Jason B. Fellman

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 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.

The origin and function of dissolved organic matter in agro‐urban coastal streams

[1] Streams draining urban and agriculture catchments are often a source of inorganic nutrients to downstream aquatic ecosystems, but little is known about how changes in land use influence the quality and biodegradability of dissolved organic matter (DOM). We used parallel factor analysis of excitation‐emission fluorescence spectroscopy and biodegradation incubations to examine how DOM composition influences bioavailable dissolved organic carbon (DOC) in surface waters of urban and agricultural catchments during summer (low flow), winter (high flow) and spring (flow recession). Percent bioavailable DOC was variable for all catchments (2–57%) and negatively related to percent humic‐like fluorescence, but positively related to percent protein‐like fluorescence and simple fluorescence metrics of DOM precursor material (fluorescence index and b:a values). Conversely, highly variable DOC concentrations (2–140 mg L −1 ) were negatively related to protein‐like fluorescence and positively related to humic‐like fluorescence. Elevated concentrations of DOC (>30 mg L −1 ) in agro‐urban streams revealed fluorescence indices (<1.3) typical of wetland and forest‐dominated ecosystems, suggesting that enriched stream DOM is either derived from the destabilization of legacy soil carbon or currently produced from remnant wetlands and patches of native vegetation. Overall, we demonstrate that fluorescence characteristics can be used to predict bioavailable DOC in human‐dominated catchments to better understand the flow of carbon and nutrients in aquatic food webs for improved monitoring and management of coastal ecosystems.

An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples

Techniques for preserving surface water samples are recently in demand because of the increased interest in quantifying dissolved organic matter (DOM) in surface waters and the frequent collection of samples in remote locations. Freezing is a common technique employed by many researchers for preserving surface water samples; however, there has been little evaluation of the effects of freezing on DOM concentrations. Ten streams were sampled in southeast Alaska with a range of dissolved organic carbon (DOC) concentrations (1.5 to 39 mg C L(-1)) to evaluate the influence of freezing (flash and standard freeze) and filter pore size (0.2 and 0.7 mum nominal pore size) on dissolved organic C, N and P concentrations. We report a significant decrease in DOC (p<0.005) and total dissolved P (p<0.005) concentrations when streamwater samples were frozen, whereas concentrations of dissolved organic N did not significantly decrease after freezing (p=0.06). We further show that when surface water samples were frozen, there was a decrease in the specific ultraviolet absorbance (SUVA) of DOC that is particularly evident with high concentrations of DOC. This finding suggests that spectroscopic properties of DOC have the potential to be used as indicators of whether surface water samples can be frozen. Our results lead us to recommend that surface water samples with high DOC concentrations (>5 mg C L(-1)) and/or samples with high SUVA values (>3.5-4 L mg-C(-1) m(-1)) should be analyzed immediately and not frozen.

Dissolved organic carbon biolability decreases along with its modernization in fluvial networks in an ancient landscape

