Chlorophyll‐Based Model to Estimate Underwater Photosynthetically Available Radiation for Modeling, In‐Situ, and Remote‐Sensing Applications

Xiaogang Xing,Emmanuel Boss

doi:10.1029/2020gl092189

Abstract

AbstractAccurate estimation of the underwater light field associated with photosynthetically available radiation (PAR) is critical to compute phytoplankton growth rate and net primary production (NPP), and to assess photo‐physiological response of phytoplankton, such as changes in cellular pigmentation. However, methods to estimate PAR used in many previous studies lack in accuracy, likely resulting in significant bias in light‐dependent products such as NPP derived from remote sensing, model simulations, or autonomous platforms. Here we propose and validate a new model for more accurate estimation of the subsurface PAR profile which uses chlorophyll concentration as its input. Validation is performed using 1,744 BGC‐Argo profiles of chlorophyll fluorescence that are calibrated with surface satellite‐derived chlorophyll concentration over their lifetime. The independent verification with the float's PAR sensors confirms the accuracy of satellite chlorophyll estimate worldwide and in the Southern Ocean in particular.

Highlights

The vertical distribution of underwater photosynthetically available radiation (PAR) is a necessary input for models of net primary production (e.g. review by Behrenfeld & Falkowski (1997)) and is key to interpret physiological response to light of phytoplankton, its change in cellular pigmentation (e.g. Behrenfeld et al 2016) or the associated solar-stimulated fluorescence (Behrenfeld et al, 2009)
Mixed-layer median iPAR requires an estimation of a representative Kd(iPAR), i.e., Kd(iPAR,MLD/2)
We find that the Lee05 model provides the most accurate iPAR(z) retrieval near sea surface and within the mixed layer, and we recommend it be employed for ocean color satellite-based mixed-layer photoacclimation and primary production models

Summary

Introduction

The vertical distribution of underwater photosynthetically available radiation (PAR) is a necessary input for models of net primary production (e.g. review by Behrenfeld & Falkowski (1997)) and is key to interpret physiological response to light of phytoplankton, its change in cellular pigmentation (e.g. Behrenfeld et al 2016) or the associated solar-stimulated fluorescence (Behrenfeld et al, 2009). In the last two decades, carbon-based primary production models (CbPM, Behrenfeld et al, 2005; Westberry et al, 2008), and photoacclimation models (Fox et al, 2020), provide novel approaches to estimate marine net primary production (NPP) from satellite measurements that have been applied to in-situ platforms (Estapa et al, 2019; Yang et al, 2020). These models derive the vertical distribution of PAR by attenuating surface PAR using an estimate of its diffuse attenuation coefficient, Kd(PAR). This approach, has not been thoroughly evaluated for the full range of MLDs in the ocean (e.g. 10-500m) beyond the evaluation within Morel et al (2007)

Methods

Results

Conclusion