Eran Hood

Glaciers as a source of ancient and labile organic matter to the marine environment

Riverine organic matter supports of the order of one-fifth of estuarine metabolism. Coastal ecosystems are therefore sensitive to alteration of both the quantity and lability of terrigenous dissolved organic matter (DOM) delivered by rivers. The lability of DOM is thought to vary with age, with younger, relatively unaltered organic matter being more easily metabolized by aquatic heterotrophs than older, heavily modified material. This view is developed exclusively from work in watersheds where terrestrial plant and soil sources dominate streamwater DOM. Here we characterize streamwater DOM from 11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage (0–64 per cent). In contrast to non-glacial rivers, we find that the bioavailability of DOM to marine microorganisms is significantly correlated with increasing 14C age. Moreover, the most heavily glaciated watersheds are the source of the oldest (∼4 kyr 14C age) and most labile (66 per cent bioavailable) DOM. These glacial watersheds have extreme runoff rates, in part because they are subject to some of the highest rates of glacier volume loss on Earth. We estimate the cumulative flux of dissolved organic carbon derived from glaciers contributing runoff to the Gulf of Alaska at 0.13 ± 0.01 Tg yr-1 (1 Tg = 1012 g), of which ∼0.10 Tg is highly labile. This indicates that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems. Moreover, because glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system, our findings indicate that climatically driven changes in glacier volume could alter the age, quantity and reactivity of DOM entering coastal oceans.

Changes in the character of stream water dissolved organic carbon during flushing in three small watersheds, Oregon

watershed. The specific UV absorbance (SUVA, 254 nm) of DOC in the three watersheds increased by 9 to 36% during the storm, suggesting that DOC mobilized from catchment soils during storms is more aromatic than DOC entering the stream during baseflow. The increase in SUVA was most pronounced in the previously harvested catchments. Chromatographic fractionation of DOC showed that the percentage of DOC composed of non-humic material decreasing by 9 to 22% during the storm. Shifts in the fluorescence properties of DOC suggest that there was not a pronounced change in the relative proportion of stream water DOC derived from allochthonous versus autochthonous precursor material. Taken together, these results suggest that spectroscopic and chemical characterization of DOC can be used as tools to investigate changing sources of DOC and water within forested watersheds.

Composition, Dynamics, and Fate of Leached Dissolved Organic Matter in Terrestrial Ecosystems: Results from a Decomposition Experiment

AbstractFluxes of dissolved organic matter (DOM) are an important vector for the movement of carbon (C) and nutrients both within and between ecosystems. However, although DOM fluxes from throughfall and through litterfall can be large, little is known about the fate of DOM leached from plant canopies, or from the litter layer into the soil horizon. In this study, our objectives were to determine the importance of plant-litter leachate as a vehicle for DOM movement, and to track DOM decomposition [including dissolve organic carbon (DOC) and dissolved organic nitrogen (DON) fractions], as well as DOM chemical and isotopic dynamics, during a long-term laboratory incubation experiment using fresh leaves and litter from several ecosystem types. The water-extractable fraction of organic C was high for all five plant species, as was the biodegradable fraction; in most cases, more than 70% of the initial DOM was decomposed in the first 10 days of the experiment. The chemical composition of the DOM changed as decomposition proceeded, with humic (hydrophobic) fractions becoming relatively more abundant than nonhumic (hydrophilic) fractions over time. However, in spite of proportional changes in humic and nonhumic fractions over time, our data suggest that both fractions are readily decomposed in the absence of physicochemical reactions with soil surfaces. Our data also showed no changes in the δ13C signature of DOM during decomposition, suggesting that isotopic fractionation during DOM uptake is not a significant process. These results suggest that soil microorganisms preferentially decompose more labile organic molecules in the DOM pool, which also tend to be isotopically heavier than more recalcitrant DOM fractions. We believe that the interaction between DOM decomposition dynamics and soil sorption processes contribute to the δ13C enrichment of soil organic matter commonly observed with depth in soil profiles.

