Most existing schemes inside NEMO treat sunlight as a single downward stream that simply fades with depth, resolved over only two to five broad wavebands. The new module, adapted from the model developed for the MITgcm-DARWIN configuration, does something fundamentally different. It separates the light field into three distinct streams: a direct downward stream that has not yet been scattered, a scattered downward stream produced by light deflected forward by particles and water molecules, and an upward stream of backscattered light heading back toward the surface. Each stream is solved independently for every wavelength, and the model runs across 33 wavebands spanning 250 to 4000 nanometres, with a fine 25-nanometre resolution in the visible range where biology matters most.

The technical heart of the module is a set of coupled equations describing how each stream is attenuated by absorption, forward scattering, and backscattering as it propagates through the water column. These processes are governed by the inherent optical properties of seawater, which are determined by what is actually in the water: pure seawater itself, phytoplankton, detritic non-algal particles, and coloured dissolved organic matter, known as CDOM. In the coupled configuration, a biogeochemical model supplies the concentrations of these optically active constituents, and the module converts them into spectral absorption and scattering coefficients. CDOM, which absorbs strongly at short wavelengths, is handled with an exponential decay law referenced at 412 nanometres, while phytoplankton optics are computed from three functional types relevant to the Black Sea: diatoms, small flagellates, and large flagellates.

What makes this development genuinely powerful is the feedback loop. The simulated irradiances are not just diagnostic outputs; they are fed back into the physics and the biology. The total scalar irradiance budget becomes the source term in the temperature equation of NEMO, replacing the crude attenuation schemes of the past, and the visible-range scalar irradiance available to phytoplankton drives primary production in the coupled ecosystem model. This two-way coupling maintains physical consistency: the same light that heats the water is the light that plankton use to grow. The researchers calibrated a tuning factor, set to 0.85 in the Black Sea configuration, that accounts for the small fraction of absorbed energy used by photosynthesis and particle degradation rather than heating.

The test case is the Black Sea, a basin whose biogeochemistry is simulated operationally by the Copernicus Marine Service using the NEMO-BAMHBI framework. BAMHBI, the BiogeochemicAl Model for Hypoxic and Benthic Influenced areas, explicitly represents the anoxic deep layer of the Black Sea and the full marine foodweb from bacteria to mesozooplankton. Previously it relied on a simple three-band optics scheme in which a single downward light stream was attenuated following Beer’s law. Substituting that scheme with the new three-stream model required careful calibration, which the team performed using more than a decade of data from Biogeochemical Argo floats drifting through the basin, measuring chlorophyll, particle backscattering, CDOM fluorescence, and downwelling irradiance at 380, 412 and 490 nanometres.

The surface boundary conditions come from an unexpected direction: the sky. The team used the MAR regional atmospheric model, running with the ECMWF ecRad radiation scheme and a high-resolution gas-optics configuration, to simulate spectral shortwave fluxes over the Black Sea in the same 33 wavebands used by the ocean module. MAR separates direct and diffuse irradiance, exactly the split the ocean model needs, and validation against ground observations showed correlations above 0.9 with low bias. Importantly, because sea surface reflectance is computed as a ratio of upwelling to downwelling irradiance, any residual bias in the atmospheric forcing largely cancels out, making the reflectance output robust.

The validation results are encouraging. Simulated downwelling irradiances at 412 and 490 nanometres agree very well with BGC-Argo profiles, with regression slopes close to one, although the model absorbs slightly too much light near the surface at shorter wavelengths where CDOM dominates. Sea surface temperature simulations remain consistent with observations, with correlations above 0.99, and the mixed layer depth, thermocline, and cold intermediate layer of the basin are preserved. Surface chlorophyll improves modestly, with the root mean square error dropping by 0.16 milligrams per cubic metre, though the model still tends to overestimate winter and spring blooms. The most striking gap appears during the May-to-July coccolithophore bloom, when the model underestimates reflectance at 490 nanometres because it does not capture the intense scattering from these calcifying plankton.

Perhaps the most transformative capability is the direct simulation of sea surface reflectance, the quantity that satellites actually measure. Rather than comparing modelled chlorophyll with satellite-derived chlorophyll through uncertain inversion algorithms, the model now produces reflectance fields that can be compared directly with remote-sensing data. The team went further and applied the same band-ratio inversion algorithm used operationally for Black Sea satellite products to their own simulated reflectance, producing a reflectance-derived chlorophyll estimate. This estimate agreed better with the satellite product than the dynamically simulated chlorophyll for most of the year, with a substantially lower error, particularly during the early spring bloom. The exercise also revealed a subtlety: the coccolithophore bloom visible in satellite reflectance does not appear in the satellite chlorophyll signal, because increases in both blue and green reflectance cancel out in the band ratio, suggesting the satellite product itself may be biased during such blooms.

