Comparison of factors controlling phytoplankton productivity in the NE and NW subarctic Pacific gyres

The subarctic North Pacific is one of the three major high nitrate low chlorophyll (HNLC) regions of the world. The two gyres, the NE and the NW subarctic Pacific gyres dominate this region; the NE subarctic Pacific gyre is also known as the Alaska Gyre. The NE subarctic Pacific has one of the longest time series of any open ocean station, primarily as a result of the biological sampling that began in 1956 on the weathership stationed at Stn P (50 degrees N, 145 degrees W; also known as Ocean Station Papa (OSP)). Sampling along Line P, a transect from the coast (south end of Vancouver Island) out to Stn P has provided valuable information on how various parameters change along this coastal to open ocean gradient. The NW subarctic Pacific gyre has been less well studied than the NE gyre. This review focuses mainly on the NE gyre because of the large and long term data set available, but makes a brief comparison with the NW gyre. The NE gyre has saturating NO, concentrations all year (winter = about 16 mu M and summer = about 8 mu M), constantly very low chlorophyll (chl) (usually < 0.5 mg m(-3)) which is dominated by small cells ( < 5 mu m). Primary productivity is low (about 300-600 mg C m(-2) d(-1) and varies little (2 times) seasonally. Annual primary productivity is 3 to 4 times higher than earlier estimates ranging from 140 to 215 g C m(-2) y(-1) Iron limits the utilization of nitrate and hence the primary productivity of large cells (especially diatoms)except in the winter when iron and light may be co-limiting. There are observations of episodic increases in chi above 1 mg m(-3), suggesting episodic iron inputs, most likely from Asian dust in the spring/early summer, but possibly from horizontal advection from the Alaskan Gyre in summer/early fall. The small cells normally dominate the phytoplankton biomass and productivity, and utilize the ammonium produced by the micrograzers. They do not appear to be Fe-limited, but are controlled by microzooplankton grazers. The NW Subarctic Gyre has higher nutrient concentrations and a shallower summer mixed depth and photic zone than Stn P in the NE gyre. Chi concentrations tend to be higher (0.5 to 1.5 mu g L-1) than Stn P, but primary productivity in the summer is similar to Stn P (similar to 600 mg C m(-2) d(-1)). There are no seasonal data from this gyre. Iron enrichment experiments in October, resulted in an increase in chi (mainly the centric diatom Thalassiosira sp.) and a draw down of nitrate, suggesting that large phytoplankton are Fe-limited, similar to Stn P. (C) 1999 Elsevier Science Ltd, All rights reserved.