James W. McClelland

Measuring 15N-NH4+ in marine, estuarine and fresh waters : An adaptation of the ammonia diffusion method for samples with low ammonium concentrations

Abstract We present a method for measuring 15 N –NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses large sample volumes (often 4 l) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured δ 15 N values.

Trajectory shifts in the Arctic and subarctic freshwater cycle.

Manifold 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 freshwater sources 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 cubic kilometers 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 cubic kilometers, and glacial melt added ∼2000 cubic kilometers. 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.

Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change

[1] Discharge 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. INDEX TERMS: 1655 Global Change: Water cycles (1836); 1803 Hydrology: Anthropogenic effects; 1833 Hydrology: Hydroclimatology; 1860 Hydrology: Runoff and streamflow; KEYWORDS: Arctic river discharge, global change Citation: McClelland, J. W., R. M. Holmes, B. J. Peterson, and M. Stieglitz (2004), Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change, J. Geophys. Res., 109, D18102,

Flow‐weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers

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. Citation: Cooper, L. W., J. W. McClelland, R. M. Holmes, P. A. Raymond, J. J. Gibson, C. K. Guay, and B. J. Peterson (2008), Flow-weighted values of runoff tracers (d 18 O, DOC, Ba, alkalinity) from the six largest Arctic rivers, Geophys. Res. Lett., 35, L18606, doi:10.1029/2008GL035007.

Nitrogen loading from watersheds to estuaries: Verification of the Waquoit Bay Nitrogen Loading Model

World-wide eutrophication of estuaries has made accurate estimation ofland-derived nitrogen loads an important priority. In this paper we verifypredictions of nitrogen loads made by the Waquoit Bay Nitrogen LoadingModel (NLM). NLM is appropriate for watersheds with mixes of forested,agricultural, and residential land uses, and underlain by coarseunconsolidated sediments. NLM tracks the fate of nitrogen inputs byatmospheric deposition, fertilizer use, and wastewater disposal, and assignslosses of nitrogen from each source as the nitrogen is transported throughthe land use mosaic on the watershed surface, then through the underlyingsoils, vadose zones, and aquifers.We verified predictions of nitrogen loads by NLM in two independent ways.First, we compared NLM predictions to measured nitrogen loads in differentsubestuaries in the Waquoit Bay estuarine system. Nitrogen loads predictedby NLM were statistically indistinguishable from field-measured nitrogenloading rates. The fit of model predictions to measurements remained goodacross the wide range of nitrogen loads, and across a broad range in size(10–10,000 ha) of land parcels. NLM predictions were most precise whenspecific parcels were larger than 200 ha, and within factors of 2 for smallerparcels.Second, we used NLM to predict the percentage of nitrogen loads toestuaries contributed by wastewater, and compared this prediction to theδ15N signature distinguishable from N derived fromatmospheric or fertilizer sources. The greater the contribution ofwastewater, the heavier the δ15N value in groundwater. Thesignificant linear relation between NLM predictions of percent wastewatercontributions and stable isotopic signature corroborated the conclusionthat model outputs provide a good match to empirical measurements. Thegood agreement obtained in both verification exercises suggests that NLMis an useful tool to address basic and applied questions about how land usepatterns alter the fate of nitrogen traversing land ecosystems, and thatNLM provides verified estimates of the land-derived nitrogen exports thattransform receiving aquatic ecosystems.

Relative importance of grazing and nutrient controls of macroalgal biomass in three temperate shallow estuaries

Macroalgal biomass and competitive interactions among primary producers in coastal ecosystems may be controlled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of bottom-up and top-down processes under different nutrient loading conditions, we estimated potential amphipod and isopod grazer impact on a dominant macroalgal species in three estuaries in Waquoit Bay, Cape Cod, Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary based on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa andCymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory grazing rates for the remaining species, and in situ macroalgal growth rates. As nitrogen loading rates increased, macroalgal biomass increased (3×), eelgrass (Zostera marina) was lost, and herbivore abundance decreased (1/4×). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gryllotalpa) and, for two of the three species investigated, were faster on algae from the high-nitrogen estuary in comparison to the low-nitrogen estuary, paralleting the increased macroalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2×) with increasing nitrogen loading rate. The comparison between estimated growth increases versus losses ofC. vagabunda biomass to grazing suggested first, that grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower nitrogen loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates and biomass, plus the diminished abundance of grazers. This study suggests the relative impact of top-down and bottom-up controls on primary producers varies depending on rate of nitrogen loading, and specifically, that the impact of herbivory on macroalgal biomass decreases with increasing nitrogen load to estuaries.

