Increased melt from Greenland’s most active glacier fuels enhanced coastal productivity

In Greenland’s coastal waters, seasonal phytoplankton blooms are critical for marine ecosystems, providing the basis for thefood web that supports Greenland’s inshore fisheries and a variety of large marine mammals. These blooms are also importantfor carbon fixation and contribute to the seasonal draw-down of carbon dioxide into the ocean. In Qeqertarsuup Tunua (DiskoBay), West Greenland, the typical Arctic spring bloom is followed by a secondary summertime bloom, elevating rates of annualprimary productivity. Recent evidence has linked the summertime bloom to melt from the Greenland Ice Sheet, which isdischarged into the ocean from beneath glaciers in the region, including Sermeq Kujalleq, the most active glacier on the icesheet. These discharge sites drive vigorous localized upwelling from the grounding line of marine-terminating outlet glaciersthat are hypothesized to alleviate nutrient limitation in the photic zone. However, this hypothesis remains largely untestedwith realistic numerical models capable of resolving variability in circulation resulting from glacier melt, inhibiting estimatesof primary productivity and atmospheric CO2 uptake. Here, we use a fjord-scale, high-resolution numerical model derivedfrom the data-assimilaltive ECCO-Darwin global-ocean biogeoechemistry model to quantify the extent to which upwelling inthe Qeqertarsuup Tunua glacier-ocean system modulates nutrient concentrations, summertime phytoplankton growth, andCO2 uptake on the continental shelf. We find that increased nutrient fluxes upwelled into the photic zone enhance net primaryproductivity by 15–40% in Qeqertarsuup Tunua during summer. However, despite this increased productivity, ocean CO2uptake only increases by approximately 3% relative to simulations without discharge when integrated annually, due to thecompeting influence of plume-driven reduction in solubility. As melt on the Greenland Ice Sheet is projected to increase inthe future, these results suggest that further climate-driven melt on large marine-terminating glaciers may drive substantialchanges in Greenland’s coastal ecosystems in coming decades.