Changes in the concentration, biodegradability, and fluorescent properties of dissolved organic matter during stormflows in-coastal temperate .watersheds

percent BDOC decreased during both storms in the upland watershed, while percent BDOC increased in the three wetland streams. Parallel factor analysis (PARAFAC) modeling of fluorescence excitation-emission matrices further showed that as stream water DOM concentrations increased during stormflows in the upland watershed, the contribution of protein-like fluorescence decreased and humic-like fluorescence increased. However, the contribution of protein-like fluorescence increased and humic-like fluorescence decreased slightly in the three wetland streams. These results indicate that shifts in the biodegradability and chemical quality of DOM are different for upland and wetland watersheds. Taken together, our findings suggest stormflows are responsible for substantial export of BDOC from coastal temperate watersheds. Moreover, we found that PARAFAC modeling of fluorescent DOM is an effective tool for elucidating shifts in the quality of stream water DOM during storms.

Sources and chemical character of dissolved organic carbon across an alpine/subalpine ecotone, Green Lakes Valley, Colorado Front Range, United States

ascending limb of the snowmelt hydrograph, with the higher concentrations at the two subalpine sites. Seasonally, the chemical fractionation of DOC showed that there was a large range (29–72%) in the fulvic acid content of DOC at the four sites, with the highest fulvic acid percentages on the ascending limb of the hydrograph. Yields of DOC at the two gauged sites ranged from 10 to 11 kg ha � 1 over the season with between 45 and 50% of the yield occurring as fulvic acid DOC and the remainder as nonhumic material. The fluorescence properties of DOC from all four sites indicated that during peak runoff, DOC was derived primarily from terrestrial precursor material. However, seasonal changes in the fluorescence properties of fulvic acids at the highest elevation sites suggest that DOC derived from algal and microbial biomass in the lakes is a more important source of DOC above tree line during late summer and fall. We hypothesize that much of the

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone, Green Lakes Valley, Colorado Front Range

AbstractHere we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH4-N), nitrate-N (NO3-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO3-N decreased downstream from 4.3 kg ha−1 in the alpine to 0.75 kg ha−1 at treeline, while yields of DON were much less variable (0.40–0.34 kg ha−1). Yields of NH4-N and PON were low and showed little variation with basin area. NO3-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R2 = 0.81, P < 0.001) Concentrations of NO3-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry.

Sublimation from a seasonal snowpack at a continental, mid-latitude alpine site

Sublimation from the seasonal snowpack was calculated using the aerodynamic profile method at Niwot Ridge in the Colorado Front Range. Past studies of sublimation from snow have been inconclusive in determining both the rate and timing of the transfer of water between the snowpack and the atmosphere, primarily because they relied on one-dimensional measurements of turbulent fluxes or short term data sets. We calculated latent heat fluxes at ten minute intervals based on measurements of temperature, relative humidity and wind speed at heights of 0.5 m, 1.0 m and 2.0 m above the snowpack for nine months during the 1994- 1995 snow season. The meteorological instruments were raised or lowered daily to maintain a constant height above the snow surface. At each ten minute time step, the latent heat fluxes were converted directly into millimeters of sublimation or condensation. Total net sublimation for the snow season was 195 mm of water equivalent, or 15% of maximum snow accumulation at the stud site. The majorit y of this sublimation occurred during the snow accumulation season. Monthly losses to sublimation during the fall and winter ranged from 27 to 54 mm of water equivalent. The snowmelt season from May through mid-July showed net condensation to the snowpack ranging from 5 to 16 mm of water equivalent. Sublimation was sometimes episodic in nature, but often showed a diurnal periodicity with higher rates of sublimation during the day.

