Diatoms, a group of unicellular algae, are limited to the upper 50 to 100 m of the water column, owing to their light requirement. Benthic diatoms, consequently, are restricted to the continental shelf, and primarily the inner shelf, coasts, and estuaries. Planktonic diatoms occur on the shelf (neritic) and in deeper waters (pelagic); certain genera are restricted to one region while others occur in both. Salinity appears to exercise an important control on these distributions, so that a salinity front frequently produces a sharp boundary between populations. The shelf-slope break is usually associated with a sharp salinity gradient, where low-salinity shelf waters encounter a high-salinity oceanic current. The result is that benthic and low-salinity planktonic diatoms ch racterize shelf sediments, while higher-salinity planktonic diatoms and an absence of benthic diatoms characterize the slope and ocean basin sediments.

A transect across the southeastern Bering shelf and slope, with stations at 25-km intervals, shows a dramatic change in species composition of the sediment assemblages at the 200-m isobath. Shallower samples are dominated by benthic marine species and a group associated with the presence of winter sea ice, while below 200 m the assemblages are dominated by a species common in the North Pacific and the Bering basin. The ratio of this pelagic species to the benthic species shows a high correlation with depth (r = .85). Other work in the literature, while not directly addressed to this question, suggests that similar transitions can be found off Peru, west Africa, and in Miocene deposits of the U.S. East Coast.

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