A threshold is an underwater ridge in a fjord or along the coast, often formed by glaciers or natural land formations. In fjords, thresholds can limit water exchange by restricting the inflow from the coast. In the deepest areas, water can become stagnant, oxygen levels decrease, and marine life may be affected.

Renewing deep water requires strong winds and ocean currents. The heavier Atlantic water flows over the threshold and sinks, bringing fresh oxygen to the depths. Additionally, freshwater from melting snow and rivers mixes with the surface layers.

In aquaculture, it is crucial to place facilities where water movement ensures oxygen renewal and protects the fjord environment.