Abstract
The shallow Pacific Arctic shelf has historically acted as an effective carbon sink, characterized by tight benthic pelagic coupling. However, the strength of the biological carbon pump in the Arctic has been predicted to weaken with climate change due to increased duration of the open-water period for primary production, enhanced nutrient limitation, and increased pelagic heterotrophy. In order and gain insights into how the biological carbon pump is functioning under the recent conditions of extreme warming and sea ice loss on the Pacific Arctic shelf, we measured sinking particulate organic carbon (POC) fluxes from drifting and moored sediment traps, as well as rates of primary production and particle-associated microbial respiration in June 2018. We measured high sinking POC fluxes similar to and/or higher than previous measurements from this region (up to 2.3 g C m-2 d-1), reaching flux levels among the highest ever documented in the global oceans. Furthermore, high export ratios averaging 82% and low rates of particle-associated microbial respiration also indicated negligible recycling of sinking POC in the water column during spring of 2018. These results highlight the extraordinary strength of biological carbon pump on the Pacific Arctic shelf during an unusually warm and low-sea ice year. While additional measurements and time are needed to confirm the ultimate trajectory of these fluxes in response to ongoing climate change, these results call into question the prevailing hypothesis that the strength of the biological carbon pump in the Pacific Arctic will weaken under these conditions.
Highlights
Arctic marine systems are currently undergoing rapid and profound changes due to the effects of climate change, including reduced sea ice extent, earlier sea ice retreat, protracted ice-free seasons, warming air and ocean temperatures, and shifts in currents and water column stratification (Vaughan et al, 2013; Richter-Menge et al, 2019)
The spring phytoplankton bloom is typically dominated by large, rapidly sinking sympagic or pelagic diatoms (Springer and McRoy, 1993; Gradinger, 1999, 2009) that contributed to an annual particulate organic carbon (POC) flux of up to 145 g C m−2 year−1 near Hanna Shoal (Lalande et al, 2020) and likely facilitate substantial carbon burial in sediments
We measured consistently lower POC flux rates in Alaska Coastal Water (ACW) than Bering Shelf/Anadyr Waters (BSAW). These results support the previously untested hypothesis that POC fluxes would be higher in the BSAW compared to ACW (Grebmeier and McRoy, 1989). This distinction in primary productivity and POC flux between the ACW and BSAW could partially be attributed to differences in stratification between these regions; we found a less stratified water column in the BSAW, which could contribute to nutrient input to the surface and allow for higher primary productivity rates
Summary
Arctic marine systems are currently undergoing rapid and profound changes due to the effects of climate change, including reduced sea ice extent, earlier sea ice retreat, protracted ice-free seasons, warming air and ocean temperatures, and shifts in currents and water column stratification (Vaughan et al, 2013; Richter-Menge et al, 2019). The shallow Pacific Arctic shelf, averaging 50 m depth, has historically acted as a strong sink of carbon (Bates, 2006; Chen and Borges, 2009) Water movement on these shelves is generally northward carrying different water masses of Pacific origin into the Arctic (Pickart et al, 2016; Danielson et al, 2017), with a significant seasonal modulation (Woodgate et al, 2015) (see Supplementary Figure 1 for visualization of currents). This active biological carbon pump supports large populations of benthic-feeding pelagic seabirds and marine mammals (Bluhm and Gradinger, 2008; Moore and Kuletz, 2019), many of which are important to Indigenous communities that rely on subsistence hunting (Hovelsrud et al, 2008)
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