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 > 200 m and ≤ 2000m) and basin (depths > 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 > 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.