Distinct Seasonal Primary Production Patterns in the Sub-Polar Gyre and Surrounding Seas

Katherine Richardson,Jørgen Bendtsen

doi:10.3389/fmars.2021.785685

Katherine Richardson, Jørgen Bendtsen

Open Access

https://doi.org/10.3389/fmars.2021.785685

Copy DOI

Abstract

Primary production (PP) in the sub-polar region appears to be important for ocean carbon uptake but how the different water masses contribute to the PP occurring here has not yet been described. Using two models based on satellite observations of surface chlorophyll, light and temperature, seasonal patterns in the distribution of PP are shown here to differ in the sub-polar gyre south of the Greenland-Scotland Ridge (GSR) and surrounding water masses. Monthly averages of PP (2003–2013) were determined. Total and seasonal PP were similar in both models. Average PP in five of the domains (0.47–0.77 g C m–2 d–1) was well above the global average (0.37 g C m–2 d–1). Over the East Greenland shelf, however, total annual PP was estimated to be only 0.19 g C m–2 d–1. The Norwegian shelf was the most productive of the regions studied. “Spring blooms” appear sporadically as spikes in the annual distribution of PP in some regions/years, but do not emerge as a dominant feature in the average annual development of PP in any of the domains. For all regions, ∼25% of the annual PP takes place in the period January-May. PP peaked over most of the study area at or around maximum insolation or temperature. PP in the study region as a whole appears to be more related to latitude or water masses than to bathymetry. In waters over the East Greenland shelf, the Norwegian shelf, and north of the GSR up to 50% of annual PP had taken place when ∼50% of the annual flux of light has reached the surface. In contrast, only about 35% of annual PP had taken place in the sub-polar gyre and waters over the southern open shelf by this time. Light-use efficiency differences may be explained by differences in mixed layer depth (MLD). Multi-model Earth System model studies have indicated that climate change may decrease the MLD in the sub-polar gyre and suggest this may lead to a decrease in the PP occurring here. The results presented here, however, suggest that a shallower MLD could lead to an increase in PP.

Highlights

The waters in and surrounding the North Atlantic sub-polar gyre are recognized as being important in atmospheric CO2 drawdown
The subpolar north-eastern North Atlantic is thereby divided into six subdomains representing either shelf or open sea areas; (1) Subpolar Gyre south of Greenland-Scotland Ridge (GSR), (2) Nordic Seas north of GSR, (3) Southern open shelf south of Iceland and including shelf and shallow areas around the Faroes, (4) Northern open shelf area north of Iceland, (5) Eastern shelf along Greenland, and (6) Western shelf along Norway
The average spring Primary production (PP) per unit area of 0.29 g C m−2 in the Nordic Seas is slightly larger than in the sub-polar gyre (0.27 g C m−2 d−1). These results suggest that open sea spring PP south of the GSR is more limited by factors other than light, e.g., mixed layer depth, nutrients or grazing, than is the case north of the GSR

Summary

Introduction

The waters in and surrounding the North Atlantic sub-polar gyre are recognized as being important in atmospheric CO2 drawdown. Primary Production in the Sub-Polar Atlantic [primary production (PP)] is an important driver in the development of the seasonal distribution of CO2 in these waters (Takahashi et al, 2002). Several studies have suggested that PP characteristics of these different water masses may differ (e.g., Astthorsson et al, 2007) and inter-annual variability has been shown to correlate with the North Atlantic Oscillation, i.e., the principal mode explaining atmospheric variability in the region (e.g., Skogen et al, 2007; Henson et al, 2009; Harrison et al, 2013). Linkages between PP and water mass distributions are indicated from the timing and distribution of phytoplankton blooms (Friedland et al, 2016), and differences in the timing of the spring bloom across the GSR have been explained by water mass characteristics, e.g., mixed layer depth (Zhai et al, 2012) or iron-depletion (Nielsdóttir et al, 2009)

Objectives

Methods

Results

Discussion

Conclusion