Particulate organic matter distributions in surface waters of the Pacific Arctic shelf during the late summer and fall season

Abstract

Climate change is leading to marked decreases in the extent, thickness and persistence of sea ice in Polar Regions, especially during the later summer and fall seasons. The expanded open water domain during this period has the potential to drastically affect the hydrography, biogeochemistry and ecology of Arctic seas. In view of these rapid changes, the distributions of particulate organic matter in surface waters from the Pacific Arctic shelf were determined during the late summer and early fall months (August–October) over four different years (2012, 2015, 2016, and 2017). Utilizing surface underway systems of research vessels in combination with a semi-automated filtration system, over a thousand individual samples from surface waters were collected along the northern Bering Sea and Chukchi Sea shelves. Particulate organic carbon (POC) concentrations and molar carbon:nitrogen ratios ([C:N]) were determined to investigate the distribution and provenance of organic matter in surface waters at a high spatial resolution along the research vessels' navigational paths. Corrections for sorption of dissolved carbon and nitrogen were possible at high frequency because of our sampling techniques. The resulting blank-corrected POC concentrations in surface waters ranged from <2 to 35 μM, with several samples collected earlier (August) in the open water season displaying much higher concentrations (>60 μM). Large spatial and temporal variability characterized measurements throughout the study area, with maxima in POC concentration often measured in association with physical features (e.g., straits, capes, fronts, shoals) and strong wind events. Hydrographically, POC distributions displayed significant variability among water masses in the region. Alaska Coastal Water, which was predominant along the southern section of the study area (between Bering Strait and Pt. Hope), displayed the highest POC concentrations whereas Bering-Chukchi Shelf Water and especially Melt Water, which occupied the northern section of the study area (Pt. Hope to Utquiagvik) were characterized by the lowest POC concentrations. Earlier periods (August–September) also were characterized by higher average POC concentrations than later periods (September–October). Overall, most of the samples collected exhibited [C:N] ratios that ranged from 6 to 8 mol:mol, and were consistent with a predominant marine provenance. Our findings indicate POC distributions reflect patterns of wind forcing and physical circulation and identify regions of localized high standing stocks that suggest spatially and temporarily variable production in this area during the late open-water season.