Responses of 20 lake-watersheds in the Adirondack region of New York to historical and potential future acidic deposition

Nitrogen and sulfur deposition reductions projected to partially restore forest soil conditions in the US Northeast, while understory composition continues to shift with future climate change

Human activities have dramatically increased nitrogen (N) and sulfur (S) deposition, altering forest ecosystem function and structure. Anticipating how changes in deposition and climate impact forests can inform decisions regarding these environmental stressors. Here, we used a dynamic soil-vegetation model (ForSAFE-Veg) to simulate responses to future scenarios of atmospheric deposition and climate change across 23 Northeastern hardwood stands. Specifically, we simulated soil percent base saturation, acid neutralizing capacity (ANC), nitrate (NO3 -) leaching, and understory composition under 13 interacting deposition and climate change scenarios to the year 2100, including anticipated deposition reductions under the Clean Air Act (CAA) and Intergovernmental Panel on Climate Change-projected climate futures. Overall, deposition affected soil responses more than climate did. Soils recovered to historic conditions only when future deposition returned to pre-industrial levels, although anticipated CAA deposition reductions led to a partial recovery of percent base saturation (60 to 72%) and ANC (65 to 71%) compared to historic values. CAA reductions also limited NO3 - leaching to 30 to 66% above historic levels, while current levels of deposition resulted in NO3 - leaching 150 to 207% above historic values. In contrast to soils, understory vegetation was affected strongly by both deposition and climate. Vegetation shifted away from historic and current assemblages with increasing deposition and climate change. Anticipated CAA reductions could maintain current assemblages under current climate conditions or slow community shifts under increased future changes in temperature and precipitation. Overall, our results can inform decision makers on how these dual stressors interact to affect forest health, and the efficacy of deposition reductions under a changing climate.

The response of streams in the Adirondack region of New York to projected changes in sulfur and nitrogen deposition under changing climate

Modeling studies project that in the future surface waters in the northeast US will continue to recover from acidification over decades following reductions in atmospheric sulfur dioxide and nitrogen oxides emissions. However, these studies generally assume stationary climatic conditions over the simulation period and ignore the linkages between soil and surface water recovery from acid deposition and changing climate, despite fundamental impacts to watershed processes and comparable time scales for both phenomena. In this study, the integrated biogeochemical model PnET-BGC was applied to two montane forest watersheds in the Adirondack region of New York, USA to evaluate the recovery of surface waters from historical acidification in response to possible future changes in climate and atmospheric sulfur and nitrogen deposition. Statistically downscaled climate scenarios on average project warmer temperatures and greater precipitation for the Adirondack by the end of the century. Model simulations suggest under constant climate, acid-sensitive Buck Creek would gain 12.8 μeq L^-1 of acid neutralizing capacity (ANC) by 2100 from large reductions in deposition, whereas acid insensitive Archer Creek is projected to gain 7.9 μeq L^-1 of ANC. However, climate change could limit those improvements in acid-base status. Under climate change, a negative offset relative to the ANC increases with no climate change are projected for both streams by 2100. In acid-insensitive Archer Creek the negative offset (-8.5 μeq L^-1) was large enough that ANC is projected to decrease by -0.6 μeq L^-1, whereas in acid-sensitive Buck Creek, the negative offset (-0.4 μeq L^-1) resulted in a slight decline of the projected future ANC increase to 12.4 μeq L^-1. Calculated target loads for 2150 for both sites decreased when future climate change was considered in model simulations, which suggests further reductions in acid deposition may be necessary to restore ecosystem structure and function under a changing climate.

Regional target loads of atmospheric nitrogen and sulfur deposition for the protection of stream and watershed soil resources of the Adirondack Mountains, USA

