The discovery of kelp forests in deep-water habitats of tropical regions

Tracing large-scale patterns of ecosystem distribution in land and marine environments is a fundamental task of field biology. The resulting patterns often have important implications in ecology, biogeography, and evolution as well as applied consequences in resource management and utilization. Classically, studies of marine ecosystems found at the sea edge, on hard substratum at levels down to 30–40 m, have distinguished three main ecological systems that replace each other at different latitudes. Coral reefs characterize lower latitudes and tropical waters (1), kelp forests replace coral reefs in most cold and temperate waters in both hemispheres (2, 3), whereas thickets of other large brown algae occupy the cold waters around Antarctica (4). Work by Graham et al. (5) included in a recent issue of PNAS changes dramatically this well accepted latitudinal pattern of marine ecosystem distribution. Using a diversity of data, Graham et al. first predicted the widespread occurrence of unexplored, submerged (30- to 200-m depth) kelp habitats in regions between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). Then, using deep-water SCUBA diving, they documented the presence of extensive kelp forests in eight localities within one of the predicted tropical regions. The emerging picture (Fig. 1A) is one of widespread occurrence of kelp forest formations in both surface and deep waters along most continents.