While both seas and lakes are expansive bodies of water, they possess fundamental distinctions that influence their physical characteristics and the life they support. The primary distinctions lie in their salinity, their connection to other water bodies, and the types of organisms that thrive within them.
Salt or Fresh: The Core Difference
Salinity is a primary characteristic distinguishing seas from lakes. Seawater contains around 3.5% (35 parts per thousand) of dissolved salts, predominantly sodium and chloride ions. In contrast, freshwater in most lakes and rivers has a salinity of less than 0.05% (0.5 ppt).
The salt in oceans originates from two main sources: runoff from land and openings in the seafloor. Rainwater erodes rocks on land, releasing ions that rivers carry to the ocean. Hydrothermal vents on the seafloor also contribute by releasing mineral-rich fluids. Rivers continuously transport dissolved salts, which accumulate over millions of years in the vast ocean, leading to its high salinity.
Most lakes remain fresh because they have outlets that allow water and dissolved salts to flow out. This continuous flushing prevents significant salt accumulation. However, some lakes, known as salt lakes, can become highly saline, sometimes even saltier than the ocean. These form in closed drainage basins where water enters but has no outlet, leading to salt concentration as water evaporates.
Open Waters vs. Enclosed Basins
Seas are large bodies of saltwater connected to the global ocean system, allowing free water flow. This connectivity means seas are subject to tides, the rise and fall of sea levels caused by the Moon and Sun’s gravitational forces. Large-scale ocean currents, driven by wind, temperature, and salinity differences, also characterize seas, distributing heat and influencing global climate patterns.
Lakes, conversely, are enclosed or inland bodies of water, primarily fed by rivers, streams, or groundwater. They have limited or no direct connection to the ocean, making them distinct from marine environments. While some lakes can be very large, seas are vast and form an integral part of the continuous global ocean. This enclosed nature impacts water circulation within lakes, often leading to thermal stratification, where distinct layers of water with different temperatures form.
Thermal stratification occurs because water density varies with temperature, with warmer, less dense water floating above cooler, denser water. This layering can affect nutrient distribution and oxygen levels, as deeper, colder layers (hypolimnion) can become isolated from the surface (epilimnion). Wind can induce mixing, particularly in the upper layers, but in deeper lakes, pronounced stratification can persist, influencing the overall lake ecosystem.
Diverse Life: Marine vs. Freshwater Habitats
The fundamental differences in salinity and connectivity profoundly influence the types of life found in seas and lakes. Marine organisms have evolved physiological adaptations to thrive in high salt concentrations. For example, marine fish excrete excess salts through specialized cells, and some marine plants have salt glands.
Marine environments support a wide array of life, including fish, corals, sharks, whales, and various forms of plankton. These organisms have developed adaptations for coping with saltwater, pressure, and vast open-water or deep-sea conditions. Some marine mammals, like whales and dolphins, possess blubber for insulation and have streamlined bodies for efficient movement.
In contrast, freshwater organisms are adapted to low salt environments, requiring mechanisms to retain salts and excrete excess water. Lakes are home to freshwater fish such as trout, bass, and catfish. Amphibians, aquatic insects, and freshwater plants also inhabit these environments. Ecological challenges in freshwater habitats include maintaining osmotic balance and adapting to seasonal changes in temperature and oxygen levels.