Nearly 35% of Greenland’s glaciers flow into the ocean, where they discharge large volumes of meltwater below sea level. This sediment-laden water then wells up along the glaciers’ calving fronts, creating buoyant plumes that deliver vital nutrients that help sustain Greenland’s highly productive coastal ecosystems. The processes that supply these nutrients to the surface waters, however, have not been rigorously quantified.
To clarify the effects of subglacial plumes on nutrient transport in Greenland fjords, Kanna et al. organized a July 2016 field campaign around Bowdoin Glacier, a tidewater glacier in northwestern Greenland. In addition to taking physical oceanographic measurements, the team conducted a suite of biogeochemical analyses on water samples collected from atop the glacier, from a buoyant plume in front of the glacier, and from multiple locations within the surrounding, 20-kilometer-long fjord.
The results indicate the plume’s composition differed substantially from the other water samples. In addition to hosting large amounts of suspended sediment, the plume water also contained several times the concentration of important nutrients, including nitrate, silicate, and phosphate, and was more saline than the other samples. These data provide strong evidence that the upwelling plume water is a mixture of subglacial meltwater and deep fjord water entrained in the glacier’s discharge.
After reaching the surface of the fjord, the plume water formed a nutrient-rich subsurface layer on whose boundary the team observed phytoplankton blooms. Because this productivity was strongly associated with the upwelling of nitrate within the plume, these data indicate that subglacial discharge plays an important role in transporting nutrients from deep in the fjord to its surface.
As one of the first studies to pair physical characteristics with comprehensive biogeochemical analyses, this research offers strong evidence that glacial meltwater plumes support phytoplankton blooms through upwelling, rather than by the direct import of nutrients. As the volume of meltwater exiting the Greenland Ice Sheet increases, studies like this one will be crucial for furthering our understanding of the effects of this change on coastal marine ecosystems. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2017JG004248, 2018)
—Terri Cook, Freelance Writer