In situ fast‐repetition‐rate fluorometric (FRRF) surveys were conducted in the North Pacific Subtropical Gyre (NPSG) at Station ALOHA (Sta. ALOHA, 22°459N, 158°009W), from September 2002 to December 2004, to assess temporal and vertical photosynthetic variability in relation to environmental conditions. The nighttime potential photosynthetic efficiency of the photoautotrophic microbial assemblage (given as the ratio of variable to maximal fluorescence, FV:FM) was low in the mixed layer and increased with depth. High FV:FM values were observed at and below the deep chlorophyll maximum layer (DCML); some values approached the theoretical maximum for prokaryotes grown under nutrient‐replete conditions in the laboratory (0.60). In contrast, the absorption cross section of photosystem II (ςPSII) was high at the surface and decreased with depth; minima (1,000 A° 2 quanta‐1) occurred around the DCML. These vertical patterns suggest photosynthetic stress conditions in surface photoautotrophic populations. No significant seasonal cycles were found for FV:FM, but surface ςPSII values peaked in winter and decreased during summer, suggesting that seasonal variations in light availability may influence the observed ςPSII variability. A significant correlation was found among surface FV:FM, ςPSII, and the distance from the mixed‐layer depth (MLD) to the top of the nutricline. Neither nutrient nor light variations were significantly related to FV:FM and ςPSII in the DCML. Within this layer, FV:FM variability was positively and negatively related to concentrations of chlorophyll b and zeaxanthin, respectively. Our results suggest that, at Sta. ALOHA, surface photosynthesis takes place under chronic nutrient limitation, while higher photosynthetic efficiency in the lower euphotic zone appears to be sensitive to community‐structure changes.

Measurements at the Hawaii Ocean Time‐series (HOT) Station ALOHA (22°45′N, 158°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 14C 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 Niño/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.

In situ 14C uptake (dawn to dusk) and fast repetition rate fluorometry (FRRF) measurements at nearly monthly intervals were compared at Station ALOHA (22°45′N, 158°00′W) between August 2002 and September 2003 in order to determine the feasibility of using FRRF profiling as a means for estimating primary production (PP). The FRRF and 14C rates were significantly correlated (r2=0.906, P value <0.05, n=70) with slopes of 2.00 and 1.90 for chl a and light normalized data, respectively. However, the relationship between 14C‐ and FRRF‐derived carbon fixation varied vertically and temporally. The FRRF: 14C ratio was >1.5 in near‐surface water (5–25 m depth) and approached 1.0 deeper in the euphotic zone. Vertical variations probably reflected the effect of different physiological processes (i.e. Mehler reaction, dark respiration, and excretion) on overall photoautotrophic respiration. In particular, the decrease in Mehler reaction rates with increasing water depth may have accounted for the decrease in difference between 14C and FRRF measurements with depth. The influence of in situ light field variability in controlling the absorption cross‐section of photosystem II (PSII) (σPSII′) may also have been responsible for some of this difference. When compared with total community respiration (R), the derived light‐driven photoautotrophic respiration (reported here as the difference between FRRF and 14C measurements) represented approximately 50% of R integrated over the euphotic zone. Our results show that FRRF and 14C measurements were well correlated in oligotrophic waters but the exact relationship between the two processes varies both temporally and vertically, such that a unique relationship between these two techniques could not be derived from first‐order principles.