Rick T. Edwards

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.

Biotic versus hydrologic control over seasonal nitrate leaching in a floodplain forest

Strong seasonal increases in aquatic (stream, ground and hyporheic water) nitrate have been observed in a variety of ecosystems. In most cases, changes in hydrological and vegetative activity occur contemporaneously, making it difficult to determine whether soil leaching is being driven by increases in the availability of leachable N or is simply due to flushing of N that has accumulated over longer periods. Three studies were conducted to better determine controls on soil nitrate leaching in a near-pristine temperate floodplain ecosystem receiving large N inputs via N-fixation by red alder: 1) an artificial rainfall experiment was conducted to estimate N-leaching potential during the summer, when plant uptake is high and new inputs of organic matter are low; 2) soil solution, groundwater and surface water were sampled during a major autumn storm to document exchanges at the seasonal transition, when plant uptake is low and inputs of senescent organic matter are high; and 3) monthly samples of soil and aquatic nitrogen were collected in 1997 and 1998 to document seasonal patterns of N exchanges. Collectively, these studies demonstrate the importance of hydrologic factors in controlling N flux. Nitrate was rapidly leached from soils during actual and simulated rainstorms. Two pathways of nitrate leaching were identified. Localized flooding and direct leaching of streamside soils into surface waters contributed to high solute concentrations in peak flows. Nitrate that leached into interstitial waters was subject to various factors that could delay or reduce its delivery to surface waters. Greater residence time may increase the influence of this component of stormflow on ecosystem productivity. While soil nitrate pools were rapidly depleted during rainstorms, accumulation of soil nitrate occurred over summer dry periods. Large differences in soil and aquatic nitrate concentrations between two years with contrasting rainfall highlight the potential for inter-annual hydrologic variability to affect ecosystem nutrient cycling.

Potential denitrification activity in the landscape of a western Alaska drainage basin

We examined denitrification potentials in six of the major landscape structure (riparian soils of both meandering and braided streams, peat lands, coniferous flats, alder slopes, and groundwater seeps) of the Lake Nerka catchment in southwest Alaska. We found significant potential denitrifying activity in all the soils of the main landscape patch types of the Lake Nerka catchment. The lowest potential denitrifying activity was measured in the peat lands. A highly significant relationship was found between soil organic matter and potential denitrification activity in three landforms— coniferous flats, groundwater seeps, and riparian soils of meandering streams. These three landscape structures also had the highest denitrifying potential. The finer soils of riparian zones along spawning streams, which corresponded to meandering streams, showed a significantly higher potential denitrification activity than the coarse riparian soils along nonspawning streams. These nonspawning streams corresponded to braided streams, where finer sediments were not as prevalent. Therefore, if this high potential denitrification measured in riparian soils of spawning streams is combined with large inputs of nitrate to anaerobic sites, it can result in 15 N signatures that mimic that of the marine-derived nitrogen provided by Pacific salmon in these Alaskan ecosystems.

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.

Biophysical controls on dissolved organic carbon concentrations of Alaskan coastal temperate rainforest streams

Coastal carbon cycling models remain incomplete in key continental margins worldwide. Large quantities of labile terrestrial DOC are transferred to the Gulf of Alaska in a flow of freshwater discharge from thousands of watersheds that equals the discharge of the Mississippi River. The coastal margin of southeast Alaska and British Columbia is a potential hotspot of worldwide DOC metabolism and the mass and reactivity of DOC in rivers and estuaries of the region make quantifying and modeling DOC export a priority. Scaling DOC export requires a well-constrained model of streamwater DOC concentrations. We established models for prediction of DOC streamwater concentrations through a broad sampling of streams across a large, diverse landscape in 61 independent watersheds. Stream DOC concentrations were significantly related to the amount of wetlands in a watershed confirming the necessity of understanding the fate of the mass of carbon stored within them. Several measures of slope were useful in predictive models of streamwater DOC concentrations suggesting that slope predicts the presence of wetland soils better than current wetland map layers. We also provide evidence that suggests watersheds with large salmon spawning runs have distinct characteristics that influence the relationship between wetlands and carbon cycling that may provide additional insights into how carbon is processed.

Hyporheic and benthic macroinvertebrate communities in glacial, clearwater, and brownwater streams in Alaska

Abstract The macroinvertebrate communities of hyporheic and benthic habitats were investigated in relation to stream physicochemistry in glacial, clearwater, and brownwater stream types (n  =  1) during the summer (June–August) of 2004 near Juneau, Alaska. Hyporheic macroinvertebrates were sampled using a series of PVC wells driven into parafluvial bars in the three stream types. Benthic invertebrates were collected using Hess samples from the stream channel adjacent to parafluvial well locations to compare surface macroinvertebrate communities to that of the hyporheic communities. Temperature, total phosphorus, dissolved organic carbon, and total nitrogen concentrations were significantly higher in the brownwater stream compared to the other stream types. Hyporheic macroinvertebrate density was 88% higher in the brownwater stream, despite significantly lower dissolved oxygen concentrations. Benthic macroinvertebrate density was 95% greater in the brownwater stream type than the benthos of the two other stream types. Benthic and hyporheic samples were collector-gatherer driven and the hyporheic zone was functionally more diverse than the benthic habitat with a similar taxa composition across all habitats and stream types. The study of aquatic ecosystems is especially important in Southeast Alaska, where the temperate rainforests are still relatively un-disturbed, and most watersheds still have healthy salmon runs. Our study provides useful data for future management and research in this unique ecoregion, which is predicted to experience substantial climatic changes in temperature and precipitation patterns in the future.