Mapping nitrate in the global ocean using remotely sensed sea surface temperature

Abstract

Nitrogen is the most broadly limiting factor for marine phytoplankton on ecological timescales. Nitrate, as the most oxidized inorganic species, plays a significant role in nitrogen availability based on nutrient flux to the euphotic zone from deeper waters, and is a major determinant of new production. Increased new production relates to higher trophic level abundance and to carbon dioxide drawdown from the atmosphere. The present work is the first stage in the development of a technique to generate a scaled index of nitrate availability in the surface waters of the global ocean using satellite-derived temperature data. The technique currently involves a fixed matrix of nitrate depletion temperatures (NDT) and remotely-sensed sea surface temperature (SST) from the monthly-average AVHRR Pathfinder series. The magnitude of the difference between these two temperatures at a given location indicates the degree of nitrate presence or absence. Graded monthly nitrate presence/absence maps over a 10-year period were created based on the size of the difference between these two temperatures. A 10-year average of these differences exhibits major nitrate distribution features similar to those observed in maps based on National Oceanic Data Center archived measurements. In contrast, monthly nitrate maps provide a unique and dynamic representation of nitrate availability in the global ocean. This nutrient monitoring capability based on remotely sensed data can contribute to the estimation of new production in the global ocean, improving management of various world fisheries and improving estimation of the atmospheric draw down of carbon dioxide, a major greenhouse gas.