Peatlands contain one-third of soil carbon (C), mostly buried in deep, saturated anoxic zones (catotelm). The response of catotelm C to climate forcing is uncertain, because prior experiments have ...focused on surface warming. We show that deep peat heating of a 2 m-thick peat column results in an exponential increase in CH
emissions. However, this response is due solely to surface processes and not degradation of catotelm peat. Incubations show that only the top 20-30 cm of peat from experimental plots have higher CH
production rates at elevated temperatures. Radiocarbon analyses demonstrate that CH
and CO
are produced primarily from decomposition of surface-derived modern photosynthate, not catotelm C. There are no differences in microbial abundances, dissolved organic matter concentrations or degradative enzyme activities among treatments. These results suggest that although surface peat will respond to increasing temperature, the large reservoir of catotelm C is stable under current anoxic conditions.
Peatlands are sources of bioaccumulating neurotoxin methylmercury (MeHg) that is linked to adverse health outcomes. Yet, the compounding impacts of climate change and reductions in atmospheric ...pollutants on mercury (Hg) export from peatlands are highly uncertain. We investigated the response in annual flow‐weighted concentrations (FWC) and yields of total‐Hg (THg) and MeHg to cleaner air and climate change using an unprecedented hydroclimatic (55‐year; streamflow, air temperature, precipitation, and peatland water tables), depositional chemistry (21‐year; Hg and major ions), and streamwater chemistry (∼17‐year; THg, MeHg, major ions, total organic carbon, and pH) data sets from a reference peatland catchment in Minnesota, USA. Over the hydroclimatic record, annual mean air temperature increased by ∼1.8°C, while baseflow and the efficiency that precipitation was converted to runoff (runoff ratio) decreased. Concurrently, precipitation‐based deposition of sulfate and Hg declined, where wet Hg deposition declined by ∼3–4 μg Hg m−2. Despite declines in wet Hg deposition over the study period, the catchment accumulated on average 0.04 ± 0.01 g Hg ha−1 yr−1 based on wet Hg deposition minus THg yield alone. Annual MeHg FWC was positively correlated with mean annual air temperatures (p = 0.03, r = 0.51), runoff ratio (p < 0.0001, r = 0.76), and wet Hg deposition concentration (p < 0.0001, r = 0.79). Decreasing wet Hg deposition and annual runoff ratios counterbalanced increased peatland MeHg production due to higher air temperatures, leading to an overall decline in streamwater MeHg FWC. Streamwater MeHg export may continue to decrease only as long as declines in runoff ratio and wet Hg deposition persistently outpace effects of increased air temperature.
Plain Language Summary
Climate change and cleaner air are unequivocally altering the movement of toxic mercury and methylmercury. Using long‐term and broad environmental measurements, a stream fed by a peatland exhibited decreased methylmercury levels that correlated with lower wet mercury deposition concentration and lower net water yields from the catchment (runoff ratio), which offsets potential increases of methylmercury due to elevated air temperatures. As such, methylmercury export from peatland catchments will likely continue to decrease over time if climate change continues to accelerate the reduction of runoff ratios and atmospheric wet mercury deposition further decreases. Adaptation to future climate and environmental changes would greatly benefit from additional and longer‐integrated multidisciplinary data sets. Without such long‐term and integrated measures critical insights, such as those gained from this study, would not be possible.
Key Points
Lower streamwater methylmercury concentrations and yields due to compounding effects of climate change and cleaner air
Lower wet atmospheric mercury deposition and runoff ratios offset higher air temperatures, resulting in lower methylmercury concentrations
Headwater peatland catchments will continue to be net sinks of total mercury; thus, we may not see a net flushing of anthropogenic mercury
Terrestrial loads of dissolved organic matter (DOM) have increased in recent years in many north temperate lakes. While much of the focus on the “browning” phenomena has been on its consequences for ...carbon cycling, much less is known about how it influences nutrient loading to lakes. We characterize potential loads of nitrogen and phosphorus to seepage lakes in northern Wisconsin, USA, based on a laboratory soil leaching experiment and a model that includes landscape cover and watershed area. In these seepage lakes, nutrient concentrations are positively correlated with dissolved organic carbon concentrations (nitrogen: r = 0.68, phosphorus: r = 0.54). Using long‐term records of browning, we found that dissolved organic matter‐associated nutrient loadings may have resulted in substantial increases in nitrogen and phosphorus in seepage lakes and could account for currently observed nutrient concentrations in the lake. “Silent” nutrient loadings to brown‐water lakes may lead to future water‐quality concerns.
