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Revisiting how the euphotic depth is computed [1]

['Emmanuel Boss', 'School Of Marine Sciences', 'University Of Maine', 'Orono', 'Maine', 'United States Of America', 'Charlotte Begouen Demeaux', 'Michael J. Behrenfeld', 'Department Of Plant Pathology', 'Oregon State University']

Date: 2026-10 

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.

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.

Data Availability: Data used in this study is all in the public domain. The BGC-Argo dataset is in Zenodo ( https://doi.org/10.5281/zenodo.7682700 ) was last accessed last in 8/2026). The KEOPS2 dataset ( https://doi.org/10.17882/103655 ) was last accessed in 5/2026. The AMT13 bottle cruise data was downloaded from the British Oceanographic Data Center in 12/2025 (see report at https://www.bodc.ac.uk/resources/inventories/cruise_inventory/report/6045/ ).

Here, we contrast the two (relative and absolute) approaches to compute the euphotic depth, attempt to identify the light horizon that best defines the layer within which net photosynthesis occurs in the ocean (i.e., the vertical extent spanned by the euphotic zone), and provide examples for why it is important to define the euphotic depth correctly.

Selected light values reported for light at the critical depth or the compensation irradiance based on lab, in-situ, and satellite studies ordered by decreasing values. Note that some of these values are for scalar irradiance and some for planar downwelling irradiance. All reported values were converted to PAR in mol photons mday. To convert to energy units [W m], multiply by 2.2 W day (mol photons) 14 ].

The use of a percent light level has been criticized, in particular by Banse [ 1 ], who traced the original definition to a series of limnological studies from the 1930s (in the oceanographic literature, see [ 13 ]). The rationale for using a fixed light level is simple; phytoplankton experience the flux of radiance at the depth where they are . Banse [ 1 ] suggested that instead of a percentage of light level, a more appropriate approach to define the euphotic depth is to use the minimum irradiance level where net primary production is positive over a 24-hour period (named compensation irradiance) and provided a list of estimates for this threshold irradiance from the literature (some of which are in Table 1 ). Recently, Davies et al. [ 27 ] expanded on Banse’s recommendation, advocating the use of constant-value light-based horizons associated with ecological processes related also to heterotrophs. Consistent with [ 1 ], ocean ecosystem and biogeochemical models use a basal respiration value for phytoplankton, that, in essence, represents the value that gross photosynthesis has to exceed for phytoplankton cells to exhibit growth.

The Ocean’s euphotic zone has been defined as the upper layer of the ocean where daily gross primary production exceeds autotrophic respiration (i.e., where net primary production is positive, e.g., [ 1 , 2 ]). The most common method to calculate the extent of this zone is based on downwelling planar irradiance ( ) of photosynthetically available radiation (PAR) and is described as the layer within which is reduced to 1% of its value just below the ocean’s surface [ 3 ]. This definition of euphotic depth (or equivalent definitions using other percentage values) has been very popular and is convenient because it can be computed with data from an uncalibrated radiometer, a Secchi disk [ 4 ], and from ocean color remote-sensing (via its diffuse attenuation product). Percentage-based definitions of the euphotic zone are used in studies of the annual dynamic of phytoplankton (phenology) in the ocean (e.g., [ 5 ]), used to determine the upper depth threshold for the calculation of carbon flux to depth (e.g., [ 6 ]), distributed as a product of ocean color remote sensing [ 7 ], and promoted in many oceanography textbooks [ 3 , 8 – 12 ].

These formulas allow one to convert from basal respiration to compensation irradiance (e.g., using values reported for biogeochemical or ecosystem models (e.g., 0.033 day –1 [ 43 ]) or to derive the likely basal respiration associated with a given compensation irradiance. For phytoplankton in cultures grown at low light, varies from 30 to 50 g/g in phytoplankton cultures and varies between 0.02–0.06 [in units of m 2 (g Chl a) –1 g C h –1 ( mol photons m –2 s –1 ) –1 ] (e.g., [ 44 ]).

where is the Carbon-specific photosynthesis rate (with units of inverse time), , the maximum value of the carbon-specific photosynthetic rate, the chlorophyll a-specific initial slope of the P vs. E curve [m 2 (g Chl a) –1 g C (mol photons) –1 ], E is irradiance (with units of mol photons m –2 day –1 ), the chlorophyll a to carbon ratio, and the Carbon-specific respiration rate (with units of inverse time). At the compensation depth where , we can rewrite Eq. 1

Cavagna et al., 2015 [ 24 ] report positive incubation-based NPP values at the 0.01% light levels at a region west of the Kerguelen Islands (between 48–49° S). We used surface and subsurface PAR data from that cruise [ 40 ] to compute the absolute daily light levels based on modeling the surface light throughout the day (surface values were consistent with the clear sky model of [ 41 ]).