Acidic deposition contributes to a range of environmental impacts across forested landscapes, including acidification of soil and drainage water, toxic aluminum mobilization, depletion of available soil nutrient cations, and impacts to forest and aquatic species health and biodiversity. In response to decreasing levels of acidic deposition, soils and drainage waters in some regions of North America have become gradually less acidic. Thresholds of atmospheric deposition at which adverse ecological effects are manifested are called critical loads (CLs) and/or target loads (TLs). Target loads are developed based on approaches that account for spatial and temporal aspects of acidification and recovery. Exceedance represents the extent to which current or projected future levels of acidic deposition exceed the level expected to cause ecological harm. We report TLs of sulfur (S) and nitrogen (N) deposition and the potential for ecosystem recovery of watershed soils and streams in the Adirondack region of New York State, resources that have been less thoroughly investigated than lakes. Regional TLs were calculated by statistical extrapolation of hindcast and forecast simulations of 25 watersheds using the process-based model PnET-BGC coupled with empirical observations of stream hydrology and established sensitivity of sugar maple (Acer saccharum) to soil base saturation and brook trout (Salvelinus fontinalis) to stream acid neutralizing capacity (ANC). Historical impacts and the expected recovery timeline of regional soil and stream chemistry and fish community condition within the Adirondack Park were evaluated. Analysis suggests that many low-order Adirondack streams and associated watershed soils have low TLs (<40 meq/m²/yr of N + S deposition) to achieve specified benchmarks for recovery of soil base saturation or stream ANC. Acid-sensitive headwater and low-order streams and watershed soils in the region are expected to experience continued adverse effects from N and S deposition well into the future even under aggressive emissions reductions. Watershed soils and streams in the western Adirondack Park are particularly vulnerable to acidic deposition and currently in exceedance of TLs. The methods used for linking statistical and process-based models to consider chemical and biological response under varying flow conditions at the regional scale in this study can be applied to other areas of concern.

The response of stream ecosystems in the Adirondack region of New York to historical and future changes in atmospheric deposition of sulfur and nitrogen

The present-day acid-base chemistry of surface waters can be directly linked to contemporary observations of acid deposition; however, pre-industrial conditions are key to predicting the potential future recovery of stream ecosystems under decreasing loads of atmospheric sulfur (S) and nitrogen (N) deposition. The integrated biogeochemical model PnET-BGC was applied to 25 forest watersheds that represent a range of acid sensitivity in the Adirondack region of New York, USA to simulate the response of streams to past and future changes in atmospheric S and N deposition, and calculate the target loads of acidity for protecting and restoring stream water quality and ecosystem health. Using measured data, the model was calibrated and applied to simulate soil and stream chemistry at all study sites. Model hindcasts indicate that historically stream water chemistry in the Adirondacks was variable, but inherently sensitive to acid deposition. The median model-simulated acid neutralizing capacity (ANC) of the streams was projected to be 55 μeq L^-1 before the advent of anthropogenic acid deposition (~1850), decreasing to minimum values of 10 μeq L^-1 around the year 2000. The median simulated ANC increased to 13 μeq L^-1 by 2015 in response to decreases in acid deposition that have occurred over recent decades. Model projections suggest that simultaneous decreases in sulfate, nitrate and ammonium deposition are more effective in restoring stream ANC than individual decreases in sulfur or nitrogen deposition. However, the increases in stream ANC per unit equivalent decrease in S deposition is greater compared to decreases in N deposition. Using empirical algorithms, fish community density and biomass are projected to increase under several deposition-control scenarios that coincide with increases in stream ANC. Model projections suggest that even under the most aggressive deposition-reduction scenarios, stream chemistry and fisheries will not fully recover from historical acidification by 2200.

Non-point source pollution risks in a drinking water protection zone based on remote sensing data embedded within a nutrient budget model

A simple, transparent and reliable method for evaluating non-point source pollution (NPSP) risks to drinking water source areas lacking observational data is proposed herein. The NPSP risks are assessed by using nutrient budget models for total nitrogen and total phosphorus, making the best use of remote sensing and field survey data. We demonstrate its potential using a case study of the Chaihe Reservoir in northeastern China. Fertilizer inputs and crop-uptake outputs were estimated based on normalized difference vegetation index, which is derived from remote sensing as indicators of crop growth and production. The nutrient balances for this area showed surpluses of both N and P within the soil system. Estimated imbalances per unit area were consistent with statistical relationships derived from all Chinese counties, demonstrating that the proposed method is reliable. The surplus P amounts were higher than the standard threshold for NPSP risks, indicating the existence of a potential contamination risk of P to this drinking water source.

