The quantitative filter technique for measuring phytoplankton absorption: Interference by MAAs in the UV waveband

Abstract

The absorption of suspended particulate material is commonly estimated with the filter pad technique, which requires a correction for pathlength amplification. The pathlength amplification factor (β) varies among different studies and phytoplankton species or communities. It remains the largest source of uncertainty in estimated absorption coefficients. Recently, several empirical models estimating this correction were developed but mostly for visible range. In this study, β was calculated empirically from the ratio of filtered to suspension absorption between 280 and 850 nm for cultures of two dinoflagellates and one diatom. Results show that in the visible waveband, β values are relatively constant and fall within the published range (averages between 2.4 and 2.8). These values remain flat over the UV waveband for the diatom tested. However, below 400 nm, the presence of mycosporine‐like amino acids (MAAs) strongly influences the absorbance measurements. For the dinoflagellates studied, the ultraviolet (UV) absorbance measured on frozen filters (stored in liquid nitrogen) reveals a large peak (Sosik 1999) caused by high concentrations of MAAs, which is much smaller on absorbance scans of suspended cells. This amplified UV peak adds to the true amplification effect caused by the extended pathlength of light in filter pads. This artifact, caused by the extracellular release of the water‐soluble MAAs during freezing, also was observed to a lower degree with measurements performed on fresh filters (immediate scanning), precluding the use of this method to estimate UV absorption in the species tested.