Distribution and trends of mercury in aquatic and terrestrial biota of New York, USA: a synthesis of 50 years of research and monitoring.

Mercury (Hg) inputs have particularly impacted the northeastern United States due to its proximity to anthropogenic emissions sources and abundant habitats that efficiently convert inorganic Hg into methylmercury. Intensive research and monitoring efforts over the past 50 years in New York State, USA, have informed the assessment of the extent and impacts of Hg exposure on fishes and wildlife. By synthesizing Hg data statewide, this study quantified temporal trends of Hg exposure, spatiotemporal patterns of risk, the role that habitat and Hg deposition play in producing spatial patterns of Hg exposure in fish and other wildlife, and the effectiveness of current monitoring approaches in describing Hg trends. Most temporal trends were stable, but we found significant declines in Hg exposure over time in some long-sampled fish. The Adirondack Mountains and Long Island showed the greatest number of aquatic and terrestrial species with elevated Hg concentrations, reflecting an unequal distribution of exposure risk to fauna across the state. Persistent hotspots were detected for aquatic species in central New York and the Adirondack Mountains. Elevated Hg concentrations were associated with open water, forests, and rural, developed habitats for aquatic species, and open water and forested habitats for terrestrial species. Areas of consistently elevated Hg were found in areas driven by atmospheric and local Hg inputs, and habitat played a significant role in translating those inputs into biotic exposure. Continued long-term monitoring will be important in evaluating how these patterns continue to change in the face of changing land cover, climate, and Hg emissions.

Human and natural impacts on the U.S. freshwater salinization and alkalinization: A machine learning approach.

Ongoing salinization and alkalinization in U.S. rivers have been attributed to inputs of road salt and effects of human-accelerated weathering in previous studies. Salinization poses a severe threat to human and ecosystem health, while human derived alkalinization implies increasing uncertainty in the dynamics of terrestrial sequestration of atmospheric carbon dioxide. A mechanistic understanding of whether and how human activities accelerate weathering and contribute to the geochemical changes in U.S. rivers is lacking. To address this uncertainty, we compiled dissolved sodium (salinity proxy) and alkalinity values along with 32 watershed properties ranging from hydrology, climate, geomorphology, geology, soil chemistry, land use, and land cover for 226 river monitoring sites across the coterminous U.S. Using these data, we built two machine-learning models to predict monthly-aggregated sodium and alkalinity fluxes at these sites. The sodium-prediction model detected human activities (represented by population density and impervious surface area) as major contributors to the salinity of U.S. rivers. In contrast, the alkalinity-prediction model identified natural processes as predominantly contributing to variation in riverine alkalinity flux, including runoff, carbonate sediment or siliciclastic sediment, soil pH and soil moisture. Unlike prior studies, our analysis suggests that the alkalinization in U.S. rivers is largely governed by local climatic and hydrogeological conditions.

Protection of forest ecosystems in the eastern United States from elevated atmospheric deposition of sulfur and nitrogen: A comparison of steady-state and dynamic model results.

Critical loads (CLs) and target loads (TLs) of atmospheric deposition of sulfur (S) and nitrogen (N) specify the thresholds of air pollution above which damage to ecosystems is expected to occur and are used to inform environmental regulation and natural resource management. Model estimates of CL and TL can vary for a given location, and these differences can be important for characterization of ecosystem effects from elevated S and N deposition. Moreover, TLs are used to evaluate associated timeframes of ecosystem recovery. We compared published CLs and TLs based on soil acidity criteria derived from steady-state versus dynamic models for terrestrial ecosystems. We examined the magnitude of differences in the CL/TL results from the two types of models for the same regions in the Eastern U.S. Results showed that CLs/TLs from dynamic models (or from steady state modeling using soil base cation weathering estimates from dynamic models) generally produce a broader range of values of acid-sensitivity, including lower CLs/TLs, as compared with a steady-state approach. Applications of dynamic biogeochemical models capable of developing CLs/TLs are relatively data intensive and typically limited to locations where measured soil and soil solution (or nearby stream water) chemistry are available for model parameterization, calibration, and testing. We recommend that CLs/TLs derived from dynamic models be used, where data permit, as they are likely more accurate and allow for evaluation of time-dependent phenomena and period needed for recovery. However, CLs derived from steady-state models remain a useful tool for understanding broad spatial patterns in soil acid-sensitivity throughout the U.S. Future work should focus on the development of more reliable model input parameters, particularly soil base cation weathering, and the extent to which CLs and TLs at a given location may vary and be altered with anticipated future climate change. In addition, dynamic models could be further developed to estimate CLs/TLs for nutrient N.

