Effects of a Phytoplankton Bloom and Photobleaching on Colored and Fluorescent Dissolved Organic Matter in the Sea-Surface Microlayer

The effects of a phytoplankton bloom and photobleaching on the colored and fluorescent dissolved organic matter (CDOM and FDOM, respectively) in the sea-surface microlayer (SML) and the underlying water (ULW) were studied in a 33-day mesocosm experiment at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven, Germany. The SML is the thin (< 100 µm) boundary layer between the ocean and the atmosphere and highly relevant to ocean biogeochemistry and climate-related exchange processes. Previous work has shown that when the SML is enriched in organic matter it can hinder gas, light, momentum, and heat exchanges between ocean and atmosphere. However, the underlying processes of organic matter enrichment in the SML are insufficiently understood. Heterogeneity and dynamics in the open sea make it difficult to differentiate between transport processes, environmental drivers, and biogeochemical processes. Hence, the mesocosm study was conducted to gain a deeper understanding of organic matter formation and degradation processes in the SML and ULW. To gain an understanding of different sources and sinks, the hypotheses tested were (1.) phytoplankton blooms result in different FDOM component compositions in the SML and ULW and (2.) photodegradation affects the component composition of the SML and the ULW differently.
Daily SML and ULW samples were collected for spectral fluorometric and photometric analysis, alternately in the morning and afternoon. Supplementary parameters like irradiance, temperature, and chlorophyll-a were also recorded within the mesocosm basin with high temporal resolution (approx. 1 min). Spectral photometric and fluorometric methods, which exhibit high sensitivity and structural specificity with respect to organic matter are used for CDOM and FDOM analysis.
The mesocosm experiment was divided into three phases (bloom onset, peak, and decay). Degradation of larger, complex molecules or production of new organic matter was assessed via the “humification index” and is dependent on the water layer (SML or ULW), the phase of the bloom, and the sampling time. As samples were taken alternatively in the morning and in the afternoon, the exposure time to UV-light and therefore photodegradation as a sink varies differently for SML and ULW. CDOM slope results showed a high variability and generally higher molecular weights and higher molecule aromaticity in the SML compared to the ULW. Protein-like component concentrations increased in both SML and ULW which indicates higher microbial activity towards the peak and decay phase of the experiment. These results suggest that photodegradation and possibly microbial activity have different effects on SML and ULW, verifying hypothesis 2. The affect of higher biological activity during the phytoplankton bloom led to the most pronounced differences between the concentration and composition of organic matter in the SML and ULW, especially in the protein-like components. This finding supports the premises of hypothesis 1.