Colored and Fluorescent DOM in the Sea-Surface Microlayer: Response to a Phytoplankton Bloom and Photodegradation in a Mesocosm Study

Abstract. A month long mesocosm study at the Institute for Chemistry and Biology of the Marine Environment (Wilhelmshaven, Germany) examined how a phytoplankton bloom and photodegradation influence colored and fluorescent dissolved organic matter (CDOM and FDOM) in the sea-surface microlayer (SML) and underlying water (ULW). The SML, a thin (< 1000 µm) interface between ocean and atmosphere, plays a key role in air-sea exchange processes, but temporal mechanisms behind organic matter enrichment remain unclear. To isolate biogeochemical processes from environmental variability, daily SML and ULW samples were analyzed using spectral fluorometric and photometric methods, with supporting data e.g. on irradiance, temperature, and chlorophyll-a. The study covered bloom onset, peak, and decay of two partially overlying phytoplankton blooms. Samples were taken alternatively in the morning and in the afternoon, varying the exposure time to UV-light. Changes in composition and quality of organic matter were tracked using CDOM and FDOM derived parameters. Changes on the FDOM component composition were investigated using PERMANOVA. Protein-like FDOM components increased in both layers during bloom progression, while humic-like FDOM components decreased throughout the study. The significant influence of the bloom phases and the layer (SML or ULW) on the component composition was confirmed, however, their interaction was not significant. It’s likely that the change in FDOM component composition is a joint result of the influences of the phytoplankton bloom and photodegradation effects. Based on the slope ratio (SR) of CDOM absorption slopes S_275-295 and S_350-400, photodegradation was confirmed as the dominant sink of organic matter over microbial alteration. Generally, photodegradation represented a major sink for aromatic DOM during the mesocosm study, yet its effects were similar in the SML and ULW. Strong vertical mixing, shallow depth, and high light penetration likely prevented surface-specific photochemical gradients from forming.