Effects of Atmospheric Nitrate on an Upland Stream of the Northeastern USA
Excess nitrogen cascades through terrestrial biogeochemical cycles and affects stream nitrate concentrations in upland forests where atmospheric deposition is an important source of anthropogenic nitrogen. We will discuss approaches including high-frequency sampling, isotopic tracers, and end-member mixing analysis that can be used to decipher the sources, transformations, and hydrological processes that affect nitrate transport through forested upland catchments to streams. We present results of studies at the Sleepers River Research Watershed in Vermont, USA, a site where we have intensively measured stream nitrate concentrations during baseflow and stormflow. Stream nitrate concentrations are typically low and nearly 75% of annual inorganic N inputs from atmospheric deposition are retained within the catchment. However, high concentrations and stream loadings of nitrate occur during storm events due to source variation and hydrological flushing of nitrate from catchment soils. Using isotopic tracers and end-member mixing analysis, we have quantified source inputs of unprocessed atmospheric nitrate and show that this stream is directly affected by nitrogen pollution. Using a long-term record of stream hydrochemistry and our findings on event- scale nitrate flushing dynamics, we then explore how stream nitrate loading may respond to anthropogenic climate forcing during the next century. Results suggest that stream runoff and nitrate loadings will change during future emission scenarios (i.e. longer growing seasons and higher winter precipitation rates). Understanding the timing and magnitude of hydrological and hydrochemical responses is important because climate change effects on catchment hydrology may alter how nitrate is retained, produced, and hydrologically flushed in headwater ecosystems with implications for aquatic metabolism, nutrient export from catchments, and downstream eutrophication.
Sources and Controls of Winter Nitrate Export at two Adjacent Forested Catchments
Winter rain-on-snow (ROS) events can account for up to 40 % of annual nitrate export in streams, and are associated with depressions in stream pH and spikes in total aluminum concentration. However, the magnitude of nitrate export in winter storms varies markedly among even neighbouring catchments that have similar deposition inputs, land use history and vegetation. In order to determine the causes of inter-catchment variability in ROS-nitrate release, we monitored winter melt and rain events at two adjacent headwater catchments (weirs ~100 m apart) where long-term (22-year) average winter nitrate export differs by more than two-fold. Results of intensive, winter storm-targeted monitoring between December 2008 and February 2009 provided insight into the sources and controls of nitrate export and changes in nitrogen cycling that could occur with winter warming.
Spatio-temporal Variation in the Characteristics of Dissolved Organic Matter in Boreal Lakes
Dissolved organic matter (DOM) is a critical component of aquatic ecosystems and has a predominant role in the complexation of contaminants, including trace metals. It regulates microbial activity, controls the toxicity and bioavailability of metals and other toxins, and serves both as an indicator and mediator of ecosystem stressors including land use and climate change. Despite the need, only very little information on its nature, composition, and spatio-temporal fluxes is currently available. This study investigates spatio-temporal changes in the nature and composition of DOM from three lakes near Dorset in south-central Ontario between 1979 and 2008. The contribution of organic acids to water acidity was determined for >1500 samples at each site based on the concentrations of major anions and cations. Significant deviation from charge balance is attributed to the known organic acids of DOM. The DOM charge densities varied by a factor of 2 with a maximum found in fall. The proportions of dissociated organic acids were also found to differ seasonally. Variability in the charge densities and dissociation proportions of organic matter could constitute a tool to monitor the impacts of changes in natural, anthropogenic, organic, and industrial inputs.
