Photodegraded Dissolved Organic Matter from Peak Freshet River Discharge as a Substrate for Bacterial Production in a Lake-rich Great Arctic Delta

Lake-rich Arctic river deltas are recharged with terrigenous dissolved organic matter (DOM) during the yearly peak water period corresponding with the solstice (24 h day−1 solar irradiance). Bacteria-free DOM collected during peak Mackenzie River discharge was exposed to sunlight for up to 14 days in June 2010. As solar exposure increased, carbon and lignin concentrations declined (10% and 42%, respectively, after 14 days), as did DOM absorptivity (62% after 14 days), aromaticity, and molecular weight. Photochemical changes were on par with those normally observed in Mackenzie Delta lakes over the entire open-water season. When irradiated freshet DOM was provided as a substrate, no significant differences were observed in community-level metabolism among five bacterial communities from representative delta habitats. However, bacterial abundance was significantly greater when nonirradiated (0 day) rather than irradiated DOM (7 or 14 days) was provided, while cell-specific metabolic measures revealed that per-cell bacterial production and growth efficiency were significantly greater when communities were provided irradiated substrate. This complex response to rapid DOM photodegradation may result from the production of inhibitory reactive oxygen species (ROS), along with shifts in bacterial community composition to species that are better able to tolerate ROS, or metabolize the labile photodegraded DOM

Photodegraded Dissolved Organic Matter from Peak Freshet River Discharge as a Substrate for Bacterial Production in a Lake-rich Great Arctic Delta

Lake-rich Arctic river deltas are recharged with terrigenous dissolved organic matter (DOM) during the yearly peak water period corresponding with the solstice (24 h day−1 solar irradiance). Bacteria-free DOM collected during peak Mackenzie River discharge was exposed to sunlight for up to 14 days in June 2010. As solar exposure increased, carbon and lignin concentrations declined (10% and 42%, respectively, after 14 days), as did DOM absorptivity (62% after 14 days), aromaticity, and molecular weight. Photochemical changes were on par with those normally observed in Mackenzie Delta lakes over the entire open-water season. When irradiated freshet DOM was provided as a substrate, no significant differences were observed in community-level metabolism among five bacterial communities from representative delta habitats. However, bacterial abundance was significantly greater when nonirradiated (0 day) rather than irradiated DOM (7 or 14 days) was provided, while cell-specific metabolic measures revealed that per-cell bacterial production and growth efficiency were significantly greater when communities were provided irradiated substrate. This complex response to rapid DOM photodegradation may result from the production of inhibitory reactive oxygen species (ROS), along with shifts in bacterial community composition to species that are better able to tolerate ROS, or metabolize the labile photodegraded DOM

Ice-out and Freshet Fluxes of CO2 and CH4 Across the Air–water Interface of the Channel Network of a Great Arctic Delta, the Mackenzie

Carbon dioxide (CO2) and methane (CH4) were monitored at five sites spanning the upstream–downstream extent of the Mackenzie Delta channel network during May 2010, capturing the historically under-sampled ice-out period that includes the rising freshet, peak water levels and the early falling freshet (flood recession). Unexpectedly, partial pressures of CO2 in the Mackenzie River were undersaturated during the rising freshet before water levels peaked, indicating net CO2 invasion at instantaneous CO2 flux rates (F-CO2) ranging from –112 to –258 mg-C m-2 d-1. Net CO2 invasion was also observed around the time of peak water levels at sites in the middle and outer delta. Following peak water levels, the Mackenzie River switched to saturation and net CO2 evasion (F-CO2 from 74 to 177 mg-C m-2 d-1). Although the Peel River (which flows into the west side of the Mackenzie Delta) was a strong emitter of CO2 (F-CO2 from 373 to 871 mg-C m-2 d-1), overall, the Mackenzie River and Delta were weak emitters of CO2 during the 2010 ice-out period. All sites were strong emitters of CH4 during ice-out, however, with the highest evasive fluxes observed in the outer delta when the extent of flooded delta landscape was greatest. Estimated aerial fluxes from Mackenzie Delta channel surfaces during May 2010 ranged from 2.1 to 4.8 Gg-C as CO2, and 186 to 433 Mg-C as CH4. These results provide critical information that can be used to refine gas flux estimates in high-latitude circumpolar river deltas during the relatively under-studied ice-out period

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

Arctic deltas as biogeochemical hotspots affecting the delivery of nutrients and dissolved organic matter to the Arctic Ocean

