Relationships between the concentration of particulate organic nitrogen and the inherent optical properties of seawater in oceanic surface waters

Abstract. The concentration of particulate organic nitrogen (PON) in seawater plays a central role in ocean biogeochemistry. Limited availability of PON data obtained directly from in situ sampling methods hinders development of thorough understanding and characterization of spatio-temporal variability of PON and associated source and sink processes within the global ocean. Measurements of seawater inherent optical properties (IOPs) that can be performed over extended temporal and spatial scales from various in situ and remote-sensing platforms represent a valuable approach to address this gap. We present the analysis of relationships between PON and particulate IOPs including the absorption coefficients of total particulate matter, a_p(λ), phytoplankton, a_ph(λ), and non-algal particles, a_d(λ), as well as the particulate backscattering coefficient, b_bp(λ). This analysis is based on an extensive field dataset of concurrent measurements of PON and particulate IOPs in the near-surface oceanic waters and shows that reasonably strong relationships hold across a range of diverse oceanic and coastal marine environments. The coefficient a_p(λ) and a_ph(λ) show the best ability to serve as PON proxies over a broad range of PON from open ocean oligotrophic to coastal waters. The particulate backscattering coefficient can also provide a good proxy of PON in open ocean environments. The presented relationships demonstrate a promising means to assess PON from optical measurements conducted from spaceborne and airborne remote-sensing platforms and in situ autonomous platforms. In support of this potential application, we provide the relationships between PON and spectral IOPs at light wavelengths consistent with those used by satellite ocean color sensors.