The Impacts of Glacial Recession on Riverine Nutrient Fluxes and the Age and Bioavailability of Riverine DOM in Gulf of Alaska Watersheds
Watersheds draining into the Gulf of Alaska (GOA) contain 75,300 km2 of glacier ice and are experiencing some of the highest rates of glacial erosion on earth, with thinning rates exceeding 5 meters of water equivalent at low elevations. This ongoing loss of glacial ice is rapidly altering landcover in GOA watersheds and has important implications for the physical and biogeochemical properties of rivers as well as the delivery of freshwater and nutrients to near-shore marine ecosystems along the GOA. We are studying the effects of changing glacial coverage on watershed biogeochemistry in eleven coastal watersheds along the GOA that vary markedly in watershed glacial coverage (range = 0-64%, mean = 36%). Our results indicate that decreased glacial coverage is strongly correlated with the temperature (r2=0.92, p<0.01) and turbidity (r2=0.87, p<0.01) of streamwater during the summer runoff season. In addition, flux modeling from three of the rivers with continuous gages suggests that riverine yields of dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) will increase with decreasing glacial coverage. In contrast, riverine fluxes of dissolved organic nitrogen (DON) appear to be decoupled from DOC fluxes and do not change with glacial coverage, while yields of soluble reactive phosphorus decrease with glacial coverage. Characterization of riverine dissolved organic matter (DOM) from our study watersheds using spectroscopic and isotopic analyses (13C and 14C) has shown that DOM is older and relatively rich in protein in watersheds with high glacial coverage. Moreover, the 14C age of DOM from these heavily glaciated watersheds exceeds 3500 years. Together these findings are consistent with the idea that glacial DOM is derived in large part from sub- and pro- glacial microbial populations that are supported by ancient carbon buried during the re-advance of glaciers along the GOA after the hypsothermal warm period ( ∼3,000-5,000 yrs bp). Interestingly, the lability of DOM was significantly positively correlated with 14C age such that the ancient DOM in glacial rivers was highly bioavailable to marine microorganisms. Using a distributed GIS model to estimate dissolved organic carbon fluxes from GOA glaciers, we conservatively estimate that glacial runoff contributes 0.14±0.01 Tg C yr- 1 to freshwater and near-shore marine ecosystems along the GOA. These findings suggest that glacial runoff from GOA watersheds, which is rapidly being altered by climate warming, may be an important source of labile DOM and nutrients to aquatic ecosystems.
What can Dissolved Oxygen Isotopes Tell us About the Metabolism of Impacted Rivers?
The cumulative effect of multiple agricultural and urban impacts on river ecology is examined using two Canadian prairie rivers with various upstream nutrient loadings and metabolic histories. While diel changes in O2 concentrations have traditionally been used to estimate biological rates, this approach suffers from a lack of information on re-aeration rates. By combining diel variation in O2 with δ18O-- O2 both metabolic and gas exchange rates can be better quantified than by O2 alone. Diel changes in O2 are driven by photosynthetic O2 production, respiratory O2 consumption, and gas exchange with the atmosphere. Each process imposes different δ18O--O2 values on the aquatic ecosystem. During photosynthesis, the photochemical oxidation of H2O produces O2 with the same δ18O value as the original H2O. This is typically <0 ‰ vs SMOW. Respiratory consumption of O2 is isotopically fractionating and the remaining δ18O-- O2 values increase as O2 is consumed. Gas exchange with the atmosphere causes the dissolved O2 pool to become progressively closer to the δ18O--O2 of the atmosphere, +23.5 ‰. Since photosynthetically produced O2 is isotopically distinct from O2 in the atmosphere, this leads to large diel changes in δ18O--O2 values. The net result is that by combining diel O2 and δ18O--O2 curves, rates of photosynthesis, respiration, and gas exchange can be determined without making the common assumptions required my night-time regression or O2-sag methods. The impact of nutrients on sewage treatment plant outfalls on the Bow River at Calgary and South Saskatchewan River at Saskatoon was assessed by 50 km longitudinal transects on both rivers in four seasons. The new dynamic PoRGy (photosynthesis--respiration--gas exchange) model was used to examine how metabolic and gas exchange rates are changed by nutrient impacts. Briefly, photosynthesis and respiration rates increased in some seasons downstream of the sewage treatment plants but the longitudinal pattern was different in both rivers. Metabolic rates declined more quickly downstream of Calgary than they did downstream of Saskatoon. Understanding the nature and duration of impacts of nutrients in Canadian rivers is an important part of river management.
