Fjords are complex bodies of water, and the simple question of whether they are freshwater or saltwater does not have a single answer. While their defining feature is a direct connection to the sea, making them fundamentally marine environments, they also receive massive inflows of freshwater from rivers and melting glaciers. This combination results in a highly variable salinity that changes dramatically from the surface to the seafloor and from the innermost reaches to the open ocean. A fjord is best described as an estuarine system where saltwater and freshwater meet, creating a unique and stratified environment.
Defining a Fjord
A fjord is a long, narrow inlet of the sea characterized by steep cliffs and sides, which often rise hundreds of meters above the water line. This distinctive geography is a remnant of the last Ice Age, formed by the immense erosive power of glaciers. As massive ice sheets flowed toward the coast, they carved out deep, U-shaped valleys, scouring the bedrock far below the present sea level.
When the glaciers retreated and global sea levels rose, these deep valleys flooded with ocean water, creating the fjords seen today. Fjords are typically much deeper than the adjacent sea, with some like Norway’s Sognefjord reaching depths of over 1,300 meters. The connection to the open ocean classifies them as marine inlets, establishing the saltwater influence. Their physical structure of a deep basin with a relatively narrow mouth sets the stage for the unique water dynamics within.
The Layered Water Column
The salinity of a fjord is not uniform; instead, the water column is highly stratified, meaning it is arranged in distinct layers. This layering is driven by the density difference between the light freshwater runoff and the dense saltwater from the ocean. Glacial meltwater and river discharge pour into the inner reaches of the fjord, creating a layer of low-salinity, or brackish, water that floats on the surface.
This surface layer, often only 5 to 10 meters deep, flows slowly out toward the sea. A counter-current of heavier, more saline ocean water flows inward beneath it. The boundary between the less dense surface water and the underlying highly saline ocean water is known as a halocline. Within this halocline, the salinity changes rapidly over a short vertical distance, creating a strong density barrier that limits mixing between the upper and lower layers. This pattern of opposing currents is known as estuarine circulation.
The Role of the Sill
A feature that maintains the fjord’s distinct water layers is the sill, a submerged ridge of bedrock or glacial debris located near the mouth where the fjord meets the open ocean. The sill is formed either by the terminal moraine deposited by the glacier as it retreated or by a section of harder bedrock that resisted the glacier’s erosive power. The sill’s depth is often significantly shallower than the main fjord basin, acting as a physical bottleneck for water circulation.
This shallow barrier restricts the free exchange of deep water between the inner basin and the open sea. Deep-water renewal—the process of bringing fresh, oxygenated ocean water into the fjord’s depths—becomes an infrequent event, sometimes occurring only every few years. The sill effectively traps the deep basin water, which can lead to the slow depletion of dissolved oxygen over time as organic matter sinks and decays. This stagnation can result in anoxic, or extremely low-oxygen, conditions in the deepest parts of the fjord basin.
Life in the Fjord Ecosystem
The layered and restricted water structure of a fjord creates a series of habitats that demand specialized biological adaptations. Organisms living near the surface must cope with the widely fluctuating salinity and temperature of the brackish layer, which changes with river flow and seasonal melt. Species like salmon and sea trout are particularly well-adapted to this challenge, as they must adjust their physiology to transition between the freshwater rivers and the highly saline ocean.
In the deeper, more stable marine environment beneath the halocline, organisms adapted to cold, dark, and highly saline conditions thrive. Fjords can host deep-sea life surprisingly close to shore, including cold-water coral reefs that rely on the inflow of nutrient-rich ocean water. However, the restricted circulation behind the sill can create a biological desert in the deepest parts where oxygen levels are severely depleted, limiting life to specialized microbes and organisms tolerant of anoxic muds.