Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in...

Plankton respiration rate is a major component of global CO₂ production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (R_light) and in the dark (R_dark) increased as the 2/3 power of R_light so that the R_light:R_dark ratio increased from an average value of 1.37 at the median R_light measured here (3.62 µmol O₂ L^-1 d^-1) to an average value of 17.56 at the highest R_light measured here (15.8 µmol O₂ L^-1 d^-1). The role of respiratory processes as a source of CO₂ in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest.