Ocean’s Sunlit Zone May Be Far Deeper Than Textbooks Say

For nearly a century, oceanographers have drawn a line in the water. Below the depth where sunlight fades to one percent of what shines at the surface, the ocean was presumed too dark for meaningful photosynthesis, and generations of textbooks have treated that horizon as the bottom of the euphotic zone, the sunlit layer where marine plants can thrive. A new study argues that this beloved convention is not just imprecise but fundamentally wrong, and that replacing it could reshape how scientists measure the ocean’s productivity and its role in the global carbon cycle.

The study, published in PLOS Ecosystems by Emmanuel Boss, Charlotte Begouen Demeaux, and Michael J. Behrenfeld, revisits how the euphotic depth should be computed. The authors revive an argument made two decades ago by the influential biological oceanographer Karl Banse, who died in 2025: phytoplankton, the microscopic algae that anchor the marine food web, do not sense light as a percentage of surface illumination. They experience only the absolute flux of photons arriving at whatever depth they happen to occupy. A definition based on relative light levels, the authors contend, is therefore physiologically meaningless.

The one-percent rule has undeniable practical appeal. It can be estimated with an uncalibrated radiometer, a simple Secchi disk, or satellite ocean-color products, and it appears in studies of phytoplankton seasonal cycles, carbon flux calculations, and remote-sensing data distributed worldwide. But the new analysis, built on nearly 21,000 profiles from autonomous BGC-Argo floats spanning October 2012 to December 2023, shows that the absolute amount of light found at the one-percent horizon varies enormously with latitude, season, and cloud cover. At any given moment, roughly seventy percent of the ocean lies beneath clouds, and seasonal swings in solar angle halve surface irradiance by thirty degrees latitude and extinguish it entirely near the poles in winter.

Instead of a percentage, the authors recommend anchoring the euphotic depth to a fixed absolute light level called the compensation irradiance: the minimum daily light at which photosynthesis can outpace algal respiration, allowing net growth. Drawing on recent Arctic field campaigns and laboratory work, they propose a value of 0.0035 plus or minus 0.002 mol photons per square meter per day, a figure orders of magnitude lower than many earlier estimates and close to the theoretical minimum of about 0.0009 mol photons per square meter per day derived from the bioenergetics of photosynthesis.

The evidence for such extraordinarily low-light growth comes from converging sources. In Baffin Bay, researchers using chlorophyll fluorescence and particle backscattering sensors below sea ice detected the onset of phytoplankton accumulation at an average light level of 0.0043 mol photons per square meter per day. A separate Arctic study coupling biomass measurements with radiocarbon-based production assays found a compensation irradiance of 0.0035 mol photons per square meter per day, essentially identical to the value now proposed. In the Southern Ocean near the Kerguelen Islands, incubation experiments previously documented positive net primary production at the 0.01 percent light level, corresponding to roughly 0.006 mol photons per square meter per day.

Critically, the new work extends the argument beyond polar waters. Analyzing cytometric counts of Prochlorococcus, the abundant photosynthetic bacterium that dominates nutrient-poor oceans, from the Atlantic Meridional Transect cruise 13, the authors found significant cell concentrations down to depths where daily light falls to the proposed 0.0035 threshold, even in subtropical latitudes. Oxygen-based measurements of gross primary production from other Atlantic transects likewise show detectable production at the 0.1 percent light level. Because the ocean is well connected and microbial strains disperse globally, the authors reason that an algal lineage capable of near-theoretical-minimum growth in the Arctic almost certainly has counterparts elsewhere.

The quantitative consequences are striking. Across the BGC-Argo dataset, the one-percent light horizon sits, on average, about 80 meters, or 48 percent, shallower than the depth of the proposed compensation isolume. When the authors integrated chlorophyll and phytoplankton carbon from the surface down to each horizon, the conventional one-percent cutoff missed more than sixty percent of the biomass inventory captured by the deeper boundary: average integrated chlorophyll was 40.9 versus 65.3 milligrams per square meter, and phytoplankton carbon 1.1 versus 1.8 grams per square meter. A substantial reservoir of phytoplankton, in other words, lives below the depth where most studies assume production stops.

That oversight matters for the biological pump, the suite of processes that transports carbon from the surface ocean into the deep sea, where it can remain sequestered for decades to centuries. Many pump estimates treat everything below the one-percent horizon as a realm of pure respiration, assuming any phytoplankton found there will simply sink. If production actually continues deeper, those assumptions inflate the apparent strength of the pump. Similar biases arise in estimates of the mixed-layer pump, the seasonal export of carbon when springtime shallowing of the mixed layer strands biomass in darkening water; using the shallower horizon overstates how much carbon that mechanism delivers to depth.

Numerical ecosystem and biogeochemical models, notably, already sidestep the percentage convention. They instead assign phytoplankton a constant basal respiration rate, typically 0.02 to 0.03 per day, and growth occurs only where photosynthesis exceeds that cost. The authors show, through a standard photosynthesis formulation, that such respiration rates imply a compensation irradiance of roughly 0.036 mol photons per square meter per day, still an order of magnitude above the lowest field estimates. Even models, it seems, do not yet allow production at the dimmest levels where real algae demonstrably persist.

The authors acknowledge open questions. Temperature might, in principle, raise respiration and thus the required light at lower latitudes, but meta-analyses suggest the temperature dependence of algal net growth weakens and effectively vanishes as light becomes limiting, lending support to a single global threshold. Deep mixed layers and internal waves complicate the light environment individual cells experience, and the appropriate isolume for a given question may differ: migrating zooplankton follow far dimmer isolumes, while nutrient and particle-attenuation horizons align with much brighter ones. Still, the recommendation is unambiguous: after nearly a century, oceanographers should retire the one-percent rule for defining the euphotic depth and adopt a physiologically grounded, absolute light threshold, a shift the authors suggest could be implemented with existing satellite products and may ultimately revise how the living ocean is measured from top to bottom.

Subject of Research: Redefining the ocean's euphotic depth using a physiologically based compensation irradiance instead of the one-percent surface light convention

Article Title: Revisiting how the euphotic depth is computed

Article References: Revisiting how the euphotic depth is computed. (n.d.). https://doi.org/10.1371/journal.pesy.0000032

Image Credits: AI Generated

DOI: 10.1371/journal.pesy.0000032

Keywords: euphotic depth, phytoplankton, compensation irradiance, oceanography, primary production, biological pump, BGC-Argo, Prochlorococcus, light attenuation, carbon export, Arctic Ocean, marine ecosystems