present in 1900 for intensive agriculture:

approximately 56 to 65 percent in Europe and North America, 27 percent in Asia, 6 percent in South America, and 2 percent in Africa. Since

freshwater ecosystems support more species per unit habitat than terrestrial or marine ecosystems, pressures affect a vast web of species, from

waterfowl resting on their long migrations to tiny invertebrates that form the base of food chains.

Related Links

EPA: Non-Point-Source Pollution »

Resources for students and educators regarding non-point-source pollution from the Environmental Protection Agency.

Global Water for Sustainability Program »

Learn about projects worldwide that are working to provide sustainable clean water to people and ecosystems.

Endangered Species »

Review the current listing of the threatened and endangered species, and discover the threats to and conservation actions for wildlife around the

world.