Some fjords develop euxinia if the connection to the open ocean is constricted, similar to the case of the Black Sea. This constriction prohibits relatively dense, oxygen-rich oceanic water from mixing with the bottom water of the fjord, which leads to stable stratified layers in the fjord. Low salinity melt water forms a lens of fresh, low density water on top of a more dense mass of bottom water. Ground sources of sulfur are also an important cause for euxinia in fjords.