Organic matter remineralization in marine sediments : A Pan-Arctic synthesis

Natural Environment Research Council (GrantNumber(s): NE/J023094/1; Grant recipient(s): Ursula Witte) ArcticNet (GrantNumber(s): Hotspot biodiversity project; Grant recipient(s): Philippe Archambault)Peer reviewedPublisher PD

A Shelf-to-Basin Examination of Food Supply for Arctic Benthic Macrofauna and the Potential Biases of Sampling Methodology

Macrofaunal samples (benthic fauna) and sediment samples were collected in association with the sampling programs of the Bering Strait Environmental Observatory (BSEO; Cooper et al. 2006, see http://arctic.bio.utk.edu/) during the summer of 2003 and 2004 and the Western Arctic Shelf-Basin Interactions (SBI; Grebmeier and Harvey 2005, see http://sbi.utk.edu for further information) during the spring (May-June) and summer (July-August) of 2004. Benthic measurements of sediment chlorophyll a, grain size, total organic carbon, C/N ratios, and macroinfaunal community composition were measured on the shelf, slope and basin of the region. The current study focuses on sediment chlorophyll a inventories of surface layer sediments and how the utilization of different sieve mesh sizes (0.5 mm and 1.0 mm) during macroinfaunal collections can impact interpretations of macroinfaunal community structure. Overall, surface sediment chlorophyll a was highest at shelf stations (depth ≤ 200 m) and decreased with increasing water depth in the slope (depths \u3e 200 m and ≤ 2000m) and basin (depths \u3e 2000 m) regions. Subsurface peaks of sediment chlorophyll a were found at stations in the northern Chukchi and western Beaufort Seas. Comparison of these downcore profiles of sediment chlorophyll a and the radioisotope 137Cs suggest that chlorophyll a that is buried in sediments could remain active for decadal time scales. At all stations sampled, macroinfaunal abundance retained on combined 0.5 mm and 1.0 mm sieve size fractions were higher than the number of animals retained on the 1.0 mm sieve alone. The increase in station abundance with addition of the 0.5 mm sieve compared to only the 1.0 mm screen was largely due to increased numbers of macrofaunal juveniles and meiofauna (e.g. foraminifera and nematodes). By comparison, approximately 97% of the total macroinfaunal carbon biomass for all stations was retained on the 1.0 mm sieve; the 0.5 mm sieve collected the remaining 3% of total carbon biomass. Since the 1.0 mm retained similar abundance and a high percentage of benthic biomass compared to the 0.5 mm sieve on the shelf and slope of the study region, I conclude that the 1.0 mm sieve provides a reasonable approximation of benthic macroinfaunal populations on the shelf and slope regions. However, in the basin (depths \u3e 2000 m) where there is a shift to meiofaunal dominance (e.g. foraminifera), the 0.5 mm sieve is clearly preferable for estimation of the benthic community abundance and biomass

The relationship between sea ice break-up, water mass variation, chlorophyll biomass, and sedimentation in the northern Bering Sea

The northern Bering Sea shelf is dominated by soft-bottom infauna and ecologically significant epifauna that are matched by few other marine ecosystems in biomass. The likely basis for this high benthic biomass is the intense spring bloom, but few studies have followed the direct sedimentation of organic material during the bloom peak in May. Satellite imagery, water column chlorophyll concentrations and surface sediment chlorophyll inventories were used to document the dynamics of sedimentation to the sea floor in both 2006 and 2007, as well as to compare to existing data from the spring bloom in 1994. An atmospherically-derived radionuclide, 7Be, that is deposited in surface sediments as ice cover retreats was used to supplement these observations, as were studies of light penetration and nutrient depletion in the water column as the bloom progressed. Chlorophyll biomass as sea ice melted differed significantly among the three years studied (1994, 2006, 2007). The lowest chlorophyll biomass was observed in 2006, after strong northerly and easterly winds had distributed relatively low nutrient water from near the Alaskan coast westward across the shelf prior to ice retreat. By contrast, in 1994 and 2007, northerly winds had less northeasterly vectors prior to sea ice retreat, which reduced the westward extent of low-nutrient waters across the shelf. Additional possible impacts on chlorophyll biomass include the timing of sea-ice retreat in 1994 and 2007, which occurred several weeks earlier than in 2006 in waters with the highest nutrient content. Late winter brine formation and associated water column mixing may also have impacts on productivity that have not been previously recognized. These observations suggest that interconnected complexities will prevent straightforward predictions of the influence of earlier ice retreat in the northern Bering Sea upon water column productivity and any resulting benthic ecosystem re-structuring as seasonal sea ice retreats in the northern Bering Sea

A mass budget for mercury and methylmercury in the Arctic Ocean

Elevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are observed throughout the Arctic Ocean, but major sources and degradation pathways in seawater are not well understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since 2004 and discuss implications and uncertainties. Our calculations show that high total mercury (Hg) in Arctic seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20 Mg a−1) occurs in the subsurface ocean (20-200 m). There it is converted to dimethylmercury (Me2Hg: 17 Mg a−1), which diffuses to the polar mixed layer and evades to the atmosphere (14 Mg a−1). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg. We postulate that most evaded Me2Hg is redeposited as MeHg and that atmospheric deposition is the largest net MeHg source (8 Mg a−1) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs account for approximately 15% of inputs to the surface ocean (2.5 Mg a−1) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice-free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations