Melting glaciers stimulate large summer phytoplankton blooms in southwest Greenland waters

Abstract

Each summer, large quantities of freshwater and associated dissolved and particulate material are released from the Greenland Ice Sheet (GrIS) into local fjords where they promote local phytoplankton growth. Whether the influx of freshwater and associated micronutrients in glacial meltwater is able to stimulate phytoplankton growth beyond the fjords is disputed, however. Here we show that the arrival of freshwater discharge from outlet glaciers from both southeast and southwest GrIS coincides with large‐scale blooms in the Labrador Sea that extend over 300 km from the coast during summer. This summer bloom develops about a week after the arrival of glacial meltwater in early July and persists until the input of glacial meltwater slows in August or September, accounting for ~40% of annual net primary production for the area. In view of the absence of a significant change in the depth of the mixed layer associated with the arrival of glacial meltwater to the Labrador Sea, we suggest that the increase in phytoplankton biomass and productivity in summer is likely driven by a greater nutrient supply (most likely iron). Our results highlight that the ecological impact of meltwater from the GrIS likely extends far beyond the boundaries of the local fjords, encompassing much of the eastern Labrador Sea. Such impacts may increase if melting of the GrIS accelerates as predicted.

Exploring the Potential Impact of Greenland Meltwater on Stratification, Photosynthetically Active Radiation, and Primary Production in the Labrador Sea

“…Very turbid meltwater plumes carrying sediment from land may reduce light availability (Hudson et al, 2014), while meltwater rich in dissolved nitrogen, phosphorus, silica, or iron may contribute essential nutrients to the coastal phytoplankton (Bhatia et al, 2013;Hawkings et al, 2014Hawkings et al, , 2015Hawkings et al, , 2016Hawkings et al, , 2017Wadham et al, 2016), although only a small fraction of sediment or nutrients may escape beyond the fjords (Hopwood et al, 2016;Meire et al, 2016). However, in our study area in the Labrador Sea, summer bloom development is correlated with the arrival of glacial meltwater from Greenland, which is suggested to be driven by increased nutrient supply (Arrigo et al, 2017). Thus, while Greenland meltwater discharge may be responsible for changes in productivity observed within fjords (Arendt et al, 2010;Calbet et al, 2011;Rysgaard et al, 2003), less is known about the mechanisms behind meltwater effects on phytoplankton productivity offshore.…”

“…Our model shows that meltwater from Greenland preserves stratification that could extend the phytoplankton growing season later into fall (Figure 4a). This effect could also explain the longer persistence of summer blooms recently observed in this region (Arrigo et al, 2017).…”

“…Averaged over all years, added meltwater had the potential to increase mixed layer GPP in the six west coast domains by 3-15 mg C m 22 d 21 (3-12%) in the summer (July-August), and 15-47 mg C m 22 d 21 (13-60%) in the fall (September-October) ( Table 4). We will note here that our mixed layer GPP calculations are considerably smaller than the net primary productivity (NPP) values found in Arrigo et al (2017), especially in summer, due to our integration over only the mixed layer, which is usually <15 m in summer, much shallower than the euphotic depth, over which the Arrigo et al (2017) NPP values were calculated. The large addition of meltwater during 2012 resulted in the potential for 31-102% higher mixed layer GPP during September-October, a much larger impact than during that summer (4-23% more mixed layer GPP) (Table 4).…”

Controls on the Transport of Meltwater From the Southern Greenland Ice Sheet in the Labrador Sea

“…Over 80% of the increase in mass loss since 2009 has been through increased surface runoff (Enderlin et al, 2014). Understanding the potential for offshore transport of meltwater from the Greenland ice sheet toward the interior of the Labrador Sea is important because increased freshwater fluxes can influence local stratification, potentially affecting deep winter convection (Böning et al, 2016;Swingedouw et al, 2013) and biological activities (Arrigo et al, 2017;Oliver et al, 2018).…”

Aluminium in glacial meltwater demonstrates an association with nutrient export (Werenskiöldbreen, Svalbard)

“…A strong correlation between filterable Al and filterable Fe (Figure ) and and Al:Fe ratio > 1 suggest a greater increase in Al‐related water toxicity associated with an increasing yield of Fe. This also suggests that amplification of primary production as expected due to high yields of Fe (Gerringa et al, ; Arrigo et al, ) may intensify the accumulation of Al in biotas as well. The median filterable Al concentration exceeds the long‐term chronic toxic concentration in seawater, showing that subsequent increase in Al yields from glacierised basins affects the Al pool of seawater (ANZECC/ARMCANZ, ; Golding et al, ).…”