The distinction between a lake and a sea is not merely a matter of size, but a fundamental difference rooted in geography, hydrology, and chemistry. Both bodies of water hold significant volumes of water and shape their surrounding landscapes, but their defining characteristics place them in separate categories of the Earth’s hydrosphere. Understanding these differences provides clarity on how these systems function and why they support such varied ecosystems.
Primary Distinction: Connection to the Global Ocean
The most significant difference between a lake and a sea lies in their relationship to the global ocean system. A sea is an expansive body of saline water that is connected to the ocean, making it an integral part of the world’s continuous saltwater network. This connection, even if narrow, allows for the exchange of water, marine life, and dissolved substances with the open ocean.
Many seas are classified as marginal seas, which are partially enclosed by landmasses, archipelagos, or peninsulas, yet still maintain a connection to a larger ocean body. The Mediterranean Sea, for example, is almost entirely surrounded by land but is linked to the Atlantic Ocean through the Strait of Gibraltar. This continuous water exchange defines it as a sea and ensures that its water dynamics and composition are tied to the broader oceanic circulation patterns.
In contrast, a lake is a body of water that is landlocked, meaning it is entirely surrounded by land and isolated from the global ocean. Water enters a lake primarily through rainfall, rivers, and streams, and leaves either through surface outflow or evaporation. Lakes situated in endorheic basins, such as the Great Salt Lake, have no surface outlet, causing water to leave only through evaporation or seepage. This isolation from the ocean is the single most consistent feature that defines a lake, regardless of its physical size or water chemistry.
Differences in Scale and Depth
While connectivity is the primary scientific classification, the physical scale of seas far exceeds that of nearly all lakes. Seas are typically much larger in surface area, volume, and depth, a direct consequence of their formation as components of the vast, interconnected world ocean. Seas often cover hundreds of thousands of square kilometers and can reach depths of several kilometers, allowing for complex, layered water circulation patterns.
Lakes, while sometimes massive, do not approach the volumetric scale of even the smaller marginal seas. The North American Great Lakes, for instance, collectively hold an enormous volume of freshwater, but their combined volume is still dwarfed by the volume of a single major sea.
The limited dimensions of lakes compared to seas mean that their internal processes, such as water circulation and temperature stratification, are fundamentally different and more susceptible to seasonal changes. The immense volume of a sea helps regulate global climate and supports deep-sea ecosystems that are entirely absent in lakes, which generally occupy localized depressions or basins.
Water Chemistry and Salinity
The chemical composition of the water, particularly its salinity, is another major differentiator. Seas are saline, containing an average concentration of dissolved salts close to that of the global ocean, typically around 35 parts per thousand (ppt). This high salt content is maintained by the continuous exchange with the ocean and the accumulation of dissolved minerals carried in by rivers over geological timescales.
Most lakes, conversely, are freshwater, defined by a very low concentration of dissolved salts, usually less than 0.5 ppt. This low salinity is maintained because most lakes have an outflowing river or stream, which constantly flushes the salts and minerals brought in by inflowing water.
However, lakes without an outlet, known as endorheic lakes, frequently become saline. The water that leaves does so only through evaporation, leaving all the dissolved minerals behind to concentrate in the remaining water. The resulting hypersaline lakes, such as the Great Salt Lake, can have salinities many times higher than the ocean, sometimes exceeding 300 ppt. Despite this high salt content, they remain classified as lakes due to their landlocked nature and separation from the ocean system. While salinity is a strong indicator, the flow and connectivity ultimately dictate the chemical profile of the water body over time.
When Geographical Names Defy Definition
The application of the terms “lake” and “sea” is not always consistent with the strict scientific criteria of connectivity and salinity, leading to notable exceptions. Historical or political naming conventions have often overridden the hydrological facts. This creates confusion when a body of water’s name suggests one classification, but its physical properties indicate another.
The most famous example is the Caspian Sea, which is geographically the world’s largest lake, yet it is named a sea. It is entirely landlocked, making it an endorheic basin with no natural connection to the global ocean. Its water is brackish, with a salinity of about 12 to 13 ppt, which is about a third of the ocean’s average, placing it outside the freshwater range.
Similarly, the Dead Sea and the Aral Sea are also technically lakes; they are closed basins with no oceanic outlet, resulting in extremely high salinity levels. The Great Salt Lake in Utah is an example of a hypersaline lake that correctly includes “lake” in its name, despite its highly saline water. These naming anomalies highlight that while scientists rely on connectivity and water balance for classification, common usage and historical convention maintain the confusing nomenclature.