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13 pages, 9 figures, 2 tables, We examined the responses of two different natural arctic plankton communities to warming in an attempt to find temperature thresholds for abrupt changes in growth or biomass. We conducted two experiments in situ using two different plankton communities; incubation temperatures represented the annual variation in these Arctic waters plus 3–4 °C to account for the potential warming expected for the Arctic Ocean during the twenty-first century. We evaluated changes in nutrient and chlorophyll a concentrations, primary production, cell abundances, and growth rates as well as cell biovolume and percentage of living cells for the main groups found during our study. Phytoplankton communities were composed of diatoms, Phaeocystis sp., Micromonas, as well as other flagellates. Results indicate a decrease in chlorophyll a concentration and growth rate at the highest temperatures tested. We also found a differing effect on cell abundance and growth rates depending on the taxonomic group or cell size: diatom abundance decreased with warming while small-sized phytoplankton showed an increasing trend at the intermediate temperature treatments. Biovolume estimates also tended to decrease with increasing temperature. We find a perceptible change in trends at intermediate temperature treatments for most of the parameters which suggests a possible thermal threshold around 5–6 °C for the arctic plankton communities examined here, This study was supported by the project Arctic Tipping Points (ATP, contract # 226248) from the European Union. A. Coello-Camba was supported by a grant BES-2007-15193 from the Spanish Ministry of Science and Innovation