Revisiting how the euphotic depth is computed
- 1School of Marine Sciences, University of Maine, Orono, Maine, United States of America
- 2Department of Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America
Abstract
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. Author summary: The method of computing the layer of the ocean where photosynthesis takes place has been debated in the literature, yet the prevailing method is based on assuming it spans from the surface to a depth of a light-level that is a fixed percent of light at the surface. Here we argue based on observations and on phytoplankton physiology, that it should be determined on the basis of a depth of a constant value of light, as phytoplankton sense absolute and not relative light. The value we propose to adopt is based on published field, laboratory, and theoretical data and is significantly lower (by orders of magnitude) than previously proposed, however, more work needs to be conducted to determine the applicability of this value throughout the World’s Ocean. We show that using a horizon based on a fixed percent of surface light value is the same as using a very wide range of thresholds of absolute light levels and that the difference in estimated depth of the productive layer in the ocean between these approaches is significant. We urge our colleagues to cease using a depth based on a fixed percentage of surface light and adopt a more physiologically-based approach.
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BibTeX
@article{boss2026revisiting,
title = {{Revisiting how the euphotic depth is computed}},
author = {Emmanuel Boss and Charlotte Begouen Demeaux and Michael J. Behrenfeld},
journal = {PLOS Ecosystems},
year = {2026},
month = oct,
doi = {10.1371/journal.pesy.0000032},
url = {https://doi.org/10.1371/journal.pesy.0000032},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Revisiting how the euphotic depth is computed
AU - Emmanuel Boss
AU - Charlotte Begouen Demeaux
AU - Michael J. Behrenfeld
PY - 2026
DA - 2026/10/06
JO - PLOS Ecosystems
DO - 10.1371/journal.pesy.0000032
UR - https://doi.org/10.1371/journal.pesy.0000032
AB - 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. Author summary: The method of computing the layer of the ocean where photosynthesis takes place has been debated in the literature, yet the prevailing method is based on assuming it spans from the surface to a depth of a light-level that is a fixed percent of light at the surface. Here we argue based on observations and on phytoplankton physiology, that it should be determined on the basis of a depth of a constant value of light, as phytoplankton sense absolute and not relative light. The value we propose to adopt is based on published field, laboratory, and theoretical data and is significantly lower (by orders of magnitude) than previously proposed, however, more work needs to be conducted to determine the applicability of this value throughout the World’s Ocean. We show that using a horizon based on a fixed percent of surface light value is the same as using a very wide range of thresholds of absolute light levels and that the difference in estimated depth of the productive layer in the ocean between these approaches is significant. We urge our colleagues to cease using a depth based on a fixed percentage of surface light and adopt a more physiologically-based approach.
SN - 3070-8141
ER -