PATTERNS OF SPECIES REPLACEMENT WITH DEPTH

Since the time of the ‘Challenger’ Expedition it has been noticed that, whereas populations change almost imperceptibly over wide horizontal areas of the great ocean basins, their composition changes rapidly with depth down the continental margin. This represents part of one of the greatest environmental gradients on this planet: that related to the depth gradient on the sloping parts of the floor of the ocean. However, the causes of this pattern remain one of the most difficult and elusive problems faced by deep-sea ecologists. In contrast to the relative ease with which depth-related gradients in standing crop and in diversity can be measured as single variables (see Chapters 7 and 8), changes in species composition with increasing depth are much more difficult to parameterize, let alone explain. As pointed out in Part II which summarizes the deep-sea fauna, some (eurybathic) species have wide vertical distributions that extend from the abyss up to quite shallow water while other (stenobathic) species seem much more tightly zoned with respect to depth. Furthermore, there are few data on the depth-related distribution of the smaller animals.

MAJOR ZONES

Early studies related the major physiographical and hydrographical features of the continental margin to boundaries in major zones in the fauna down the depth gradient. The shelf break, which constitutes the major physiographical feature of the Earth's surface, marks the boundary between the shelf and the deep-sea fauna.