Plain Language Summary
The color of many temperate lakes is changing; some lakes are becoming more darkly stained brown. The tea‐colored stain is due to dissolved organic matter from the surrounding landscape. Much of the research related to the causes and consequences of increased staining, or “brownification,” relate to its connection to the carbon cycle. However, by examining long‐term lake chemical records, analyzing the properties of the organic compounds, and modeling potential flows of the compounds, we find that carbon is not the only element that is influenced by browning. Nitrogen and phosphorus, two nutrients important to growth of organisms at the base of the food web, may also be increasing in lakes due to brownification.
Key Points
Terrestrial loads of dissolved organic matter to lakes are an underappreciated nutrient load to surface waters
Browning in lakes in the U.S. upper Midwest shows a delayed response compared to other temperate regions
Forest harvesting leads to changes in soil moisture, temperature and incident solar radiation, all strong environmental drivers of soil–air mercury (Hg) fluxes. Whether different forest harvesting ...practices significantly alter Hg fluxes from forest soils is unknown. We conducted a field-scale experiment in a northern Minnesota deciduous forest wherein gaseous Hg emissions from the forest floor were monitored after two forest harvesting prescriptions, a traditional clear-cut and a clearcut followed by biomass harvest, and compared to an un-harvested reference plot. Gaseous Hg emissions were measured in quadruplicate at four different times between March and November 2012 using Teflon dynamic flux chambers. We also applied enriched Hg isotope tracers and separately monitored their emission in triplicate at the same times as ambient measurements. Clearcut followed by biomass harvesting increased ambient Hg emissions the most. While significant intra-site spatial variability was observed, Hg emissions from the biomass harvested plot (180±170ngm−2d−1) were significantly greater than both the traditional clearcut plot (−40±60ngm−2d−1) and the un-harvested reference plot (−180±115ngm−2d−1) during July. This difference was likely a result of enhanced Hg2+ photoreduction due to canopy removal and less shading from downed woody debris in the biomass harvested plot. Gaseous Hg emissions from more recently deposited Hg, as presumably representative of isotope tracer measurements, were not significantly influenced by harvesting. Most of the Hg tracer applied to the forest floor became sequestered within the ground vegetation and debris, leaf litter, and soil. We observed a dramatic lessening of tracer Hg emissions to near detection levels within 6months. As post-clearcutting residues are increasingly used as a fuel or fiber resource, our observations suggest that gaseous Hg emissions from forest soils will increase, although it is not yet clear for how long such an effect will persist.
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•We investigated forestry impacts on gaseous Hg flux in a field experiment.•We measured ambient Hg fluxes and fluxes from added enriched Hg isotope tracers.•Biomass harvesting following clearcut had the greatest impact on ambient fluxes.•These impacts were seasonally restricted to the leaf-out growing season.•Isotope results suggest emissions dominated by legacy Hg pools.
The denitrifier method to determine the dual isotopic composition (δ15N and δ18O) of nitrate is well suited for studies of nitrogen contributions to streams during runoff events. This method requires ...only 70 nmol of NO3− and enables high throughput of samples. We studied nitrate sources to a headwater stream during snowmelt by generating a high‐temporal resolution dataset at the Sleepers River Research Watershed in Vermont, USA. In the earliest phase of runoff, stream NO3− concentrations were highest and stream discharge, NO3− concentrations, and δ18O of NO3− generally tracked one another during diurnal melting. The isotopic composition of stream NO3− varied in‐between atmospheric and groundwater NO3− end members indicating a direct contribution of atmospherically‐derived NO3− from the snow pack to the stream. During the middle to late phases of snowmelt, the source shifted toward soil NO3− entering the stream via shallow subsurface flow paths.
