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 (14)C age. Moreover, the most heavily glaciated watersheds are the source of the oldest ( approximately 4 kyr (14)C 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 = 10(12) g), of which approximately 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.

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM inXuences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation-emission Xuorescence spectroscopy, speciWc UV absorbance (SUVA 254 ) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four diVerent wetland and forest soils: bog, forested wetland, fen and upland forest. There were signiWcant diVerences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA 254 and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9-32 mg C l ¡1 and between 23-42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation-emission Xuorescence spectroscopy showed that protein-like Xuorescence was positively correlated (r 2 = 0.82; P < 0.001) with BDOC for all soil types taken together. This Wnding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARA-FAC modeling conWrmed that the four soil types contain DOM with distinct chemical properties and have unique Xuorescent Wngerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes diVerently by inXuencing temporal patterns in stream heterotrophic productivity.

[1] Dissolved organic matter (DOM) transport during storms is studied because it is important in the annual watershed export budget for dissolved organic carbon (DOC). We sampled stream water from two watersheds (upland and wetland-dominated) and three subcatchments (bog, forested wetland, and mineral forest) located within the wetland-dominated watershed during a fall and summer storm to investigate changes in the magnitude and chemical quality of DOM during stormflows. Stormflow export of DOC ranged from 2.3 kg C ha À1 in the upland watershed to 13.9 kg C ha À1 in the bog subcatchment. Biodegradable DOC (BDOC) export for these same storms ranged from 0.6 kg C ha À1 in the upland watershed to 4.2 kg C ha À1 in the bog subcatchment. The 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 composition and biodegradability of streamwater dissolved organic matter (DOM) varies with source material and degree of transformation. We combined PARAFAC modeling of fluorescence excitation-emission spectroscopy and biodegradable dissolved organic carbon (BDOC) incubations to investigate seasonal changes in the lability of DOM along a soil-stream continuum in three soil types: bog, forested wetland and upland forest. The percent BDOC ranged from 7 to 38% across all sites, and was significantly greater in soil compared to streamwater in the bog and forested wetland, but not in the upland forest. The percent BDOC also varied significantly over the entire sampling period in soil and streamwater for the bog and forested wetland, as BDOC peaked during the spring runoff and was lowest during the summer months. Moreover, the chemical quality of DOM in wetland soil and streamwater was similar during the spring runoff and fall wet season, as demonstrated by the similar contribution of protein-like fluorescence (sum of tyrosine and tryptophan fluorescence) in soil water and in streams. These findings suggest that the tight coupling between terrestrial and aquatic ecosystems is responsible for the delivery of labile DOM from wetland soils to streams. The contribution of protein-like fluorescence was significantly correlated with BDOC (p \ 0.001) over the entire sampling period indicating DOM is an important source of C and N for heterotrophic microbes. Taken together, our findings suggest that the production of protein-rich, labile DOM and subsequent loss in stream runoff might be an important loss of labile C and N from coastal temperate watersheds.

The extensive mortality of yellow-cedar along more than ¡000 kilometers of the northern Pacific coast of North America serves as a leading example of climate effects on a forest tree species. In this article, we document our approaches to resolving the causes of tree death, which we explain as a cascade of interacting topographic, forest-structure, and microclimate factors that act on a unique vulnerability of yellow-cedar to fine-root freezing. The complex causes of tree mortality are reduced to two risk factors-snow depth and soil drainage-which are then used to model present and future cedar habitat suitability. We propose a dynamic, comprehensive conservation strategy for this valuable species on the basis of zones created by shifting climate, cedar's ecological niche, and observed risk factors. Research on yellow-cedar decline is offered as a template for understanding and adapting to climate change for other climate-forest issues.