The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

Stephan Krisch,Jaw-Chuen Yong,Stéphane Roig,Mark J Hopwood,Pablo Lodeiro,Eric P Achterberg,Torsten Kanzow,Kai-Uwe Ludwichowski,Thomas J Browning,Martin Graeve

doi:10.1038/s41598-020-72100-9

Abstract

Climate change has led to a ~ 40% reduction in summer Arctic sea-ice cover extent since the 1970s. Resultant increases in light availability may enhance phytoplankton production. Direct evidence for factors currently constraining summertime phytoplankton growth in the Arctic region is however lacking. GEOTRACES cruise GN05 conducted a Fram Strait transect from Svalbard to the NE Greenland Shelf in summer 2016, sampling for bioessential trace metals (Fe, Co, Zn, Mn) and macronutrients (N, Si, P) at ~ 79°N. Five bioassay experiments were conducted to establish phytoplankton responses to additions of Fe, N, Fe + N and volcanic dust. Ambient nutrient concentrations suggested N and Fe were deficient in surface seawater relative to typical phytoplankton requirements. A west-to-east trend in the relative deficiency of N and Fe was apparent, with N becoming more deficient towards Greenland and Fe more deficient towards Svalbard. This aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in + N treatment near Greenland and + N + Fe near Svalbard. Collectively these results suggest primary N limitation of phytoplankton growth throughout the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. We suggest that the supply of Atlantic-derived N and Arctic-derived Fe exerts a strong control on summertime nutrient stoichiometry and resultant limitation patterns across the Fram Strait region.

Highlights

The Fram Strait region (76–82°N, 20°W–15°E) forms the northern boundary of the Greenland Sea and is the major gateway for water exchange between the Arctic and North Atlantic Oceans (Fig. 1)[20,21,22]
Towards the Arctic Ocean[22,23]; and (ii) the southward flowing East Greenland Current (EGC), which originates in the Arctic Ocean, includes a contribution of NE Greenland Shelf waters, and constitutes low-salinity Polar Surface Water (S < 31.40) and more-saline sub-surface waters (S < 34.80) below 50–250 m23–25
Samples were collected on the RV Polarstern (PS100) as part of the GEOTRACES programme

Summary

Introduction

The Fram Strait region (76–82°N, 20°W–15°E) forms the northern boundary of the Greenland Sea and is the major gateway for water exchange between the Arctic and North Atlantic Oceans (Fig. 1)[20,21,22]. About half of the Fram Strait region is covered by seasonal sea ice, leading to strong year-round light limitation of phytoplankton growth[24,26,27]. Fram Strait has experienced surface freshening that has driven enhanced water column s tratification[28,29], potentially linked to variations in the advection and recirculation of Atlantic-derived waters[16,30,31]. It remains unclear how these on-going physical changes are affecting nutrient supply to surface waters and thereby primary production in the region. We (i) performed a detailed investigation into nutrient and micronutrient concentration distributions and stoichiometry across Fram Strait as part of the GEOTRACES GN05 cruise, and (ii) conducted nutrient addition bioassay experiments as direct assessments of spatial patterns in limiting (micro-)nutrients

Objectives

Methods

Results