Changes in Terminal Electron Acceptors During Experiment Mixing of a Dystrophic Lake
Changing climate conditions are associated with decreased duration of ice-on periods in lakes worldwide. This trend, coupled with likely increases in extreme weather events, is predicted to disrupt patterns of stratification and mixing in lakes and may result in unexpected, episodic mixing events. Because the mixing regime is a key component of a lake's physical template, regime changes should affect all aspects of the ecosystem. To understand the magnitude and nature of potential ecosystem changes, we experimentally mixed a small dystrophic lake during summer stratification in the Northern Highlands Lake District of Wisconsin (USA). The lake was mixed in July 2008 using a gradual entrainment lake inverter (GELI), a novel apparatus that uses alternating buoyancy stages to oscillate large volumes of water without introducing gas into the water column. Chemical conditions were monitored from spring to fall mixis (April-Sept). Here, we report on changes in the lake's thermal structure and responses of terminal electron acceptors (TEAs) as indicators of microbial activity and biogeochemical dynamics in the water column. Prior to experimental mixing, the lake was completely stratified. As expected, the surface layer (epilimnion) was well-oxygenated and had high concentrations of SO4. Deeper waters of the hypolimnion were anoxic, with detectable levels of H2S, FeII, and CH4. The GELI successfully homogenized the entire water column; the hypolimnion became oxygenated (3-4 mg DO/L), and reduced TEA concentrations declined to detection limits. After mixing ceased, the lake re- stratified but the hypolimnion was 14 degrees warmer than immediately prior to mixing. Re-establishment of chemical profiles was rapid as CH4 and H2S returned to pre-mixing concentrations within 1-2 weeks. The post-mixing stratification period was best characterized by accumulation of NH4-N in the hypolimnion; concentrations reached 1.5 mg N/L by the end of the summer, 4X greater than the pre-mixing level in July. Rapid accumulation of reduced TEAs and NH4-N enrichment both indicate elevated rates of respiration in the hypolimnion. Thus, episodic mixing is expected to increase whole-ecosystem respiration via both re- introducing oxygen needed for aerobic respiration, and by supporting higher anaerobic rates of respiration due to hypolimnetic warming. In turn, by increasing respiration, episodic mixing has the potential to increase the role of lakes as CO2 sources to the atmosphere.
Response of Mercury to Forest Management Activities and Extreme Weather Events
Mercury (Hg) levels are alarmingly high in fish from lakes across Fenno-Scandia and northern North America. Few studies exist as to how one of the primary agents of land use change in northern forests, forest harvesting influences this problem. The studies that do exist all indicate increased transport of Hg in harvested forests and increases in bioaccumulation in downstream aquatic ecosystems. The reported Hg runoff response in these studies is extremely variable and persistent. Recent studies in northern Sweden and Canada indicate that increased runoff Hg fluxes are largely tied to the increase in water yield in harvested catchments while earlier harvest studies in Sweden and Finland also documented significant increases in THg and MeHg runoff concentration along with increased water yield driven transport fluxes. Subsequent data collection from these earlier forest harvest studies have continued to document elevated leaching of MeHg from these catchments well after the initial catchment disturbance. From these studies we estimate that 9-23% of the Hg now in the fish of forested, high-latitude landscapes in Fenno-Scandia can be attributed to forest harvesting operations. In conjunction with the impacts of forestry driven land use change, recent studies in the boreal forest have also documented significant increases in Hg fluxes following the occurrence of drought and large storms. We discuss the potential impacts of an increasing variable climate and the recommendation for the large scale restoration of wetlands and reduced forest drainage in Scandinavia on the Hg/MeHg loadings to aquatic ecosystems in northern forests.