© 2008 Macmillan Publishers Limited. All rights reserved. © 2008 Macmillan Publishers Limited. All rights reserved. LETTERS Riverine organic matter and nutrients in southeast Alaska affected by glacial coverage ERAN HOOD 1 * AND DURELLE SCOTT 2 1 Environmental Science Program, University of Alaska Southeast, 11120 Glacier Hwy, Juneau, Alaska 99801, USA 2 Virginia Tech, Biological Systems Engineering, 200 Seitz Hall, Blacksburg, Virginia 24061, USA *e-mail: eran.hood@uas.alaska.edu Published online: 24 August 2008; doi:10.1038/ngeo280 The delivery of fresh water, carbon, nitrogen and phosphorous from high-latitude regional watersheds is important to the ecology and nutrient balance of coastal marine ecosystems in the Northern 1 and Southern 2 hemispheres. Bioavailable dissolved organic matter from rivers can support microbes in near-shore environments, and may also stimulate primary production 3,4 . Recent studies suggest that impacts of climate change, such as thawing permafrost, may affect nutrient yields in large northern rivers 5 . Here we analyse riverine dissolved organic matter and nutrient loads in three adjacent coastal watersheds along the Gulf of Alaska. We find that different levels of glacial coverage can alter the timing and magnitude of fresh water, dissolved organic matter and nutrient yields. Our results suggest that a lower extent of glacial coverage within a watershed can lead to higher amounts of dissolved organic matter, but decreased phosphorous yields. Moreover, an abundance of early successional plant species following deglaciation can increase riverine nitrogen levels. We conclude that changes in riverine yields of dissolved organic matter and nutrients due to reductions in glacier extent in coastal watersheds may affect the productivity and function of near-shore coastal ecosystems. Mountain glaciers are currently thinning and retreating throughout the globe as a result of climate warming 6 . However, glacial retreat is particularly acute along the Gulf of Alaska (GOA), where there is abundant ice at elevations close to tide water. In the last decade of the twentieth century, glacier ice was lost from this region at a rate of approximately 90 km 3 yr −1 , which constituted a greater contribution to sea level rise (∼0.25 mm yr −1 ) than runoff from the Greenland ice sheet 7 . The melting of glacier ice is increasing stream flow in coastal glacial watersheds in southeastern Alaska 8 , and has important implications for circulation in both coastal fjords 9 and the greater GOA 10 . However, there are very few reports of nutrient yields from glacial watersheds 11 ; thus, it is difficult to assess the consequences of glacial recession for riverine biogeochemical fluxes into glacially influenced, near-shore marine ecosystems along the GOA. We used measured discharge and frequent year-round sampling to quantify riverine yields of C, N and P for three adjacent watersheds in southeastern Alaska with glacial coverages of 0%, 25% and 55%. The differences in glacial coverage across the watersheds enable us to evaluate how changing glacial coverage alters yields of organic and inorganic nutrients from coastal temperate watersheds. Our three study watersheds, Montana Creek, Lemon Creek and the Mendenhall River, are located near Juneau, Alaska and are representative of the thousands of moderately sized coastal watersheds (30–500 km 2 ) along the GOA. Each watershed contains high-elevation reaches with alpine tundra, exposed bedrock and relatively thin (<1 m) soils, whereas the lower elevations are largely forested with wetlands present in low-gradient terrain and along slope breaks. The three watersheds vary in both watershed area and glacial coverage (Fig. 1), but share similar bedrock lithology. We found that daily specific yields of dissolved organic matter (DOM) and nutrients during the dominant runoff season between May–November are strongly influenced by glacial melt water. In 2006, daily yields of dissolved organic carbon (DOC) were typically lowest in the glacier-dominated Mendenhall River watershed (Fig. 2), which is consistent with the very low DOC concentrations found in glacial rivers during the summer runoff season 12,13 . Yields of DOC from non-glacial Montana Creek tracked more closely with precipitation, whereas daily DOC yields in Lemon Creek were intermediate in both magnitude and variability. All three watersheds showed multiple transient increases in yields of DOC in response to large frontal storms from the GOA, particularly during the autumn rainy season. However, differences in land cover and hydrology mediated very different DOC responses across the sequence of watersheds. The sharp, pulsed increases in daily DOC yields in Montana Creek are consistent with rapid flushing of allochthonous DOC from terrestrial sources into the stream channel 14 , including enhanced DOC mobilization from abundant peatlands and forested wetlands 15 . In contrast, water flow through temperate glaciers is channelized and occurs through englacial pathways 16 that provide little opportunity to mobilize DOM through contact with carbon-rich organic soils. As a result, the peaks for daily fluxes of DOC in response to storm events were broader and less pronounced in the glacial watersheds. On monthly and annual timescales, specific runoff and yields of C, N and P vary widely in their responses to land-cover differences across the three watersheds. Specific runoff is substantially higher in the glaciated watersheds, particularly during May–October, when the bulk of glacier melting occurs (Fig. 3a). In Mendenhall River, well over half the summer stream flow is derived from glacial melt water, and the high water yields in both Mendenhall River and Lemon Creek are augmented by the ongoing rapid loss of ice nature geoscience VOL 1 SEPTEMBER 2008 www.nature.com/naturegeoscience 583