What factors are driving summer phytoplankton blooms in the North Pacific Subtropical Gyre?

[1] Annually recurrent summer to fall surface blooms of the dinitrogen (N-2) fixing genera Trichodesmium and Richelia have a significant impact on biogeochemical cycling in the North Pacific Subtropical Gyre (NPSG). Yet the environmental determinants of these blooms have not been thoroughly resolved. Here, we combine remote sensing of ocean color, sea surface temperature (SST), sea surface height anomalies (SSHa), wind forcing, and integrated irradiance with the vessel-based time series of the Hawaii Ocean Time-series (HOT) program at Station ALOHA ( 22.75 degrees N, 158.00 degrees W) and mooring data derived from the National Data Buoy Center (NDBC) buoy 51001 (23.42 degrees N, 162.2 degrees W). With these data sets we attempt to constrain the environmental window under which blooms of large cell-sized N-2 fixing organisms increase in abundance in NPSG surface waters using phycoerythrin (PE) as a proxy. For identified blooms, our analyses indicate that these events are confined to the months of June-October, SST in the range of 25 degrees-27 degrees C, and mixed layer depths less than 70 m. Neither wind forcing nor SSHa are correlated (directly or time-lagged) with increases in PE concentrations. Furthermore, blooms do not consistently result in increases of in situ or remotely sensed chlorophyll a. Additional higher-resolution data sets of physical forcing, diazotroph abundance, and biochemical properties, sampled on the timescale of bloom development (days-weeks), will be necessary to the environmental conditions supporting annual summer-fall blooms.