An Interaction between Light and Temperature on the Growth of Arctic Phytoplankton

Arctic environments are changing rapidly, with some of the most dramatic changes in temperature on the globe. Temperature changes lead to changes in ice cover and mixing regime, which are resulting in dramatic changes in the underwater light field. Not much is known about the effects of temperature, irradiance, and their interaction on Arctic phytoplankton. We examined the effect of realistic temperature and irradiance changes on the growth rate and photosynthetic physiology of the widely distributed Arctic green flagellate Micromonas sp. and diatom Attheya septentrionalis. The growth rate of A. septentrionalis increased with temperature throughout the range of experimental temperatures (3-14°C) but Micromonas exhibited a marked decrease in growth rate at elevated temperatures (about 11°C). The effect of temperature on growth was altered by irradiance: at saturating irradiances, the effect of temperature was consistent with the widely observed Q₁₀ or Eppley temperature curve, but with decreasing irradiance the effect of temperature on growth rate becomes increasingly muted. Models commonly assume a uniform effect of increasing temperature on all species across all other environmental conditions. Our results indicate this assumption must be reexamined, particularly for systems experiencing rapid temperature increases.