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

The subarctic North Pacific is one of the three major high nitrate low chlo rophyll (HNLC) regions of the world. The two gyres, the NE and the NW subar ctic Pacific gyres dominate this region; the NE subarctic Pacific gyre is a lso known as the Alaska Gyre. The NE subarctic Pacific has one of the longe st time series of any open ocean station, primarily as a result of the biol ogical sampling that began in 1956 on the weathership stationed at Stn P (5 0 degrees N, 145 degrees W; also known as Ocean Station Papa (OSP)). Sampli ng 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 ch ange along this coastal to open ocean gradient. The NW subarctic Pacific gy re 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 m akes a brief comparison with the NW gyre. The NE gyre has saturating NO, co ncentrations all year (winter = about 16 mu M and summer = about 8 mu M), c onstantly very low chlorophyll (chl) (usually < 0.5 mg m(-3)) which is domi nated by small cells ( < 5 mu m). Primary productivity is low (about 300-60 0 mg C m(-2) d(-1) and varies little (2 times) seasonally. Annual primary p roductivity 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 wint er when iron and light may be co-limiting. There are observations of episod ic 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 horiz ontal 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-limi ted, but are controlled by microzooplankton grazers. The NW Subarctic Gyre has higher nutrient concentrations and a shallower summer mixed depth and p hotic 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 d own of nitrate, suggesting that large phytoplankton are Fe-limited, similar to Stn P. (C) 1999 Elsevier Science Ltd, All rights reserved.