Particulate scattering and backscattering in relation to the nature of particles in the Red Sea

Abstract

Measurements of light scattering can be used to quantify the concentration and composition of oceanic particles, and resolve biogeochemical processes spanning different time and space scales. In this paper, we analyze the first dataset, collected over wide spatial scales in the Red Sea, of particulate scattering (urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0001), particulate backscattering (urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0002), particulate absorption and chlorophyll-a concentration [Chl_a]. We fit a three-component conceptual model relating urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0003 to [Chl_a], assuming a non-algal background component (urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0004), and two additional components driven by small (<2urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0005m) and large phytoplankton (>2urn:x wiley:21699275:media:jgrc24395:jgrc24395-math-0006m) (urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0007 and urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0008, respectively). We extend the approach, for the first time, to the modelling of total particulate scattering (urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0009), allowing us to retrieve the backscattering ratio for each component in the model. We observe a high backscattering ratio for the background component which, when analyzed alongside measurements of particulate absorption, suggests it is likely dominated by non-algal (rather than algal) particles. The high contribution of non-algal particles to urn:x-wiley:21699275:media:jgrc24395:jgrc24395-math-0010 at low [Chl_a] may be related to the unique conditions in the Red Sea, or more broadly, characteristic of other oceanic conditions. The work illustrates how we can combine optical measurements with conceptual models, to understand better the composition of oceanic particles and ultimately, improve monitoring of marine biogeochemical processes. Our work will also be useful for developing regional ocean-colour models for the Red Sea.