Multiple tracers demonstrate distinct sources of dissolved organic matter to lakes of the Mackenzie Delta, western Canadian Arctic

Author Posting. © American Society of Limnology and Oceanography, 2011. This article is posted here by permission of American Society of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 56 (2011): 1297-1309, doi:10.4319/lo.2011.56.4.1297.Lakes of the Mackenzie Delta occur across a gradient that contains three clear end members: those that remain connected to river-water channels throughout the summer; those that receive only brief inputs of river water during an annual spring flood but contain dense macrophyte stands; and those that experience significant permafrost thaw along their margins. We measured dissolved organic carbon (DOC) concentration, dissolved organic matter (DOM) absorption and fluorescence, and stable isotopes of DOM, DOM precursor materials, and bacteria to elucidate the importance of river water, macrophytes, and thermokarst as DOM sources to Mackenzie Delta lakes. Despite standing stocks of macrophyte C that are sevenfold to 12-fold greater than those of total DOC, stable isotopes indicated that autochthonous sources contributed less than 15% to overall DOM in macrophyte-rich lakes. Instead, fluorescence and absorption indicated that the moderate summertime increase in DOC concentration in macrophyte-rich lakes was the result of infrequent flushing, while bacterial δ13C indicated rapid bacterial removal of autochthonous DOC from the water column. In thermokarst lakes, summertime increases in DOC concentration were substantial, and stable isotopes indicated that much of this increase came from C released as a result of thermokarst-related processes. Our results indicate that these distinct sources of DOM to neighboring arctic Delta lakes may drive between-lake differences in C cycling and energy flow. Rapidly assimilated macrophyte DOM should be an important contributor to microbial food webs in our study lakes. In contrast, the accumulation of thermokarst-origin DOM allows for a significant role in physico-chemistry but indicates a lesser contribution of this DOM to higher trophic levels.This study was supported by a Discovery Grant and Northern Research Supplement from the Natural Sciences and Engineering Research Council of Canada (NSERC) to L.F.W.L.; funds from the Science Horizons Youth Internship Program, Northern Scientific Training Program, and NSERC Northern Research Internship. Personal financial support to S.E.T. was provided by a Simon Fraser University CD Nelson Memorial Graduate Scholarship, an NSERC Canada Graduate Scholarship-Doctoral, and a Garfield Weston Award for Northern Research

Methane emission dynamics among CO2-absorbing and thermokarst lakes of a great Arctic delta

Lake-rich Arctic deltas differ biogeochemically from tundra lakes, and their role as sources and sinks of greenhouse gases remains poorly understood. Under-ice and open-water changes in methane (CH4) storage (43 lakes, 2014), floating chamber measurements of total and diffusive CH4 evasion to the atmosphere (6 lakes, 2014-2015), and water-column CH4 oxidation (MOX) (6 lakes, 2014-2015) permitted evaluation of how CH4 emissions vary among lakes with differing river-to-lake connection times within the Mackenzie Delta. CH4 emissions during ice-out were considerable, followed by substantial declines as open-water progressed. Water-column MOX rates were highest after ice-out, and declined throughout open-water. After accounting for a strong effect of CH4 substrate levels, MOX rates were inversely related to pH, which can increase to high levels during open-water because of high macrophyte production. Comparisons of water-column CH4 storage versus open-water fluxes (6 lakes) showed that diffusive evasion plus MOX removed most CH4 in the water columns every 1-2 days with only modest changes in storage, suggesting that counter-balancing water-column replenishment is substantial. Lakes with short river-connection times (i.e. most strongly autotrophic and strongly CO2- absorbing in this delta) and thermokarst lakes contribute disproportionately to CH4 flux, relative to lakes with long river-connection times. Thus, this great Arctic delta represents an important system of greenhouse-gas emitting lakes despite prior work showing their net absorption of CO2 during open-water, and having a low landscape area of CO2-saturated thermokarst lakes. Autotrophically absorbed CO2 becomes labile carbon substrate, and is microbially shunted back to the atmosphere as the more potent greenhouse gas CH4

Appendix D. Effect of elevated pH on specific bacterial production and respiration in short-term incubations.

Effect of elevated pH on specific bacterial production and respiration in short-term incubations

Appendix C. Assessment of the effect of increased ionic strength and alkalinity, and raising pH through NaOH addition vs. dissolved CO2 depletion.

Assessment of the effect of increased ionic strength and alkalinity, and raising pH through NaOH addition vs. dissolved CO2 depletion

Appendix A. Calculated changes in alkalinity and ionic strength in response to short-term and long-term experiments.

Calculated changes in alkalinity and ionic strength in response to short-term and long-term experiments

Appendix B. Additional methodological details.

Additional methodological details

Mercury Export to the Arctic Ocean from the Mackenzie River, Canada

Circumpolar rivers, including the Mackenzie River in Canada, are sources of the contaminant mercury (Hg) to the Arctic Ocean, but few Hg export studies exist for these rivers. During the 2007–2010 freshet and open water seasons, we collected river water upstream and downstream of the Mackenzie River delta to quantify total mercury (THg) and methylmercury (MeHg) concentrations and export. Upstream of the delta, flow-weighted mean concentrations of bulk THg and MeHg were 14.6 ± 6.2 ng L^–1 and 0.081 ± 0.045 ng L^–1, respectively. Only 11–13% and 44–51% of bulk THg and MeHg export was in the dissolved form. Using concentration–discharge relationships, we calculated bulk THg and MeHg export into the delta of 2300–4200 kg yr^–1 and 15–23 kg yr^–1 over the course of the study. Discharge is not presently known in channels exiting the delta, so we assessed differences in river Hg concentrations upstream and downstream of the delta to estimate its influence on Hg export to the ocean. Bulk THg and MeHg concentrations decreased 19% and 11% through the delta, likely because of particle settling and other processes in the floodplain. These results suggest that northern deltas may be important accumulators of river Hg in their floodplains before export to the Arctic Ocean