TIdewater Glacier Retreat Impact on Fjord circulation and ecosystems

Informations

Funding country

Norway

Acronym

TIGRIF

URL

-

Start date

1/1/2015

End date

12/31/2019

Budget

798,270 EUR

Fundings

Abstract

Tidewater glaciers terminate directly into the sea, often into fjords. Global warming has led to the retreat of most tidewater glaciers around the world. Tidewater glaciers have a significant influence on fjord circulation. During the summer, rivers at the bottom of the glacier discharge into the fjord below the seawater surface. The low density of this water causes it to rise rapidly in the denser fjord water, forming a meltwater 'plume'. Plumes have two effects on fjord ecosystems. First, phytoplankton, zooplankton and nutrients entrained in the rising plume are brought to the surface. Second, the rising plume promotes sub-surface circulation toward the tidewater front, drawing in warmer and often nutrient- and organism-rich water from the outer fjord or shelf. Thus glacier front areas are important feeding areas for seabirds and marine mammals. Given that tidewater glaciers around the world are retreating, an important question is what the impact on fjord circulation will be once tidewater glaciers retreat so much that they terminate on dry land, and further, what the impact of these circulation changes will be on fjord ecosystems. With full retreat, freshwater will flow out onto the surface of the denser seawater, and fjord circulation will be significantly reduced. Less nutrients and food will be brought towards the surface, which will directly impact bird, seal, and other animal populations. Provided the inner fjord basins are sufficiently sheltered from exchanges with outer fjord areas, the reduction of mixing can lead to retention of low salinity surface water, which will promote sea ice formation in the fjords. In the TIGRIF project we are modelling fjord circulation in the Kongsfjorden system in NW Svalbard, the location of the Ny-Ålesund Research Station. Kongsfjorden has five tidewater glacier fronts, which together drain a glacier area of over 1,100 km2. The large number of ongoing international research programs in Ny-Ålesund, covering a wide variety of disciplines, means many relevant datasets are available, both geophysical and ecological. We set up a numerical ocean model to simulate 1) the present-day circulation in Kongsfjord, and 2) the future circulation once the glacier fronts have retreated onto dry land. To model future fjord circulation, we had to first map the subglacial topography of the tidewater glaciers, using ice-penetrating radar. Since these glaciers are heavily crevassed, the radar mapping had to be done from helicopter. The resultant subglacial topography shows that the three largest tidewater glaciers each have the potential to retreat by up to 10 km before they become land-terminating. Furthermore, the deep subglacial troughs of these glaciers means sediment infilling of the inner basin will not happen in the near future. Fjord modelling results show that the submerged runoff emerging at the bases of tidewater glacier fronts contributes significantly to large-scale circulation within the inner fjord. We find that a transition to a fjord with only land-terminating glaciers will affect the fjord circulation. Removal of subglacial discharge due to retreating tidewater glaciers causes substantial reduction of subsurface volume fluxes, and hence exchange rates, in the inner part of the fjord, and results in enhanced stratification during summer months. This has further implications for the biogeochemistry and ecosystem in Kongsfjorden. Our results suggest that the retreat onto land will significantly affect dissolved inorganic nitrogen concentrations in the whole fjord, during peak discharge periods, with the maximal impact in the inner fjord. Moreover, nutrient concentration decreases will impact primary production, leading to a reduction in phytoplankton concentration and possible effects further up the food web. Differences in fjord biogeochemistry between summer (high discharge) and spring or autumn (low discharge) will decrease in the future scenario.