McClelland
James W.
McClelland
James W.
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ArticleRecent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska(American Geophysical Union, 2007-11-08) McClelland, James W. ; Stieglitz, Marc ; Pan, Feifei ; Holmes, Robert M. ; Peterson, Bruce J.Export of nitrate and dissolved organic carbon (DOC) from the upper Kuparuk River between the late 1970s and early 2000s was evaluated using long-term ecological research (LTER) data in combination with solute flux and catchment hydrology models. The USGS Load Estimator (LOADEST) was used to calculate June–August export from 1978 forward. LOADEST was then coupled with a catchment-based land surface model (CLSM) to estimate total annual export from 1991 to 2001. Simulations using the LOADEST/CLSM combination indicate that annual nitrate export from the upper Kuparuk River increased by ~5 fold and annual DOC export decreased by about one half from 1991 to 2001. The decrease in DOC export was focused in May and was primarily attributed to a decrease in river discharge. In contrast, increased nitrate export was evident from May to September and was primarily attributed to increased nitrate concentrations. Increased nitrate concentrations are evident across a wide range of discharge conditions, indicating that higher values do not simply reflect lower discharge in recent years but a significant shift to higher concentration per unit discharge. Nitrate concentrations remained elevated after 2001. However, extraordinarily low discharge during June 2004 and June–August 2005 outweighed the influence of higher concentrations in determining export during these years. The mechanism responsible for the recent increase in nitrate concentrations is uncertain but may relate to changes in soils and vegetation associated with regional warming. While changes in nitrate and DOC export from arctic rivers reflect changes in terrestrial ecosystems, they also have significant implications for Arctic Ocean ecosystems.
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ArticleIncreasing river discharge in the Eurasian Arctic : consideration of dams, permafrost thaw, and fires as potential agents of change(American Geophysical Union, 2004-09-17) McClelland, James W. ; Holmes, Robert M. ; Peterson, Bruce J. ; Stieglitz, MarcDischarge from Eurasian rivers to the Arctic Ocean has increased significantly in recent decades, but the reason for this trend remains unclear. Increased net atmospheric moisture transport from lower to higher latitudes in a warming climate has been identified as one potential mechanism. However, uncertainty associated with estimates of precipitation in the Arctic makes it difficult to confirm whether or not this mechanism is responsible for the change in discharge. Three alternative mechanisms are dam construction and operation, permafrost thaw, and increasing forest fires. Here we evaluate the potential influence of these three mechanisms on changes in discharge from the six largest Eurasian Arctic rivers (Yenisey, Ob', Lena, Kolyma, Pechora, and Severnaya Dvina) between 1936 and 1999. Comprehensive discharge records made it possible to evaluate the influence of dams directly. Data on permafrost thaw and fires in the watersheds of the Eurasian Arctic rivers are more limited. We therefore use a combination of data and modeling scenarios to explore the potential of these two mechanisms as drivers of increasing discharge. Dams have dramatically altered the seasonality of discharge but are not responsible for increases in annual values. Both thawing of permafrost and increased fires may have contributed to changes in discharge, but neither can be considered a major driver. Cumulative thaw depths required to produce the observed increases in discharge are unreasonable: Even if all of the water from thawing permafrost were converted to discharge, a minimum of 4 m thawed evenly across the combined permafrost area of the six major Eurasian Arctic watersheds would have been required. Similarly, sensitivity analysis shows that the increases in fires that would have been necessary to drive the changes in discharge are unrealistic. Of the potential drivers considered here, increasing northward transport of moisture as a result of global warming remains the most viable explanation for the observed increases in Eurasian Arctic river discharge.
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PreprintMacrophyte abundance in Waquoit Bay : effects of land-derived nitrogen loads on seasonal and multi-year biomass patterns( 2008-01) Fox, Sophia E. ; Stieve, Erica ; Valiela, Ivan ; Hauxwell, Jennifer ; McClelland, James W.Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for six years in three estuaries subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen loads over the broad range of 12 to 601 kg N ha-1 y-1. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries, but remained abundant in the estuary with the lowest load. Seasonal changes in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass over the six years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy. The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses. The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes in these temperate estuaries.