To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected ...from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchments at a site and among sites across the USA. We found both increasing and decreasing trends in monthly flow-weighted stream nitrate and ammonium concentrations. At a subset of the catchments, we found that the length and period of analysis influenced whether trends were positive, negative or non-significant. Trends also differed among neighboring catchments within several Experimental Forests, suggesting the importance of catchment-specific factors in determining nutrient exports. Over the longest time periods, trends were more consistent among catchments within sites, although there are fewer long-term records for analysis. These findings highlight the critical value of long-term, uninterrupted stream chemistry monitoring at a network of sites across the USA to elucidate patterns of change in nutrient concentrations at minimally disturbed forested sites.
Prior research has demonstrated the importance of water limitations and increasing temperatures on upland black spruce (Picea mariana Mill. B.S.P.) tree growth, which is a dominant component of the ...North American boreal forest. However, little work has been done to investigate the connectivity between growth and hydro‐climate in peatland black spruce systems. The boreal forest is the largest global terrestrial biome and is highly threatened due to current and projected increases in temperatures for the northern latitudes. Here we explore the dynamics among annual black spruce growth, climate, and water table elevations using 45 years of in situ precipitation, temperature, and water table elevation coupled with dendrochronological analysis from six research peatlands at the Marcell Experimental Forest, MN, USA. From 1963 to 2010, we found weak relationships between water table elevation and black spruce growth at the six study sites. Instead, annual black spruce growth was most favourable during three climatic periods: (a) cool, moist conditions in mid‐summer; (b) warm mid‐spring temperatures; and (c) cool temperatures in the fall prior to the current growing season. The disconnect between black spruce growth and water table dynamics was surprising and suggests that either annual black spruce growth is minimally responsive to hydrological fluctuations at the timescale we analysed or there is great elasticity of black spruce growth to peatland water table and evapotranspiration dynamics under the range of hydrological fluctuations contained in our record.
This study documented linkages between lakeshore seepage fluxes, pore water chemistry, and aquatic plants in several lakes of the Adirondack Mountains region of New York, USA. Three replicate ...stations were set up along each of four different lake shorelines. From June through September 1998 and from snowmelt in April through August 1999, seepage flux was measured with seepage meters. Throughout this time period, lake surface water and pore water chemistry were monitored weekly to biweekly. At each station, leaf tissue chemistry of the water lily Nuphar variegatum was measured once in each year. Sediment chemistry and plant abundance were also measured once in 1998. We found that pore water concentrations of base cations, iron, and zinc were related to the direction, magnitude, and variability of seepage fluxes. Concentrations of base cations, iron, and zinc were both highest and most variable where seepage was low (0 to 50 mL m-2 h-1) in contrast to being more stable where seepage was highest and variable (-608 to 612 mL m-2 h-1). Leaf tissue chemistry and plant abundance were also related to seepage patterns. N. variegatum leaves had elevated zinc content at stations with low average discharge. Knowledge of seepage patterns helped to explain spatial patterns of elevated trace metal content in both pore water and plant tissues. Our work suggests that the hydrological process of lakeshore seepage exerts important controls on lakeshore biogeochemistry.
Although seepage in lakes is known to vary as a function of precipitation and watershed characteristics, temporal patterns of seepage flux over daily and weekly time scales have not been extensively ...studied with a concentrated effort of direct measurement of seepage in nearshore areas. In this study, seepage was intensively measured with seepage meters over two summers and during snowmelt in Lower Sylvan Pond, a small lake in the Adirondack Mountains region of New York State, USA. A consistent pattern of slight discharge (never exceeding 105
mL
m
−2
h
−1) was observed at three stations along a segment of the shoreline near the inlet stream. A distinct temporal pattern of seepage was observed at three stations along another portion of the shoreline. Seepage discharged (>400
mL
m
−2
h
−1) for a portion of the summer in 1998 and then shifted to recharge (<−500
mL
m
−2
h
−1) for the remainder of the summer. A similar temporal pattern was observed again in 1999 but the transition to recharge occurred earlier and higher rates of discharge (610
mL
m
−2
h
−1) and recharge (−608
mL
m
−2
h
−1) were recorded. Additionally, seepage at these three stations increased after large rainfall events and a pronounced episodic increase of seepage was measured during snowmelt. Distinct seepage patterns along portions of the shoreline show that seepage can vary considerably within seasons and suggests a need to consider changes of seepage over time periods of days to weeks as well as among seasons and years. This study demonstrates that the resolution of temporal seepage data can be important to the understanding of water fluxes to aquatic ecosystems.
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