Water pollution acts as a powerful agent of change in stream ecosystems, systematically reducing the variety of life forms that can survive and thrive. When contaminants enter a stream, they trigger a series of physical, chemical, and biological impacts that fundamentally alter the habitat. This alteration favors a limited number of pollution-tolerant species, and the resulting reduction in biological complexity is a clear indicator of ecosystem degradation. Diversity declines through habitat destruction, direct poisoning of sensitive organisms, and the simplification of the entire food web.
What Biodiversity Means in a Stream Ecosystem
Stream biodiversity represents the range of different species present and the relative abundance of individuals among those species. This is typically measured using two main components: species richness and species evenness. Species richness is the total count of unique species found in a particular stretch of the stream.
Species evenness refers to how equally distributed the individual organisms are among the different species. Ecologists often monitor groups like fish, periphyton (algae), and especially aquatic macroinvertebrates—insects, snails, and worms—to gauge the health of the ecosystem.
Certain groups of macroinvertebrates, such as the insect orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies), collectively known as EPT, are highly sensitive indicators of water quality. The presence of a high number of EPT taxa signifies a healthy, unpolluted stream. When pollution enters the system, a decline in these sensitive species is one of the first observed changes.
Habitat Alteration: Physical and Chemical Stressors
Pollution often changes the physical and chemical structure of the stream habitat, making it unsuitable for diverse life. Runoff from construction or agricultural areas introduces fine sediment particles that physically alter the stream bottom. This sedimentation smothers the rocky substrate, filling the interstitial spaces where many macroinvertebrates live and where fish lay their eggs.
The suspended sediment also increases the water’s turbidity, reducing light penetration into the water column. Less light inhibits photosynthesis by periphyton and aquatic plants, which are the base of the stream food web. Sediment can also directly impair aquatic life by clogging the gills of fish and invertebrates, reducing their ability to breathe.
Another major habitat stressor is nutrient loading, a process called eutrophication, caused by excess nitrogen and phosphorus from sources like fertilizers and sewage. These nutrients trigger rapid growth of algae, known as an algal bloom. When the dense algae eventually die, their decomposition consumes large amounts of dissolved oxygen (DO). This consumption leads to hypoxic conditions, killing or severely stressing organisms that require high oxygen levels, such as many species of fish and sensitive insects.
Selective Pressure: Direct Toxicity and Sensitive Species Loss
Beyond altering the habitat, specific chemical pollutants act as direct poisons, exerting a selective pressure that removes the most vulnerable species. Heavy metals like copper, zinc, and lead enter streams from industrial discharge and mining runoff. These metals can bioaccumulate in the tissues of invertebrates and fish, interfering with physiological processes like respiration and enzyme function.
Even low concentrations of these contaminants can be acutely toxic to sensitive organisms, causing death, or leading to chronic effects like reduced growth and reproduction. Macroinvertebrates, particularly mayflies, are often more sensitive to metal contamination than many fish species.
Pesticides and herbicides, primarily from agricultural runoff, introduce another layer of toxicity. These chemicals are designed to be lethal to invertebrates or plants, and their presence can eliminate specialist species that lack the physiological defenses to cope. This chemical pressure acts as a filter, leaving behind only pollution-tolerant generalist species, which significantly reduces the total species richness.
Thermal pollution, such as warm or cold water released from industrial processes or dam bottoms, also stresses certain organisms. Warm-water discharges reduce the amount of oxygen the water can hold and can raise the temperature above the optimal range for cold-water species like trout, disrupting their spawning cycles and causing local extinctions.
Ecosystem Simplification: Disruption of the Food Web
The combined effects of habitat alteration and direct toxicity ultimately lead to a loss of complexity and stability within the stream ecosystem. As sensitive species are removed, the functional groups they represent are also lost. Functional groups are sets of species that perform similar ecological roles, such as shredders that break down leaves or filter feeders that clean the water.
The elimination of specific grazers or predators destabilizes the entire food web structure. For example, the loss of filter-feeding invertebrates due to sedimentation can reduce the stream’s natural ability to process organic matter. The community then shifts toward a state dominated by a few pollution-tolerant generalists, such as certain aquatic worms or midges (Chironomidae).
These generalist species thrive because their competitors and predators have been removed by pollution, leading to an increase in their abundance. This results in low species evenness, where a few species are highly abundant, despite the overall number of species being low. The resulting simplified ecosystem is less resilient to further disturbances and has diminished ecological function.