Abstract
Experimental work with cultures of the red tide dinoflagellate Lingulodinium polyedrum suggested mycosporine‐like amino acids (MAAs) are a component of the organic matter excreted by cells. MAAs in dissolved organic matter (DOM) also may have a large influence on absorption of ultraviolet (UV) light through the water column. To test these hypotheses in a natural setting, data were collected from March 1995 through April 1996 in coastal waters off California, U.S.A. During this time, a large red tide of L. polyedrum occurred in March and April 1995. In this field study, we show that MAAs are a quantifiable component of DOM, particularly during the period following the red tide events. Absorption spectra of particulate organic matter (POM) and DOM samples revealed disproportionately high levels of UV absorption relative to visible wavelengths. UV absorption by POM at 330 nm was linearly related to phytoplankton biomass (between 0–10 µg chlorophyll [Chl] a L −1 ), measured as Chl a , accounting for 71% of the variability in absorption. Chromatographic analyses revealed the presence of various MAAs in both POM and DOM pools. MAAs were observed in 83% ( n = 53) and 47% ( n = 13) of the samples analyzed with concentrations ranging from 0 to 2.75 µM (0–696.00 µmol µg −1 Chl a) in the POM and 0 to 111.40 nM in the DOM fraction. Absorption by dissolved MAAs, as calculated from the measured concentrations, correlated with DOM UV absorption ( r 2 = 0.77) and accounted for up to 10% of the total DOM absorption at 330 nm. Thus, MAAs are a small but quantifiable component of the DOM pool in the field and contribute to UV absorption.