The classically pristine fjords of Norway are darkening as global
warming is causing more and more freshwater to run into them from
melting snow pack and glaciers. The increased runoff into rivers and
streamsJellyfish (Photo by Brocken Inaglory/Creative Commons via Wikimedia)Jellyfish (Photo by Brocken Inaglory/Creative Commons via Wikimedia) takes more organic material with it -- causing the waters to
become darker. The result could be a drastic reduction of many fish
species, except for jellyfish which would thrive.

The darkened water means that less sunlight can prevent photosynthesis in algae so that it won't grow and create food. Fish that hunt by sight will have more trouble tracking down their daily meal. For jellyfish, who basically hunt by running into their prey, the darker water would not interrupt their hunting at all. 

Researchers studied two different fjords, one more affected by desalination than the other. In the one where little fresh water had been diluting its natural waters, the fish balance remained virtually the same. In the other fjord where there had been a much larger influx of fresh water fish numbers had declined and the jellyfish had taken over. 

While researchers are not yet certain that this will affect the entire coastline, they do know that should it happen it would be extremely difficult to reverse.

Source: NBC News