ALBERT

Description: Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen : carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2:CO:NH4+ molar ratio of 165:11:1 in the estuary, 60:3:1 on the shelf, and 18:2:1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85 % of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4 % of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a~DIN source corresponding to 7 % of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities would increase correspondingly to 6 %, 11 %, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.

Description: In the summer of 2010, atmosphere–ice–ocean interaction was studied aboard the icebreaker R/V Xuelong during the Chinese National Arctic Research Expedition (CHINARE), in the sea ice zone of the Pacific Arctic sector between 150° W and 180° W up to 88.5° N. The expedition lasted from 21 July to 28 August and comprised of ice observations and measurements along the cruise track, 8 short-term stations and one 12-day drift station. Ship-based observations of ice thickness and concentration are compared with ice thickness measured by an electromagnetic induction device (EM31) mounted off the ship's side and ice concentrations obtained from AMSR-E. It is found that the modal thickness from ship-based visual observations matches well with the modal thickness from the mounted EM31. A grid of 8 profiles of ice thickness measurements (four repeats) was conducted at the 12-day drift station in the central Arctic (~ 86°50´ N–87°20´ N) and an average melt rate of 2 cm day−1, primarily bottom melt, was found. As compared with the 2005 data from the Healy/Oden Trans-Arctic Expedition (HOTRAX) for the same sector but ~ 20 days later (9 August to 10 September), the summer 2010 was first-year ice dominant (vs. the multi-year ice dominant in 2005), 70% or less in mean ice concentration (vs. 90% in 2005), and 94–114 cm in mean ice thickness (vs. 150 cm in 2005). Those changes suggest the continuation of ice thinning, less concentration, and younger ice for the summer sea ice in the sector since 2007 when a record minimum sea ice extent was observed. Overall, the measurements provide a valuable dataset of sea ice morphological properties over the Arctic Pacific Sector in summer 2010 and can be used as a benchmark for measurements of future changes.

Description: The Chinese National Arctic Research Expedition (CHINARE) in the summer 2010, primarily from 21 July to 28 August, in the ice zone of Arctic Pacific Sector, between 150° W to 180° W to 88.5° N, conducted comprehensive scientific studies on atmosphere-ice-ocean interactions, using icebreaker R.V. Xuelong. Measurements made included underway visual observations of snow and ice conditions at half-hourly time scale and EM31-measured ice thickness at one 12-day and eight short-term (3–4 h each) ice stations. The visual observation data are compared with AMSR-E ice concentration, ice thickness measured by a hanging EM31 from the vessel, and video-recorded image-derived ice concentration and pond coverage. A grid of 8 profiles of ice thickness measurements (four repeats) was conducted at the 12-day ice station (∼86°50' N–87°20' N) in the central Arctic and an average 2 cm day−1 melt rate, primarily bottom melt, was found, after surface melting had almost ceased. The high bottom melt rate, as compared with previous results from other studies, is consistent with the high floe speed (mean 0.17 m s−1) that is also larger than that previously reported. We also found that the daily AMSR-E ice concentration data can be used to map the marginal ice zone (MIZ) and its change. There are clear differences between the MIZ and pack ice zone in terms of ice concentration, thickness, ice type, floe size, as well as pond coverage. Results indicate that, as compared with the 2005 data from the Healy/Oden Trans-Arctic Expedition for the Arctic Pacific Sector (9 August to 10 September), the 2010 was first-year ice dominant (vs. the multiyear ice dominant in 2005), 70% or less in ice concentration (vs. 90% in 2005), 94–114 cm in ice thickness (vs. 150 cm in 2005). No snow cover was observed on the ice south of 78° N and 8–10 cm mean snow depth due to new snowfall events, which melted away after 17 August (vs. no snow cover south of 84.3° N or ~10 cm snow depth further north in 2005). Those changes indicate the continuation of ice thinning, less concentration, and younger ice after the 2007 shift, when a record minimum sea ice extent was observed. Overall, the measurements provided a valuable dataset of sea ice morphological properties over the Arctic Pacific Sector in summer 2010, which confirms, by comparison with previous data, that a "new normal" of Arctic sea ice is now present and is a benchmark for measurements of possible future changes.

