Abstract
We investigated the size-dependent temperature response of natural phytoplankton communities from a lake and a river in the Canadian subarctic. Photosynthesis by total, 2 um size fractions was determined at 11 irradiances (1-109% of ambient solar radiation) and five temperatures (5-25°C) in outdoor solar incubators. Temperature had no effect on photosynthesis at low irradiance, but strongly regulated the photosynthetic response at saturating and inhibiting irradiances. For the river phytoplankton, low temperatures lowered EK values (onset of light satur- ation) and shifted photosynthesis in the water column from light dependence to temperature depend- ence. A photosynthesis-temperature-irradiance (P-T-E) model was developed to describe the varied temperature response of photosynthesis across the full range of limiting, saturating and inhibiting irra- diances. The P-T-E model explained 74-95% of the variation in photosynthesis for all size fractions (total community, >2 um fraction and 2 um communities. The picoplankton fraction was also more responsive to increasing temperature than larger cells, implying a greater sensi- tivity to diumal or longer term changes in lake water temperature.
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