Phytoplankton performance is influenced by environmental variation at multiple scales. However, experimental information on the response to multifactorial variation is still limited, particularly for polar environments. We investigated the response in maximum growth rate (r<sub>max/sub>), biomass and stoichiometry of an assembled community consisting of three Antarctic phytoplankton species from key groups exposed to a 5 x 5 gradient of nitrogen (N) and phosphorus (P) supply combined with two amplitudes of temperature fluctuation (± 0.5 and 1°C) around two different means (2°C and 5°C). Overall, phytoplankton growth rate and biomass was higher with increased N and P supply, indicating co-limitation of both nutrients. Biomass and growth rate decreased at elevated mean temperature under imbalanced nutrient supply conditions, indicating strong interactive effects of warming and nutrient conditions (ratios). At 2 °C, higher amplitude resulted in higher growth rate and biomass compared to low amplitude. At 5°C, increased amplitude resulted in lower growth and biomass at suboptimal N conditions. Phytoplankton N:P followed the supply ratio but decreased with increasing mean temperature and higher amplitude at N:P > 40. Overall, our results indicate that increasing thermal fluctuation (amplitude) around an increasing mean will push polar phytoplankton closer to their maximum thermal tolerance. Balanced nutrient conditions may compensate for the negative effect but under suboptimal nutrient supply, this may result in decreasing primary production and altered community composition.

Primary Presenter: Christoph Plum, University of Oldenburg (c.plum@uni-oldenburg.de)

Authors:

Christoph Plum, University of Oldenburg/Institute for Chemistry and Biology of the Marine Environment  (c.plum@uni-oldenburg.de)

Eva Klumb, University of Oldenburg/Institute for Chemistry and Biology of the Marine Environment  (eva.lisbeth.klumb@uni-oldenburg.de)

Miriam Gerhard, Universidad de la República/Departamento de Ecología y Gestión Ambiental  ()

Mike Smykala, University of Oldenburg/Institute for Chemistry and Biology of the Marine Environment  (mike.jan.smykala@uni-oldenburg.de)