To make the case that a significantly lower light level (on the same order as theoretically argued by [ 26 ]) is sufficient for NPP at mid-latitude regions, we use data collected on the Atlantic Meridional Transect cruise 13 (2003). PAR data from the rosette (equipped with downwelling and upwelling scalar PAR sensors) were downloaded from the British Oceanographic Data Center. Prochlorococcus data were kindly provided by Dr. Zachary Jonson who analyzed them in [ 39 ]. We performed the analysis for all the cytometrically enumerated Prochlorococcus cells. Light data were interpolated to the depth where cells were collected, converted from values measured with the rosette to daily values assuming similar sun/cloud conditions throughout the day and taking into account time of sampling and daylength (a function of date and latitude) and a semi-sinusoidal daily light evolution with a maxima at local noon. We confirmed, using remotely sensed MODIS data, that many of the observations took place under sunny skies (at least near the time the satellite passed).

A similar analysis method to chlorophyll was applied for , including the subtraction of a dark value obtained from the fifth percentile at depths greater than 200 m. A trapezoidal integration was performed between the surface and the target depth (after linearly interpolating at equally spaced depths) using all data points. The integrated was then transformed to phytoplankton carbon ( ) by first converting to using ([ 37 ]) and then applying the relationship: [ g L –1 ] [ 38 ]. The calculated ratios of were consistent with values for phytoplankton cultures in the literature (details in [ 36 ]).

The relationship between and Chl is known to vary with biogeochemical drivers such as species composition, seasonality, and nutrient availability, with reported :Chl ratios ranging from 0.4 to 3.4 [ 34 ]). To account for this variability, the profiles were divided among open-ocean biomes [ 35 ], and a biome-specific calibration factor between and Chl was calculated and applied to all profiles within a biome, following the radiometric approach described in [ 36 ]. This procedure yields biome-adjusted, float-based calibrated Chl. To integrate over depth, Chl was linearly interpolated over 100 equally spaced depths between the surface and the target depth ( ). Chl integration was performed using a trapezoidal scheme. Note that the presence of clouds could reduce significantly compared to sunny skies (while not significantly changing the euphotic depth computed based on the percent of light levels). However, note that when conditions are cloudy, phytoplankton photo-acclimate rapidly, as recently reported [ 36 ].

The depth-integrated chlorophyll (Chl) and phytoplankton carbon (based on the particulate back-scattering coefficient, ( ), see below) were calculated for each BGC-Argo float profile. On-board BGC-Argo floats, Chl is measured using Chlorophyll Fluorescence ( ) [ 32 ]. Each profile was processed by subtracting a dark value (for a given float, the mean of below the 5th percentile at depths greater than 200 m). The resulting profile was then corrected for Non-Photochemical-Quenching (NPQ) following [ 33 ].

To find the depth at which the float reached a given light level, a second-degree polynomial was fitted to the PAR profile and, by extrapolation, used to compute . A fourth-degree polynomial fit to the PAR data was then used to interpolate the depth at which reached 1%, 0.5%, 0.1%, or 0.0035 mol m –2 day –1 (denoted as ).

We analyze data obtained with BGC-Argo profiling floats to highlight the importance of the chosen horizon for the euphotic depth. We show how the amount of light experienced by phytoplankton varies at a given proposed % light horizon as a function of latitude. Similarly to [ 28 ], we show the impact of the choice of euphotic depth on integrated phytoplankton biomass and pigment concentration within this layer. We used a data set [ 29 ] of nearly 20,900 BGC-Argo profiles from October 2012 to December 2023 that included sensors measuring light, chlorophyll, and backscattering (for locations of all in-situ measurements used see Fig 1 ). Irradiance on these floats was measured with Seabird OCR-504 radiometers measuring at 3 wavelengths and available photosynthetic radiation (PAR). This data set was quality controlled following [ 31 ] and consists of profiles unaffected by perturbations to the light field, such as wave focusing or a passing cloud. Instantaneous PAR values measured with floats were converted to daily PAR values, taking into account day length (based on latitude and date of measurement), local time of measurement, assuming similar sun/cloud conditions at measurement and throughout the day, and a semi-sinusoidal daily light evolution with a maximum at local noon.

