POSTER SESSION 3
15:30–17:30, Wednesday, September 16
Poster Session | 1 | 2 | 3 | 4 | Schedule at a Glance
ABSTRACT 1259 | POSTER 19
REVISITING HOW THE EUPHOTIC DEPTH IS COMPUTED
The 1% surface PAR depth (Z(1%)), or similarly defined depths based on relative light levels, has long been used to define the ocean euphotic depth, despite a weak physiological basis. Accurately setting this boundary, i.e., the compensation depth where autotrophic production balances autotrophic respiration, is essential as it sets the lower boundary of the layer over which we compute our estimates of phytoplankton biomass, net primary production, and carbon export.
Synthesizing field, laboratory, and theoretical estimates from the literature (including recent Arctic observations approaching the theoretical lower limit), we propose to shift towards a fixed compensation irradiance value of 0.0035 mol photons m–2 day–1 as the basis for defining the euphotic depth (Z(CD)). AMT transects (40°N–40°S) support this choice, showing significant picophytoplankton populations extending down to the Z(CD) horizon at low and mid latitudes as well as positive net primary production growth. We quantify the implications of adopting Z(CD) using ~20,900 quality-controlled BGC-Argo profiles (2012–2023) with concurrent measurements of PAR, chlorophyll fluorescence, and particulate backscattering.
The conventional Z(1%) is, on average, 80 m (48%) shallower than Z(CD), with the largest discrepancies at high latitudes, i.e., in regions of high primary productivity. Vertically integrating to Z(1%) underestimates depth-integrated chlorophyll and phytoplankton carbon by ~35% and ~37%, respectively. These biases will propagate into estimates of net primary production, biological carbon pump strength, and the seasonal mixed-layer pump. We provide a practical recipe for computing Z(CD) from surface PAR and Kd(PAR), directly applicable to ocean color remote sensing.
Charlotte Begouen Demeaux*, University of Maine, [email protected], https://orcid.org/0000-0003-0471-5022
Emmanuel Boss, University of Maine, [email protected], https://orcid.org/0000-0002-8334-9595
Michael Behrenfeld, Oregon State University, [email protected]
Poster Session | 1 | 2 | 3 | 4 | Schedule at a Glance
