High concentrations of mycosporine‐like amino acids and colored dissolved organic matter in the sea surface microlayer off the Iberian Peninsula

Abstract

Mycosporine‐like amino acids (MAAs), phytoplankton pigments, and inherent optical properties were analyzed in sea surface microlayer (SSM), near‐surface (0‐2 m), and subsurface (0–110 m) samples from stations off the Iberian Peninsula in June‐July 2005. During a visible surface slick, MAA concentrations reached 290 µg L−1 in the SSM, which correlated with an increase in abundance of the dinoflagellate, Prorocentrum micans (> 4000 cells mL−1) and covaried with low Ekman transport. High levels of colored dissolved organic material (CDOM) were also found in the SSM, and a prominent absorption shoulder between 300 and 340 nm was evident in the CDOM absorption spectra, indicative of MAAs in the dissolved fraction. Low CDOM slope ratios (SCDOM(275‐295) : (350‐400)) during the development of the surface slick suggest microbial production of CDOM, possibly exuded from the sudden proliferation in P. micans. In the absence of a well‐defined SSM, higher SCDOM(275–295) : (350–400) ratios were found, suggesting photobleaching of CDOM at the surface. The particulate absorption spectra of MAAs (aPMAA(λ)) were effective at absorbing ultraviolet (UV) radiation in a narrow spectral band between 310 and 350 nm. Since aCDOM(λ) exhibits an exponential rise in absorption in the UV and was high in the SSM and the subsurface compared with aPMAA (λ), CDOM would be the principal mechanism of UVA and B attenuation in the water column, providing an additional sunscreen to phytoplankton.