Relative depths of the subsurface peaks of phytoplankton abundance conserved over ocean provinces

Flow cytometric data collected from more than 250 stations in the Pacific and Indian Oceans were analyzed to determine the factors affecting the depth and magnitude of the subsurface abundance maximum of Synechococcus, Prochlorococcus, and small eukaryotes (< 6 mu m). The peak depth of each phytoplankton population estimated by curve fitting was strongly correlated with the depth of the subsurface chlorophyll maximum (SCM). The slope of the regression line demonstrated that the peak depths of Synechococcus, Prochlorococcus, and small eukaryotes were 74% +/- 1%, 88% +/- 1%, and 104% +/- 1% of the SCM depth (with confidence interval of 95%), respectively. This trend was largely conserved across the different ocean provinces of the Pacific and Indian Oceans. The peaks of Synechococcus frequently appeared in the nitrate-depleted layer of subtropical waters, suggesting their high affinity for regenerated and/or organic nutrients. The estimated daily irradiance received at the peak depths of Synechococcus and Prochlorococcus did not show a distinct latitudinal trend and fluctuated among neighboring stations, whereas that of small eukaryotes slightly increased toward the subarctic region. The present results show that the peak depths of Synechococcus, Prochlorococcus, and small eukaryotes relative to SCM were globally conserved on average, which is largely in line with the difference in their ability to acquire light and nutrients. However, the absolute light level and nutrient concentrations at the peak depths varied dramatically among neighboring stations, likely affected by physical movements.