We performed a review of the different methods reported to estimate the euphotic depth using constant light as well as relative light levels, providing a series of literature-based estimates for the minimal amount of light necessary for either phytoplankton accumulation (e.g., blooming) or for net growth. The former always overestimates the latter, but the two cannot be separated well in field observations because it is impossible to fully remove the impact of grazing and viral lysis from field measurements. Although not exhaustive, our review spans the different approaches that have been applied to compute the euphotic depth.

The euphotic depth in models is an emergent property. Models include an assigned, constant, loss-rate (called basal respiration) for each phytoplankton group (e.g., [ 43 , 52 ]). For net accumulation to occur, the photosynthetically driven growth rate (a function of taxonomic group, intercellular chlorophyll, nutrients, and light) must exceed the basal respiration rate (and all other loss processes). Typically, the basal respiration is on the order of 0.02–0.03 day –1 [ 43 , 52 , 53 ]. Applying this basal respiration rate to Eq. 3 and selecting parameters that will minimize the computed (e.g., =30 =0.06 [in units of m 2 (g Chl a) –1 g C h –1 ( mol photons m –2 s –1 ) –1 ] [ 44 ]), results in a value =0.036 mol photons m –2 day –1 . While this value is low compared to many in Table 1 , it is larger by an order of magnitude than the lowest values reported. Note that the specific values of are typically dynamic in models, depending on the formulation of phytoplankton light acclimation.

Comparing the distributions of vertically integrated chlorophyll (Chl) and integrated phytoplankton carbon (i.e., the water column pigment and biomass inventories, respectively) from the surface to the different depth horizons of Z 1% and , we find that the biomass inventory based on integration down to Z 1% horizon significantly underestimates that to ( Fig 5 ). In other words, the phytoplankton pigment and biomass inventories below Z 1% are significant. The average depth-integrated Chl is, respectively, 40.9 and 65.3 mg Chl m –2 for Z 1% and , while the average is 1.1 and 1.8 mg C m –2 , respectively, a difference >60%. Similar results have been calculated by [ 28 ] when integrating from the surface to 500 m worldwide.

Using the BGC-Argo data set (which is not seasonally biased but is biased in space, see Fig 1 ), we compare the depth of the 1% (Z 1% ) and 0.1% (Z 0.1% ) light levels to the depth of the 0.0035 mol photons m –2 day –1 isolume ( ). We observe a significant underestimation of the compensation depth for both relative criteria, as well as a significant spread around the relationships to ( Fig 4 ). On average, Z 1% is 79.5 meters, or 47.6% shallower than , with the largest percentage difference occurring at high latitudes ( S1 Table ).

Additionally, Cavagna et al. [ 24 ] report significant positive NPP at three stations at the 0.01% light level. At two of them it occurs below the mixed-layer (F-L & E4-W) corresponding to an in-situ light level of about 0.006 mol photons m –2 day –1 using light measurement they made (see methods) which, at the surface, matched model prediction for sunny skies [ 41 ]).

Analyzing data on the number of cells of the genus Prochlorococcus from the AMT13 cruise, we find a significant number of cells down to the depth horizon 0.0035 mol photons m –2 day –1 from N to S ( Fig 3 ). We note that below 0.0035 mol photons m –2 day –1 , the light intensity flattens out as a function of depth, suggesting the existence of a small, non-zero, dark current present in the data. These data are consistent with those from Serret et al. [ 48 ] for the AMT-22 cruise (their), where cell numbers of Prochlorococcus and picoeukaryotic cells are significant at certain stations below the 0.1% horizon, and where at most stations GPP > 0 at this level (they did not sample at deeper depths).

Maybe surprisingly, the yearly maximal daily PAR light level just below the ocean surface on a sunny day varies only from 60–75 mol photons m –2 day –1 (Fig 4 in [ 14 ]), being lowest near the equator. However, seasonal variability in the Earth’s tilt relative to the sun causes this value to be highly modulated away from the equator even for sunny skies, decreasing by half by the 30°N and 30°S, and further decreasing to zero as far as 66°N and 66°S, (Fig 11a in [ 14 ]). This causes the light level associated with a percent horizon to be highly variable throughout the year and a function of latitude. Variability due to clouds further modulates the intensity of light (at any given time, about 70% of the ocean is under clouds [ 51 ]). The distribution of daily PAR at different % horizons based on the BGC-Argo dataset ( Fig 2 ) is highly variable, with variability increasing as a function of latitude.