Atmospheric deposition and exceedances of critical loads from 1800-2025 for the conterminous United States

Atmospheric deposition of nitrogen (N) and sulfur (S) has increased dramatically over pre-industrial levels, with many potential impacts on terrestrial and aquatic ecosystems. Quantitative thresholds, termed "critical loads" (CLs), have been developed to estimate the deposition rate above which damage is thought to occur. However, there remains no comprehensive comparison of when, where, and over what time periods individual CLs have been exceeded. We addressed this knowledge gap by combining several published data sources for historical and contemporary deposition, and overlaying these on six CL types from the National Critical Loads Database (NCLDv2.5; terrestrial acidification, aquatic acidification, lichen, nitrate leaching, plant community composition, and forest-tree health) to examine exceedances from 1800 to 2011. We expressed CLs as the minimum, 10th, and 50th percentiles within 12-km grid cells. Minimum CLs were relatively uniform across the country (200-400 eq·ha^-1 ·yr^-1 ), and have been exceeded for decades beginning in the early 20th century. The area exceeding minimum CLs peaked in the 1970s and 1980s, exposing 300,000 to 3 million km² (depending on the CL type) to harmful levels of deposition, with a total area exceeded of 5.8 million km² (~70% of the conterminous United States). Since then, deposition levels have dropped, especially for S, with modest reductions in exceedance by 2011 for all CL types, totaling 5.2 million km² in exceedance. The 10th and 50th percentile CLs followed similar trends, but were not consistently available at the 12-km grid scale. We also examined near-term future deposition and exceedances in 2025 under current air quality regulations, and under various scenarios of climate change and additional nitrogen management controls. Current regulations were projected to reduce exceedances of any CL from 5.2 million km² in 2011 to 4.8 million km² in 2025. None of the additional N management or climate scenarios significantly affected areal exceedances, although exceedance severity declined. In total, it is clear that many CLs have been exceeded for decades, and are likely to remain so in the short term under current policies. Additionally, we suggest many areas for improvement to enhance our understanding of deposition and its effects to support informed decision making.

A machine learning approach to estimation of downward solar radiation from satellite-derived data products: An application over a semi-arid ecosystem in the U.S

Shortwave solar radiation is an important component of the surface energy balance and provides the principal source of energy for terrestrial ecosystems. This paper presents a machine learning approach in the form of a random forest (RF) model for estimating daily downward solar radiation flux at the land surface over complex terrain using MODIS (MODerate Resolution Imaging Spectroradiometer) remote sensing data. The model-building technique makes use of a unique network of 16 solar flux measurements in the semi-arid Reynolds Creek Experimental Watershed and Critical Zone Observatory, in southwest Idaho, USA. Based on a composite RF model built on daily observations from all 16 sites in the watershed, the model simulation of downward solar radiation matches well with the observation data (r2 = 0.96). To evaluate model performance, RF models were built from 12 of 16 sites selected at random and validated against the observations at the remaining four sites. Overall root mean square errors (RMSE), bias, and mean absolute error (MAE) are small (range: 37.17 W/m2-81.27 W/m2, -48.31 W/m2-15.67 W/m2, and 26.56 W/m2-63.77 W/m2, respectively). When extrapolated to the entire watershed, spatiotemporal patterns of solar flux are largely consistent with expected trends in this watershed. We also explored significant predictors of downward solar flux in order to reveal important properties and processes controlling downward solar radiation. Based on the composite RF model built on all 16 sites, the three most important predictors to estimate downward solar radiation include the black sky albedo (BSA) near infrared band (0.858 μm), BSA visible band (0.3-0.7 μm), and clear day coverage. This study has important implications for improving the ability to derive downward solar radiation through a fusion of multiple remote sensing datasets and can potentially capture spatiotemporally varying trends in solar radiation that is useful for land surface hydrologic and terrestrial ecosystem modeling.

Impacts of Acidification and Potential Recovery on the Expected Value of Recreational Fisheries in Adirondack Lakes (USA)

We estimated the potential economic value of recreational fisheries in lakes altered by acid pollution in the Adirondack Mountains (USA). We found that the expected value of recreational fisheries has been diminished because of acid deposition but may improve as lakes recover from acidification under low emissions scenarios combined with fish stocking. Fishery value increased with lake pH, from a low of $4.41 angler day^-1 in lakes with pH < 4.5, to a high of $38.40 angler day^-1 in lakes with pH > 6.5 that were stocked with trout species. Stocking increased the expected fishery value by an average of $11.50 angler day^-1 across the entire pH range of the lakes studied. Simulating the future long-term trajectory of a subset of lakes, we found that pH and expected fishery value increased over time in all future emissions scenarios. Differences in estimated value among pollution reduction scenarios were small (<$1 angler day^-1) compared to fish stocking scenarios (>$4 angler day^-1). Our work provides a basis for assessing the costs and benefits of emissions reductions and management efforts that can hasten recovery of the economic and cultural benefits of ecosystems degraded by chronic pollution.