A circumpolar perspective on fluvial sediment flux to the Arctic ocean

[1] Quantification of sediment fluxes from rivers is fundamental to understanding land-ocean linkages in the Arctic. Numerous publications have focused on this subject over the past century, yet assessments of temporal trends are scarce and consensus on contemporary fluxes is lacking. Published estimates vary widely, but often provide little accessory information needed to interpret the differences. We present a pan-arctic synthesis of sediment flux from 19 arctic rivers, primarily focusing on contributions from the eight largest ones. For this synthesis, historical records and recent unpublished data were compiled from Russian, Canadian, and United States sources. Evaluation of these data revealed no long-term trends in sediment flux, but did show stepwise changes in the historical records of two of the rivers. In some cases, old values that do not reflect contemporary fluxes are still being reported, while in other cases, typographical errors have been propagated into the recent literature. Most of the discrepancy among published estimates, however, can be explained by differences in years of records examined and gauging stations used. Variations in sediment flux from year to year in arctic rivers are large, so estimates based on relatively few years can differ substantially. To determine best contemporary estimates of sediment flux for the eight largest arctic rivers, we used a combination of newly available data, historical records, and literature values. These estimates contribute to our understanding of carbon, nutrient, and contaminant transport to the Arctic Ocean and provide a baseline for detecting future anthropogenic or natural change in the Arctic.

An analysis of the carbon balance of the Arctic Basin from 1997 to 2006

This study used several model-based tools to analyse the dynamics of the Arctic Basin between 1997 and 2006 as a linked system of land-ocean-atmosphere C exchange. The analysis estimates that terrestrial areas of the Arctic Basin lost 62.9 Tg C yr-1 and that the Arctic Ocean gained 94.1 Tg C yr-1. Arctic lands and oceans were a net CO2 sink of 108.9 Tg C yr-1, which is within the range of uncertainty in estimates from atmospheric inversions. Although both lands and oceans of the Arctic were estimated to be CO2 sinks, the land sink diminished in strength because of increased fire disturbance compared to previous decades, while the ocean sink increased in strength because of increased biological pump activity associated with reduced sea ice cover. Terrestrial areas of the Arctic were a net source of 41.5 Tg CH4 yr-1 that increased by 0.6 Tg CH4 yr-1 during the decade of analysis, a magnitude that is comparable with an atmospheric inversion of CH4. Because the radiative forcing of the estimated CH4 emissions is much greater than the CO2 sink, the analysis suggests that the Arctic Basin is a substantial net source of green house gas forcing to the climate system.

The Arctic Ocean Estuary

Large freshwater contributions to the Arctic Ocean from a variety of sources combine in what is, by global standards, a remarkably small ocean basin. Indeed, the Arctic Ocean receives ∼11% of global river discharge while accounting for only ∼1% of global ocean volume. As a consequence, estuarine gradients are a defining feature not only near-shore, but throughout the Arctic Ocean. Sea-ice dynamics also play a pivotal role in the salinity regime, adding salt to the underlying water during ice formation and releasing fresh water during ice thaw. Our understanding of physical–chemical–biological interactions within this complex system is rapidly advancing. However, much of the estuarine research to date has focused on summer, open water conditions. Furthermore, our current conceptual model for Arctic estuaries is primarily based on studies of a few major river inflows. Future advancement of estuarine research in the Arctic requires concerted seasonal coverage as well as a commitment to working within a broader range of systems. With clear signals of climate change occurring in the Arctic and greater changes anticipated in the future, there is good reason to accelerate estuarine research efforts in the region. In particular, elucidating estuarine dynamics across the near-shore to ocean-wide domains is vital for understanding potential climate impacts on local ecosystems as well as broader climate feedbacks associated with storage and release of fresh water and carbon.

Role of Salt Marshes as Part of Coastal Landscapes

Salt marshes are located between land and coastal water environments, and nutrient and production dynamics within salt marshes interact with those of adjoining ecosystems. Salt marshes tend to export materials to deeper waters, as shown by mass balance and stable isotopic studies. Salt marshes also intercept land-derived nutrients, and thus modify the potential response of phytoplankton, macroalgae, and seagrasses in the receiving estuarine waters. In particular, the maintenance of eelgrass meadows seems to depend on the ability of fringing salt marshes to intercept land-derived nitrogen. The bulk of the interception of land-derived nitrogen is likely to be the result of relatively high rates of denitrification characteristic of salt marshes. Thus, through exports of energy-rich materials, and interception of limiting nutrients, salt marsh parcels interact in quantitatively important ways with adjoining units of landscape. These interactions are of importance in understanding the basic functions of these mosaics of different coastal systems, as well as provide information needed to manage estuaries, as for example, in conservation of valuable eelgrass meadows.