Exploring the Subtropical Seas by Scuba Diving
Although there is no strict definition, subtropical seas can be described as waters too consistently warm to support temperate marine life, but too cold for coral reefs to flourish. Also known as warm temperate waters, subtropical seas are found in all of the world’s oceans except the Arctic.
Between Two Worlds
Subtropical waters are sandwiched between tropical ecosystems (generally identified by the presence of coral reefs and mangroves), and the temperate zone (identified by cold, rich waters supporting complex systems such as kelp forests). In subtropical marine systems, the water temperature is too high to contain large amounts of oxygen and nutrients, and yet too low-to promote the recycling properties of coral reefs.
The most popular subtropical areas for diving are the Mediterranean and parts of the Gulf of California (also known as the Sea of Cortez). Other key-sites include South Africa’s Aliwal Shoal, Fernando de Noronha oil the Brazilian coast, and the islands of Cocos, Malpelo, and Roea Partida in the eastern Pacific.
In terms of their marine life, there is a stark contrast between the Gulf of California and the Mediterranean. The former is famed throughout the diving world for its big-animal encounters, while the latter is a somewhat barren system. The cause of this difference is their topography. The Mediterranean is in effect an isolated bowl of deep water, whereas the Gulf of California is less enclosed and relatively shallow, with water and animals able to move freely to and from the Pacific.
Plankton, which are at the base of the food chain, thrive in waters in winch mixing of the water layers brings nutrient-rich deep water to the surface. Most of the cold, nutrient-rich water from the Atlantic that enters the Mediterranean does so through the narrow Straits of Gibraltar, creating a plume of colder water that extends to the Sicilian Channel. The best dive sites in the Mediterranean lie along this plume. Much of the rest of this sea experiences very little mixing of the water column, and while the result is very clear water, there is little to support life.
The Gulf of California is open at the southern end, with the Baja peninsula to the west and the Mexican mainland over 125 miles (200 km) to the east. Much of it is less than 985 ft (300 m) deep, and these shallow waters are regularly mixed by water entering from the Pacific to the south and are rich in marine life. This is one of the finest locations to encounter large animals such as humpback whales and manta rays.