Eutrophication

Eutrophication describes rising nutrient availability and biological productivity in an aquatic ecosystem. Nitrogen and phosphorus are essential for growth, but an excess can upset the balance of a lake, river, estuary, or coastal sea. Nutrients enter naturally through processes such as runoff and the breakdown of organic matter. Over long periods, this can contribute to a lake's natural ecological succession. The term cultural, or human-caused, eutrophication is used when pollution and land use accelerate nutrient enrichment beyond the natural rate. The distinction matters: eutrophication is not always a sign of human pollution, although human activity has intensified it in many waters.

Human-driven enrichment often begins in the surrounding watershed. Fertilizers and animal manure can wash from fields during rain or snowmelt, while nutrients may also reach water in wastewater, urban stormwater, and other runoff. Nitrogen compounds can also be transported through the atmosphere and deposited onto land or water. Which nutrient most constrains algal growth differs among water bodies, so local conditions influence which sources matter most and which reductions will be effective. Nutrients can travel downstream, allowing activities far from a lake or coast to affect its water quality.

When additional nutrients stimulate rapid growth, dense populations of algae or cyanobacteria can form. A bloom may shade submerged vegetation and make water cloudy. When this biomass dies, bacteria decompose it and consume dissolved oxygen. If oxygen falls far enough, the water becomes hypoxic; in severe cases, oxygen levels become too low for aquatic life to survive, creating what is often called a dead zone. Some kinds of algal bloom produce toxins that threaten people and wildlife, but a bloom is not necessarily toxic. The effects can include disrupted food webs, lost habitat, fish kills, and difficulties for fisheries, recreation, or drinking-water treatment.

Scientists and water managers assess eutrophication using indicators such as nutrient concentrations, algal abundance, chlorophyll levels, water clarity, and dissolved oxygen. The European Environment Agency, for example, tracks oxygen depletion and unusual chlorophyll levels in marine waters as indicators of nutrient enrichment. Because nutrient sources and limiting nutrients vary by watershed, management typically focuses on reducing the loads most relevant to a particular water body. Measures include applying fertilizer and manure in appropriate amounts and at suitable times, preventing soil and nutrient runoff, and improving wastewater treatment. Monitoring helps determine whether these actions are reducing blooms and improving oxygen conditions.