Revisiting how the euphotic depth is computed

The method of computing the euphotic depth, the depth horizon in the ocean above which light is sufficient for net primary production, has lacked consensus in the literature. Although many still use a % light level as a definition, a practice which dominates textbooks in oceanography, the modeling community uses a constant basal respiration to set the threshold. Following Banse (2004), we remind the readers that phytoplankton only sense absolute irradiance level, and recommend the use of a fixed value, named the ’compensation irradiance’, to define this layer. A value based on field (Arctic Ocean) and laboratory data is now available (0.0035 ± 0.002 mol photons m –2 day –1 ), and is supported by physiological arguments. Here we argue that a value of a similar order of magnitude is likely applicable for more southerly regions of the ocean and should be used to determine the depth of the surface ocean layer where autotrophic production exceeds autotrophic respiration (if such a layer exists). Additionally, using field observations, we show that the suggested % light horizons are associated with a wide range of irradiance values (even in low latitudes) and that the 1% light horizon, the value most commonly used for the computation of the euphotic depth, is, on average in the open ocean, 80 m (48%) shallower than that based on the 0.0035 mol photons m –2 day –1 isolume. We find that a substantial fraction of the phytoplankton biomass lies below the conventional 1% light threshold, which is likely to affect biological pump estimates that assume no production occurs below that threshold. Additional work is needed to determine if indeed a single light threshold applies throughout the ocean and at all seasons and, if not, to what degree does this threshold varies.