Four sediment cores (50 cm length, 10 cm diameter) were collected from the Isla D station (located in Potter Cove) by SCUBA divers in December 2016. The sediment cores were stored in a water bath at in situ temperature of 0.5°C. A magnetic stirrer was inserted into the core and the overlying water was permanently aerated. Thereby, the overlying water was kept homogeneous and oxygen saturated. Cold-light lamps (Osram Lumilux Cool Daylight L36W/865, Osram, Munich, Germany) were installed above the sediment cores and the emitted photosynthetically active radiation (PAR) was permanently controlled with a spherical PAR-sensor (US-SQS/L and ULM-500, Walz, Germany). The spherical PAR-sensor was placed in the water bath, adjusted to the lowest height of the sediment surface of the sediment cores, covered with sea-water and treated similar to the sediment cores.Sediment cores were exposed to PAR intensities of 0, 5, 10, 15, 20, 25, 35, 47 and 70 µmol photons/m² s, starting with the lowest PAR. In order to enable the benthic microalgae community (microphytobenthos, MPB) to adjust to the experimental light conditions, the sediment cores were pre-incubated at each PAR intensity for 4 h. The sediment cores were closed airtight with no air bubbles in the overlying water and the volume of the overlying water was determined afterward. An optical oxygen microsensor (OXR50, Pyroscience, Aachen, Germany) with a tip size diameter of 50 µm was installed in the lid, which allowed a continuous measurement of the oxygen concentration in the overlying water. On beforehand, the microsensor was calibrated at in situ temperature with a two-point calibration using air saturated and anoxic waters (by adding sodium dithionite).The sediment cores were incubated at each PAR for ≥3 h. Measurements of oxygen concentrations were performed at a 2 s temporal resolution, while the overlying water was kept homogeneous by rotating magnets. The total oxygen flux over the period of each PAR exposure was calculated using the formula:Total oxygen flux= -(δO_2×V)/(δt×A)in which δO2, δt, V and A represent the difference in oxygen concentration, the difference in time, the volume of the overlying water and the enclosed surface area, respectively.After the incubation, the sediment core was exposed to the next higher PAR by adjusting the height of the cold-light lamps and the procedure for total oxygen flux measurement was repeated. To avoid an oxygen oversaturation at the highest PAR, which would lead to an underestimation of the total oxygen flux, the overlying water of the sediment cores was aerated with helium until an oxygen concentration of 240 µmol O2/L was reached (70% oxygen saturation, controlled by above-mentioned oxygen microsensors).