Empirical nitrogen and sulfur critical loads of U.S. tree species and their uncertainties with machine learning.

Critical loads (CLs) of atmospheric deposition for nitrogen (N) and sulfur (S) are used to support decision making related to air regulation and land management. Frequently, CLs are calculated using empirical methods, and the certainty of the results depends on accurate representation of underlying ecological processes. Machine learning (ML) models perform well in empirical modeling of processes with non-linear characteristics and significant variable interactions. We used bootstrap ensemble ML methods to develop CL estimates and assess uncertainties of CLs for the growth and survival of 108 tree species in the conterminous United States. We trained ML models to predict tree growth and survival and characterize the relationship between deposition and tree species response. Using four statistical methods, we quantified the uncertainty of CLs in 95 % confidence intervals (CI). At the lower bound of the CL uncertainty estimate, 80 % or more of tree species have been impacted by nitrogen deposition exceeding a CL for tree survival over >50 % of the species range, while at the upper bound the percentage is much lower (<20 % of tree species impacted across >60 % of the species range). Our analysis shows that bootstrap ensemble ML can be effectively used to quantify critical loads and their uncertainties. The range of the uncertainty we calculated is sufficiently large to warrant consideration in management and regulatory decision making with respect to atmospheric deposition.

Long-Term Ecological Research on Ecosystem Responses to Climate Change.

In this article marking the 40th anniversary of the US National Science Foundation's Long Term Ecological Research (LTER) Network, we describe how a long-term ecological research perspective facilitates insights into an ecosystem's response to climate change. At all 28 LTER sites, from the Arctic to Antarctica, air temperature and moisture variability have increased since 1930, with increased disturbance frequency and severity and unprecedented disturbance types. LTER research documents the responses to these changes, including altered primary production, enhanced cycling of organic and inorganic matter, and changes in populations and communities. Although some responses are shared among diverse ecosystems, most are unique, involving region-specific drivers of change, interactions among multiple climate change drivers, and interactions with other human activities. Ecosystem responses to climate change are just beginning to emerge, and as climate change accelerates, long-term ecological research is crucial to understand, mitigate, and adapt to ecosystem responses to climate change.

Contrasting Impacts of Photochemical and Microbial Processing on the Photoreactivity of Dissolved Organic Matter in an Adirondack Lake Watershed.

Photochemical and microbial processing are the prevailing mechanisms that shape the composition and reactivity of dissolved organic matter (DOM); however, prior research has not comparatively evaluated the impacts of these processes on the photoproduction of reactive intermediates (RIs) from freshly sourced terrestrial DOM. We performed controlled irradiation and incubation experiments with leaf and soil samples collected from an acid-impacted lake watershed in the Adirondack Mountain region of New York to examine the effects of DOM processing on the apparent quantum yields of RIs (Φapp,RI), including excited triplet states of DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH). Photodegradation led to net reductions in Φapp,1O2, Φapp,3DOM*, and Φapp,•OH, whereas (photo-)biodegradation resulted in increases in Φapp,1O2 and Φapp,3DOM*. Photodegradation and (photo-)biodegradation also shifted the energy distribution of 3DOM* in different directions. Multivariate statistical analyses revealed the potential relevance of photo-biodegradation in driving changes in Φapp,1O2 and Φapp,3DOM* and prioritized five bulk DOM optical and redox properties that best explained the variations in Φapp,1O2 and Φapp,3DOM* along the watershed terrestrial-aquatic continuum. Our findings highlight the contrasting impacts of photochemical and microbial processes on the photoreactivity of freshly sourced terrestrial DOM and invite further studies to develop a more holistic understanding of their implications for aquatic photochemistry.

Amazon forests capture high levels of atmospheric mercury pollution from artisanal gold mining.