Role of organic nitrogen in the nitrogen cycle of a high‐elevation catchment, Colorado Front Range

Here we report on 3 years (1996 -1998) of measurements of organic and inorganic nitrogen (N) fluxes to and from Green Lakes Valley, a high-elevation ecosystem in the Colorado Front Range of the Rocky Mountains. Nitrate-N (NO3-N) was the dominant form of N in both precipitation and stream water. Annual precipitation contained 52% NO3-N, 32% ammonium-N (NH4-N), 9% dissolved organic N (DON), and 7% particulate organic N (PON). Annual export of N in streamflow was composed of 70% NO3-N, 4% NH4-N, 14% DON, and 12% PON. Thus the percentage of organic N increased from 16% of total N in precipitation to 26% of total N in streamflow. Subtracting inputs from outputs, Green Lakes Valley always shows net retention of inorganic N. The only form of N that showed net export was DON. DON export was low (0.18 to 0.13 kg ha 1 yr 1 ), with net export recorded in 2 years and basin retention recorded in 1 year. There was a seasonal pattern in the concentrations of inorganic N (NO3-N NH4-N) and organic N (DON PON). Concentrations of inorganic N were 15-25 mol L 1 during base flow, increased to 30 mol L 1 on the rising limb of the hydrograph during snowmelt runoff, then decreased to 5 mol L 1 on the recession limb of late summer, with a return to base flow values in the autumn. In contrast, organic N was 7-15 mol L 1 during base flow and decreased to near or below detection limits on the rising limb of the hydrograph, with a gradual but consistent increase on the recession limb and on into the autumn. The amount of N in dissolved organic matter changed over time, with the dissolved organic carbon (DOC):DON ratio decreasing from 45 on the rising limb of the hydrograph to 20 in the autumn. Spatially, there was a striking difference in the ratios of NO3-N and DON between talus and tundra areas. Nitrate concentrations in surface water draining talus areas were always greater than DON. In contrast, DON concentrations in surface water draining tundra areas were always greater than NO3-N. Concentrations of DON were not significantly correlated with DOC (R 2 0.04, p 0.05), indicating that controls on DON export may be different than controls on DOC export. Our results suggest that the ratio of the annual mass flux of inorganic N to organic N in stream waters may provide a novel index to evaluate the N status of terrestrial ecosystems from various biomes.

Contribution of glacier runoff to freshwater discharge into the Gulf of Alaska

annual freshwater discharge to the GOA at 870 km 3 yr −1 . Small distributed coastal drainages contribute 78% of the freshwater discharge with the remainder delivered by larger rivers penetrating coastal ranges. Discharge from glaciers and icefields accounts for 47% of total freshwater discharge, with 10% coming from glacier volume loss associated with rapid thinning and retreat of glaciers along the GOA. Our results indicate the region of the GOA from Prince William Sound to the east, where glacier runoff contributes 371 km 3 yr −1 , is vulnerable to future changes in freshwater discharge as a result of glacier thinning and recession. Changes in timing and magnitude of freshwater delivery to the GOA could impact coastal circulation as well as biogeochemical fluxes to near‐shore marine ecosystems and the eastern North Pacific Ocean. Citation: Neal, E. G., E. Hood, and K. Smikrud (2010), Contribution of glacier runoff to freshwater discharge into the Gulf of Alaska, Geophys. Res. Lett., 37, L06404, doi:10.1029/ 2010GL042385.

Effects of changing glacial coverage on the physical and biogeochemical properties of coastal streams in southeastern Alaska

concentrations of dissolved organic carbon (DOC) were typically low (0.5–3.0 mg C L 1 ) and showed a significant trend toward higher concentrations as watershed glacier coverage decreased. Concentrations of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen also increased significantly with decreasing glacial coverage. In contrast, concentrations of soluble reactive phosphorus decreased with lower glacial coverage. Interestingly, we found that the DOC:DON ratio of stream water dissolved organic matter (DOM) decreased with increasing glacier coverage, suggesting that glaciers may be a source of N-rich DOM. During winter low flows (November–April) there were few differences in stream water physical and biogeochemical conditions across the six watersheds as glacial inputs diminished and streamflow was dominated by groundwater. Our findings suggest that in southeastern Alaska ongoing glacial recession and the associated land cover change will impact physical and biogeochemical conditions in coastal streams, with implications for salmon spawning habitat, aquatic ecosystem productivity, and fluxes of reactive nutrients to downstream nearshore marine ecosystems.