Fine scale phytoplankton community structure across the Kuroshio Front

Concurrent physical, chemical and biological observations across the Kuroshio Front collected in October 2009 provide a detailed view of the relationship between the physical environment and the phytoplankton community. Depth profiles were taken at stations 9 km apart along five 70 km transects. With a combination of flow cytometry, microscopy and high-pressure liquid chromatography pigment analysis, we characterized the phytoplankton community structure across the front. The observed phytoplankton community fell into two distinct assemblages, largely separated by the front, but which also reflected patterns in the distribution of Kuroshio and Oyashio water masses shaped by mesoscale lateral mixing. Phytoplankton biomass was elevated where there was a positive vertical flux of nitrate towards the surface, and the frontal circulation drove a lateral transport of nutrients southwards into the subtropical gyre. The observations showed that the phytoplankton respond to forcing on several scales: the phytoplankton community across the front was shaped by a combination of the large scale biogeography of the region, mesoscale mixing of populations and finer scale modification of the light and nutrient environment.