Preprints
https://doi.org/10.5194/egusphere-2024-3502
https://doi.org/10.5194/egusphere-2024-3502
29 Nov 2024
 | 29 Nov 2024
Status: this preprint is open for discussion.

Simulating vertical phytoplankton dynamics in a stratified ocean using a two-layered ecosystem model

Qi Zheng, Johannes J. Viljoen, Xuerong Sun, and Robert J. W. Brewin

Abstract. Phytoplankton account for around half of planetary primary production and are instrumental in regulating ocean biogeochemical cycles. Around 70 % of our ocean is characterised by either seasonal or permanent stratification. In such regions, it has been postulated that two distinct planktonic ecosystems exist, one that occupies the nutrient-limited surface mixed layer, and the other that resides below the mixed-layer in a low-light, nutrient-rich environment. Owing to challenges observing the planktonic ecosystem below the mixed layer, less is known about it. Consequently, it is rarely characterised explicitly in marine ecosystem models. Here, we develop a simple, two-layered box model comprising of an ecosystem (Nutrient, Phytoplankton and Zooplankton, NPZ) in the surface mixed layer and a separate one (NPZ) in a subsurface layer below it. The two ecosystems are linked only by dynamic advection of nutrients between layers and controls on light attenuation. The model is forced with surface light (modelled from top-of-atmosphere) and observations of mixed layer depth. We run our model at the Bermuda Atlantic Time-series Study site (BATS) and compare results with a 30+ year time-series of phytoplankton and nutrient observations. When compared with observations, the model simulates contrasting seasonal and interannual variability in phytoplankton in the two layers, reproducing trends post 2011 caused by ocean warming and explaining the drivers. Results lend support to the hypothesis that the euphotic zone of stratified systems can be described using two vertically separated planktonic ecosystems. Nevertheless, simulating the ecosystem in the subsurface layer was more challenging than the ecosystem in the surface mixed-layer, suggesting more work is needed to study controls on subsurface planktonic communities.

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Qi Zheng, Johannes J. Viljoen, Xuerong Sun, and Robert J. W. Brewin

Status: open (until 10 Jan 2025)

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Qi Zheng, Johannes J. Viljoen, Xuerong Sun, and Robert J. W. Brewin

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Simulating vertical phytoplankton dynamics in a stratified ocean using a two-layered ecosystem model Qi Zheng https://github.com/Qicodediary/two-layered-ecosystem-model

Qi Zheng, Johannes J. Viljoen, Xuerong Sun, and Robert J. W. Brewin
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Short summary
Phytoplankton contribute to half of Earth’s primary production, but not a lot is known about subsurface phytoplankton, living at the base of the sunlit ocean. We develop a two-layered box model to simulate phytoplankton seasonal and interannual variations in different depth layers of the ocean. Our model captures seasonal and long-term trends of the two layers, explaining how they respond to a warming ocean, furthering our understanding of how phytoplankton are responding to climate change.