Phytoplankton competition during the spring bloom in four Plankton Functional Type Models
Abstract
We investigated the mechanisms of phytoplankton competition during the spring bloom,one of the most dramatic seasonal events in lower-trophic level ecosystems, in fourstate-of-the-art Plankton Functional Type (PFTs) models: PISCES, NEMURO, Plank-TOM5 and CCSM-BEC. In particular, we investigated the relative importance of dif-5ferent ecophysiological processes on the determination of the community structure,focusing both on the bottom-up and the top-down controls. The models reasonablyreproduced the observed global distribution and seasonal variation of phytoplanktonbiomass. The fraction of diatoms with respect to the total phytoplankton biomass in-creases with the magnitude of the spring bloom in all models. However, the governing10mechanisms differ between models, despite the fact that current PFT models repre-sent ecophysiological processes using the same types of parameterizations. The in-creasing trend in the percentage of diatoms with increasing bloom magnitude is mainlycaused by a stronger nutrient dependence of photosynthesis for diatoms compared tonanophytoplankton (bottom-up control). The difference in the maximum photosynthesis rate plays an important role in NEMURO and PlankTOM5 and determines the abso-lute values of the percentage of diatoms during the bloom. In CCSM-BEC, the lightdependency of photosynthesis plays an important role in the North Atlantic and theSouthern Ocean. The grazing pressure by zooplankton (top-down control), however,strongly contributes to the dominance of diatoms in PISCES and CCSM-BEC. The re-20gional differences in the percentage of diatoms in PlankTOM5 are mainly determinedby top-down control. These differences in the mechanisms suggest that the responseof marine ecosystems to climate change could significantly differ among models, evenif the present-day ecosystem is reproduced to a similar degree of confidence. For fur-ther understanding of plankton competition and for the prediction of future change in marine ecosystems, it is important to understand the relative differences in each phys-iological rate and life history rate in the bottom-up and the top-down controls betweenPFTs Show more
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https://doi.org/10.3929/ethz-b-000060387Publication status
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Biogeosciences DiscussionsVolume
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CopernicusOrganisational unit
03731 - Gruber, Nicolas / Gruber, Nicolas
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