Drivers of phytoplankton blooms in Hawaii: A regional model study

The region around the main Hawaiian Islands (MHI) is characterized by a permanent thermocline, and numerous processes have been proposed to facilitate phytoplankton blooms in this oligotrophic province. Here, we use a coupled physical-biogeochemical model of the MHI to elucidate some of the different dynamics behind phytoplankton blooms. The model permits submesoscale processes and is integrated for the years 2010-2017 embedded in a physical state-estimate reanalysis using nearly 50 million observations. Model results exhibit good agreement between simulated values and observations at Station ALOHA for physical and biogeochemical parameters. The overall levels and the amplitude of the seasonal cycles are well captured for many variables. We show that variations in net primary production are mainly driven by domain-wide seasonal cycles of light and nitrogen fixers, respectively, as well as short-lived, stochastic bloom events resulting from the formation of eddies to the west of the island of Hawaii. Furthermore, sporadic wind- and current-driven upwelling is resulting in ephemeral enhancements of nearshore phytoplankton blooms mainly on the northeastern side of the islands.