Anomalous conditions in the south-eastern Bering Sea, 1997: nutrients, phytoplankton and zooplankton

Anomalies in the regional weather over the south-eastern Bering Sea during spring and summer of 1997 resulted in significant differences in nutrient a vailability, phytoplankton species composition, and zooplankton abundance o ver the continental shelf as compared with measurements in the 1980s. Calm winds and the reduction of cloud cover in spring and summer produced a very shallow mixed layer in which nitrate and silicate were depleted after an A pril diatom bloom. High submarine light levels allowed subsequent phytoplan kton growth below the pycnocline and eventual depletion of nitrate from the water column to depths of 70 m or more. Thus, total new production during 1997 may have exceeded that of previous years when nitrate was not depleted below the pycnocline. A bloom of the coccolithophorid, Emiliania huxleyi, was observed in early July in the warm, nutrient-depleted waters over the m iddle and inner shelf. Emiliania huxleyi concentrations reached 4.5 x 10(6) cells L-1 by September, and the bloom persisted through the autumn. There was evidence for increased abundance of some species of copepods in 1997 as compared with data from the middle domain in June 1981. The abundance of a dult and juvenile euphausiids in 1997 was statistically similar to values m easured in 1980 and 1981. However, near-surface swarms were rarely observed on the inner shelf in August-September 1997. Lack of euphausiid availabili ty in the upper water column may partially explain the August-September mas s mortality of planktivorous short-tailed shearwaters (Puffinus tenuirostri s) observed on the inner shelf.