Mercury emissions from artisanal and small-scale gold mining throughout the Global South exceed coal combustion as the largest global source of mercury. We examined mercury deposition and storage in an area of the Peruvian Amazon heavily impacted by artisanal gold mining. Intact forests in the Peruvian Amazon near gold mining receive extremely high inputs of mercury and experience elevated total mercury and methylmercury in the atmosphere, canopy foliage, and soils. Here we show for the first time that an intact forest canopy near artisanal gold mining intercepts large amounts of particulate and gaseous mercury, at a rate proportional with total leaf area. We document substantial mercury accumulation in soils, biomass, and resident songbirds in some of the Amazon's most protected and biodiverse areas, raising important questions about how mercury pollution may constrain modern and future conservation efforts in these tropical ecosystems.

Energy policy and coastal water quality: An integrated energy, air and water quality modeling approach.

Federal policy changes in the management of carbon emissions from power plants offer a potent real-world example for examining air-land-water interactions and their implications for coastal water quality. We integrate models of energy (Integrated Planning Model (IPM)), air quality (Community Multiscale Air Quality (CMAQ) and water quality (SPAtially Referenced Regression On Watershed attributes (SPARROW)) to investigate the potential water quality impacts of policy-driven changes in total nitrogen deposition in watersheds draining to US coastal areas. We estimate the combined effects of three recently proposed energy policy scenarios, population growth, and climate change. We decompose the combined effects into the roles of the individual components on the supply of riverine nitrogen for the entire US and eight coastal regions. We find that population growth is the most important driver of changes in coastal nitrogen flux. Energy policies play a minor role in offsetting the negative effects of population growth, although the effect varies by energy policy and region. The greatest population and policy effects are projected for the Gulf of Mexico. Given limited reductions in nitrogen emissions and deposition associated with energy policies, the net effect of policy and population changes is an increase in total nitrogen flux to all estuaries relative to the 2010 baseline. While population growth increases flux, and energy policies decrease flux in all regions, climate change can either increase or decrease flux depending on the region. That is because the relatively large individual effects of temperature and precipitation on watershed nitrogen processes work in opposing directions. The net result of the offsetting nature of individual climate processes varies in both magnitude and direction by coastal region. Further research is needed to sort out individual temperature and precipitation effects in different regions.

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.

Simulation of the effects of forest harvesting under changing climate to inform long-term sustainable forest management using a biogeochemical model.

Process ecosystem models are useful tools to provide insight on complex, dynamic ecological systems, and their response to disturbances. The biogeochemical model PnET-BGC was modified and tested using field observations from an experimentally whole-tree harvested northern hardwood watershed (W5) at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. In this study, the confirmed model was used as a heuristic tool to investigate long-term changes in hydrology, biomass accumulation, and soil solution and stream water chemistry for three different watershed cutting intensities (40%, 60%, 80%) and three rotation lengths (30, 60, 90 years) under both constant (current climate) and changing (MIROC5-RCP4.5) future climate scenarios and atmospheric CO2 through the year 2200. For the no future cutting scenario, total ecosystem stored carbon (i.e., sum of aboveground biomass, woody debris and soil) reached a maximum value of 207 t C ha-1 under constant climate but increased to 452 t C ha-1 under changing climate in 2200 due to a CO2 fertilization effect. Harvesting of trees decreased total ecosystem stored carbon between 7 and 36% for constant climate and 7-60% under changing climate, respectively, with greater reductions for shorter logging rotation lengths and greater watershed cutting intensities. Harvesting under climate change resulted in noticeable losses of soil organic matter (12-56%) coinciding with loss of soil nutrients primarily due to higher rates of soil mineralization associated with increases in temperature, compared with constant climate conditions (3-22%). Cumulative stream leaching of nitrate under climate change (181-513 kg N ha-1) exceeded constant climate values (139-391 kg N ha-1) for the various cutting regimes. Under both climate conditions the model projected greater sensitivity to varying the length of cutting period than cutting intensities. Hypothetical model simulations highlight future challenges in maintaining long-term productivity of managed forests under changing climate due to a potential for a deterioration of soil fertility.

Measurement of the Vertical Distribution of Gaseous Elemental Mercury Concentration in Soil Pore Air of Subtropical and Temperate Forests.

