Phytoplankton competition during the spring bloom in four plankton functional type models

[摘要] We investigated the mechanisms of phytoplankton competition during thespring bloom, one of the most dramatic seasonal events in lower-trophic-levelecosystems, in four state-of-the-art plankton functional type (PFT)models: PISCES, NEMURO, PlankTOM5 and CCSM-BEC. In particular, weinvestigated the relative importance of different ecophysiological processeson the determination of the community structure, focusing both on thebottom-up and the top-down controls. The models reasonably reproduced theobserved global distribution and seasonal variation of phytoplanktonbiomass. The fraction of diatoms with respect to the total phytoplanktonbiomass increases with the magnitude of the spring bloom in all models.However, the governing mechanisms differ between models, despite the factthat current PFT models represent ecophysiological processes using the sametypes of parameterizations. The increasing trend in the percentage ofdiatoms with increasing bloom magnitude is mainly caused by a strongernutrient dependence of diatomgrowth compared to nanophytoplankton(bottom-up control). The difference in the maximum growth rate plays animportant role in NEMURO and PlankTOM5 and determines the absolute values ofthe percentage of diatoms during the bloom. In CCSM-BEC, the lightdependency of growth 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 andCCSM-BEC. The regional differences in the percentage of diatoms in PlankTOM5are mainly determined by top-down control. These differences in themechanisms suggest that the response of marine ecosystems to climate changecould significantly differ among models, even if the present-day ecosystemis reproduced to a similar degree of confidence. For further understandingof plankton competition and for the prediction of future change in marineecosystems, it is important to understand the relative differences in eachphysiological rate and life history rate in the bottom-up and the top-downcontrols between PFTs.

[学科分类] 地球化学与岩石