- The North Pacific subtropical gyre (NPSG), once considered to be a biological desert due to low primary production (PP) and its associated variability, has been found more productive and variable than previously thought. The environmental conditions controlling this relatively high PP variability are yet to be elucidated, despite important implications regarding the role of this large oceanic region in the global C- cycle and marine trophic dynamics. In this context, the present investigation aimed to further elucidate PP patterns and their underlying causes of variability in the NPSG, across different vertical (meters) and temporal (decadal to monthly) scales. Physical and biogeochemical data collected at Station ALOHA (Sta. ALOHA, 22º 45' N 158º 00' W), as part of the on-going Hawaii Ocean Time-Series (HOT) program, were used to address these research questions. Over the last two decades, PP, based on in situ 14C measurements, has increased by approximately 50 %. This increase has not been linear, but punctuated by sudden temporal changes in relation to variations in the physical stability of the upper water column. These physical changes are related to large scale, ocean-atmosphere interactions occurring during the El Niño/ Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Concomitant with these bio-physical variations, a significant change in community structure was also observed; Prochlorococcus spp. cells abundance declined, while picoeukaryotes and prymnesiophytes concentrations increased.
In order to understand the seasonal efficiency and variability of the energy flow in the NPSG, the relationship between gross vs. net primary production (GPP vs. NPP, respectively) was determined by comparing in situ Fast Repetition Rate Fluorometry (FRRF) derived PP and 14C measurements, respectively. The GPP and NPP were significantly related across vertical (10-20 m) and temporal (months) scales, but the GPP:NPP ratio decreased as a function of depth and peaked during fall. Monthly and vertical PP variability was assessed by photosynthetic efficiency as determined by in situ FRRF. Photosynthetic efficiency was low at the surface, but increased with depth peaking in the Deep Chlorophyll Maximum Layer (DCML). Chronic nutrient limitation seemed to control photosynthetic processes in surface layers, while community structure changes appeared to contribute to the observed variability in photosynthetic efficiency in the lower euphotic zone.
This investigation supports the current view of NPSG as a more productive and variable ecosystem than previously thought. It also provides some insights regarding the controlling mechanisms of the biological (i.e. PP) variability in the NPSG. Over decadal scales, the ecosystem PP and variability appear sensitive to large-scale, atmosphere-ocean interactions in relation to ENSO and PDO events. However, a predictive relationship was not found between PP and ENSO-PDO in the NPSG. In addition, the efficiency of the ecosystem in storing fixed C seems to vary in relation to season’s development. Finally, nutrients availability and community structure could control the observed vertical variability in photosynthetic efficiency, and in turn in PP, in the NPSG.