Beaver ponds with deeper sediments store more nitrogen, simple mapping reveals

Newswise — WASHINGTON —

Equine lakes enclose phosphorus, a vital element that may turn into a contaminant upon excessive accumulation. Administrators of land necessitate determining whether equine lakes are retaining or emitting phosphorus, however, chemical analysis can be costly. Recent research demonstrates how uncomplicated measurement of an equine lake's profundity and sediment can apprise administrators if it's a phosphorus origin or repository.

The research was printed in the Journal of Geophysical Research: Biogeosciences, a scholarly publication that disseminates scientific investigations concerning the interplay among biological, geological, and chemical mechanisms within the Earth's ecosystems.

Nitrogen, which is present in synthetic fertilizers and naturally occurring in manure, has been introduced to rivers in the western United States due to grazing and agriculture activities, resulting in downstream eutrophication. Past research indicates that downstream nitrogen levels can either increase or decrease in the presence of beaver ponds. However, only a limited number of studies have scrutinized the fate of nitrogen within a beaver pond, leaving unresolved queries about whether beaver ponds have a beneficial or detrimental effect on nitrogen pollution and whether reintroducing beavers to the ecosystem is advisable.

Desneiges Murray, a biogeochemist who spearheaded the study while affiliated with Utah State University and now based at the University of New Hampshire, asserted that "It depends on the river, but for sites like the one we studied, reintroducing beavers could be a wise decision." She further added, "These ecosystems evolved with beavers in the first place. So, the combined effects of less erosion, better resistance to forest fires, more water storage during droughts, and now the benefit of long-term nitrogen storage — there are lots of reasons humans should be facilitating beaver recolonization into their natural habitats."

Between the 1600s and 1800s, approximately 25 to 160 million beavers were relentlessly hunted and nearly eradicated in the contiguous United States. However, their population gradually recovered to around 6 to 12 million beavers by the late 1980s. Presently, initiatives to reintroduce beavers are underway throughout the United States, with dissimilar levels of achievement and acceptance.

In the recent study, the researchers mapped out distinct areas within a beaver pond system, known as a complex, situated in the Bear River Mountains to the north of Salt Lake City. The team led by Murray defined five zones within the system based on factors such as water movement, pond depth, and sediment thickness and grain size. They collected water samples from various zones and measured their nitrogen and oxygen content. Furthermore, they obtained sediment samples from the pond and conducted an in-depth analysis of nutrient transformations over time using long sediment cores. Finally, they created a comprehensive map of the complex, with particular attention given to the sediment depth to water depth ratio.

Emily Fairfax, a hydrologist and self-proclaimed "beaver-ologist" at California State University-Channel Islands, who was not part of the study, commented that "This new method of examining geomorphic units in a beaver complex will prove useful in comprehending how beavers mitigate nitrogen, heavy metals, or acid mine drainage."

The majority of nitrogen enters beaver ponds as dissolved and particulate nitrogen. When it reaches the pond's sediments, nitrogen undergoes chemical changes and can transform into various forms, including ammonium, inert nitrogen gas, or reactive nitrogen dioxide gas. Reactive nitrogen dioxide gas can cause ozone depletion in the atmosphere.

The research team discovered that the beaver pond system was capable of storing up to 15% of the incoming nitrogen, primarily in the sediments of the backwater zone. The backwater zone contains thicker, organic-rich sediments and lower oxygen levels, creating an environment that promotes the conversion of nitrogen to inert nitrogen gas, which can then be stored. Other zones with less sediment or higher oxygen levels were not as efficient at nitrogen storage. This finding is the first of its kind, demonstrating a correlation between a beaver pond's small-scale geomorphology and nitrogen storage, and it provides new avenues for assessing nutrient levels in beaver ponds.

Murray noted that mapping the zones of beaver ponds is a valuable tool in itself. Land managers could bypass chemical analyses if they only need an approximation of nitrogen cycling. Ponds with thicker, organic-rich sediments and shallow, low-oxygen water are more inclined to store nitrogen. Conversely, faster-moving ponds with more oxygen and less sediment are more likely to release nitrogen.

"Performing the experiments was arduous, costly, and time-consuming," Murray explained. "As a more affordable and straightforward option, we suggest using mapping. The strong correlation we discovered between a beaver pond's shape and its chemical composition is highly persuasive, and these are fundamental assessments that anyone with a scientific background could execute."

The new research is a case study of a specific site, but Fairfax thinks the mapping approach will be useful wherever beavers are found. 

Fairfax emphasized the significance of localized investigations, stating that "by identifying the key areas responsible for denitrification and nutrient processing, there is immense potential here, especially considering that beavers construct unique dams."

"Decision makers require access to localized data for policies to evolve," Fairfax stated. "As there are few studies on the impact of beavers on nutrient loads in the western region, this case study holds immense value as an addition to the literature."

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