[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-05] Instantaneous Acclimation allows computationally efficient modelling of the biogeochemical impacts of plankton ecophysiology

*S. Lan Smith¹, Onur Kerimoglu², Prima Anugerahanti¹, Yoshio Masuda³, Yasuhiro Yamanaka³, Yoshikazu Sasai¹ (1.Marine Ecosystem Dynamics Research Group, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2.ICBM, University of Oldenburg, Germany, 3.Faculty of Environmental Earth Science, Hokkaido University)

Keywords:nutrients, photo-acclimation, phytoplankton, Earth System Modelling, marine ecosystems, biogeochemistry

Many coupled physical-biogeochemical models reproduce large-scale patterns of chlorophyll, primary

production and biogeochemistry, but they often underestimate observed variability and gradients.

One common reason is insufficient representation of systematic variations in the elemental

composition and chlorophyll (Chl) content of phytoplankton. Chl is widely taken as a proxy

for phytoplankton biomass, despite well known variations in Chl:biomass ratios as an acclimative

response to changing environmental conditions. However, for the sake of simplicity and computational

efficiency, many large scale biogeochemical models ignore this flexibility, compromising

their ability to capture phytoplankton dynamics. Although some models account for the dynamics

of phytoplankton composition by adding state variables (one for each element or pigment considered),

that approach substantially increases computational requirements in spatially explicit (1-D

and 3-D) setups. The Instantaneous Acclimation (IA) approach addresses these challenges by

assuming that Chl:C:nutrient ratios are instantly optimized locally (within each modelled grid cell,

at each timestep), such that they can be resolved as diagnostic variables.However, the IA approach

was developed in a 0-D model and has not yet been rigorously tested in spatially explicit

models. Here we present tests of IA in various spatially explicit models, including: an idealized,

1D vertical setup in the Framework for Aquatic Biogeochemical Models (FABM) coupled with the

General Ocean Turbulence Model (GOTM), as well as a 3-D regional model and another 3-D

global model. We show that the IA model and a fully dynamic, otherwise equivalently acclimative

(DA) variant with an additional state variable behave similarly, and that both resolve chlorophyll

patterns and nutrient and growth dynamics not captured by the typical fixed-stoichiometry (FS)

models [e.g., 1].

(1) Kerimoglu, O.; Anugerahanti, P.; Smith, S. Geosci. Model Dev. Discuss., in review 2021 , DOI:

10.5194/gmd-2020-396 .