The photosynthetic performance of sea ice microalgae, benthic microalgae, coralline red algae (Phymatolithon foecundum) and brown macroalgae (Laminaria saccharina, Fucus evanescens and Desmarestia aculeata) in an ice-covered high-Arctic fjord (Young Sound, NE Greenland) was studied with in situ instruments for oxygen microprofiling and active chlorophyll fluorescence measurements. In situ measurements of the effective quantum yield of Photosystem II (PSII) under ambient low irradiance ranged between 0.3 and 0.7. Net oxygen production of benthic microalgae and sea ice microalgae was measured in situ at low irradiance of -2 to 30 μmol photons m-2 s-1. The compensation irradiance determined in the laboratory for coralline algae and brown macroalgae was 1.6 and 1.9 μmol photons m-2 s-1 respectively. The same experiments showed irradiances at onset of saturation, Ek, for coralline red algae and brown macroalgae of 17 and 12.8 μmol photons m-2 s-1 respectively. In situ measurements of rapid light curves, measured over 1 to 2 min by the saturation pulse method, enabled snap shots of the in situ adaptation of the photosynthetic apparatus in the different phototrophs under ambient irradiance conditions. These measurements showed Ek values for PSII related electron transport (relative ETR) at 7.9 (sea ice microalgae), 4.6 (benthic microalgae), 11 (coralline red algae) and 4.4 to 7.3 (brown macroalgae) μmol photons m-2 s-1. At higher irradiance levels, relative ETR decreased, indicating inhibition and/or downregulation of photosynthesis. When light curves were measured with longer application of the different actinic light levels, a less pronounced or zero decrease of relative ETR was found at higher irradiance, and Ek values were shifted to higher irradiance, i.e. 6.9 μmol photons m-2 s-1 (benthic microalgae) and 11.5 to 16.1 μmol photons m-2 s-1 (brown macroalgae). This apparent short-term acclimation to increasing irradiance reversed to the initial characteristics of the light curve within 15 to 20 min after irradiance returned to ambient intensity. Our measurements add to the small database on in situ photosynthetic performance of surface-associated algae below ice cover. All phototrophs were well adapted to low irradiance under the ice, and active fluorescence measurements indicated that the phototrophs are able to adapt reversibly within minutes to moderate changes in irradiance levels.