NASA

© 2008 Ellesmere Ice Shelves

A micrograph of Limnocalanus macrurus a species of zooplankton found in Disraeli Fiord before the drainage of the epishelf lake.  © Patrick Van Hove 2001.

Fiords and Epishelf Lakes

When ice shelves completely block the mouth of a fiord, a unique type of lake is created called an epishelf lake.  This is caused by meltwater that flows into the fiord every summer but is impounded behind the ice shelf.  Since freshwater is less dense than seawater, it floats on top of the ocean in a layer that is as deep as the ice shelf.  A thick perennial ice-cover on the surface of the fiord helps to prevent the wind from mixing the two layers together. 

The interesting thing about these kinds of lakes is that there is no physical barrier at the "bottom" of the freshwater layer.  That means that organisms, like fish and plankton can go between the ocean and freshwater by simply moving across this layer.  Researchers have found freshwater species of zooplankton in the top portion of an epishelf lake and marine species of zooplankton in the ocean below (Van Hove et al. 2001). 

 

Between 1999 and 2002, the largest epishelf lake in the Northern Hemisphere (located in Disraeli Fiord) drained into the Arctic Ocean through a crack in the Ward Hunt Ice Shelf (Mueller et al. 2003).  The amount of freshwater that was released is roughly equivalent to what flows over Niagara Falls in a few weeks time.  As the freshwater drained away from the fiord, the seawater replaced it from below, drastically altering the epishelf lake ecosystem.

We can monitor changes in epishelf lakes by taking a profile of the water column.  This amounts to lowering an instrument into the fiord that measures the conductivity (from which we calculate salinity), the temperature and the depth.  This has been done every year since the drainage of the epishelf lake in Disraeli Fiord and no freshwater has returned to the fiord. 

We are actively monitoring freshwater layers in other fiords, which are trapped behind thick multiyear sea ice.  These are ice-dammed lakes, like epishelf lakes but their freshwater layers are not as deep. 

A cartoon of an epishelf lake in cross-section (and plan view in the upper-right inset)