Nitrogen Budget of a subartic stream altered by beaver

by Robert J. Naiman and Jerry M. Melillo

Oecologia 1984 62:150-155

Naiman and Melillo introduce the beaver as the species that once numbered "about 60 million individuals ranging over 15.5 million square kilometers. The density was about 4 beaver per square kilometer, which is in accord with densities in remote areas today." Now that beaver numbers are increasing, they "are again becoming a significant component of aquatic ecosystems." Little work has been done quantifying that effect, and so these authors tried to "quantify the influence of beaver activities on the nitrogen dynamics of a small, nutrient poor, subartic stream and to examine this influence in reference to current perspectives in running water ecology."

They chose to study a stream near Sept Isles in Eastern Quebec, a region that's Precambian shield granite and the stream water is low in nutrients and somewhat acidic. They began by mapping the entire stream and measuring the nitrogen content in everything they could. Then they compared this "nitrogen budget" of the stream riffle and beaver ponds. For the pond they assumed an average pond typical of the region: the beavers changed 100 meters of stream about 1 meter wide and 15 centimeters deep into a pond with an average width of 7 meters and average depth of 150 centimeters.

They estimated that "the water column in the beaver pond contains 37 times more nitrogen than the riffle and the wood [in the pond] sequesters at least six times more nitrogen." As for the sediment "the beaver pond stores approximately 1000 times more nitrogen in sediments, per linear meter of stream channel, than does the riffle."

Naiman and Melillo were also careful to measure other sources of nitrogen, including that from falling rain and leaves. Needless to say, those sources accounted for little change in the nitrogen budget. In beaver ponds 66.2% of nitrogen fixation is "associated with accumulated sediment." This accumulation is also stable, "the turnover time for nitrogen is a slow 72.4 years."

In the estuaries at the end of watersheds serving populated areas, an excess of nitrogen from fertilizer run-off is a problem often overwhelming an ecosystem. However, the storage of so much nitrogen at the beginning of watersheds is quite another matter. Naiman and Melillo note "the fact that large amounts of sediment and nitrogen were sequestered high up in the watershed, rather than being eroded downstream, suggests that watercourses throughout North America were significantly altered by the removal of beaver long before extensive research began." In the remote area where this research was done in the 1980s researchers estimated that 30% of the streams were impacted by beavers. Historically the storage of nitrogen and subsequent enrichment of stream water with nitrogen also effected the ecology of downstream areas, especially those that are nitrogen poor. So by its wood harvests, the beaver is fertilizing areas far beyond its pond.

Finally the authors come up with a nice analogy. The ponds created by beavers in a watershed are not unlike "the small 'disturbance' zones analogous to those in marine intertidal areas." Just as those intertidal areas are places we find enhanced biodiversity, so it is with a beaver pond.


by Bob Arnebeck

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