Impact of physiology, ecology, and trait trade-offs on marine plankton communities and the carbon cycle

Abstract

Plankton modeling helps us understand how aquatic ecosystems and marine biogeochemistry interact. With observations and eco-physiological rules, it is possible to create simulations where plankton assemble their communities and shed light on the mechanisms that regulate the carbon cycle. Past literature assumed that plankton can be neatly categorized, according to the trophic strategy, into "phytoplankton'' and "zooplankton''. Autotrophic phytoplankton use photosynthesis and inorganic nutrients, while heterotrophic zooplankton consume other organisms or organic substances. However, in recent years, it has become apparent that trophic strategies exist on a spectrum, with many organisms occupying a continuous spectrum between strictly autotrophy and strictly heterotrophy. "Mixotrophs'' are organisms capable of using both autotrophy and heterotrophy, and their role in the ecosystems increases size and carbon export. One of the main questions about mixotrophs is the role of their trophic "trade-off'', where a trade-off is a compromise between different traits in the same organism (the autotrophic and heterotrophic traits in this case). In this thesis, through plankton modelling, I show that the trophic trade-off is slightly penalizing for mixotrophs, which decreases the capacity of mixotrophs to increase plankton average size and carbon export in a marine community. Another finding is that mixotrophs, through their flexible trophic strategy, survive better than zooplankton during the polar winter, which then increases the coupling between phytoplankton and mixotrophs blooms in the early spring, leading to a decrease in the strength of the carbon pump in the high latitudes. Furthermore, I explored the response of plankton to climate changes showing that mixotrophs increase community resistance to a warmer ocean by increasing average primary production, plankton size and carbon export. However, these increases are not sufficient to stop a possible positive feedback loop between carbon sequestration and climate warming, especially in the light of a sublinear trade-off that erodes the impact of the mixotrophic eco-physiology compared to the impact of phytoplankton and zooplankton. In summary, the higher the global average temperature will rise, the lower the carbon export will become, which in turn will decrease the capacity of plankton to sequester carbon from the atmosphere, and which ultimately feeds back on climate warming itself. This research also shows that the resolution of mixotrophs in computer models could improve the representation of biogeochemical cycles and community structure, which is relevant to the biogeochemistry field and the production of climate changes trajectories.

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