Research
Effects of variable stoichiometry of phytoplankton on the marine biogeochemical cycles and climate
The amount of carbon fixed by marine phytoplankton is largely influenced by their carbon:nitrogen:phosphorus (C:N:P) ratio. In this topic, we aim to understand how variability in the C:N:P ratios of phytoplankton affects marine carbon and nutrient cycles. We conduct model experiments primarily using an optimality-based ecosystem model within an Earth system framework (UVic-OPEM) to explore the impacts on processes such as primary production, carbon export, and nitrogen fixation.
(Fig. 3 in Chien et al., 2023)
The nitrogen-to-phosphorus (N:P) ratio in phytoplankton plays a key role in determining the strength of the marine biological carbon pump and nutrient cycles. In a sensitivity experiment, we applied different subsistence nitrogen and phosphorus quotas for phytoplankton and found that these changes can significantly alter the N:P ratio in both particulate organic matter and the dissolved inorganic pool. Net primary production (NPP), nitrogen fixation, and denitrification are also influenced by perturbations in the phytoplankton physiological parameters. Our results highlight the importance of particulate N:C and P:C ratios for regulating dN:P on the global scale, with marine oxygen level being an important control. Our analysis provides further insight into the potential interdependence of phytoplankton physiology and global climate conditions.
Ecological stoichiometry of marine phytoplankton
The elemental ratios of marine phytoplankton can deviate from the Redfield ratio (C:N:P = 106:16:1); however, the factors driving these deviations and their mechanisms remain largely unknown. Our goal is to better understand the physiological and biogeochemical processes that influence phytoplankton elemental composition. For this topic, we combine laboratory and field experiments with model simulations to investigate these dynamics.
(Photo: Growth of Thalassiosira pseudonana and Thalassiosira weissflogii under different nutrient levels.)
One of the factors affecting the C:N:P ratio of phytoplankton is their subsistence nitrogen quota. We can determine this quota through laboratory incubation experiments by cultivating phytoplankton under nitrogen-limited conditions.
Marine biogeochemical cycles in a changing world
Anthropogenic inputs of pollutants and carbon dioxide into the atmosphere and ocean are impacting our environment and the climate. In this topic, we conduct model experiments using different boundary conditions and parameter combinations to explore how the marine ecosystem and biogeochemical cycles respond to a changing world.
(Fig. 22 in Chien et al., 2022)
Sea surface temperature (SST), surface pH anomaly, upper-layer oxygen, and surface nitrate are all sensitive to global warming. By conducting transient simulations, we can investigate how those variables respond to anthropogenic perturbations.