The metabolism of dissolved organic carbon (DOC) along fluvial networks determines what fraction of organic matter is exported to the ocean. Although it is thought fresh rather than older DOC is preferred by bacteria, old DOC can also be highly bioavailable to stream bacterial communities. In strongly seasonal and oligotrophic regions, we argue that groundwater inputs of old DOC may increase the bioavailability of stream organic matter. We sampled 22 streams along a gradient of size (wetted widths from 1 to 60 m) and one groundwater spring in the Kimberley region of northwest Australia to determine how the age and bioavailability of streamwater DOC varied with stream size. Our hypothesis was that stream DOC would become more enriched in 14C (younger) and less bioavailable as streams increased in size and depleted 14C-DOC was metabolized by stream microbial communities. We also used fluorescence characterization of DOC, ultraviolet absorbance at 254nm (SUVA254), δ13C-DOC and lignin phenol yields to assess how these indicators of DOC character influenced the bioavailability and age of stream DOC. Stream evaporation/inflow ratios (E/I, used as a proxy for catchment water residence time), determined from changes in stream δ18O along the gradient of stream size, were positively related to DOC concentration and carbon-normalized lignin yields, while δ13C-DOC became more depleted with increasing E/I. Stream Δ14C-DOC varied from −452.1‰ (groundwater) to 48.9‰ and showed progressive enrichment as streams increased in size and accumulated DOC mainly from terrestrial plant material. Older DOC corresponded to higher bioavailability (R2 = 0.67, P < 0.01), suggesting that old bioavailable DOC, which has escaped from subterranean food webs utilizing 14C-depleted carbon, is common to one of the oldest landscapes on earth. Therefore, rapid biotic uptake of old bioavailable DOC originating in groundwater springs and the accumulation of modern, terrestrially derived DOC work in opposite directions affecting DOC dynamics along fluvial networks. We suggest the metabolism of old DOC along fluvial networks provides a biogeochemical link between non-contemporary carbon fixation and modern river productivity.

Return of Salmon-Derived Nutrients from the Riparian Zone to the Stream during a Storm in Southeastern Alaska

Spawning salmon deliver nutrients (salmon-derived nutrients, SDN) to natal watersheds that can be incorporated into terrestrial and aquatic food webs, potentially increasing ecosystem productivity. Peterson Creek, a coastal watershed in southeast Alaska that supports several species of anadromous fish, was sampled over the course of a storm during September 2006 to test the hypothesis that stormflows re-introduce stored SDN into the stream. We used stable isotopes and PARAFAC modeling of fluorescence excitation–emission spectroscopy to detect flushing of DOM from salmon carcasses in the riparian zone back into a spawning stream. During the early storm hydrograph, streamwater concentrations of NH4–N and total dissolved phosphorus (TDP), the fluorescent protein tyrosine and the δ15N content of DOM peaked, followed by a rapid decrease during maximum stormflow. Although δ15N has previously been used to track SDN in riparian zones, the use of fluorescence spectroscopy provides an independent indicator that SDN are being returned from the riparian zone to the stream after a period of intermediate storage outside the stream channel. Our findings further demonstrate the utility of using both δ15N of streamwater DOM and fluorescence spectroscopy with PARAFAC modeling to monitor how the pool of streamwater DOM changes in spawning salmon streams.

Nitrogen and phosphorus mineralization in three wetland types in southeast Alaska, USA

To improve our ability to predict how different wetland soils cycle nutrients, it is necessary to gain an understanding of N and P net mineralization rates. Since information on mineralization rates in southeast Alaska is limited, this study will improve our ability to predict how different wetlands affect soil nutrient processing. Net N and P mineralization rates were measured both in situ and via lab incubations to evaluate both actual and potential mineralization rates in three wetland types: bogs; forested wetlands; and riparian wetlands. Soil pH was an important controlling variable for both net N and P mineralization rates and soil phosphorus content significantly influenced net P mineralization rates. In situ net mineralization rates ranged from 410–1,710 μg N kg soil −1 day−1 for N and from 2–27 μg P kg soil−1 day−1 for P after 56 days. Lab incubations revealed mineralization potentials were 2–3 times greater than in situ rates. Net N and P mineralization potentials were greatest in the riparian wetlands and were significantly different from the bogs and forested wetlands. In contrast, the bogs mineralized a greater proportion of the total N and P soil pool (μg nutrient mineralized per gram nutrient) and indicates greater internal nutrient cycling within bogs. These results suggest that different wetland types of southeast Alaska process N and P differently and these wetland types should be evaluated separately in future evaluations of wetland ecosystem function.

