Over the past hundred years, synthetic fertilizer use on crops and lawns has increased exponentially, flooding coastal and inland waterways with unprecedented levels of nitrogen and phosphorus. Generally speaking, these fertilizers keep our grass green and harvests bountiful, but an excess of these nutrients fosters the growth of dense algae blooms, which consume large amounts of oxygen. This phenomenon, called eutrophication, harms—and often exterminates—other plants and animals in the ecosystem.
Still-water (or lentic) ecosystems, such as lakes and wetlands, play an important role in regulating the flow of nutrients into downstream waters. For example, in U.S. freshwater systems, wetlands are responsible for 64% of nitrogen retention, whereas rivers and streams retain just 8%.
Although this process is well documented, less is known about how nutrient retention varies between differently sized lentic ecosystems. This is an important sticking point in the United States, where the Supreme Court has ruled that small, remote wetlands are protected by the Clean Water Act only if they can be proven to be a significant link to nearby waterways.
In a recent study Cheng and Basu analyzed data from 600 lentic ecosystems across the globe to better understand these variations.
The researchers compiled a database from previously published studies, using keywords such as “nutrients,” “nitrogen,” “nitrate,” “phosphorus,” and “phosphate,” as well as “wetland,” “lake,” “pond,” and “reservoir,” to narrow their search. The sites, scattered across the globe but primarily concentrated in North America and Europe, ranged from small wetlands (less than roughly 40 square meters) to lake systems on the magnitude of roughly 83,000 square kilometers.
Comparing all 600 sites, the researchers found that the benefit of small wetlands is disproportionately large, as half of all nitrogen removal occurs in wetlands smaller than 315 square meters. Similarly, they found that when smaller wetlands are removed versus comparably sized areas of larger wetlands, the potential losses (in terms of nutrient removal potential) are far greater.
This study illustrates and brings much-needed attention to the important role that small bodies of water play in processing nutrients across the landscape. (Water Resources Research, https://doi.org/10.1002/2016WR020102, 2017)
—Sarah Witman, Freelance Writer