Phytoplankton blooms affect microscale gradients of oxygen and temperature across the sea surface microlayer

Abstract. The sea surface microlayer (SML) is the thin layer on top of the ocean that is in direct contact with the atmosphere and is crucial for air–sea interactions. Its properties are influenced in particular by surface-active substances (surfactants), mainly produced by phytoplankton and bacteria. Thus, phytoplankton blooms and their decay can have a considerable influence on the SML. A mesocosm study was conducted to assess the impact of a phytoplankton bloom on the SML using a multidisciplinary approach, which enabled in situ measurements under controlled yet natural conditions. A phytoplankton bloom was induced within a mesocosm facility filled with seawater, resulting in three phases of the study: the pre-bloom, bloom, and post-bloom phases. During all phases, microsensors measured in situ microprofiles of oxygen and temperature with a 125 µm vertical resolution through the air, SML, and underlying water. Oxygen and temperature gradients were determined from the profiles, as well as the thicknesses of the oxygen diffusion boundary layer (DBL) and thermal boundary layer (TBL). The night-time oxygen gradients (Ø_{ΔO2, pre-bloom} = –2.16 ± 5.53 µmol L^–1, Ø_{ΔO2, bloom} = +24.90 ± 14.51 µmol L^–1, Ø_{ΔO2, post-bloom} = +2.07 ± 4.82 µmol L^–1) correlated highly with the chlorophyll a concentration (r = 0.755, p < 0.001), while the DBL thickness (Ø_{DBL, overall} = 937 ± 369 µm) showed a moderate correlation to the SML surfactant concentration (r = 0.490, p = 0.014). Both indicate the phytoplankton bloom's influence on oxygen gradients across the SML. Night-time temperature gradients (Ø_{ΔT, overall} = –0.133 ± 0.079 °C) and the TBL thickness (Ø_{TBL, overall} = 1300 ± 392 µm) were not correlated to the chlorophyll a or surfactant concentration. The mesocosm study and the microprofiling approach provide in situ data on the air–sea exchange processes in the SML, reflecting the distinct interplay of the SML and phytoplankton blooms in the exchange of oxygen and heat. This has implications for future studies on air–sea gas and heat exchange between the ocean and the atmosphere.