Estimating the compensation irradiance in the ocean: The importance of accounting for non-photosynthetic uptake of inorganic carbon

The compensation irradiance, the irradiance at which net photosynthesis is zero over a 24-h period, was estimated at station ALOHA (22 degrees 45'N, 158 degrees W) from analysis of C-14 uptake rates measured from 8 January 1989 to 13 June 1990 at depths ranging from 5 to 175 m. The estimates were made on the basis of linear regressions of the difference between light bottle and dark bottle C-14 uptake in the light-limited region of the euphotic zone and determination of the depth at which the difference between the uptake rates was zero. About half of the non-photosynthetic C-14 uptake at the compensation irradiance could be attributed to chemolithoautotrophy; the remainder was presumably due to anaplerotic processes. Deriving the compensation irradiance by extrapolating dawn-to-dawn light-bottle uptake above the compensation irradiance to zero resulted in underestimation of the compensation irradiance by a factor of 2. We estimated the compensation irradiance at station ALOHA to be 0.054 mol-photons m(-2) d(-1), about 0.11% of surface 400-700 nm radiation and 1% of surface 475-nm (blue) light. (C) 2014 Published by Elsevier Ltd.