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.
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 δ
13C 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.
Large circumpolar rivers influence the biogeochemistry of coastal shelf ecosystems and Arctic Ocean circulation. From 2007 to 2010, Mackenzie River nutrients were measured immediately upstream of the ...Mackenzie Delta. Total suspended sediment (TSS) and the dissolved and particulate fractions of carbon, nitrogen, and phosphorus were measured throughout the water year, stratified by hydrologically defined seasonal periods. Sampling was most frequent during the ice breakup and freshet periods to capture changes in sediment and nutrient concentrations that occurred with rapidly changing discharge. Our 4 year sampling coverage yielded constituent‐specific concentration‐discharge relationships that differed among seasonal hydrological periods for 6 of 12 constituents. Results revealed that having data from the rising‐water freshet, when ice effects drive much higher water levels for a given discharge than during open water, had a surprisingly modest effect on annual flux calculations for most constituents. The rising freshet, however, was dominated by high relative levels of C‐rich dissolved organic matter, P‐rich particles, and N‐rich inorganic nutrients. Comparisons with fluxes based on volume‐weighted means of sample concentrations revealed that most, though not all, river fluxes were strongly driven by short‐term concentration‐discharge dynamics rather than by total annual discharge. Comparisons with fluxes derived for the Mackenzie discharge record, where results from other years are available, suggest that concentration‐discharge relations may not be robust beyond the observation period in this system. Constituent fluxes herein differ from previously published fluxes for the Mackenzie (e.g., lower TSS, higher dissolved organic carbon), likely reflecting more frequent measurements, more representative characterization of differing hydrological periods, and possible changes in concentration‐discharge relations during the longer‐term Mackenzie record.
Key Points
Nutrient fluxes were measured in an ice‐affected great Arctic river based on stratified sampling within hydrologically defined seasons
The effect of having rising freshet data on flux estimates was smaller than the effect of dividing flows into hydrologically defined seasons
Fluxes herein differed from prior work, and a method comparison revealed a strong effect of short‐term concentration‐discharge dynamics
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.
In 2004, ultraviolet‐B (UVB) and ultraviolet‐A (UVA) attenuation were measured in Mackenzie Delta lakes spanning gradients in water renewal rate, dissolved organic carbon (DOC) concentration, and ...dissolved organic matter (DOM) composition. DOM compositions (ratio of chromophoric DOM (CDOM) to non‐chromophoric DOM) in Delta lakes are complex, evolving seasonally via flooding, dilution, macrophyte production, photobleaching, and bacterial metabolism. Attenuation was more strongly related to CDOM absorption coefficients (a330; UVB r2 = 0.69, p < 0.0001; UVA r2 = 0.58, p < 0.0001) than to DOC concentrations due to variable DOM compositions. Attenuation in one set of lakes was well related (linear models) to a330 and total suspended solids (UVB R2 = 0.80, p < 0.0001; UVA R2 = 0.81, p < 0.0001). When these models were applied to other Delta lakes, however, attenuation of UVB was overestimated in 17 of 19 cases and attenuation of UVA was overestimated in all 18 cases. This bias indicates that models are not transferrable among Delta lakes, and likely cannot be applied in other circumpolar delta lakes with similarly complex DOM compositions. Although attenuation is high in Delta lakes (KdUVB 17.1–33.4 m−1; KdUVA 7.7–19.2 m−1), plankton and photoreactive solutes may be exposed to high levels of ultraviolet radiation (UVR) because Delta lakes are shallow, UVB and UVA penetrate the top 19% and 31% of water columns, respectively, and day lengths are extended during open water. Thus, climate change effects on DOM compositions may significantly alter in situ UVR environments in circumpolar delta lakes.
