AbstractThe sea-surface microlayer is the boundary layer between the water of the oceans and the atmosphere occupying approximately “1000 micrometer” uppermost part of the sea-surface microlayer (SML) (Liss and Duce, 1997). Nutrients and pollutants are known to be concentrated in the SML compared to the water column. The unique physical and chemical features of the SML may provide a habitat for a large number and diverse groups of neuston (microorganisms living in the SML) such as ciliates and phytoplankton. However, these microorganisms in the SML may be exposed to the higher concentrations of nutrients and pollutants, compared to the water column, probably affecting their diversities and species composition. The diversity and species composition of the ciliates and phytoplankton were assessed in the sea-surface microlayer (SML) and sub-surface layer during an experiment at three locations in the outer Oslofjord during July 2009. Verdens Ende was expected to have low amounts of nutrients and oil pollution. Ferjeodden (a harbor) and Bustangen (close to a farm) were expected to have elevated concentrations of oil and nutrients, respectively. The results showed that there were variations in the number of individuals of ciliates and phytoplankton between replicates collected from each habitat and location. Measures of diversity indicated that there were not any differences in ciliate and phytoplankton between the SML and the sub-surface layer and between the SML samples at the three locations. The ciliate and phytoplankton species compositions in the two layers were not significantly different but separations were observed between the two layers. Significant differences were found in ciliate and phytoplankton species compositions in the SML samples from the three locations. Oligotrich ciliates and diatoms were identified as the most responsible taxonomic groups contributing to separations between neuston and plankton and between the neuston at the three locations. Adaptation to eutrophication, oil pollution, and ultraviolet radiation (UVR), predator pressure reduction and food availability could be possible explanations for this observation. Finally, species composition could not be related to the environmental factors (temperature and salinity) because the environmental factors did not vary substantially at the three locations.