Aquatic particulate absorption coefficient combining extraction and bleaching optimized for inland waters

Abstract

The particulate absorption coefficient is one of the fundamental inherent optical properties describing interactions of light with material in water. Its spectral properties contain important information about chemical and biological constituents. It is often partitioned into algal and non‐algal fractions which provide useful information describing phytoplankton. Particulate absorption coefficient has been routinely measured in the ocean particularly to calibrate remote sensing algorithms. However, the methods to measure marine algal and non‐algal absorbing fractions might fail in freshwaters due to difficulties extracting green‐algae pigments and cyanobacterial phycocyanin and the high organic content of the non‐algal particles, making direct bleaching biased. In this work, we describe a method with sequential extraction, bleaching, and post‐processing to obtain unbiased pigments and non‐algal absorption fractions in freshwater environments, and we compare it against the resulting fractions obtained by only extraction or bleaching, using samples collected from 649 lakes across Canada. The resulting non‐algal particles spectra from our method appear free of interfering pigments while maintaining spectral shapes, as verified by the higher correlation coefficient between the 400 and 700 nm exponential coefficient (S, often referred to as slope) of the non‐algal particles spectra and the organic fraction of total suspended solids, and by having a better correlation between the ratio of absorption coefficient of phytoplankton at 620 and 676 nm and cyanobacterial biomass percentage. Overall, this method solves the two problems in freshwater particulate absorption partitioning associated with (1) unextracted pigments with methanol extraction methods and (2) bias introduced to non‐algal absorption spectra from NaOH bleaching.

Bio-optical properties of the Brazilian Abrolhos Bank’s shallow coral-reef waters

“…The highest in situ values of a NAP (440) were found in the inner arc, possibly related to terrigenous inputs of particles and bottom resuspension in those shallower areas that are restricted to this arc due to the presence of a hydrodynamic barrier. Note that the a NAP (440) measured in ABR in this study might be underestimated due to the extraction method used, since using sodium hypochlorite to depigment the algae fraction could modify the organic fraction from NAP (Cremella et al, 2022). The a dg (440) values retrieved from PRISMA data confirmed the higher values of a NAP (440) in the inner arc.…”