No abstract

Understanding spatial and temporal dynamics of non‐algal particles in open ocean is of the utmost importance to improve estimations of carbon export and sequestration. These particles covary with phytoplankton abundance but also accumulate independently of algal dynamics. The latter likely represents an important fraction of organic carbon, but it is largely overlooked. A possible way to study these particles is via their optical backscattering properties (bbp) and relationship with chlorophyll‐a (Chl). To this aim, we estimate the fraction of bbp associated with the non‐algal particle portion ( bbpk) that does not covary with Chl by using a global Biogeochemical‐Argo data set. We quantify the spatial, temporal, and vertical variability of bbpk. In the northern productive areas, bbpk is a small fraction of bbp and shows a clear seasonal cycle. In the Southern Ocean, bkbp is a major fraction of total bbp. In oligotrophic areas, bbpk has a smooth annual cycle.

The Forel-Ule (FU) color comparator scale is the oldest set of optical water types (OWTs). This scale was originally developed for visual comparison and generated an immense amount of data, with hundreds of thousands of observations being gathered from the last 130 years. Since recently, the FU scale is also applicable to remote sensing data. This has been possible thanks to an optical characterization of the 21 FU colors in terms of the (x,y) CIE standards and new algorithms that convert remote-sensing reflectances (R rs ) from satellite-borne ocean color sensors to FU. R rs -derived hue angle and FU have been recently applied with success in the assessment of color variability of lakes and specific shelf areas, but an evaluation over global oceanic waters is still missing. By clustering global climatological ESA-OC-CCI v2.0 R rs with the derived FU, we obtain a set of R rs to be used as optical water types (OWTs). Diffuse attenuation coefficient, Secchi disk depth and chlorophyll concentration are also associated to the FU classes. The angular distances of a given R rs to the two nearest FU classes are proposed as simple and robust membership indexes, adding up to one. We also evaluate the advantages and limitations of FU and the hue angle as monitoring tools over the full marine range, from the most oligotrophic areas to the turbid and productive coastal zones. The first 7 FU indexes cover 99 % of global surface waters. Unlike the hue angle, that resolves all spatiotemporal color variations, the FU scale is coarse as a monitoring tool for oligotrophic waters as all the subtropical gyres saturate to FU=1, while the color of other seas varies across 2, 3 or even 4 FU classes. We illustrate the introduction of a new "zero" FU class that increases monitoring resolution at the blue end of the color range. Finally, we show how optical diversity varies across the color range and compare several sets of OWTs from a color perspective. Overall, we provide a valuable and self-consistent dataset that enhances the usefulness of the FU scale by converting it to useful information for the oceanographic community. This OWT scheme keeps the advantages of other datasets, like being useful to study ocean color product quality and characterize the uncertainties, but also allows to continue to monitor long-term change in optical diversity over the global ocean color. Integration into the optical modules of ecosystem models can help verify past simulations that predate the satellite age, through comparisons with in-situ FU data collected at the time. *Revised Manuscript with no Changes Highlighted Click here to download Revised Manuscript with no Changes Highlighted: manuscript_R2_accepted_changes.docx