1Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan
2Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
3Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Tokyo, Japan
4Institute for Biogeochemistry and Pollutant Dynamics, CHN 23.2, Universitaetsstrasse 16, 8092 Zürich, Switzerland
5Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
6LOG, Univers ité Lille Nord de France, ULCO, CNRS, Wimereux, France
7Laboratoire du Climat et de l'Environnement (LSCE), L'Orme des Merisiers Bât. 712, 91191 Gif sur Yvette, France
8Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
*now at: Plymouth Marine Laboratory, Plymouth, UK
Received: 20 Nov 2012 – Discussion started: 13 Dec 2012
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 four state-of-the-art plankton functional type (PFT) models: PISCES, NEMURO, PlankTOM5 and CCSM-BEC. In particular, we investigated the relative importance of different ecophysiological processes on the determination of the community structure, focusing both on the bottom-up and the top-down controls. The models reasonably reproduced the observed global distribution and seasonal variation of phytoplankton biomass. The fraction of diatoms with respect to the total phytoplankton biomass increases with the magnitude of the spring bloom in all models. However, the governing mechanisms differ between models, despite the fact that current PFT models represent ecophysiological processes using the same types of parameterizations. The increasing trend in the percentage of diatoms with increasing bloom magnitude is mainly caused by a stronger nutrient dependence of diatom growth compared to nanophytoplankton (bottom-up control). The difference in the maximum growth rate plays an important role in NEMURO and PlankTOM5 and determines the absolute values of the percentage of diatoms during the bloom. In CCSM-BEC, the light dependency of growth plays an important role in the North Atlantic and the Southern Ocean. The grazing pressure by zooplankton (top-down control), however, strongly contributes to the dominance of diatoms in PISCES and CCSM-BEC. The regional differences in the percentage of diatoms in PlankTOM5 are mainly determined by top-down control. These differences in the mechanisms suggest that the response of marine ecosystems to climate change could significantly differ among models, even if the present-day ecosystem is reproduced to a similar degree of confidence. For further understanding of plankton competition and for the prediction of future change in marine ecosystems, it is important to understand the relative differences in each physiological rate and life history rate in the bottom-up and the top-down controls between PFTs.
Revised: 27 Aug 2013 – Accepted: 17 Sep 2013 – Published: 02 Nov 2013
Hashioka, T., Vogt, M., Yamanaka, Y., Le Quéré, C., Buitenhuis, E. T., Aita, M. N., Alvain, S., Bopp, L., Hirata, T., Lima, I., Sailley, S., and Doney, S. C.: Phytoplankton competition during the spring bloom in four plankton functional type models, Biogeosciences, 10, 6833-6850, doi:10.5194/bg-10-6833-2013, 2013.