Table 2. Selected % light values at the compensation depth based on in-situ data. Note that some of these values are for scalar irradiance and some for planar down-welling irradiance, some relative to surface measurement above the surface and some for just below the surface.

Two recent studies conducted in the Arctic Ocean [ 25 , 30 ] have provided new estimates for the compensation irradiance that are lower than any previous field study and are remarkably close to the theoretical limit proposed by [ 26 ] (i.e., 0.0009 mol photons m –2 day –1 ). Randlhoff et al. [ 25 ] used a method similar to [ 50 ] to detect the time when phytoplankton start to accumulate in winter. They used both chlorophyll fluorescence and particulate backscattering measurements below the ice in Baffin Bay to determine when the mixed-layer-integrated phytoplankton biomass begins to accumulate and associated this event with an average mixed-layer light level of 0.0043 0.0025 mol photons m –2 day –1 . Hoppe et al.[ 30 ], coupled measurements of chlorophyll accumulation below the arctic ice with 14 C-based NPP measurements and found that both approaches yield a similar (and extremely low) compensation irradiance of 0.0035 0.0017 mol photons m –2 day –1 . We note that such a value is about 0.005% of the highest yearly expected value of scalar PAR at low latitudes just below the ocean’s surface [ 14 ]. In the following subsection we compare some of the above fixed light level values to light values observed at fixed percent light horizon ( Table 2 ).

Gran [ 49 ] proposed the existence of a critical light threshold below which the biomass of mixed-layer phytoplankton does not accumulate (i.e., ‘bloom’), a concept later termed the ‘critical depth hypothesis’ (see review in [ 23 ]). Gran originally assumed that phytoplankton losses to grazing are minimal (which we now know to be incorrect) and thus viewed this critical light threshold to primarily represent the balance between phytoplankton respiration and photosynthesis (i.e., an approximation of the compensation irradiance [ 23 ],). Siegel et al. [ 15 ] used satellite retrievals of surface chlorophyll concentrations in the North Atlantic to estimate a critical light threshold for phytoplankton accumulation (blooming) of 0.96–1.75 mol photons m –2 day –1 . Another accumulation-based approach was suggested by [ 21 ], consisting of finding the irradiance value at depth when the concentration of [chl] (used as a proxy for phytoplankton biomass) begins to decline following the spring accumulation. The argument is based on assuming that the decline in biomass starts when phytoplankton NPP becomes lower than phytoplankton respiration, interpreting it as the time when the compensation irradiance occurs. Based on this approach [ 21 ], estimated the compensation irradiance to be between 0.06–0.3 mol photons m –2 day –1 .

Another field-based approach is to use the difference between oxygen production in light and dark 24h incubation bottles to obtain gross primary production (GPP). Serret et al. [ 48 ] analyzed data from 10 AMT cruises, particularly the AMT 22 dataset, and found that cell concentration and GPP were significantly higher than zero at the 0.1% PAR level and argued that this horizon should be used as the appropriate community-based euphotic depth.

An alternative field-based approach for estimating the depth of the euphotic zone has been the employment of 14 C uptake measurements with phytoplankton collected through the water column and incubated in situ at the depths of collection. Following this method [ 2 ], estimated the compensation irradiance at 0.1–0.3 mol photons m –2 day –1 , significantly deeper than the 1% surface PAR depth horizon during their experiment. A recent compilation of similar measurements [ 47 ] confirms these findings, suggesting a level of 0.5% PAR to be the most appropriate, and also stating, based on the data they compiled, that compensation irradiance levels are inherently variable and that this relative light level is a better predictor of the euphotic depth at low- and mid-latitudes. We note that all the measurements in their compilation are limited to surface PAR levels higher than 35 mol photons m –2 day –1 (NB: some of their reported surface PAR values are unrealistically high). Some additional analyses of 14 C uptake measurements yielded lower values for compensation irradiance. For the Hawaii Ocean Time (HOT) series site [ 22 , 23 ] computed a compensation irradiance of about 0.05 mol photons m –2 day –1 while [ 24 ] found significant NPP at the 0.01% light level.

One field-based approach to estimating the euphotic depth has been to use the isolume associated with the nutricline as indicative of the compensation irradiance. Based on this approach, Letelier et al. [ 18 ] suggested a compensation irradiance of 0.415 mol photons m –2 day –1 which coincided with the annual averaged light at the 1% depth horizon.