Solid-gas-water phase partitioning of mercury (Hg) and the processes governing its diffusivity within soils are poorly studied. In this study, landscape and forest species dependences of gaseous elemental Hg (Hg(0)) in soil profiles (0-50 cm) were investigated over four seasons in eight subtropical (130 days) and temperate (96 days) forest plots. The vertical soil pore Hg(0) concentrations differed between subtropical (Masson pine, broad-leaved forest, and open field) and temperate (Chinese pine, larch, mixed broad-leaf forests, and open field) catchments, with annual averages ranging from 6.73 to 15.8 and 0.95 to 2.08 ng m-3, respectively. The highest Hg(0) concentrations in soil gas consistently occurred in the upper mineral or organic horizons, indicating immobilization of Hg(0) in mineral soils. A strongly positive relationship between pore Hg(0) concentrations and ratios of Hg to organic matter (SOM) in soils suggests that the vertical distribution of Hg(0) is related to soil Hg(0) formation by Hg(II) reduction and sorption to SOM. Temperature was also an important driver of Hg(0) production in soil pores. Based on measurements of soil-air Hg(0) exchange, diffusion coefficients (Ds) of Hg(0) between soil and atmosphere were calculated for field sites, providing a foundation for future development and validation of terrestrial Hg models.

Experimental approach and initial forest response to a simulated ice storm experiment in a northern hardwood forest.

Ice storms are a type of extreme winter weather event common to north temperate and boreal forests worldwide. Recent climate modelling studies suggest that these storms may become more frequent and severe under a changing climate. Compared to other types of storm events, relatively little is known about the direct and indirect impacts of these storms on forests, as naturally occurring ice storms are inherently difficult to study. Here we describe a novel experimental approach used to create a suite of ice storms in a mature hardwood forest in New Hampshire, USA. The experiment included five ice storm intensities (0, 6.4, 12.7 and 19.1 mm radial ice accretion) applied in a single year, and one ice storm intensity (12.7 mm) applied in two consecutive years. Results demonstrate the feasibility of this approach for creating experimental ice storms, quantify the increase in fine and coarse woody debris mass and nutrients transferred from the forest canopy to the soil under the different icing conditions, and show an increase in the damage to the forest canopy with increasing icing that evolves over time. In this forest, little damage occurred below 6.4 mm radial ice accretion, moderate damage occurred with up to 12.7 mm of accretion, and significant branch breakage and canopy damage occurred with 19.1 mm of ice. The icing in consecutive years demonstrated an interactive effect of ice storm frequency and severity such that some branches damaged in the first year of icing appeared to remain in the canopy and then fall to the ground in the second year of icing. These results have implications for National Weather Service ice storm warning levels, as they provide a quantitative assessment of ice-load related inputs of forest debris that will be useful to municipalities creating response plans for current and future ice storms.

Photochemical Characterization of Surface Waters from Lakes in the Adirondack Region of New York.

The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φapp,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), for surface waters collected from 16 representative Adirondack lakes. Φapp,3DOM* and Φapp,1O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φapp,•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S290-400) as the most effective predictor of Φapp,RI among measured water chemistry parameters and bulk DOM properties. Φapp,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.

Simulating Impacts of Ice Storms on Forest Ecosystems.

Ice storms can have profound and lasting effects on the structure and function of forest ecosystems in regions that experience freezing conditions. Current models suggest that the frequency and intensity of ice storms could increase over the coming decades in response to changes in climate, heightening interest in understanding their impacts. Because of the stochastic nature of ice storms and difficulties in predicting when and where they will occur, most past investigations of the ecological effects of ice storms have been based on case studies following major storms. Since intense ice storms are exceedingly rare events it is impractical to study them by waiting for their natural occurrence. Here we present a novel alternative experimental approach, involving the simulation of glaze ice events on forest plots under field conditions. With this method, water is pumped from a stream or lake and sprayed above the forest canopy when air temperatures are below freezing. The water rains down and freezes upon contact with cold surfaces. As the ice accumulates on trees, the boles and branches bend and break; damage that can be quantified through comparisons with untreated reference stands. The experimental approach described is advantageous because it enables control over the timing and amount of ice applied. Creating ice storms of different frequency and intensity makes it possible to identify critical ecological thresholds necessary for predicting and preparing for ice storm impacts.

