Interannual mesoscale physical and biological variability in the North Pacific Central Gyre

Springtime composites of surface chlorophyll a (chi) from the SeaWiFS instrument show a sharp increase in chi standing stock (from similar to0.1 to 0.3 mg m(-3)) occurring between 30 degrees and 40 degreesN in the eastern North Pacific Ocean. To investigate the spatial and interannual variability of this springtime feature, multi-platform surveys (23 degrees -33 degreesN, 158 degreesW), including shipboard and satellite measurements, were conducted in April 1998 and 1999 to characterize the hydrographic conditions associated with the large chi gradient. Irradiance and chi data collected during the cruise periods were used in an optical model to compute depth-integrated rates of primary production. The southern portion of both transects resembled the climatological conditions at the Hawai'i Ocean Time-series (HOT) study site, Station ALOHA (22.75 degreesN, 158 degreesW), with two major physical and biological frontal features encountered: 1) the South Subtropical Front (SSTF) in 1998 at 27 degrees -28 degreesN and in 1999 at 32.5 degreesN; and 2) the Subtropical Front (STF) in 1998 at 32 degreesN, which in 1999 had migrated northward out of our study area to similar to 34 degreesN. Integrated chi and primary production both increased at the frontal locations in both years. Increases in surface chi from SeaWiFS and shipboard measurements were not apparent at the SSTF, yet the subsurface chi maximum both shallowed and doubled at this front. In addition, primary production modeled from the satellite chi data did not reflect the smaller mesoscale variability of the SSTF. The increase in surface chi at the STF, however, was clearly recorded in both the satellite and shipboard chi measurements. HPLC pigment analysis was performed for select samples collected during the 1998 cruise and revealed a change in phytoplankton composition at all frontal locations. Specifically, there were distinct increases in the dinoflagellate marker, peridinin, and the diatom marker, fucoxanthin, at the SSTF. The same patterns at the SSTF were observed during both cruises, except all gradients were shifted to the north, as a result of changes in the large-scale circulation of the gyre in 1999. While the eastern equatorial Pacific experienced a dramatic decrease in chi concentrations during El Nino events, the subtropical Pacific appeared to be more 'green' with a southward shift of the circulatory and high surface chl fronts. (C) 2001 Elsevier Science Ltd. All rights reserved.