Observations of a summertime phytoplankton bloom in the northeastern subtropical Pacific

Summer blooms appear regularly in the northeastern subtropical Pacific. However, their physical and biogeochemical dynamics remain enigmatic. Here, we consider remote-sensing and in-situ observations of a bloom that appeared near 29^° N and 140^° W from mid-July to the end of September 2014, Satellite ocean color images show a region of elevated surface chlorophyll a (0.2 µg L^-1 compared to < 0.1 µg L^-1) about 100 km by 300 km in size. The bloom occurred in the presence of modest mesoscale eddy activity and exhibited spatial variability characteristic of eddy stirring. In situ observations show that the bloom was characterized by a thick layer of elevated chlorophyll a from the surface down to 100 m. The sub-surface chlorophyll maximum (SCM, ~0.5 µg L^-1) occurred near the seasonal pycnocline (~50 m), shallower than SCMs elsewhere in the region (>100 m). Above the seasonal pycnocline, the bloom was also characterized by low transmissivity and two-fold increases in particulate organic carbon, particulate organic nitrogen, and primary production. A few observations showed modest nitrogen fixation rates in the bloom (~2 nmol L^-1 d^-1), suggesting physical nutrient input from below was also necessary. The physics of the bloom region was anomalous in that the layer above the seasonal pycnocline was stratified (N²~ 10^-5 s^-2 to 10^-4 s^-2) up to <15 m below the surface. Nevertheless, Richardson numbers (inferred diapycnal diffusivities) in the seasonal pycnocline were lower (larger) than elsewhere. Prior observations and models will be used to interpret these results.