The Sargasso Sea is a region within the North Atlantic Ocean. It is the only named sea on Earth that is not bounded by any landmasses or coastlines. This two-million square mile area is defined by circulating ocean currents that enclose it. The sea’s existence and its name are directly linked to the large, golden-brown seaweed that floats upon its surface.
The Floating Seaweed That Inspired the Name
The namesake of this sea is a genus of free-floating brown algae known as Sargassum. Unlike most seaweeds attached to the ocean floor or coastal rocks, the two primary species here (Sargassum natans and Sargassum fluitans) are holopelagic, living their entire lives floating in the open sea. These species do not rely on a holdfast for attachment or reproduce sexually.
Instead, Sargassum reproduces asexually through fragmentation, where pieces break off and grow independently, allowing the population to proliferate across the ocean surface. The algae forms vast, dense mats that can stretch for miles, creating a complex, floating ecosystem. This structure is maintained by numerous small, spherical, gas-filled bladders called pneumatocysts.
These pneumatocysts are filled with gas and act as flotation devices, keeping the algae buoyant and near the surface where sunlight is plentiful for photosynthesis. The Sargassum mats function as a nursery ground and habitat for a host of specialized marine life, including fish, crabs, and sea turtles.
Historical Context of the Naming
The discovery and naming of the Sargasso Sea are attributed to early European explorers, Portuguese navigators who were sailing the North Atlantic in the 15th century. These mariners were the first to document the immense, floating fields of seaweed that covered the water. Navigating through the thick, slow-moving mats of algae led to the sea’s eventual designation.
The name “Sargasso” comes from the Portuguese word sargaço. Navigators chose this term because the small, berry-like pneumatocysts on the algae resembled grapes or a grape cluster. This provided a simple, relatable name for the strange phenomenon they encountered far from any shore.
Records from Christopher Columbus’s 1492 voyage also describe the thick seaweed, which he feared would trap his ships and impede their progress. The sheer volume of the floating algae led early sailors to distinguish this particular area from the rest of the Atlantic Ocean.
How Ocean Currents Contain the Sea
The reason Sargassum seaweed accumulates and remains in this location is a function of the sea’s oceanography. The Sargasso Sea is contained and defined by the North Atlantic Gyre, a clockwise-circulating system of ocean currents. This gyre acts like a giant oceanic conveyor belt, trapping the floating material in its relatively calm center.
The boundaries of this oceanic enclosure are delineated by four major currents. The western edge is formed by the Gulf Stream; the North Atlantic Current flows along the northern boundary. The eastern side is contained by the Canary Current, and the North Equatorial Current completes the circulation.
The rotation of these currents creates an area of high pressure and slow-moving water at the center of the gyre. This lack of turbulent mixing and the stability of the water column result in a sea with exceptionally clear conditions, often exhibiting underwater visibility of up to 200 feet, and a distinctive deep blue color. The gyre continuously sweeps the pelagic Sargassum into this central region, effectively containing the seaweed and ensuring the sea remains true to its name.