An inland sea is a large body of saline water that is partially or entirely enclosed by landmasses. While the term may sound like a large lake, these water bodies are geologically distinct due to their origin and chemical composition, which is often tied to the ocean. Their semi-isolated nature creates unique environments where water exchange is limited, leading to a host of specialized physical and chemical properties. Understanding these systems requires looking at how they are formed and how their isolation affects their internal balance.
Defining Inland Seas and Their Geological Classification
A sea is defined by its past or present connection to the global ocean, distinguishing it from a large lake. Inland seas are broadly grouped into two primary classifications based on their degree of connection to the open ocean. The first category is the marginal or epicontinental sea, which is a shallow body of water located on a continental shelf and partially enclosed by land, islands, or archipelagos. This type maintains a restricted, but continuous, exchange of water with the main ocean basin, preventing complete isolation.
The second classification is the landlocked or remnant sea, representing a former part of the ocean that has become entirely cut off from any oceanic connection. This isolation usually results from tectonic uplift or a significant drop in global sea levels over geological timescales. These remnant seas exist within endorheic basins, meaning they have no natural outflow and lose water primarily through evaporation. The Caspian Sea, for example, is the world’s largest body of water in an endorheic basin, representing an ancient remnant of the Tethys Ocean.
Distinct Physical and Chemical Characteristics
The limited water exchange results in dramatic variations in salinity profiles, which deviate significantly from the average salinity of the open ocean. Where evaporation rates greatly exceed freshwater inflow, such as in the landlocked Dead Sea, the result is a hypersaline environment many times saltier than seawater. Conversely, marginal seas or remnant basins with large river inputs, like the Baltic Sea, are often brackish, featuring a mixture of fresh and saltwater with a much lower salinity.
Water balance is maintained by the ratio between evaporation and the inflow from major river systems, which dictates the sea’s volume and chemistry. The restricted circulation in many of these basins also leads to stratification, where layers of water with different densities fail to mix vertically. This stratification is often haline, meaning it is driven by differences in salt content, with denser, saltier water settling permanently on the bottom.
This permanent layering prevents oxygen from reaching the deep water, leading to the formation of anoxic zones where oxygen is completely depleted. In the Baltic Sea, this lack of deep-water oxygen creates vast “dead zones” on the seafloor where most marine life cannot survive. When oxygen levels drop, chemical processes cause the release of nutrients like phosphorus from the seafloor sediments. This creates a feedback loop that fuels surface algae blooms and worsens the oxygen deficit in a process called internal loading.
Global Examples and Current Status
The Caspian Sea, classified as a landlocked remnant sea, is the largest enclosed body of water on Earth, bordered by five nations. Its water balance is dominated by the Volga River, which provides approximately 80% of its annual inflow. Today, the Caspian Sea is experiencing an environmental crisis, with water levels reaching historic lows, falling to less than 29 meters below sea level in recent years due to climate-driven evaporation and reduced river flow. This dramatic shrinkage threatens the unique ecosystems of the northern shallow regions, including the spawning grounds for sturgeon and the habitats of the endemic Caspian seal.
The Baltic Sea serves as an example of a marginal or epicontinental sea, with its connection to the North Sea restricted by narrow, shallow straits. Its immense freshwater input gives it a low, brackish salinity, which results in a strong, permanent haline stratification. This stratification has contributed to the expansion of anoxic areas, with nearly one-fifth of its deep seafloor currently lacking oxygen. The deep waters must rely on rare, massive influxes of highly saline, oxygen-rich water from the North Sea, known as Major Baltic Inflows, to periodically refresh the stagnant basins.
The Aral Sea presents a stark example of human impact on a landlocked sea, having been nearly desiccated due to massive Soviet-era irrigation projects that diverted its main feeder rivers. What was once the world’s fourth-largest lake has largely disappeared, leaving behind vast salt flats and a regional ecological disaster. The fate of the Aral Sea serves as a global warning about the vulnerability of closed-basin water systems to unsustainable human activity and changing climate patterns.