The module also ships with a stochastic mode designed to confront one of the field’s thorniest problems: uncertainty in optical properties. Using first-order autoregressive processes with roughly 75-kilometre spatial correlation and one-month temporal correlation, the model perturbs absorption and scattering coefficients with standard deviations of up to 50 percent, following log-normal distributions consistent with the statistics of optical variables. A 50-member ensemble compared against in situ reflectance measurements from the AERONET-OC Galata tower off the Romanian coast showed that observations generally fell within the ensemble spread, with uncertainty widening during bloom periods. This quantification of model uncertainty is precisely what is needed for future data assimilation of reflectance, a step that could revolutionise how satellite ocean colour constrains ecosystem forecasts.

There are trade-offs. Running the radiative transfer model at every time step across 33 wavebands roughly doubles the computation time compared with the simple optics scheme, a cost that matters for long-term or global simulations, though it can be reduced by lowering spectral resolution or simplifying the scattering treatment. The authors also note limitations: the assumption of no bottom reflection restricts some coastal applications, phytoplankton optical properties were derived partly from Mediterranean observations, and suspended minerals near river mouths are not yet explicitly represented. Still, the implications reach well beyond the Black Sea. With hyperspectral satellite missions such as NASA’s PACE and the Italian Space Agency’s PRISMA now delivering reflectance data at unprecedented spectral resolution, models capable of resolving light propagation in fine wavebands are becoming essential. By making sophisticated radiative transfer a pluggable component of NEMO, this work opens a path toward ocean models that speak the same optical language as the satellites watching them, bringing simulation and observation into direct, quantitative conversation.

Subject of Research: Three-stream spectral radiative transfer modelling coupled with biogeochemical simulation of the Black Sea in the NEMO ocean modelling framework

Article Title: Three-stream modelling of radiative transfer for the simulation of Black Sea biogeochemistry in a NEMO framework

Article References: Macé, L., Vandenbulcke, L., Brankart, J.-M., Grailet, J.-F., Brasseur, P., & Grégoire, M. (2026). Three-stream modelling of radiative transfer for the simulation of Black Sea biogeochemistry in a NEMO framework. Geoscientific Model Development, 19(18), 9077-9102. https://doi.org/10.5194/gmd-19-9077-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9077-2026

Keywords: radiative transfer, NEMO, Black Sea, biogeochemical modelling, ocean optics, sea surface reflectance, phytoplankton, CDOM, BGC-Argo, remote sensing, Copernicus Marine Service, stochastic modelling

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

The study, published in Discover Oceans by Madhurima Paul, Rashmi R. Nayak, and Arun Chakraborty of the Indian Institute of Technology Kharagpur, draws on 263 depth-resolved profiles collected between March 2015 and December 2020 by a biogeochemical Argo float (WMO ID 2902174) deployed by the Indian National Centre for Ocean Information Services. Every ten days, the float dove to 2000 meters and ascended, measuring temperature, salinity, dissolved oxygen, and chlorophyll-a fluorescence along the way. Because the float stayed within a compact box between roughly 70.3°E and 73.35°E and 13.2°N and 17.2°N, the researchers could build a clean, location-specific climatology of how plankton biomass is distributed through the water column across the entire annual cycle.

The headline finding is deceptively simple. Surface chlorophyll-a in the CEAS climbs through the southwest monsoon months, peaking at about 0.31 milligrams per cubic meter in August, then plummets sharply in September before surging back to roughly 0.36 milligrams per cubic meter in October. Satellite ocean-color data and the float measurements agree closely, with a correlation of 0.86, and the September dip and October rebound appeared in every single year of the record. That consistency rules out a one-off anomaly and marks the pattern as a genuine climatological signature of the region, one that neither the persistently upwelling southeastern Arabian Sea to the south nor the freshwater-dominated northern waters display in the same way.

Why does September fail? The answer, according to the study, lies in the physics of the retreating monsoon. During the peak monsoon, strong winds associated with the low-level Findlater Jet push wind stress to around 0.13 to 0.14 newtons per square meter, driving turbulent mixing and positive Ekman pumping velocities of 1.5 to 2.5 times ten to the minus six meters per second. This Ekman suction draws cold, salty, nutrient-rich water upward, feeding the phytoplankton and deepening the mixed layer to about 40 meters. In September, however, wind stress collapses to roughly 0.06 newtons per square meter, the wind stress curl weakens, and the Ekman pumping velocity drops sharply. Vertical mixing falters, the upward nutrient conveyor stalls, and the phytoplankton bloom runs out of fuel despite the water still being relatively cool.

