Seasonal cycle of phytoplankton UV absorption at the Bermuda Atlantic Time‐series Study (BATS) site

Abstract

Measurements of the phytoplankton absorption coefficient, aph(Λ), at the Bermuda Atlantic Times‐series Study (BATS) site demonstrated a seasonal pattern of absorption in the ultraviolet (UV) range. This was evidenced as peaks between 313 and 335 nm in aph(Λ) from the surface waters (≪50 m) in the summer months (April‐October) that were smaller or absent in the winter (November‐March). These peaks were most probably caused by pigments such as mycosporine‐like amino acids. UV pigment expression in the surface samples, approximated from aph(319)/aph(365), was linearly related to the irradiance exposure at 324 nm (r2 = 0.81). Irradiance exposure was estimated by a layered mixing model parameterized with the calculated mixed layer depth and modeled surface irradiance. Modeled UV irradiance, calculated with satellite‐derived atmospheric data and an atmospheric radiative transfer model, was not significantly different from the measured monthly mean surface irradiance. These results suggest that UV pigments are potentially ubiquitous throughout the world’s oceans and could be an important source of highly colored dissolved organic matter. A peak in the monthly averaged aCDOM for both surface and deeper waters occurred during the period of maximal UV pigment expression at the surface, suggesting either a direct link or similar physical forcing of the two. UV pigments can also be important in higher trophic levels if bioaccumulation, which has been demonstrated in other regions, occurs.