Log response ratio of specific growth rates of Antarctic phytoplankton in monocultures versus mixed cultures in response to temperature and nitrogen:phosphorus supply interactions

Data were obtained from a laboratory growth experiment conducted from April to May 2021 at the Institute for Chemistry and Biology of the Marine Environment (University of Oldenburg). Three Antarctic phytoplankton species (Chaetoceros simplex, Phaeocystis antarctica (single cell culture) and Geminigera cryophila) were grown in monocultures and a mixture of all three species. We used a factorial design including four different N/P treatments (control plus three different N/P levels) and 10 different temperatures (-2, -1, 0, 1, 2, 3, 4, 6, 8 and 10°C). The detailed experimental setup is explained in doi:10.1594/PANGAEA.971329. Differences in growth rates of the individual species between monocultures and the assemblage were assessed by comparing their specific growth rates. Therefore, the biovolume of single species in the mixture was transformed to optical density (OD) based on biovolume-OD calibrations, and specific growth rates were calculated for each species in monoculture and the mixture as the regression between the start conditions and the exponential growth phase using the formula ln(Nt/N0)/t) were N0 is the OD at the start of the experiment and Nt the OD at the exponential growth phase of the mixture. In order to evaluate the effect size of the temperature and nutrient treatments on the differences in the specific growth rate of the individual species between monocultures and in the mixed assemblage, we calculated a log response ratio for each species and treatment as ln(µmix/µmono). Positive deviations from 0 indicate higher species-specific growth in the assemblage compared to the monoculture. Negative deviations from 0, in turn, indicate higher specific growth rates in monocultures.