Description: This study develops a model for early warning of snow-caused livestock disasters on a county basis and proposes a method of qualitative risk assessment of snow disasters at 500 m resolution for pastoral areas on the Tibetan Plateau (TP). Data used for the model development include remote sensing data, statistical data of weather, livestock, and social economy, and 45 typical snow disaster cases from 2000 to 2010. The principal component analysis (PCA) approach is used to choose 7 crucial factors that contribute over 85% of information for early warning snow disasters on the TP. They are mean annual probability of snow disaster, number of snow-covered days, livestock stocking rate, continual days of mean daily temperature below −10 °C, grassland burial index, rate of snow-covered grassland, and per livestock gross domestic product. The chosen 411 cases from 2008 to 2010 are used to validate the prediction results from the developed early warning model, with an overall accuracy of 85.64% in predicting snow disasters and no disasters. This suggests that the early warning approach developed in the study has operational potential for predicting snow disasters on the TP.

Description: Irrigation development is rapidly expanding in mostly rainfed Sub-Saharan Africa. This expansion underscores the need for a more comprehensive understanding of water resources beyond surface water. Gravity Recovery and Climate Experiment (GRACE) satellites provide valuable information on spatio-temporal variability in water storage. The objective of this study was to calibrate and evaluate a semi-distributed regional-scale hydrologic model based on the Soil and Water Assessment Tool (SWAT) code for basins in Sub-Saharan Africa using seven-year (July 2002–April 2009) 10-day GRACE data and multi-site river discharge data. The analysis was conducted in a multi-criteria framework. In spite of the uncertainty arising from the tradeoff in optimising model parameters with respect to two non-commensurable criteria defined for two fluxes, SWAT was found to perform well in simulating total water storage variability in most areas of Sub-Saharan Africa, which have semi-arid and sub-humid climates, and that among various water storages represented in SWAT, water storage variations in soil, vadose zone and groundwater are dominant. The study also showed that the simulated total water storage variations tend to have less agreement with GRACE data in arid and equatorial humid regions, and model-based partitioning of total water storage variations into different water storage compartments may be highly uncertain. Thus, future work will be needed for model enhancement in these areas with inferior model fit and for uncertainty reduction in component-wise estimation of water storage variations.

Description: Irrigation development is rapidly expanding in mostly rainfed Sub-Saharan Africa. This expansion underscores the need for a more comprehensive understanding of water resources beyond surface water. Gravity Recovery and Climate Experiment (GRACE) satellites provide valuable information on spatio-temporal variability of water storage. The objective of this study was to calibrate and evaluate a semi-distributed regional-scale hydrological model, or a large-scale application of the Soil and Water Assessment Tool (SWAT) model, for basins in Sub-Saharan Africa using seven-year (2002–2009) 10-day GRACE data. Multi-site river discharge data were used as well, and the analysis was conducted in a multi-criteria framework. In spite of the uncertainty arising from the tradeoff in optimizing model parameters with respect to two non-commensurable criteria defined for two fluxes, it is concluded that SWAT can perform well in simulating total water storage variability in most areas of Sub-Saharan Africa, which have semi-arid and sub-humid climates, and that among various water storages represented in SWAT, the water storage variations from soil, the vadose zone, and groundwater are dominant. On the other hand, the study also showed that the simulated total water storage variations tend to have less agreement with the GRACE data in arid and equatorial humid regions, and the model-based partition of total water storage variations into different water storage compartments could be highly uncertain. Thus, future work will be needed for model enhancement in these areas with inferior model fit and for uncertainty reduction in component-wise estimation of water storage variations.

Description: Spectrally resolved efficiency (i.e. apparent quantum yield, AQY) of carbon monoxide (CO) photoproduction is a useful indicator of substrate photoreactivity and a crucial parameter for modeling CO photoproduction rates in the water column. Recent evidence has suggested that CO photoproduction from particles in marine waters is significant compared to the well-known CO production from chromophoric dissolved organic matter (CDOM) photodegradation. Although CDOM-based CO AQY spectra have been extensively determined, little is known of this information on the particulate phase. Using water samples collected from the Mackenzie estuary, shelf, and Canada Basin in the Southeastern Beaufort Sea, the present study for the first time quantified the AQY spectra of particle-based CO photoproduction and compared them with the concomitantly determined CDOM-based CO AQY spectra. CO AQYs of both particles and CDOM decreased with wavelength but the spectral shape of the particulate AQY was flatter in the visible regime. This feature resulted in a disproportionally higher visible light-driven CO production by particles, thereby increasing the ratio of particle- to CDOM-based CO photoproduction with depth in the euphotic zone. In terms of depth-integrated production in the euphotic zone, CO formation from CDOM was dominated by the ultraviolet (UV, 290–400 nm) radiation whereas UV and visible light played roughly equal roles in CO production from particles. Spatially, CO AQY of bulk particulate matter (i.e. the sum of organics and inorganics) augmented from the estuary to shelf to basin while CO AQY of CDOM trended inversely. Water from the deep chlorophyll maximum layer revealed higher CO AQYs than did surface water for both particles and CDOM. CO AQY of bulk particulate matter exceeded that of CDOM on the shelf and in the basin but the sequence reversed in the estuary. Mineral absorption-corrected CO AQY of particulate organic matter (POM) was, however, greater than its CDOM counterpart in all three sub-regions and displayed magnitudes in the estuary that were no inferior to those in shelf and offshore waters. In terms of CO photoproduction, POM was thus more photoreactive than CDOM, irrespective of the organic matter's origins (i.e. terrigenous or marine). Riverine CDOM exhibited higher photoreactivity than marine CDOM and land-derived POM appeared similarly or more photoreactive than marine POM. AQY-based modeling indicates that CO photoproduction in the study area is underestimated by 13–48 % if the particulate term is ignored.

