Optical proxy for particulate organic nitrogen from BGC-Argo floats.

Area of Science:

• Oceanography
• Biogeochemistry
• Marine Optics

Background:

• Particulate organic nitrogen (PON) is a key nutrient influencing marine ecosystems.
• Accurate measurement of PON is crucial for understanding oceanic productivity and biogeochemical cycles.
• The Western Tropical South Pacific is an important region influenced by dinitrogen (N₂) fixation.

Purpose of the Study:

• To develop and validate an optical proxy for particulate organic nitrogen (PON) concentration.
• To assess the relationship between the backscattering coefficient and PON in the Western Tropical South Pacific.
• To enable autonomous, large-scale monitoring of phytoplankton biomass dynamics using biogeochemical-Argo floats.

Main Methods:

• Utilized biogeochemical-Argo float measurements in the Western Tropical South Pacific.
• Measured the backscattering coefficient at 700 nm (b_bp).
• Correlated b_bp with particulate organic nitrogen (PON) concentrations determined by wet oxidation.

Main Results:

• Established a significant relationship (R²=0.87) between b_bp and PON, particularly with wet oxidation measurements.
• Developed an optical proxy (PON^opt) capable of estimating PON concentrations between 0.02 and 0.95 µM.
• Demonstrated PON^opt as a reliable proxy for phytoplankton biomass in the study area.
• Observed significant increases in PON^opt (0.16 to 0.80 µM) linked to N₂ fixation during stratification.

Conclusions:

• The backscattering coefficient (b_bp) serves as a viable optical proxy for PON concentration (PON^opt).
• This optical proxy enables unprecedented autonomous monitoring of PON and phytoplankton biomass dynamics.
• The findings provide insights into new production linked to N₂ fixation events in the Melanesian Archipelago.