The impact of lime additions on mercury dynamics in stream chemistry and macroinvertebrates: a comparison of watershed and direct stream addition management strategies.

Acid deposition has declined across eastern North America and northern Europe due to reduced emissions of sulfur and nitrogen oxides. Ecosystem recovery has been slow with limited improvement in surface water chemistry. Delayed recovery has encouraged acid-neutralization strategies to accelerate recovery of impaired biological communities. Lime application has been shown to increase pH and dissolved organic carbon (DOC), which could also drive increased mobilization of mercury (Hg) to surface waters. A four-year study was conducted within Honnedaga Lake's watershed in the Adirondack region of New York to compare the effects of watershed and direct channel lime additions on Hg in stream water and macroinvertebrates. All treatments sharply increased stream pH and DOC concentrations, but large differences in the duration of impacts were apparent. The watershed treatment resulted in multi-year increases in concentrations and loads of total Hg (150%; 390%), DOC (190%; 350%) and nutrients, whereas total Hg and DOC increased for short periods (72-96 h) after channel treatments. No response of Hg in macroinvertebrates was evident following the watershed treatment, but a potential short-term and spatially constrained increase occurred after the channel treatment. Our observations indicate that both treatment approaches mobilize Hg, but that direct channel liming mobilizes considerably less than watershed liming over any period longer than a few days. During the final study year, increased methyl Hg concentrations were observed across reference and treated streams, which may reflect an extended dry period, highlighting that climate variation may also affect Hg dynamics.

Mercury exposure in songbird communities along an elevational gradient on Whiteface Mountain, Adirondack Park (New York, USA).

Mercury (Hg) is a potent neurotoxin that biomagnifies within food webs. Adverse effects have been documented for avian species related to exposure of elevated Hg levels. High elevation, boreal forests generally receive higher atmospheric Hg deposition and regional studies have subsequently identified elevated blood Hg concentrations in songbird species inhabiting these montane habitats. The overall goal of this study was to investigate spatial and seasonal Hg exposure patterns in songbird species along an elevational gradient on Whiteface Mountain in the Adirondack Park of New York State. Songbird blood samples were collected from June-July in 2009 and 2010 along an elevational gradient at 13 study plots (450-1400 m) with a focus on Catharus thrushes, including the hermit thrush, Swainson's thrush, and Bicknell's thrush. The main results of this study documented: (1) an overall linear pattern of increasing blood Hg concentrations with increasing elevation, with additional analysis suggesting a nonlinear elevational pattern of increasing blood Hg concentrations to 1075 m, followed by decreasing concentrations thereafter, for all Catharus thrush species across the elevational gradient; and (2) an overall nonlinear seasonal pattern of increasing, followed by decreasing blood Hg concentrations across target species. Avian exposure patterns appear driven by elevated atmospheric Hg deposition and increased methylmercury bioavailability within high elevation habitats as compared with low elevation forests. Seasonal patterns are likely influenced by a combination of complex and dynamic variables related to dietary selection and annual molting cycles. Considering that few high elevation analyses have been conducted within the context of regional songbird research, this project complements the results from similar studies and highlights the need for further monitoring efforts to investigate environmental Hg contamination within avian communities.

Mercury exposure in songbird communities within Sphagnum bog and upland forest ecosystems in the Adirondack Park (New York, USA).

