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

Springtime composites of surface chlorophyll a (chi) from the SeaWiFS instr ument 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 Nor th 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 conduct ed in April 1998 and 1999 to characterize the hydrographic conditions assoc iated with the large chi gradient. Irradiance and chi data collected during the cruise periods were used in an optical model to compute depth-integrat ed rates of primary production. The southern portion of both transects rese mbled the climatological conditions at the Hawai'i Ocean Time-series (HOT) study site, Station ALOHA (22.75 degreesN, 158 degreesW), with two major ph ysical and biological frontal features encountered: 1) the South Subtropica l Front (SSTF) in 1998 at 27 degrees -28 degreesN and in 1999 at 32.5 degre esN; and 2) the Subtropical Front (STF) in 1998 at 32 degreesN, which in 19 99 had migrated northward out of our study area to similar to 34 degreesN. Integrated chi and primary production both increased at the frontal locatio ns in both years. Increases in surface chi from SeaWiFS and shipboard measu rements were not apparent at the SSTF, yet the subsurface chi maximum both shallowed and doubled at this front. In addition, primary production modele d from the satellite chi data did not reflect the smaller mesoscale variabi lity of the SSTF. The increase in surface chi at the STF, however, was clea rly recorded in both the satellite and shipboard chi measurements. HPLC pig ment analysis was performed for select samples collected during the 1998 cr uise and revealed a change in phytoplankton composition at all frontal loca tions. Specifically, there were distinct increases in the dinoflagellate ma rker, peridinin, and the diatom marker, fucoxanthin, at the SSTF. The same patterns at the SSTF were observed during both cruises, except all gradient s were shifted to the north, as a result of changes in the large-scale circ ulation of the gyre in 1999. While the eastern equatorial Pacific experienc ed a dramatic decrease in chi concentrations during El Nino events, the sub tropical 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.