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ArticleLability of DOC transported by Alaskan rivers to the Arctic Ocean(American Geophysical Union, 2008-02-09) Holmes, Robert M. ; McClelland, James W. ; Raymond, Peter A. ; Frazer, Breton B. ; Peterson, Bruce J. ; Stieglitz, MarcArctic rivers transport huge quantities of dissolved organic carbon (DOC) to the Arctic Ocean. The prevailing paradigm is that DOC in arctic rivers is refractory and therefore of little significance for the biogeochemistry of the Arctic Ocean. We show that there is substantial seasonal variability in the lability of DOC transported by Alaskan rivers to the Arctic Ocean: little DOC is lost during incubations of samples collected during summer, but substantial losses (20–40%) occur during incubations of samples collected during the spring freshet when the majority of the annual DOC flux occurs. We speculate that restricting sampling to summer may have biased past studies. If so, then fluvial inputs of DOC to the Arctic Ocean may have a much larger influence on coastal ocean biogeochemistry than previously realized, and reconsideration of the role of terrigenous DOC on carbon, microbial, and food-web dynamics on the arctic shelf will be warranted.
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PreprintTrajectory shifts in the Arctic and Subarctic freshwater cycle( 2006-06-22) Peterson, Bruce J. ; McClelland, James W. ; Curry, Ruth G. ; Holmes, Robert M. ; Walsh, John E. ; Aagaard, KnutManifold changes in the freshwater cycle of high-latitude lands and oceans have been reported in the past few years. A synthesis of these changes in sources of freshwater and in ocean freshwater storage illustrates the complementary and synoptic temporal pattern and magnitude of these changes over the past 50 years. Increasing river discharge anomalies and excess net precipitation on the ocean contributed ~20,000 km3 of fresh water to the Arctic and high latitude North Atlantic oceans from lows in the 1960s to highs in the 1990s. Sea ice attrition provided another ~15,000 km3, and glacial melt added ~2000 km3. The sum of anomalous inputs from these freshwater sources matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995. The changes in freshwater inputs and ocean storage occurred in conjunction with the amplifying North Atlantic Oscillation and rising air temperatures. Fresh water may now be accumulating in the Arctic Ocean and will likely be exported southward if and when the North Atlantic Oscillation enters into a new high phase.
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ArticleA pan-arctic evaluation of changes in river discharge during the latter half of the 20th century(American Geophysical Union, 2006-03-30) McClelland, James W. ; Dery, Stephen J. ; Peterson, Bruce J. ; Holmes, Robert M. ; Wood, Eric F.Several recent publications have documented changes in river discharge from arctic and subarctic watersheds. Comparison of these findings, however, has been hampered by differences in time periods and methods of analysis. Here we compare changes in discharge from different regions of the pan-arctic watershed using identical time periods and analytical methods. Discharge to the Arctic Ocean increased by 5.6 km3/y/y during 1964-2000, the net result of a large increase from Eurasia moderated by a small decrease from North America. In contrast, discharge to Hudson/James/Ungava Bays decreased by 2.5 km3/y/y during 1964-2000. While this evaluation identifies an overall increase in discharge (~120 km3/y greater discharge at the end of the time period as compared to the beginning for Hudson/James/Unvaga Bays and the Arctic Ocean combined), the contrasting regional trends also highlight the need to understand the consequences of adding/removing freshwater from particular regions of the arctic and subarctic oceans.
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ArticleMacrophytes as indicators of land-derived wastewater : application of a δ15N method in aquatic systems(American Geophysical Union, 2005-01-25) Cole, Marci L. ; Kroeger, Kevin D. ; McClelland, James W. ; Valiela, IvanWe measured δ15N signatures of macrophytes and particulate organic matter (POM) in six estuaries and three freshwater ponds of Massachusetts to assess whether the signatures could be used as indicators of the magnitude of land-derived nitrogen loads, concentration of dissolved inorganic nitrogen in the water column, and percentage of N loads contributed by wastewater disposal. The study focused specifically on sites on Cape Cod and Nantucket Island, in the northeastern United States. There was no evidence of seasonal changes in δ15N values of macrophytes or POM. The δ15N values of macrophytes and POM increased as water column dissolved inorganic nitrogen concentrations increased. We found that δ15N of macrophytes, but not of POM, increased as N load increased. The δ15N values of macrophytes and groundwater NO3 tracked the percent of wastewater contribution linearly. This research confirms that δ15N values of macrophytes and NO3 can be excellent indicators of anthropogenic N in aquatic systems.