Mercury (Hg) is a potent neurotoxin that biomagnifies within both aquatic and terrestrial food webs resulting in adverse physiological and reproductive effects on impacted wildlife populations, including songbird communities. Due to reducing conditions, wetland ecosystems promote the formation of methylmercury. Regional studies have documented elevated blood mercury concentrations in songbird species within these habitat types. The overall goal of this research was to examine spatial and seasonal patterns of Hg exposure for targeted songbird species within Sphagnum bog wetland systems and compare these patterns with adjacent upland forests in the Adirondack Park of New York State. Project sampling was conducted at study plots within four Sphagnum bog and associated upland forest sites from May - August during the 2008, 2009, and 2011 field seasons. The overall results documented: (1) blood Hg concentrations were elevated in songbird species inhabiting Sphagnum bog habitats as compared to nearby upland forest species; (2) target species within each habitat type exhibited consistent species-level patterns in blood Hg concentrations at each study site; and (3) no seasonal change in blood Hg concentrations within Sphagnum bog habitats was documented, but an increasing, followed by a decreasing seasonal pattern in mercury exposure was detected for upland forest species. Habitat type was demonstrated to influence avian Hg exposure levels. Moreover, Sphagnum bog ecosystems may be contributing to elevated Hg concentrations in biota within the surrounding environment. Seasonal patterns for blood Hg concentrations were found to vary between habitat type and are likely related to a combination of variables including habitat-driven Hg concentrations in prey items, seasonal dietary shifts, and annual molting cycles. This project emphasizes the importance of prioritizing future research efforts within identified high Hg habitat types, specifically wetland systems, to better characterize associated avian exposure levels, estimate the spatial extent of wetland systems on the surrounding environment, and identify locations of potential biological hotspots across the Adirondack Park.

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.

Total and methylmercury concentrations in ground and surface waters in natural and restored freshwater wetlands in northern New York.

Nearly half of freshwater wetlands have been lost due to human disturbance. In response, wetlands are being restored to retain their ecosystem services. A potentially adverse consequence of wetland function is the production of methylmercury (MeHg). We measured concentrations of mercury (Hg) species and ancillary parameters in groundwaters and surface waters from four natural and 16 restored wetlands in northern New York State, USA to investigate differences in concentrations of Hg species among wetlands. We found no obvious differences in concentrations of total mercury (THg) and methylmercury in pond waters between natural and restored wetlands. High values of %methylmercury were evident in both ground (38.8 ± 27.6%) and surface waters (43.4 ± 25.6%) suggesting these wetland complexes are highly efficient in converting ionic Hg to methylmercury, regardless if restored or natural. High methylation efficiency may be due to observed drying and rewetting cycles. Hg in pond waters is likely derived from direct atmospheric deposition or by mobilization from near-wetland shallow sediments, in addition to groundwater inflows. Water flow of groundwaters from the associated watershed into pond waters resulted in increases in concentrations of THg and methylmercury. Dissolved organic matter likely plays an important role in the supply of Hg to pond waters. Relationships between methylmercury and %methylmercury with sulfate and nitrate in groundwaters may suggest some chemical limitation on Hg methylation at higher concentrations of these anions. Because of the similarity in Hg dynamics for natural and restored wetlands, the most effective strategy to mitigate methylmercury production would be to decrease atmospheric Hg deposition.

Temporal trends in fish mercury concentrations in an Adirondack Lake managed with a continual predator removal program.

Mercury is a neurotoxic pollutant and contamination in remote ecosystems due to atmospheric mercury deposition coupled with watershed characteristics that influence mercury bioavailability. Biological interactions that affect mercury bioaccumulation are especially relevant as fish assemblages change in response to species introductions and lake management practices. We studied the influence of shifting food web dynamics on mercury in fisheries of Little Moose Lake in the southwestern Adirondack Mountains of New York, USA. Annual removal of non-native Smallmouth Bass (Micropterus dolomieu) has been used as a management strategy since 2000 to restore the native fish assemblage and food web in favor of Lake Trout (Salvelinus namaycush). Changes in total mercury, stable carbon (13C/12C) and nitrogen (15N/14N) isotopes, and growth were evaluated for Lake Trout and Smallmouth Bass. Growth rates increased for both predators and trophic position increased for Lake Trout post-removal. Mercury concentrations in Lake Trout increased over the 16-year study period influenced by a diet shift from invertebrates to higher trophic level prey fish, regardless of increased growth. Smallmouth Bass mercury concentrations decreased with compensatory growth from a reduced population size. These contrasting trends indicate that changes in mercury deposition were not the primary driver for mercury bioaccumulation responses in Little Moose Lake. Stable isotope values changed for both predators and for several lower trophic level organisms, likely reflecting changes in nutrient cycling and/or inputs. Our findings emphasize the potential role of fisheries management on whole-lake and predatory fish responses to mercury contamination in temperate lakes.