Microscale profiles of oxygen, pH and temperature through the sea surface microlayer in a mesocosm experiment during an algal bloom

The sea surface microlayer (SML) is the boundary layer on top of all oceans and is crucial for all exchange processes between the ocean and atmosphere. This less than 1 mm thick layer is heavily influenced by biological processes and events like algal blooms. To quantify the influence of an algal bloom in a controlled environment, we conducted a mesocosm study at the Sea sURface Facility (SURF) of the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven, Germany (53.5148 °N, 8.1463°E). SURF is an 8.5 m long, 2 m wide and 1 m deep water basin, which can directly be filled with seawater from the Jade Bay, North Sea. The facility is equipped with a retractable roof, pumps for water circulation and dedicated mounts for multiple sensor systems. The mesocosm experiment was conducted from 18 May to 16 June 2023 as part of the project BASS (Biogeochemical processes and Air-sea exchange in the Sea-Surface microlayer). SURF was filled with seawater a few days before the start of the experiment (water depth 0.7 m). The water was then filtered and the surface skimmed to remove initial pollution. To prevent particle and microbial sedimentation during the experiment, the pumps operated at low speed to maintain gentle mixing of the water column. The roof of SURF was closed during the night, while it was open during the day except when it rained. To induce an algal bloom, a mix of nutrients (nitrogen, phosphorus and silicate) was added on 26 May, 30 May and 01 June. Based on the chlorophyll measurements which show the development of the bloom, three phases of the experiment were determined: the pre-bloom phase (18 May to 26 May), the bloom phase (27 May to 04 June) and the post-bloom phase (05 June to 16 June). Several physical, chemical and biological parameters were measured, which will be published in other datasets.