Outflow from north-flowing circumpolar rivers has a strong influence on the Arctic Ocean. The Peel and Arctic Red Rivers are tributaries of the Mackenzie Delta, a large, lake-rich floodplain that ...forms the interface between the Mackenzie River and the Beaufort Sea basin of the Arctic Ocean. Here, we present water quality data that were collected from the Peel and Arctic Red Rivers between 2007 and 2010 as part of an International Polar Year project that investigated the seasonal hydrology and biogeochemistry of the Mackenzie River and its delta. The Peel River was sampled 57 times between May 2007 and September 2010 upstream of the community of Fort McPherson, Northwest Territories (NT), while the Arctic Red River was sampled 32 times between May 2007 and August 2008 (with one additional sample in June 2010) approximately 0.5 km upstream of its confluence with the Mackenzie River near the community of Tsiigehtchic, NT. Each water sample was analyzed for up to 22 water quality parameters, including water temperature, specific conductivity, pH, chlorophyll a, total suspended sediments, particulate nutrients (carbon, nitrogen, and phosphorus), soluble reactive silica, major ions (calcium, magnesium, potassium, sodium, chloride, and sulfate), dissolved carbon (inorganic and organic fractions), and dissolved nutrients (three nitrogen and two phosphorus fractions). This data set, which is available for download and reuse, provides important baseline information about water quality in the Peel and Arctic Red Rivers, complements other data that have been collected in these watersheds, and will be of interest to researchers, resource managers, Indigenous organizations, and governments that are active in the region.
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.
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 (H
2
O
2
) in lake waters. Here we assessed the patterns of H
2
O
2
using three approaches: (i) H
2
O
2
levels were tracked in 40 lakes during the open water season from the Arctic summer solstice (24 h sunlight) to late summer; (ii) diurnal dynamics of in situ H
2
O
2
were tracked in a pair of lakes with contrasting DOC regimes (coloured vs. noncoloured DOC); and (iii) buildup of H
2
O
2
was tracked in experimental microcosms of lake water exposed to manipulated UV levels. H
2
O
2
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 H
2
O
2
during multiple cloudless days. Cumulatively, H
2
O
2
was highest in the Delta lake with a higher DOC concentration and low in colour. H
2
O
2
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 H
2
O
2
production.
While many instructors have reservations against Wikipedia use in academic settings, editing Wikipedia teaches students valuable writing, editing, and critical thinking skills. Wikipedia assignments ...align with the community of inquiry framework, which focuses on the elements needed for a successful online learning experience. We report on a faculty mentoring network, created by WikiProject Limnology and Oceanography, which helped 14 instructors with little to no prior experience implement a Wikipedia assignment in their classes. We found that Wikipedia assignments increase students’ motivation to produce high quality work and enhance their awareness of reliable scientific sources. Wikipedia assignments can be comparable to other writing assignments in length and complexity, but have a far wider audience than a traditional research paper. Participants in our mentoring network reported challenges with implementing this new type of assignment, and here, we share resources and solutions to those reported barriers.
Abstract
Large circumpolar rivers influence the biogeochemistry of coastal shelf ecosystems and Arctic Ocean circulation. From 2007 to 2010, Mackenzie River nutrients were measured immediately ...upstream of the Mackenzie Delta. Total suspended sediment (TSS) and the dissolved and particulate fractions of carbon, nitrogen, and phosphorus were measured throughout the water year, stratified by hydrologically defined seasonal periods. Sampling was most frequent during the ice breakup and freshet periods to capture changes in sediment and nutrient concentrations that occurred with rapidly changing discharge. Our 4 year sampling coverage yielded constituent‐specific concentration‐discharge relationships that differed among seasonal hydrological periods for 6 of 12 constituents. Results revealed that having data from the rising‐water freshet, when ice effects drive much higher water levels for a given discharge than during open water, had a surprisingly modest effect on annual flux calculations for most constituents. The rising freshet, however, was dominated by high relative levels of C‐rich dissolved organic matter, P‐rich particles, and N‐rich inorganic nutrients. Comparisons with fluxes based on volume‐weighted means of sample concentrations revealed that most, though not all, river fluxes were strongly driven by short‐term concentration‐discharge dynamics rather than by total annual discharge. Comparisons with fluxes derived for the Mackenzie discharge record, where results from other years are available, suggest that concentration‐discharge relations may not be robust beyond the observation period in this system. Constituent fluxes herein differ from previously published fluxes for the Mackenzie (e.g., lower TSS, higher dissolved organic carbon), likely reflecting more frequent measurements, more representative characterization of differing hydrological periods, and possible changes in concentration‐discharge relations during the longer‐term Mackenzie record.
Key Points
Nutrient fluxes were measured in an ice‐affected great Arctic river based on stratified sampling within hydrologically defined seasons
The effect of having rising freshet data on flux estimates was smaller than the effect of dividing flows into hydrologically defined seasons
Fluxes herein differed from prior work, and a method comparison revealed a strong effect of short‐term concentration‐discharge dynamics
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