Description: The Arctic Ocean (AO) undergoes profound changes of its physical and biotic environments due to climate change. The greater light exposure and stratification alter its plankton ecosystem structure, functioning and productivity promoting oligotrophy in some areas as the Beaufort Sea. A one-dimension (1-D) physical-biological coupled model based on the large multiparametric database of the Malina project in the Beaufort Sea was used (i) to infer the functioning and nitrogen fluxes within the summer plankton ecosystem and (ii) to assess the model sensitivity to key light-associated processes involved in nutrient recycling and phytoplankton growth. The coupled model suggested that ammonium photochemically produced from photosensitive dissolved organic nitrogen (i.e. photoammonification process) was a necessary nitrogen source to achieve the observed levels of microbial biomass and production. It contributed to ca. two-thirds and one-third of the simulated surface (0–10 m) and depth-integrated primary and bacterial production, respectively. The model also suggested that carbon to chlorophyll ratios for small (〈 5 μm) phytoplankton (ca. 15–45 g g−1) lower than those commonly used in biogeochemical models applied to the AO were required to simulate the observed herbivorous versus microbial food web competition and realistic nitrogen fluxes in the Beaufort Sea oligotrophic waters. In face of accelerating Arctic warming, more attention should be paid in the future to the mechanistic processes involved in food webs and functional groups competition, nutrient recycling and primary production in poorly productive waters of the AO as they are expected to expand rapidly.

Description: Photolysis of dimethylsulfide (DMS), a secondary photochemical process mediated by chromophoric dissolved organic matter (CDOM), has previously been demonstrated to be an important loss term of DMS in the surface layer of warm seas and the Southern Ocean. The role of photolysis in regulating the DMS dynamics in the Arctic Ocean, however, remains obscure. This study for the first time determined the apparent quantum yield (AQY) spectra of DMS photooxidation in northern polar marine milieus covering the Baffin Bay in the eastern Canadian Arctic and the Mackenzie River estuary, Mackenzie Shelf and Canada Basin in the western Canadian Arctic. The DMS AQY was fairly invariant at salinities 〈 25 but rose rapidly with further increasing salinity, which is well defined by a three-parameter exponential decay equation with a constant intercept. Salinity can therefore be used as a quantitative indicator of the DMS AQY. The DMS AQY in the ultraviolet (UV) wavelengths was linearly and positively correlated with the spectral slope coefficient (275–295 nm) of the CDOM absorption spectrum, suggesting that marine CDOM photosensitizes the degradation of DMS more efficiently than does terrestrial CDOM. High concentrations of nitrate (~12 μmol L−1) in deep water samples boosted DMS photooxidation by 70–80%, due likely to radical chemistry of nitrate photolysis. Coupled optical-photochemical modeling, based on the obtained DMS AQY spectra, shows that UV-A (320–400 nm) accounted for 60–75% of the DMS photolysis in the sunlit surface layer and that photochemistry degraded DMS on an e-folding time from 9 to 100 d (mean: 29 d). The photooxidation term on average accounted for 21% of the DMS gross loss rate and was comparable to the atmospheric DMS ventilation rate estimated for the same geographic regions.

Description: Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen:carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2 : CO : NH4+ molar ratio of 165 : 11 : 1 in the estuary, 60 : 3 : 1 on the shelf, and 18 : 2 : 1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85% of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4% of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a DIN source corresponding to 7% of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities could increase correspondingly to 6%, 11%, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.