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ArticleModeling transport and fate of riverine dissolved organic carbon in the Arctic Ocean(American Geophysical Union, 2009-10-07) Manizza, Manfredi ; Follows, Michael J. ; Dutkiewicz, Stephanie ; McClelland, James W. ; Menemenlis, Dimitris ; Hill, C. N. ; Townsend-Small, Amy ; Peterson, Bruce J.The spatial distribution and fate of riverine dissolved organic carbon (DOC) in the Arctic may be significant for the regional carbon cycle but are difficult to fully characterize using the sparse observations alone. Numerical models of the circulation and biogeochemical cycles of the region can help to interpret and extrapolate the data and may ultimately be applied in global change sensitivity studies. Here we develop and explore a regional, three-dimensional model of the Arctic Ocean in which, for the first time, we explicitly represent the sources of riverine DOC with seasonal discharge based on climatological field estimates. Through a suite of numerical experiments, we explore the distribution of DOC-like tracers with realistic riverine sources and a simple linear decay to represent remineralization through microbial degradation. The model reproduces the slope of the DOC-salinity relationship observed in the eastern and western Arctic basins when the DOC tracer lifetime is about 10 years, consistent with published inferences from field data. The new empirical parameterization of riverine DOC and the regional circulation and biogeochemical model provide new tools for application in both regional and global change studies.
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PreprintEffects of watershed land use on nitrogen concentrations and δ15 Nitrogen in groundwater( 2005-07-18) Cole, Marci L. ; Kroeger, Kevin D. ; McClelland, James W. ; Valiela, IvanEutrophication is a major agent of change affecting freshwater, estuarine, and marine systems. It is largely driven by transportation of nitrogen from natural and anthropogenic sources. Research is needed to quantify this nitrogen delivery and to link the delivery to specific land-derived sources. In this study we measured nitrogen concentrations and δ15N values in seepage water entering three freshwater ponds and six estuaries on Cape Cod, Massachusetts and assessed how they varied with different types of land use. Nitrate concentrations and δ15N values in groundwater reflected land use in developed and pristine watersheds. In particular, watersheds with larger populations delivered larger nitrate loads with higher δ15N values to receiving waters. The enriched δ15N values confirmed nitrogen loading model results identifying wastewater contributions from septic tanks as the major N source. Furthermore, it was apparent that N coastal sources had a relatively larger impact on the N loads and isotopic signatures than did inland N sources further upstream in the watersheds. This finding suggests that management priorities could focus on coastal sources as a first course of action. This would require management constraints on a much smaller population.
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ArticleThe Arctic freshwater system : changes and impacts(American Geophysical Union, 2007-11-20) White, Daniel ; Hinzman, Larry ; Alessa, Lilian ; Cassano, John ; Chambers, Molly ; Falkner, Kelly ; Francis, Jennifer ; Gutowski, William J. ; Holland, Marika M. ; Holmes, Robert M. ; Huntington, Henry ; Kane, Douglas ; Kliskey, Andrew ; Lee, Craig M. ; McClelland, James W. ; Peterson, Bruce J. ; Rupp, T. Scott ; Straneo, Fiamma ; Steele, Michael ; Woodgate, Rebecca ; Yang, Daqing ; Yoshikawa, Kenji ; Zhang, TingjunDramatic changes have been observed in the Arctic over the last century. Many of these involve the storage and cycling of fresh water. On land, precipitation and river discharge, lake abundance and size, glacier area and volume, soil moisture, and a variety of permafrost characteristics have changed. In the ocean, sea ice thickness and areal coverage have decreased and water mass circulation patterns have shifted, changing freshwater pathways and sea ice cover dynamics. Precipitation onto the ocean surface has also changed. Such changes are expected to continue, and perhaps accelerate, in the coming century, enhanced by complex feedbacks between the oceanic, atmospheric, and terrestrial freshwater systems. Change to the arctic freshwater system heralds changes for our global physical and ecological environment as well as human activities in the Arctic. In this paper we review observed changes in the arctic freshwater system over the last century in terrestrial, atmospheric, and oceanic systems.
