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

• We estimated the compensation depth at station ALOHA from C-14 uptake.
• We subtracted dark bottle counts from light bottle counts.
• The irradiance at the compensation depth was about 0.11% of surface irradiance.
• The compensation irradiance was about 1% of surface blue light (475 nm) irradiance.
• The compensation irradiance was about 0.054 mol-photons m−2 d−1

The compensation irradiance, the irradiance at which net photosynthesis is zero over a 24-h period, was estimated at station ALOHA (22°45′N, 158°W) from analysis of 14C 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 14C 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 14C 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.