The float data add a crucial vertical dimension that satellites simply cannot capture. During the spring intermonsoon from March to May, intense solar heating stratifies the upper ocean, shoaling the mixed layer to around 20 meters. Surface nutrients are rapidly exhausted, and near-surface chlorophyll falls below 0.1 milligrams per cubic meter. Yet below the mixed layer, chlorophyll thrives, reaching about 0.2 milligrams per cubic meter in a pronounced subsurface chlorophyll maximum between 30 and 50 meters, where light from the deeply penetrating euphotic zone, extending to 100 to 140 meters, meets residual nutrients. Integrated biomass during this period is modest at about 17 milligrams per square meter, but most of it sits beneath the layer that satellites would count.

This vertical seesaw between the mixed layer and the water beneath it is the study’s central structural insight. Correlation analysis shows that mixed layer depth is positively linked with chlorophyll in the near-surface and mixed layers (a correlation coefficient of 0.48) and strongly negatively correlated with chlorophyll below the mixed layer (minus 0.87). In other words, when monsoon winds deepen the mixed layer and pump nutrients upward, biomass concentrates near the surface and thins at depth; when the ocean calms and stratifies, the opposite occurs, and phytoplankton migrate downward to a depth where light and nutrients overlap. The subsurface chlorophyll maximum, which the authors note can account for nearly one-fifth of total euphotic-zone biomass basin-wide, is a hidden reservoir that satellite chlorophyll maps systematically miss.

The October rebound has its own mechanism, and it is not simply the mirror image of the September collapse. As the southwest monsoon winds die away, sea surface temperature climbs to a secondary peak of about 29.2 degrees Celsius, the mixed layer shoals to roughly 17 meters, and sea surface salinity drops sharply to about 35.4 practical salinity units. That freshening reflects the poleward reversal of the West India Coastal Current, which begins transporting low-salinity water from the Bay of Bengal along the coast, alongside increased discharge from rivers such as the Periyar, Vaigai, and Tambraparni. These freshwater inputs, combined with northward-flowing surface currents, deliver nutrients and displace nutrient-rich water northward, driving the sharp rise in near-surface chlorophyll, concentrated mainly in the top ten meters of the water column.

The study also highlights how salinity structure shapes the region’s biology in less obvious ways. A barrier layer, the salt-stratified zone between the isothermal layer depth and the mixed layer, thickens markedly during the winter monsoon months of December through February, insulating the mixed layer from cooler subsurface water and influencing sea surface temperature evolution. Meanwhile, the depth of the 20 degree Celsius isotherm, a standard proxy for thermocline depth, deepens during winter, a feature the authors link to heat trapped by the Findlater Jet during the previous summer and redistributed by westward-propagating downwelling Rossby waves. As spring approaches, these waves decay, the thermocline shoals, and the trapped heat is ventilated upward, resetting the upper-ocean structure for the next annual cycle.

Ecologically, the findings help explain why the CEAS sits in an awkward middle ground. To the south, the southeastern Arabian Sea’s vigorous upwelling supports large phytoplankton, rich fisheries, and planktivorous fish communities; to the north, winter convective mixing sustains an extended growing season. The CEAS, by contrast, experiences weaker winds, reduced upwelling, and a deeper thermocline, resulting in lower chlorophyll and integrated biomass during winter and a weaker fishery dominated by different trophic structures. The region also acts as a perennial source of carbon dioxide to the atmosphere in some seasons, while river-fed productivity during the southwest monsoon can temporarily tip it toward being a carbon sink, underscoring how sensitive its biogeochemical balance is to the timing of winds, currents, and runoff.

Perhaps the study’s most important message is methodological. A single float, profiling faithfully every ten days for six years, exposed a seasonal pattern invisible to two decades of satellite ocean color and decades of shipboard sampling. As climate change alters monsoon winds, river discharge, and ocean stratification, the biological response of regions like the CEAS will depend on fine-scale vertical processes that only autonomous biogeochemical sensors can resolve. The September dip and October rebound documented here are more than a regional curiosity; they are a demonstration that the ocean’s hidden interior still holds surprises, and that the robotic observers drifting silently through it are our best hope of catching them before the baseline itself begins to shift.

Subject of Research: Seasonal variability of chlorophyll-a and phytoplankton biomass in the central eastern Arabian Sea observed by biogeochemical Argo floats

Article Title: Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations

Article References: Paul, M., Nayak, R. R., & Chakraborty, A. (2026). Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations. Discover Oceans, 3(1), Article 15. https://doi.org/10.1007/s44289-026-00128-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00128-2

Keywords: chlorophyll-a, Arabian Sea, BGC-Argo, phytoplankton, monsoon, upwelling, mixed layer depth, subsurface chlorophyll maximum, West India Coastal Current, ocean biogeochemistry, satellite ocean color, marine productivity