Seasonal variability of organic matter composition in an Alaskan glacier outflow: insights into glacier carbon sources

Glacier ecosystems are a significant source of bioavailable, yet ancient dissolved organic carbon (DOC). Characterizing DOC in Mendenhall Glacier outflow (southeast Alaska) we document a seasonal persistence to the radiocarbon-depleted signature of DOC, highlighting ancient DOC as a ubiquitous feature of glacier outflow. We observed no systematic depletion in Δ 14C-DOC with increasing discharge during the melt season that would suggest mobilization of an aged subglacial carbon store. However, DOC concentration, δ 13C-DOC, Δ 14C-DOC and fluorescence signatures appear to have been influenced by runoff from vegetated hillslopes above the glacier during onset and senescence of melt. In the peak glacier melt period, the Δ 14C-DOC of stream samples at the outflow (−181.7 to −355.3‰) was comparable to the Δ 14C-DOC for snow samples from the accumulation zone (−207.2 to −390.9‰), suggesting that ancient DOC from the glacier surface is exported in glacier runoff. The pre-aged DOC in glacier snow and runoff is consistent with contributions from fossil fuel combustion sources similar to those documented previously in ice cores and thus provides evidence for anthropogenic perturbation of the carbon cycle. Overall, our results emphasize the need to further characterize DOC inputs to glacier ecosystems, particularly in light of predicted changes in glacier mass and runoff in the coming century.

Allochthonous dissolved organic matter controls bacterial carbon production in old-growth and clearfelled headwater streams

Abstract.  We investigated how the source and composition of stream dissolved organic matter (DOM) influenced rates of benthic bacterial C production (BCP) in 20 forested, headwater streams in southern Tasmania. We also assessed whether the source and composition of stream DOM was influenced by clearfell forest harvesting (1–19 y after harvest). Stream DOM was dominated by humic- and fulvic-like fluorescence (86.3–95.5%) as measured by parallel factor (PARAFAC) analysis of DOM fluorescence. Several reach-scale environmental variables showed significant positive (leaf-area index, sediment total N, organic C) or negative (stream temperature) linear relationships with BCP. However, an increasing contribution of terrestrial DOM, as measured by a decreasing fluorescence index (FI), was the strongest variable driving in situ benthic BCP (R2  =  0.38, p  =  0.004, n  =  20). Forest harvesting did not significantly affect DOM source despite the major reach-scale disturbance that clearfell forestry represents. Nevertheless, conflicting evidence was found for changes in DOM composition after harvesting. Catchment-scale processes probably are more important than reach-scale processes in determining stream DOM biogeochemistry because clearfelled areas are small relative to the total catchment area. Our results demonstrate that freshly leached, terrestrial DOM can influence stream ecosystem processes through the tight biogeochemical linkage that exists between forested, headwater streams and their surrounding terrestrial environment.

Spatial Variation in the Origin of Dissolved Organic Carbon in Snow on the Juneau Icefield, Southeast Alaska.

Dissolved organic carbon (DOC) plays a fundamental role in the biogeochemistry of glacier ecosystems. However, the specific sources of glacier DOC remain unresolved. To assess the origin and nature of glacier DOC, we collected snow from 10 locations along a transect across the Juneau Icefield, Alaska extending from the coast toward the interior. The Δ(14)C-DOC of snow varied from -743 to -420‰ showing progressive depletion across the Icefield as δ(18)O of water became more depleted (R(2) = 0.56). Older DOC corresponded to lower DOC concentrations in snow (R(2) = 0.31) and a decrease in percent humic-like fluorescence (R(2) = 0.36), indicating an overall decrease in modern DOC across the Icefield. Carbon isotopic signatures ((13)C and (14)C) combined with a three-source mixing model showed that DOC deposited in snow across the Icefield reflects fossil fuel combustion products (43-73%) and to a lesser extent marine (21-41%) and terrestrial sources (1-26%). Our finding that combustion aerosols are a large source of DOC to the glacier ecosystem during the early spring (April-May) together with the pronounced rates of glacier melting in the region suggests that the delivery of relic DOC to the ocean may be increasing and consequently impacting the biogeochemistry of glacial and proglacial ecosystems in unanticipated ways.