Reconciling Between Optical and Biological Determinants of the Euphotic Zone Depth

Abstract

The euphotic zone constitutes the upper layer of aquatic ecosystems, where ambient light levels are sufficient to support primary production. At any depth within this layer, net primary production (NPP, mg C m −3 d −1 ) rates are always greater than zero (Falkowski, 1994). The base of the euphotic zone (Z eu , m) where the rate of photosynthesis equals that of autotrophic respiration, that is, NPP = 0, has been termed the compensation depth (Z c , m) (Kirk, 1994;Ryther, 1955). Both Z eu and Z c have frequently been used in aquatic ecology as benchmarks for describing and comparing integrated phytoplankton biomass and pigment content, nutrient inventories, NPP, quotients of total water-column light utilization index, and export production processes across different water types and different aquatic ecosystems (Ducklow et al., 2001;

A Multivariable Empirical Algorithm for Estimating Particulate Organic Carbon Concentration in Marine Environments From Optical Backscattering and Chlorophyll-a Measurements

“…The euphotic zone depth (EZD) was determined as the depth at which PAR values are 0.5% of PAR(0 -). Most of the actively photosynthesizing and productive phytoplankton are present within this euphotic zone (Wu et al, 2021). The EZD is a more reliable depth for assessing strength of POC export flux as opposed to constant depths (Buesseler et al, 2020); therefore, emphasis is placed on metrics related to EZD.…”

A Dual-Band Model for the Vertical Distribution of Photosynthetically Available Radiation (PAR) in Stratified Waters

“…The dynamics of phytoplankton biomass would in turn adjust the light penetration via bio-optical feedbacks (Manizza et al, 2005). Second, the euphotic layer depth, defined as the depth where PAR is attenuated to 1% (z 1% ;Ryther, 1956) or 0.5% of sea-surface value (z 0.5% ; Wu et al, 2021), can be derived directly through the USRGR model from a Chl profile (Figure 2). In this case, the seasurface PAR is not required, as the euphotic layer depth refers to the relative change of PAR in the water column, which actually represents the transmittance of PAR.…”

“…Here we adopted the conventional definition of 1% surface PAR (z 1% ) to represent the euphotic layer depth, although a depth of 0.5% of surface PAR better matches the compensation depth of phytoplankton photosynthesis (Wu et al, 2021). To evaluate the performance of the USRGR model in estimating z 1% , the BIOSOPE cruise data were employed (Figure 6).…”

“…In addition, we found that the USRGR model also showed good performance for z 0.5% , with RMSD of 7.6 m and MPD of 0% (figure not shown here). It should be noted, similarly, that the USRGR model also admits to estimate euphotic layer depths by other definitions: e.g., z 0.1% and z 0.9%USR (the depth where USR decreases to 0.9% of seasurface USR; Wu et al, 2021).…”

“…For example, Liu et al (2020) found that the uses of different PAR models would lead to differences up to 1.5°C-2°C in sea surface temperature. An accurate estimate of the PAR profile is the prerequisite to determine the euphotic layer, which is typically used as a proxy for the compensation depth of the phytoplankton ecosystem (Ryther, 1956;Wu et al, 2021). The model-estimated euphotic layer depth is critical not only in carbon export assessment of the ocean biological carbon pump (Carlson et al, 1994;Siegel et al, 2014;Buesseler et al, 2020) but also in marine ecosystem modeling (e.g., Chai et al, 2002;Aumont et al, 2015), as well as in studies on the mechanisms of seasonal phytoplankton blooms (e.g., Behrenfeld, 2010;Mignot et al, 2018).…”