The Mackenzie River and Delta were sampled during hydrologically-defined seasons in four consecutive years to assess 1) the importance of sampling during the rising limb of the flood hydrograph (rising freshet) for accurately characterizing constituent fluxes and quality, and 2) how floodplain processes affect discharge to the Arctic Ocean. Including rising freshet samples had a modest effect on annual sediment and nutrient flux estimates for the Mackenzie River (-9 to +26% difference). Nutrient quality was very different during the rising freshet, however, with relatively high concentrations of carbon-rich dissolved organic matter (DOM), phosphorus-rich particles, and nitrogen-rich inorganic nutrients. Mackenzie River DOM quality was relatively fresher, more terrigenous, and younger (radiocarbon values suggesting ages < 15 years) during the rising freshet, indicating a high proportion of recently-fixed vascular plant material. The Mackenzie was also a net absorber of carbon dioxide during the rising freshet (-112 to -258 mg-C m-2 d-1), switching to net emission after peak flood. Open water (freshet through summer) fluxes of dissolved organic carbon (1.4 Tg) and lignin (7.1 Gg) in the Mackenzie River were greater than previously reported total annual fluxes, likely due to the inclusion of rising freshet data herein. Optical parameters, and statistical relations between fluorescence components derived from Parallel Factor Analysis (PARAFAC; six in delta channels, five in delta lakes) and chemical biomarkers (e.g. lignin phenols), suggest substantial modification of DOM in delta lakes and on the floodplain during downstream transport. When incubated (14 days) under solar conditions similar to those on the floodplain, Mackenzie River DOM (isolated during peak flood) experienced photochemical changes on par with those observed in delta lakes over the entire open water period. Photodegraded DOM significantly reduced abundances but fueled per-cell growth in bacterial populations from delta habitats, indicating rapid shifts in community composition. Gradients in chemical biomarkers were related to the delta-wide gradient of lake hydrological connectivity. These results emphasize the importance of the rising freshet in accurately characterizing Mackenzie River DOM quality and carbon fluxes, and the need to sample downstream sites in lake-rich circumpolar deltas to constrain flux estimates and characterize discharge to the Arctic Ocean

Underwater irradiance attenuation and photobleaching of chromophoric dissolved organic matter in shallow Arctic lakes of the Mackenzie Delta, NWT.

In situ attenuation of ultraviolet-B (UVB; 310-320 nm subset), ultraviolet-A (UVA) and photosynthetically active radiation (PAR) generally declined over the 2004 open-water season in a set of shallow Mackenzie Delta floodplain lakes. Average 1% photic depths were < 30 cm for UVB and < 50 cm for UVA, indicating that part of the water column remained sunscreened from UV. Average euphotic and lake depths were approximately equal (~2 m), indicating sufficient PAR for photosynthesis at all depths. In 2005, dissolved organic matter (DOM) quality was examined using optical indices. Both spectral slope (decline in DOM absorbance as wavelength increases) and specific ultraviolet absorbance (average DOM aromaticity) were strongly correlated with sill elevation. UVA dominated in situ chromophoric DOM photobleaching, indicating that future increases in UVB fluxes are unlikely to exert as great an influence on in situ Mackenzie Delta optical environments as will increased chromophoric DOM from climate change

Patterns of hydrogen peroxide among lakes of the Mackenzie Delta, western Canadian Arctic

Dissolved organic carbon (DOC) in Mackenzie Delta lakes varies in composition and concentration, ranging from low concentrations and high colour in frequently flooded lakes to high concentrations in clear lakes that are infrequently flooded. DOC is a precursor to the photochemical production of hydrogen peroxide (H2O2) in lake waters. Here we assessed the patterns of H2O2 using three approaches: (i) H 2O2 levels were tracked in 40 lakes during the open water season from the Arctic summer solstice (24 h sunlight) to late summer; (H) diurnal dynamics of in situ H2O2 were tracked in a pair of lakes with contrasting DOC regimes (coloured vs. noncoloured DOC); and (JH) buildup of H2O2 was tracked in experimental microcosms of lake water exposed to manipulated UV levels. H2O2 levels were highest at the solstice and in intermittently flooded lakes. During 24 h daylight, lakes with contrasting DOC regimes showed cumulative build up of H2O2 during multiple cloudless days. Cumulatively, H 2O2 was highest in the Delta lake with a higher DOC concentration and low in colour. H2O2 buildup in lake water shielded from UVB exposure was not significantly lower from microcosms under full sunlight. UVA was the most important in coloured DOC photobleaching and H2O2 production. © 2006 NRC

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