Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre

Abstract

Continuous records of upper water column (0–150 m) temperature profiles, spectral distribution of down welling irradiance, and phytoplankton solar‐induced fluorescence at 25 m depth were obtained during the inaugural deployment of the Hawaii Air‐sea Logging Experiment, A Long‐term Oligotrophic Habitat Assessment (HALE ALOHA) mooring, near the Hawaii Ocean Time‐series (HOT) Station ALOHA (22°45′N; 158°00′W). The temperature record showed a strong upwelling event in March–April 1997, displacing the thermocline by 120 m. Remote sensing satellite (NSCAT and TOPEX/ERS 2) analyses suggest that the observed upwelling was a result of strong wind divergence and the passage of a cyclonic eddy through the HOT program study area. At the onset of the upwelling event increases in colored dissolved organic matter (CDOM) and chlorophyll fluorescence efficiency in the upper water column were detected by changes in the spectral distribution of the down welling irradiance. The 0–25 m mean chlorophyll a (chl a) concentration increased threefold toward the end of the upwelling period. Water column samples collected during the monthly HOT cruises also indicate that the relative contribution of diatoms to total chl a increased twofold inside the eddy. The long‐term temporal variability in frequency and intensity of these poorly resolved mesoscale events might be key factors determining the structure of the pelagic ecosystem in the North Pacific Subtropical Gyre. Integrating multi‐year remote sensing satellite, moored, and vessel‐based time series records permits a quantification of the spatial and temporal scale of upper water column perturbations and the characterization of the pelagic ecosystem response at various timescales.