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ArticleFlow-weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers(American Geophysical Union, 2008-09-20) Cooper, Lee W. ; McClelland, James W. ; Holmes, Robert M. ; Raymond, Peter A. ; Gibson, J. J. ; Guay, Christopher K. ; Peterson, Bruce J.We present new flow-weighted data for δ 18OH2O, dissolved organic carbon (DOC), dissolved barium and total alkalinity from the six largest Arctic rivers: the Ob', Yenisey, Lena, Kolyma, Yukon and Mackenzie. These data, which can be used to trace runoff, are based upon coordinated collections between 2003 and 2006 that were temporally distributed to capture linked seasonal dynamics of river flow and tracer values. Individual samples indicate significant variation in the contributions each river makes to the Arctic Ocean. Use of these new flow-weighted estimates should reduce uncertainties in the analysis of freshwater transport and fate in the upper Arctic Ocean, including the links to North Atlantic thermohaline circulation, as well as regional water mass analysis. Additional improvements should also be possible for assessing the mineralization rate of the globally significant flux of terrigenous DOC contributed to the Arctic Ocean by these major rivers.
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ArticleLinkages among runoff, dissolved organic carbon, and the stable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean(American Geophysical Union, 2005-12-07) Cooper, Lee W. ; Benner, Ronald ; McClelland, James W. ; Peterson, Bruce J. ; Holmes, Robert M. ; Raymond, Peter A. ; Hansell, Dennis A. ; Grebmeier, Jacqueline M. ; Codispoti, Louis A.Concentrations of dissolved organic carbon (DOC) and δ18O values have been determined following sampling of runoff from a number of major arctic rivers, including the Ob, Yenisey, Lena, Kolyma, Mackenzie and Yukon in 2003-2004. These data are considered in conjunction with marine data for DOC, δ18O values, nutrients, salinity, and fluorometric indicators of DOC that were obtained as part of the Shelf-Basin Interactions program at the continental shelf-basin boundary of the Chukchi and Beaufort Seas. These marine data indicate that the freshwater component is most likely derived from regional sources, such as the Mackenzie, the Bering Strait inflow and possibly eastern Siberian rivers, including the Kolyma, or the Lena but not rivers further west in the Eurasian arctic. Contributions of freshwater from melted sea ice to marine surface waters appeared to be insignificant over annual cycles compared to runoff, although on a seasonal basis, freshwater from melted sea ice was locally dominant following a major sea-ice retreat into the Canada Basin in 2002. DOC concentrations were correlated with the runoff fraction, with an apparent meteoric water DOC concentration of 174 ± 1 μM (standard error). This concentration is lower than the flow-weighted concentrations measured at river mouths of the five largest Arctic rivers (358 to 917 μM), indicating that removal of terrigenous DOC during transport through estuaries, shelves and in the deep basin. DOC data indicate that flow-weighted concentrations in the two largest North American arctic rivers, the Yukon (625μM) and the Mackenzie (382 μM), are lower than in the three largest Eurasian arctic rivers, the Ob (825 μM), the Yenesey (858 μM) and the Lena (917 μM). A fluorometric indicator of chromophoric dissolved organic matter (CDOM) that has provided estimates of terrigenous DOC concentrations in the Eurasian Arctic was not correlated with DOC concentrations in the Amerasian marine waters studied, except below the upper Arctic Ocean halocline. Nutrient distributions and concentrations as well as derived nutrient ratios suggest the CDOM fluorometer may be responding to the release of chromophoric materials from continental shelf sediments. Shipboard incubation experiments with undisturbed sediment cores indicate that continental shelf sediments on the Bering and Chukchi Sea shelves are likely to be a net source of DOC to the Arctic Ocean.