Review Article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?

Abstract

Abstract. Freshwater discharge from glaciers is increasing across the Artic in response to anthropogenic climate change, which raises questions about the potential downstream effects in the marine environment. Whilst a combination of long-term monitoring programmes and intensive Arctic field campaigns have improved our knowledge of glacier-ocean interactions in recent years, especially with respect to fjord/ocean circulation in the marine environment, there are extensive knowledge gaps concerning how glaciers affect marine biogeochemistry and productivity. Following two cross-cutting disciplinary International Arctic Science Committee (IASC) workshops addressing ‘The importance of glaciers for the marine ecosystem’, here we review the state of the art concerning how freshwater discharge affects the marine environment with a specific focus on marine biogeochemistry and biological productivity. Using a series of Arctic case studies (Nuup Kangerlua/Godthåbsfjord, Kongsfjorden, Bowdoin Fjord, Young Sound, and Sermilik Fjord), the interconnected effects of freshwater discharge on fjord-shelf exchange, nutrient availability, the carbonate system, and the microbial foodweb are investigated. Key findings are that whether the effect of glacier discharge on marine primary production is positive, or negative is highly dependent on a combination of factors. These include glacier type (marine- or land-terminating) and the limiting resource for phytoplankton growth in a specific spatiotemporal region (light, macronutrients or micronutrients). Glacier fjords therefore often exhibit distinct discharge-productivity relationships and multiple case-studies must be considered in order to understand the net effects of glacier discharge on Arctic marine ecosystems.

Pan-Arctic Ocean Primary Production Constrained by Turbulent Nitrate Fluxes

“…Tidal mixing over the two shallow sills in concert with isopycnal mixing may contribute to overall upward nitrate supply (see e.g., Fer and Drinkwater, 2014), but terrestrial runoff may also contribute significantly to the nutrient cycling (Rysgaard et al, 2003) as nitrate concentrations in run-off water are higher than those measured in the sea surface (Paulsen et al, 2017). This scenario is likely specific to this fjord and cannot be generalized around Greenland as nitrate concentrations in Greenland Ice Sheet runoff often act to dilute surface nitrate concentrations (Meire et al, 2016;Hopwood et al, 2019).…”

Bioavailable iron produced through benthic cycling in glaciated Arctic fjords (Svalbard)

“…Again, Dicksonfjorden is in contrast to Kongsfjorden and Lilliehöökfjorden where sediment sulfate reduction was active and dFe(II) and/or dMn, could be detected within the upper 2 cm of the sediment at all stations ( Figure 5 and 6). The deeper penetration of oxidants in Dicksonfjorden sediments is likely caused by the generally lower primary productivity in fjords with land-terminating glaciers as they lack glacial upwelling, which is known to entrain nutrient-rich bottom water and transport it up to the photic zone where it supports primary productivity [44][45][46][47] . The smaller increase in TOC is likely due to diminished primary productivity in the Dicksonfjorden water column and leads to a smaller increase in TOC content of the sediment with distance from the head of the fjord compared to Kongsfjorden and Lilliehöökfjorden (Figure 7a, Table S6).…”

“…With ongoing warming, glacier termini are retreating from the sea onto land, which will lead to changes in the export, processing, and delivery of glacially-derived material 43 . Glacial retreat also causes changes in water circulation and primary productivity in the fjord ecosystem [44][45][46][47] . Despite the fact that fjords are significant sinks of carbon 11 and hotspots of biogeochemical cycles 48,49 , it remains unknown how glacial retreat onto land will impact the processing of glacially-sourced iron in fjord sediments along with its speciation, transport and bioavailability.…”

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“…Over a 2-month discharge period, this would produce a NO 3 flux of 40-160 Mmol NO 3 , with 2-6% of the NO 3 flux arising from meltwater discharge and 94-98% from plume entrainment. Complete utilization of this NO 3 by phytoplankton according to the Redfield ratio (106 C:16 N) (Redfield, 1934), would correspond to a biological sink of 0.27-1.0 Gmol C.…”