The paradox of pelagic food webs on the Bering-Chukchi continental shelf

Abstract

Prolific primary production and spectacular populations of marine birds and mammals in the northern Bering Sea were for many years considered to be a paradox of an environment that should have had low production, as is typical of shallow continental shelves elsewhere. However, a "river" of oceanic water, Anadyr Water, originating along the continental slope of the Bering Sea carries a perpetual supply of nutrients and biota onto this northern shelf that transforms part of the region into one that is extremely productive at all trophic levels. Diatoms grow profusely throughout the ice-free season and, together with oceanic zooplankton advected in the Anadyr stream, provide the energy base for rich pelagic and benthic food webs. Contrasting with the highly productive pelagic regime is one associated with Bering Shelf Water and Alaskan Coastal Water. Both of these water masses originate over the shallow shelf of the northern and eastern Bering Sea, and are typically nutrient-poor following the spring phytoplankton bloom. Terriginous nutrients introduced by the Yukon and other rivers are not sufficient to elevate primary production above a low level typical of inner shelf regions. The oceanic zooplankton are excluded from this environment, and populations at higher trophic levels are small. The consequence of these contrasting physical regimes is that discrete oceanic and inner shelf food webs coexist in a small geographic region where only a coastal ecosystem is expected.