Satellite Chlorophyll Bloom Dynamics in the North Eastern Pacific Subtropical Gyre

Large chlorophyll blooms spanning hundreds of square kilometers and persisting for weeks-months, have been repeatedly observed in satellite records of ocean color in the North Pacific Subtropical Gyre (NPSG). These blooms, typically been defined as chlorophyll concentrations exceeding some threshold (generally ~0.15 mg m-3), occur uniquely within the late summer/fall months of June-October and are often attributed to the activity of N2 fixing organisms (diazotrophs). Elevated phytoplankton biomass can lead to enhanced export of carbon to the deep ocean yet the physical and chemical drivers of these anomalies are not well understood nor are the events well defined. Here we aim to characterize the magnitude, frequency, and geographic location of satellite-derived chlorophyll blooms in the NPSG near the monitoring site of the Hawaiʻi Ocean Time-series program between 1997-2022. We present an anomaly approach to defining ‘blooms’ and find a significant linear slope in the magnitude of the summer chlorophyll anomalies over time in large areas of our study region, indicating that bloom intensity is increasing within this region in summer months. Despite the increase in the summer bloom intensity, we find no significant trend in bloom area or duration. We also present a case-study time-series of a large bloom observed in 2018, a year of strong volcanic activity in the region, to illustrate the time-resolved change in phytoplankton biomass, and provide an assessment of relationships to potential determinants of enhanced biomass in the region. This effort is intended to better characterize chlorophyll anomalies in the subtropical NPSG.