Research at Bear Brook and similar locations in the U.S. was sparked by an EPA effort in the mid- to late-1980s to determine the ecological effects of acid precipitation. For Norton and Fernandez, the focus at the time was on determining how nitrogen, base cations and, especially, sulfur would affect their watersheds at Bear Brook. But UMaine researchers had unexpected results over the long term, including some that deviated substantially from model projections.
For example, nitrogen levels did not behave as expected. For two or three years after Bear Brook had been identified, nitrogen was increasing in the watershed runoff. Researchers assumed levels would continue that way, but in the fourth year, nitrogen in untreated East Bear Brook decreased to virtually zero and has remained that way ever since.
“It was a climate signal that was altering the watershed we haven’t been tinkering with, and it’s been seen now all over the Northeast,” says Fernandez, who earned his Ph.D. at UMaine and returned to the faculty after working in industry, and has been involved in Bear Brook from the beginning. “At the same time, the nitrogen in the watershed we treated has gone up, but not as much as we thought it would.”
Most of the nitrogen being added is still accumulating in the soils.
The combination of acidification and nitrogen in the treated watershed has changed the way phosphorus cycles through the watershed. Fernandez says the prediction was that, as more nitrogen was added to West Bear, phosphorus would become a limiting nutrient. However, after 10 years, researchers found some evidence that the treated watershed cycled phosphorus faster than expected. But while phosphorus was more available, this effect varied in the different forest types in the watershed.
Those changes also resulted in an increased loss of phosphorus from the ecosystem through time. An increase in the rate of phosphorus cycling means trees — which need a certain amount of phosphorus in order to store energy from photosynthesis and other functions — are taking in more phosphorus, which ends up in the leaves.
When the leaves fall, they enrich the upper levels of the soil with phosphorus. But it also leaches into streams at a rate that can sometimes be 10 times as fast as normal, compared to other watersheds in the region.
Research is now focused on understanding if this is a temporary shift in phosphorus cycling and if the ecosystem ends up in a new nutrient status because evidence suggests some of these processes are transient. If so, then only through this type of long-term project would researchers see these changes — an argument for continued research to determine how these processes cycle through time on a decadal scale.