Linking Organic Carbon Sources with Meiobenthic Abundance and Diversity Across the Coastal Shelf of the Mackenzie Delta

During the Arctic spring freshet, the Mackenzie River releases large amounts of dissolved and particulate terrestrial organic carbon into the Canadian Beaufort Shelf Sea. The accompanying pulse of nutrients fuels phytoplankton blooms in late spring and summer, and together with under ice production, contributes a pulse of marine derived carbon to both the water column and the sediment. Regional carbon budgets suggest both may contribute to benthic production. This component of the Marine Arctic Ecosystem Study (MARES) examined a transect collected across the western Mackenzie coastal shelf to determine the relative contribution of marine and terrestrial carbon to marine sediments and its relationship with meiobenthic diversity and abundance. Organic biomarker analysis via tandem mass spectrometry was used to constrain the amount and type of organic sources together with total hydrolysable amino acids (THAA) as markers of organic matter lability and cycling. Organic carbon sources (lipids and amino acids) were then compared to meiofaunal abundance and diversity, as well as sedimentary stable isotopes. The distribution of organic markers suggest that both terrestrial and marine carbon are important sources to sediments, but are not uniform. Phytol, total fatty acid concentrations, and algal-associated neutral lipids were all highest in the mid shelf region while the site nearest the River Delta showed highest terrestrial organic inputs as expected from river transport. Amino acid and diatom frustules further support that diatoms were a major source of marine carbon at mid-shelf stations, suggesting that water column productivity peaks in the mid-shelf due to early polynyas or significant under ice production. Benthic meiofaunal abundance was also highest in the mid shelf, reflecting more labile carbon reaching sediments annually in this region. Sediment texture was likely a limiting factor to benthic abundance in the shallow station, while lower organic carbon input reduced benthic production in the deepest location. These unique sea ice thermodynamics and water circulation along the mid-shelf may contribute to this unexpected site for carbon disposition and benthic activity.