Biogeochemical Links in the Sea-Surface Microlayer: A Multidisciplinary Mesocosm Study

The sea-surface microlayer (SML) represents the thin (< 1000 µm) uppermost layer of the ocean. Due to its unique position between ocean and atmosphere, the SML plays a central role in marine biogeochemical cycles. Changes in the phytoplankton biomass and community composition are linked to profound changes in the physical, chemical, and biological properties of the SML. And this influences air-sea interaction such as heat and gas exchange, organic matter composition, and surface-active substances in the SML and underlying water (ULW). Dynamic interactions between the SML and the ULW and the connectivity of the biogeochemical processes in the SML remain unclear. To fill this knowledge gap, we conducted a multidisciplinary mesocosm study. Here we report the general setup in a 17 m³ mesocosm facility, the progression of an induced phytoplankton bloom, and the general description and coupling of the changes in biogeochemical properties of the SML and the ULW.

SML and ULW samples were collected daily to analyze inorganic nutrients (NO₃^-, NO₂^-, PO₄^3-, SiO₃^2-), turbidity, solar radiation, phytopigments, surfactants, dissolved and particulate organic carbon (DOC, POC), total dissolved and particulate nitrogen (TDN, PN), phytoplankton and bacterial abundance, and their utilized substrates.

A self-organizing map (SOM) configuration revealed a clear temporal segregation of nutrient samples in SML and ULW. Based on nutrient levels, phytoplankton bloom progression over the time of the mesocosm experiment could be clearly classified into pre-bloom, bloom, and post-bloom phases. During this time, Chla concentrations varied from 1.0 to 11.4 μg L^-1 with an average of 7.3 µg L^-1. POC and PN exhibited a strongly positive relationship (r = 0.95) and followed the trend of Chla. Turbidity demonstrated a peak during bloom phase, which was associated with a high biological activity. Phytopigment composition data showed that haptophytes were the dominant phytoplankton group, followed by diatoms which could be confirmed by optical methods.

The daily average solar irradiance aligned with the local weather variability. Surfactants were enriched in the SML compared to the ULW. A discrepancy between the onset of increases in phytoplankton biomass and surfactant concentrations was observed with a lag of five days. This mismatch suggests a physiological acclimation of phytoplankton towards less favorable growth conditions, for example, nutrient limitations after the bloom phase. The high surfactant concentrations were also mirrored as DOC and TDN enrichment in the SML compared to ULW. A distinct slick formation with high turbidity was observed, indicating a biofilm-like SML habitat during the bloom and post-bloom phases. This biofilm was characterized by higher bacterial cell counts in SML. Bacterial metabolic profiles assessed by Biolog EcoPlates showed that the bacterial community utilized amino acids as key substrates in both water layers.

The main findings of our study emphasize that changes in biological parameters were linked to changes in chemical and physical parameters in SML. Our study provides deeper insights into the biogeochemical controls of the SML at a mechanistic level. Further spatio-temporal studies are needed to investigate the coupling of biogeochemical processes between the SML and ULW at both regional and global scales.