The effects of temperature and light on ecosystem metabolism in a Japanese stream

Ecosystem metabolism is a fundamental process that governs biogeochemical C cycling through the fixation and mineralization of C by all the organisms in a system; however, the functional responses of ecosystem metabolism to light and temperature variability remain unknown in streams and rivers. In this study, we assessed the effects of temperature and light on ecosystem metabolism, namely gross primary production (GPP) and ecosystem respiration (ER), in a Japanese stream by monitoring the seasonal diel changes in dissolved oxygen. We found that the temperature dependence of metabolism, which is expressed as activation energy, was higher for ER (0.48 eV) than for GPP (0.39 eV), supporting the metabolic theory of ecology. Both daily GPP and ER increased with the daily integrated photosynthetically active radiation (PAR), peaking at moderate PAR values and decreasing at higher irradiance levels. This unimodal relationship between daily GPP and PAR was due to the rapid increase of instantaneous PAR during the early morning, suppressing the maximum rate of area-specific productivity, which was a main determinant of daily GPP. Moreover, daily GPP and ER were closely associated in this study stream, resulting in a unimodal pattern of daily ER with daily integrated PAR. Our results suggest that both temperature and light regulate stream ecosystem metabolism but that the sunlight regime (i.e., the rapid increase of solar irradiance in the early morning) rather than light intensity per se may be a critical factor for both GPP and ER in the study stream.