High nutrient levels in runoff waters can cause significant ecosystem damage through eutrophication. Eutrophication is the process by which excessive amounts of nutrients in rivers and lakes lead to massive growth of algae. As these algae grow, die, and decay, oxygen is removed from the water. When oxygen levels fall below certain thresholds, fish and other organisms begin to die.
As a result of the potential disastrous effects of nutrient pollution, nutrients must be carefully controlled in nursery runoff waters. Wetlands are a perfect way to accomplish this task of nutrient removal.
Two main nutrients are of greatest concern with relation to eutrophication. These are nitrogen and phosphorous. We will discuss each of them in the following paragraphs.
Adequate nitrogen is critical for plant growth. As a result, nitrogen containing fertilizers are constantly applied to nursery production areas.
Fortunately, wetlands are quite capable of removing nitrogen from runoff water. These biological water processing plants are home to many processes that consume nitrogen. First of all, we must understand that nitrogen is present in many forms in wetlands. These compounds all have special characteristics. Some are volatile and can escape into the atmosphere from the surface of the water. Some are incorporated into plant or bacterial organisms as part of their structures. Some forms precipitate or fall out of solution and are mixed with the sediments of the wetland.
As plant and bacterial organisms die, nitrogen may be released back into the environment, but some of their organic material will be compressed beneath sediments. This burial of nitrogen makes it unavailable and prevents release.
The specifics of nitrogen removal by constructed wetlands are not entirely understood by science. Many factors affect the quantity of nitrogen removed. Despite some uncertainties regarding nitrogen removal mechanisms and the effects of environmental factors, wetlands have been validated time and time again by scientific studies showing efficient nitrogen removal.
A second critical nutrient for removal is phosphorous. This nutrient can also be a cause of eutrophication if concentrations become elevated in lakes or streams. Wetlands are also capable of removing phosphorous from runoff waters.
Phosphorous in wetlands is subjected to a complicated cycle that results in its absorption, use, storage, release, permanent storage, and reuse over time. Through this cycle, phosphorous may be used many different times in the wetland before finding a more permanent storage site. Like nitrogen, phosphorous can be present in many forms, and all forms are acted upon differently in the wetland. Significant amounts of phosphorous can be permanently stored in buried plant material at the bottom of the wetland and attached to soil particles.
Like nitrogen, all of the processes with regard to phosphorous removal by wetlands are not entirely understood by science. The complex biological system is, however, capable of significant phosphorous removal. Phosphorous removal requires large areas of wetlands to provide adequate time and opportunity for various processes to act on the phosphorous in solution.
While the major nutrient removal concerns center around nitrogen and phosphorous, many other nutrients are also impacted by constructed wetlands. The following lists of nutrients provide some idea of the nutrients that can be removed using a constructed wetland.
Not typically removed by constructed wetlands:
Typically removed by constructed wetlands: