The Effects of Algal Blooms on Oxygen Concentration and Temperature in the Sea Surface Microlayer – a Mesocosm Study

The sea surface microlayer (SML) is of global importance as all exchange processes of heat and gases between the ocean and the atmosphere have to pass through it and are regulated by the features of the SML. These exchanges occur not only permanently between the SML and the atmosphere, but also between the SML and the underlying water (ULW). The properties of the SML are strongly influenced by surface-active substances known as surfactants, which are mostly of biological origin. Events such as algal blooms can produce large amounts of surfactants, thus changing the properties of the SML and the ULW. Obtaining in situ data of the SML proved very difficult in the past, due to its small thickness. Using microsensors gives the opportunity to close this gap by obtaining in situ data of the SML and to directly show the influence an algal bloom has on the SML.

A mesocosm experiment was conducted to obtain a more mechanistic understanding of the effect of an algal bloom on the physicochemical properties of the SML. An algal bloom was artificially induced in a seawater basin and physiochemical changes in the SML and ULW were investigated over time by applying multiple techniques. To directly study changes in temperature and oxygen, very precise microsensors (UNISENSE) were used for continuous in situ profiling, measuring from the air, through the SML, and into the ULW on a scale of tens of micrometers. We conducted the experiment over a continuous 30-day period during the algal bloom, allowing us to gain insights into the boundary layer, including the formation of oxygen and temperature gradients and the thickness of the SML.

The microsensor data showed, that the oxygen gradient in the SML is strongly correlated to the chlorophyll a concentration (r = 0.76, p < 0.01) and thus the algal bloom, while the thickness of the oxygen diffusion boundary layer, however, only shows a weak correlation to the surfactant concentration (r = 0.47, p = 0.01). The oxygen measurements deliver the in situ data to verify previous assumptions on oxygen gradients (-10 – 50 µmol L^-1) and the thickness of the oxygen diffusion boundary layer (500 – 1500 µm) at the sea surface. The temperature gradient in the SML and the thickness of the thermal boundary layer were not influenced by the algal bloom, but the in situ measurements also confirm previous assumptions on temperature gradients (0.05 – 0.2 °C) and the thermal boundary layer thickness (750 – 2000 µm).

Obtaining gradients of gases or temperature in the SML and calculating the SML thickness was in the past only possible via indirect methods like measuring gas concentration differences between air and ULW or with computing surface temperatures from the emitted longwave irradiance. The in situ microsensor measurements now enable us to directly investigate processes inside the SML without relying on indirect measurements. Overall, we investigated the effect of an algal bloom on the SML and demonstrated a new in situ approach using microsensors to investigate physicochemical changes in and across the SML.