Measurements at the Hawaii Ocean Time-series (HOT) Station ALOHA (22 degrees 45'N, 158 degrees W) have revealed a significant, approximately 50% increase in euphotic zone depth-integrated rates of primary production ( PP; mol C fixed m(-2) d(-1)) based on in situ C-14 experiments. The character of the nearly two-decade increasing trend in PP was punctuated by several abrupt episodes that coincided with changes in the El Nino/ Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO) climate indices, or both. In contrast to the observed increase in rates of PP, the PP per unit chlorophyll a ( mol C fixed mol chl a(-1) d(-1)), a measure of the biomass-normalized production, was relatively constant, whereas PP per unit solar radiation ( mol C fixed mol quanta(-1)), a measure of the efficiency of light utilization, varied in synchrony with the temporal trend in PP. Coincident with variations in PP, the HOT program core data sets also revealed changes in mixed-layer depth, upper ocean stratification, inorganic nutrients, phototrophic microbial abundances and pigment inventories. These time-series data suggest that the ENSO/PDO may control upper ocean stratification and vertical nutrient delivery into the euphotic zone at Sta. ALOHA, thereby influencing the composition of the plankton assemblage and altering rates of PP and particulate matter export. The present research was supported by NSF grant OCE03-26616 (awarded to D.M.K.), OCE-0117919 and OCE-0327513 (awarded to R.L.), and by a grant from the Gordon and Betty Moore Foundation, Marine Microbiology Initiative (awarded to D.M.K.).