Non-linear response of summertime marine productivity to increased meltwater discharge around Greenland

M J Hopwood,S Krisch,D Carroll,J Mortensen,E P Achterberg,T J Browning,L Meire

doi:10.1038/s41467-018-05488-8

Abstract

Runoff from the Greenland Ice Sheet (GrIS) is thought to enhance marine productivity by adding bioessential iron and silicic acid to coastal waters. However, experimental data suggest nitrate is the main summertime growth-limiting resource in regions affected by meltwater around Greenland. While meltwater contains low nitrate concentrations, subglacial discharge plumes from marine-terminating glaciers entrain large quantities of nitrate from deep seawater. Here, we characterize the nitrate fluxes that arise from entrainment of seawater within these plumes using a subglacial discharge plume model. The upwelled flux from 12 marine-terminating glaciers is estimated to be >1000% of the total nitrate flux from GrIS discharge. This plume upwelling effect is highly sensitive to the glacier grounding line depth. For a majority of Greenland’s marine-terminating glaciers nitrate fluxes will diminish as they retreat. This decline occurs even if discharge volume increases, resulting in a negative impact on nitrate availability and thus summertime marine productivity.

Highlights

Runoff from the Greenland Ice Sheet (GrIS) is thought to enhance marine productivity by adding bioessential iron and silicic acid to coastal waters
Less well understood are the physical mixing processes induced by subglacial discharge plumes, which may lead to a complex non-linear relationship between meltwater discharge volume and the magnitude of the induced nutrient fluxes from upwelling[22]
The upwelling of macronutrient-rich bottom waters entrained within subglacial discharge plumes has recently been shown to constitute the dominant source of NO3 supplied to the photic zone downstream of two Greenlandic marine-terminating glaciers[15,23] and this upwelling mechanism may be responsible for maintaining unusual patterns of seasonal primary production in these systems

Summary

Introduction

Runoff from the Greenland Ice Sheet (GrIS) is thought to enhance marine productivity by adding bioessential iron and silicic acid to coastal waters. The upwelling of macronutrient-rich bottom waters entrained within subglacial discharge plumes has recently been shown to constitute the dominant source of NO3 supplied to the photic zone downstream of two Greenlandic marine-terminating glaciers[15,23] and this upwelling mechanism may be responsible for maintaining unusual patterns of seasonal primary production in these systems.

Results

Conclusion