Department Seminar: Taketoshi Kodama

Title: Revealing nanomolar nutrient patterns in the open oligotrophic ocean

Abstract: The strong relationship between nutrient concentration and phytoplankton biomass has been recognized for about a century. Today, it is widely accepted that surface concentrations of macronutrients (nitrate, nitrite, ammonium, and phosphate) regulate phytoplankton growth and thus drive biogeochemical cycles. Nutrient concentrations are also used as proxies for ocean circulation; nutrient measurements are routine in ocean-going observations, resulting in more than a million available data points.

However, most nutrient studies and datasets have two major caveats: one is the insufficient detection limits, and the other is the detection of ammonium. About 60% of the global oceans, particularly in low-latitude regions, are oligotrophic. In these regions, surface nutrient concentrations, particularly nitrate, seem to vary at the nanomolar level, which is below the detection limits of standard methods, making the values unreliable. Ammonium, along with nitrate and nitrite, is a major nitrogenous nutrient and is considered the primary nitrogen source in oligotrophic oceans. Nevertheless, its concentrations have been reported far less frequently than nitrate and phosphate, and its dynamics remain poorly understood.

This seminar will begin with a brief review of my published work on the marginal sea of the western North Pacific, and will then focus on the spatial patterns of nanomolar-level nutrient concentrations using highly sensitive colorimetry in the low-latitude Pacific and Indian Oceans. I will present the processes regulating these spatial patterns and their biological impacts. These results suggest that macronutrients primarily limit phytoplankton biomass and primary production in the oligotrophic oceans, but unresolved processes also constrain them. In the last part of the seminar, I will introduce a newly developed method for elemental analysis.