15.5: Sargasso Sea

The Sargasso Sea is the region of the North Atlantic Ocean surrounded by the North Atlantic subtropical gyre. The permanent thermocline in the Sargasso Sea is steep, persistent, and deep (Chap. 8). Hence, upwelling does not occur, and the surface layer is depleted of nutrients. Furthermore, there is little exchange between Sargasso Sea surface waters and adjacent surface waters from which nutrients could be resupplied. Consequently, primary productivity is very low, and we would expect the Sargasso Sea to be an ocean “desert.” The phytoplankton-based food chain in the Sargasso Sea is indeed very limited. However, parts of the Sargasso Sea surface are covered by dense mats of floating brown macroalgae (Fig. 15-15a,b), within which lives a diverse community of animals. These mats often cover areas of many square kilometers.

Figure 15-15. The Sargassum community. (a) Sargassum weed floats on the Sargasso Sea surface in rafts that can exceed hundreds of meters in diameter. (b) Sargassum weed. Note how thick the mat of algae fronds is. This makes Sargassum an ideal place for many small fishes and invertebrates to hide.

Why do these vast quantities of algae grow in what should be a desert? The answer illustrates the infinite adaptability of nature. The brown algae in the Sargasso Sea are species of the genus Sargassum. These species grow extremely slowly and are thought to have lifetimes of decades or even centuries. The very small amounts of nutrients available in Sargasso Sea surface waters are adequate to sustain the very slow growth of Sargassum. However, the critical factors that enable Sargassum to populate the Sargasso Sea so heavily are very weak currents and very limited wind mixing. Because the surface currents are weak and flow in directions that are oriented partially toward the center of the subtropical gyre, they tend to retain the floating algae within the Sargasso Sea, and the mats of the long-lived algae tend to stay together. The algal population therefore can develop a large biomass despite its very slow growth and reproduction rate. In addition, because wind mixing is very limited, nutrients released into the water column by the consumers and decomposers that live in the Sargassum are not rapidly diluted, and are available to be recycled back into new Sargassum growth.

Within the Sargassum mats lives a community of many different species of fishes, crabs, snails, and other animals, including a frogfish called the sargassumfish. Many in this community have a body that mimics the structure and color of the Sargassum, and they can cling to the branches of the algae unnoticed by predators or prey that swim by. In contrast to many other ocean ecosystems, the Sargassum community has relatively few grazers and a relatively small animal biomass in relation to the algal biomass. This composition reflects the evolution of the community to a stable condition. If there were large populations of grazers or other animals, the algal biomass would be rapidly reduced to the point where the animal populations could not be sustained. The animals that have survived in this community include many long-lived, slow-growing species with sedentary lifestyles. Each of these characteristics represents an adaptation to minimize food intake needs.

Although some species similar to those found in the Sargasso Sea are found within the interior of other subtropical gyres, for reasons as yet not fully understood, none of these other areas sustain the high biomass found in the Sargasso Sea.