Since the 1930s, there have been several approaches to determine the minimal light level necessary for net photosynthesis. Both Riley’s [ 45 ] pioneering modeling of the annual cycle of nutrients, phytoplankton, and zooplankton in the Gulf of Maine, as well as Sverdrup’s [ 46 ] investigation into the conditions necessary for phytoplankton to start accumulating in the Nordic Sea, required a threshold light level below which net phytoplankton photosynthesis is not possible (referred to as compensation irradiance). These studies used different values (0.09 and 0.15 g cal cm –2 h –1 , respectively, which are equivalent to approximately 0.4 and 0.6 mol photon m –2 day –1 ) that were taken from the works of [ 16 ] and [ 17 ]. Clark [ 16 ] and Jenkins [ 17 ] determined the compensation irradiance based on oxygen production in the Western North Atlantic and the English Channel, respectively, using diatom cultures isolated locally and incubated at different depths.

Discussion

Contrasting fixed or a percent light level as estimates of the euphotic depth The use of percent light level has been dominating the oceanographic literature for decades. It is easy to compute, as it differs little in value between instantaneous and daily integrated measurements [54], can be estimated with a secchi disk, and does not require a well-calibrated radiometer. Additionally, near the longest day of the year and on a sunny day, surface irradiance is constrained ocean wide (e.g., between 55–70 mol photons m–2 day–1). Hence, under such conditions, a fixed percent light value is associated with a relatively constrained light level (within 14%). However, while % light horizon takes into account the optical properties of the water, it does not take into account changes in surface illumination due to clouds (most of the ocean is under clouds in a given day [51]) and seasonal changes in surface illumination. Thus, when analyzing the variability of light at a given % light horizon, the distribution is so wide (Fig 2) as to make its utility extremely questionable. The different absolute light levels we report in Table 1 suffer from a variety of problems when used to determine the value of the critical irradiance. All definitions that are based on accumulation rates underestimate the compensation irradiance due to unknown losses to processes such as grazing and viral lysis. Those based on in-situ incubation of cultures suffer from using cultures not necessarily adapted to the depth where they are incubated. Lab cultures, while very informative, may not be representative of in-situ communities. Incubations on vessels suffer from handling, photo-acclimation within the bottle (chlorophyll is typically used as a proxy of biomass), and incorrect spectral matching with the location where cells come from. Finally, the use of the nutricline as a measure of euphotic depth removes the deeper part of the water column where nutrient uptake is smaller than the sum of the turbulent diffusive flux and local remineralization. However, considering the converging tendency between field observations [24,25,30] and theoretical estimates [26], we suggest adopting 0.0035 0.002 mol photons m–2 day–1 as the value for the compensation irradiance. This value, based on [30], bridges the gap between the physiological limits for low-light net primary production predicted by theory [26] and the most recent in situ determinations from polar environments [25,30]. Therefore, it provides a realistic observation-based estimate of the irradiance level below which phytoplankton net photosynthesis cannot support accumulation of phytoplankton. Our simple analysis of model formulation above suggests that current models do not allow for positive NPP at such low light levels.

Cases when phytoplankton are not at a fixed depth In our treatment up to here, we have only addressed, for simplicity, cases where the water column is stratified and phytoplankton stay at a tight depth range throughout the 24hr period of interest. This is a simplification. Deep mixed-layers do occur in the ocean, and the critical irradiance may occur within them, if the light at the base of the mixed layer is below the critical irradiance. Phytoplankton in such a layer experience a highly variable light environment. The time-averaged daily light experienced by cells could be estimated from the depth and time-averaged light level in the mixed-layer as cells travel up and down in an active mixed-layer on a time scale on the order of tens of minutes to an hour [55,56]. Additionally, in stratified environments, cells may experience significant changes in light due to internal waves [22]. The above processes complicate our ability to estimate the light level phytoplankton are exposed to in the environment, but could be addressed with appropriate measurements (see S1 text for the case of a deep mixed-layer). Whether the community of phytoplankton in such cases is dominated by the organisms specializing in growth at extremely low-light may also be a factor needed to be taken into consideration.

