Organic biomarkers and Meiofauna diversity reflect distinct carbon sources to sediments transecting the Mackenzie continental shelf

Abstract During the Arctic spring freshet, the Mackenzie River delivers large amounts of dissolved and particulate terrestrial organic carbon to the Canadian Beaufort Sea shelf. This pulse of material and nutrients fuels phytoplankton blooms in late spring and summer and together with under ice primary production contributes to the organic carbon reservoir of the coastal shelf sediments. As a component of the Marine Arctic Ecosystem Study (MARES), samples transecting the Canadian Mackenzie Trough were collected in fall before ice cover to examine the balance of marine and terrestrial carbon in sediments and its relationship with sediment meiobenthic diversity and abundance. Organic biomarker analysis of lipid proxies together with total hydrolyzable amino acids (THAA) defined organic matter sources and potential lability. Across the shelf transect, consistent values observed in sedimentary total organic carbon (1.09%–1.62%) belied a shift in the balance of terrestrial and marine carbon sources. Predominately terrestrial inputs at the shallow site near the delta outflow appeared to constrain meiofaunal populations while the deepest location off shelf reflected lower labile marine organic carbon reaching the sediments that limited benthic production. Surface sediments at the mid-shelf, however, showed a near 3-fold increase (to 1.16 mg C OC-1) in contributions by diatoms and dinoflagellates markers plus a peak in total amino acids (9.53 mg g OC-1) which corresponded with higher meiofauna abundance. The combination of detailed organic analysis together with meiofaunal community measures suggest that mid-shelf sediments receive higher contributions of labile marine carbon which drive increased meiofauna diversity and abundance despite the large background of terrestrial organic material exiting the Mackenzie River outflow.