The Deep Sea: Desert and Rainforest
Snelgrove, Paul V. R., Grassle, J. Frederick, Oceanus
Debunking the Desert Analogy
The title of this article may surprise some and offend others, but we chose it to highlight a common misconception. In sources ranging from the popular press to university textbooks, the deep sea is often likened to a desert with large expanses of monotonous landscape devoid of life. Most panoramic photographs of the deep sea bottom are indeed reminiscent of deserts, with gently rolling contours of mud or sand and little visible life.
Until the 1960s, most impressions of the deep sea were based on photographic observations and ineffective sampling techniques, and both supported the view of life in the deep oceans as species poor. Thus, there arose the analogy of the "ocean desert," a perspective that persists even today among most people who do not actually study deep-sea biology. In the 1960s, WHOI biologists Howard Sanders and Robert Hessler (Hessler is now at the Scripps Institution of Oceanography) began to use a sampling device called an epibenthic sled [ILLUSTRATION FOR PHOTO OMITTED]. This device was dragged across the bottom to provide more quantitative and complete samples of bottom-living organisms (benthos). They sampled a number of deep-sea sites between Martha's Vineyard and Bermuda, and provided the first evidence that deep-sea communities are actually extremely varied.
A tremendous diversity of tiny invertebrates (macrofaunal benthos) lives within the bottom sediment. This community includes polychaetes, crustaceans, and mollusks that had been missed in photographs and by the relatively primitive sampling equipment used up until that time. The magnitude of this diversity was not fully appreciated until extensive sampling of the Atlantic continental slope of the United States was undertaken in the 1980s by author Grassle's lab at WHOI and Nancy Maciolek's and Jim Blake's lab at Battelle Ocean Sciences. These samples were collected using a device called a box corer, (see photo on page 27), which collects quantitative samples of benthic organisms, including fauna that were not effectively sampled using previous gear. This sampling revealed that the deep sea may, in fact, rival tropical rainforests in terms of numbers of species present. Thus, the deep sea may physically resemble a desert, but in terms of species composition it is more like a tropical rainforest!
The continental slope and rise from New England to South Carolina is the most extensively sampled region of the deep sea. On eight cruises during the period from 1983 to 1985 we collected 556 box-core samples at depths ranging from 600 to 3,500 meters. Each encompassed a 30-by-30-centimeter-square section of ocean bottom and included the sediment to 10 centimeters depth. At a single sampling site off Charleston, South Carolina, at about 800 meters depth, 436 species were taken from an area of less than one square meter of seafloor (nine samples pooled). A total of 1,597 species were identified in the 556 box cores combined. These sampling squares together total a little over 7 by 7 meters, an area about the size of a large living room, but nevertheless represent (by far) the most extensive sample collection from an area of the deep sea!
The figure overleaf summarizes the diversity of taxa present in a subset of these samples (collected from between 1,500 and 2,500 meters in a 180-kilometer section of continental slope). Very few individuals are qualified to undertake identification of species in even one of the groups of deep-sea animals, and we were fortunate to have a high proportion of the world's deep-sea taxonomic experts working on this project. It is very difficult to find support for deep-sea systematists, and this is one reason for a critical shortage of trained taxonomists. The numbers of species in this sampling area alone invite comparison with rainforests. Initial estimates by Terry Erwin (Smithsonian Institution) of tens of millions of species of insects and spiders in rainforests were based on his finding 1,080 species of beetles from 50-meter transects in four different types of forest within a 70-kilometer radius of Manaus, Brazil. …