Should the same critical irradiance be used ocean wide? One may wonder if the idea of a constant-value light horizon for the whole ocean and at all seasons is useful, given that different phytoplankton taxa and strains exhibit very different minimal irradiance for NPP and that compensation irradiances based on 14C incubations provide highly variable results [47]. We think it is. If a species of phytoplankton has managed to accumulate at extremely low Arctic light at near theoretical values [25,30], it is very likely that a strain of the same species or another species have evolved to do the same (the ocean, unlike some terrestrial environments, is well connected [57]). Indeed, when we look at distributions of Prochlorococcus cells from N to S as function of light, they are found at significant concentrations at horizons with similar light levels. While this is not evidence of NPP > 0 at those depths (there are phytoplankton cells all winter under the ice in the Arctic), it is suggestive. Furthermore, GPP measurements from the same transect (different cruises) show significant oxygen production at similar light levels. Finally [24], has reported positive NPP at the 0.01% horizon north of the 49°S, with absolute light values similar to the compensation irradiance recommended here and observed by [30]. The evidence provided here supports the adoption of a similar value of compensation irradiance throughout the ocean, though further studies will be needed, particularly regarding the possible temperature dependence of basal respiration and the phytoplankton communities inhabiting such low light level habitats. Further, one may wonder if temperature does not increase basal respiration to values significantly higher than those in the Arctic making the spring data from the Arctic effectively irrelevant for lower latitudes. The idea of a temperature effect on phytoplankton respiration can be traced back to [58] who assigned four factors that the compensation irradiance should depend on: surface illumination, its attenuation in the water, the phytoplankton assembly present, and temperature. They wrote: “We have no observations in regard to the photosynthesis and respiration of the plankton algae in relation to temperature; but it may be supposed that their respiration has a higher temperature coefficient than that of their photosynthesis and that at low temperatures a lower light intensity would be sufficient to give balance between photosynthesis and respiration than at higher temperatures.” Indeed, it has been observed that under light and nutrient replete conditions, phytoplankton respiration increases as a function of temperature. However, it has been found that the influence of temperature on autotrophic respiration is suppressed under nutrient [59] or light [60] limitations. In particular, Edwards et al.[60], through a meta-analysis of a large compilation of field data, found that the lower the light available for phytoplankton growth the weaker was the dependence of autotrophic net-growth on temperature (see their Fig 4F), to the point of no significant temperature dependency as growth-rates approached zero (e.g., when photosynthesis balances exactly basal respiration) suggesting that basal respiration is not significantly temperature sensitive. Below, we ask whether the actual depth where positive NPP occurs matters and bring into view different processes where the depth horizon of the compensation depth is used.

Extent of regions where photosynthesis can occur all year round Behrenfeld and Boss [23] argued that across nearly the entire global ocean, mixed layer light levels are sufficient to sustain NPP throughout the year (the only exception is immediately before, during, and after polar night, where it occurs) and, accordingly, mixed layer phytoplankton essentially always have the potential for accumulation (blooming). Their assessment was based on the median mixed-layer light level of 0.065 mol photons m–2 day–1 the compensation irradiance reported by Geider [20] for low-light-acclimated phytoplankton cultures. The compensation irradiance of 0.0035 mol photons m–2 day–1 that we propose here, implies that an even greater fraction of the global ocean has positive NPP throughout the year, and thus the potential for blooming year round.

Biological pump computations Most biological pump studies using field data use the 1% surface irradiance PAR light-level as the depth horizon below which one can assume there is no NPP and hence only community respiration [6]. The use of a high compensation irradiance (e.g., when using Z 1% ) overestimates the strength of the biological pump, as the phytoplankton biomass below this horizon (which is shallower than ) is all assumed to be exported to depth. Additionally, when the mixed-layer begins to shallow in the spring at high latitude regions, the stratified layer left below may not have sufficient light for NPP, resulting in carbon export to depth (a process named the mixed layer pump [61]). Knowledge of the compensation depth is necessary to assess the magnitude of the carbon loss caused by this process. For example, recently [62] used Z 1% to define the upper boundary of the layer below which carbon is exported. By using this definition of euphotic depth, they overestimate the role of the mixed-layer pump and, more generally, carbon export.

Relating the euphotic depth to migrating organisms The deep scattering layer, detected with a 150kHz acoustic sensor and associated with micro-zooplankton and micro-nekton migration, has been shown to follow, near station Papa in the N. Pacific, the instantaneous 0.00009 mol photons m–2 day–1 isolume [63] with migration up to 60 m following this isolume when clouds obscure the sun. On the other hand, Flores et al., 2023 [64], found in the Arctic, that the 0.044 mol photons m–2 day–1) isolume was followed by the scattering layer detected with a 455kHz acoustic sensor (sensitive to smaller organisms than those detected by lower frequency acoustics). For all such migrating organisms, the revised definition of the euphotic zone ( ) implies that they spend significantly more time within the euphotic zone compared to that based on Z 1% .

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