Ecosystems in these deep, narrow inlets were mostly formed by glaciers and subsequently flooded (e.g. fjords). They have some features of open oceans, but are strongly influenced by freshwater inflows and the surrounding coast. Productivity by phytoplankton is seasonal and limited by cold, dark winters. Oxygen may be limited in the deepest parts of these systems. The diverse biota includes invertebrates and fish, such as jellyfish and salmon, and predatory marine mammals such as killer whales.