Abstract

Unusually warm conditions recently observed in the Pacific Arctic region included a dramatic loss of sea ice cover and an enhanced inflow of warmer Pacific-derived waters. Moored sediment traps deployed at three biological hotspots of the Distributed Biological Observatory (DBO) during this anomalously warm period collected sinking particles nearly continuously from June 2017 to July 2019 in the northern Bering Sea (DBO2) and in the southern Chukchi Sea (DBO3), and from August 2018 to July 2019 in the northern Chukchi Sea (DBO4). Fluxes of living algal cells, chlorophyll a (chl a), total particulate matter (TPM), particulate organic carbon (POC), and zooplankton fecal pellets, along with zooplankton and meroplankton collected in the traps, were used to evaluate spatial and temporal variations in the development and composition of the phytoplankton and zooplankton communities in relation to sea ice cover and water temperature. The unprecedented sea ice loss of 2018 in the northern Bering Sea led to the export of a large bloom dominated by the exclusively pelagic diatoms Chaetoceros spp. at DBO2. Despite this intense bloom, early sea ice breakup resulted in shorter periods of enhanced chl a and diatom fluxes at all DBO sites, suggesting a weaker biological pump under reduced ice cover in the Pacific Arctic region, while the coincident increase or decrease in TPM and POC fluxes likely reflected variations in resuspension events. Meanwhile, the highest transport of warm Pacific waters during 2017–2018 led to a dominance of the small copepods Pseudocalanus at all sites. Whereas the export of ice-associated diatoms during 2019 suggested a return to more typical conditions in the northern Bering Sea, the impact on copepods persisted under the continuously enhanced transport of warm Pacific waters. Regardless, the biological pump remained strong on the shallow Pacific Arctic shelves.

Highlights

  • The Pacific Arctic marine ecosystem is extremely productive due to the persistent flow of nutrient-rich waters fueling high primary production on the shallow northern Bering Sea and Chukchi Sea shelves [1]

  • While sea ice was first detected in December 2017 at DBO2 and DBO3, a lasting sea ice cover only formed in January 2018 and sea ice concentrations temporarily decreased in February 2018 at both sites (Fig 2)

  • Sea ice breakup was observed in early May 2019 at DBO2, a few days later at DBO3, and mid-May at DBO4

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Summary

Introduction

The Pacific Arctic marine ecosystem is extremely productive due to the persistent flow of nutrient-rich waters fueling high primary production on the shallow northern Bering Sea and Chukchi Sea shelves [1]. The region showed signs of a warming trend, including. Export fluxes in the Pacific Arctic region and A91-00a) awarded to AMD and SLD. The CEO project was funded under NPRB Long Term Monitoring program (NPRB grant numbers 1426 and L36) awarded to SLD. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

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