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

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.

Automated summary

The particulate absorption coefficient is an important optical property describing the interaction of light with material in water, and its measurement in freshwater environments has presented challenges. This study proposes a method that successfully extracts unbiased pigments and non-algal absorption fractions in freshwater environments, providing valuable information for studying phytoplankton in lakes.