Subsurface ammonium maximum in the northern North Pacific and the bering sea in summer

Abstract

Ammonium data collected on three summer cruises of the R.V. Hakuho Maru, University of Tokyo, in offshore shallow waters (200 m or less) of the northern North Pacific and the Bering Sea are summarized. Three types can be distinguished with respect to vertical distribution of ammonium: (1) the maximum concentration of ammonium appears in subsurface layer of 30 to 50 m, (2) the maximum appears near the sea floor, and (3) ammonium concentration is low throughout the water column and shows no vertical trend. The subsurface ammonium maximum is commonly observed in the areas where the depth is > 100 m. It is associated with a sharp pycnocline near the bottom of the euphotic zone. The regeneration of ammonium by zooplankaton and/or bacteria together with the stratification of the water column in summer are responsible for the formation of the subsurface ammonium maximum. Even with the utilization of this ammonium by phytoplankaton, the supply is sufficient for the maximum to persist. The second type of ammonium distribution is found in the midshelf region of the eastern Bering Sea. Different processes are responsible for the formation of the subsurface and sea-floor ammonium maxima. The ammonium near the sea floor is probably produced by benthic organisms. Implications of the subsurface ammonium ammonium to the fertility of the subarctic Pacific and the Bering Sea are discussed.