Interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s

Surface chlorophyll a concentrations were measured at seven stations located at 1° latitude intervals between 52°30′ N and 58°30′ N along longitude 179°30′ W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993–95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179°30′ W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991–2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 μg l−1 in the Bering Sea basin and the values were usually <1 μg l−1. The results suggest that phytoplankton biomass in the basin is relatively stable and rather low; in other words, the Bering Sea basin may be considered to be a high nitrate, low chlorophyll (HNLC) region.