Northeastern mountain ponds as sentinels of change: current and emerging research and monitoring in the context of shifting atmospheric deposition and climate interactions

Sarah J. Nelson1, Rachel A. Hovel2, Julia Daly3, Amanda Gavin4, Stephanie Dykema5 and William H. McDowell6

Mountain ponds in the northeastern US have undergone acidification and subsequent recovery due to changes in atmospheric deposition, and now additionally reflect physical and biological responses to climate change. These ponds are distinct from other lakes and ponds in the region as they are higher in elevation, often forested, have a legacy of significant acidification followed by recovery, and have little direct impact from human land use in their watersheds. They are relatively small and could be vulnerable to the increases in extreme weather events, overall warming, and shortening of winter that are occurring throughout the northeastern US. We compiled data for mountain ponds >560 meters elevation and <60 ha surface area across the northeastern US (Northern Appalachians and Adirondack Mountains) spanning 1978-2019. The full dataset includes 257 ponds, most sampled intermittently, and 36% of sites sampled at sufficient frequency to conduct trend analyses. We found strong and widespread significant declining trends in sulfate (95% of sites, ~45% decline) and increasing ANC at 53% of sites. Declining conductivity at 97% of sites (32% decline) suggested dilution approaching hypothetical baseline concentrations (13.9 µS/cm by the 2010s), and also reflected low chloride concentrations at these sites compared to others in the region experiencing road salt contamination. DOC increased in 54% of ponds. Increasing DOC has been attributed to both changing sulfate and climate, specifically related to increases in precipitation and air temperature. Although these lakes are typically colder and experience more persistent winter snowpack than lower elevation lakes, their thermal structure is sensitive to warming air temperatures, which impacts water temperature, stratification, and ice duration. Zooplankton communities of eastern mountain lakes may be slowly shifting towards pre-acidification community composition; however, densities and community composition are sensitive to a suite of water chemistry conditions, and the trajectories of primary and secondary production are confounded by increasing DOC and the multiple impacts of climate change. Based on this synthesis and case studies documenting insights from integrating chemical, physical, and biological research, we developed a conceptual model and framework for research and monitoring to support broader baseline data collection and measurements of change in these ecosystems. The records in our assembled dataset indicate the importance of eastern mountain ponds for understanding the combined impacts of atmospheric deposition and climate change in lake ecosystems, and highlight current gaps and areas for future research directions.

 

1Appalachian Mountain Club, snelson@outdoors.org
2University of Maine-Farmington, Farmington, ME, rachel.hovel@maine.edu
3University of Maine-Farmington, Farmington, ME, dalyj@maine.edu
4FB Environmental, Portland, ME, amandagavin88@gmail.com
5University of Maine, Orono, ME; INSTAAR, University of Colorado, Boulder, CO, stephanie.dykema@maine.edu
6University of New Hampshire, Durham, NH, Bill.McDowell@unh.edu