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Coccolithophores are one of the important groups of phytoplankton in the global oceans, which makes it important to know how this group will react to changes in their environment due to climate change. Modellers already recognized their importance and included this group independently in global biogeochemical models. This study assesses the effect of light, temperature and nutrient availability on five coccolithophores, performing a range of laboratory experiments. The results of these experiments were then used to change the parameterisation of coccolithophores in the global biogeochemical model PlankTOM10. Furthermore, the model was validated in two ways, using a database of coccolithophore biomass measurements from the field and measurements of surface calcium carbonate derived from satellite data. Temperature effects on growth depend a great deal on the coccolithophore species. E. huxleyi (both, a subtropical and a temperate strain) and P. carterae grew best around 20°C, whereas G. oceanica and C. leptoporus had optimum temperatures above 25°C and still grew well at the maximum temperature tested in the experiments. E. huxleyi was the species with the highest growth rates (μmax=0.98 for the subtropical strain and μmax=0.97 for the temperate), followed closely by G. oceanica and C. leptoporus (μmax=0.91 in both species). P. carterae (μmax=0.77) had a noticeably lower maximum growth rate than the other coccolithophores. An inverse relationship with growth rate was found for all measured cellular components (POM, PIC, Chl a) as well as for cell volume in P. carterae. Coccolithophores are good competitors at high light intensities, having optimum growth light intensities above 180 μmol photons m-2 s-1. The temperate strain of E. huxleyi and the species G. oceanica showed the lowest optima at 350 μmol photon III m-2 s-1. C. leptoporus (Iopt=500 μmol photon m-2 s-1) and P. carterae (Iopt=600 μmol photon m-2 s-1) had higher optimum growth light intensities and the subtropical strain of E. huxleyi (Iopt=900 μmol photon m-2 s-1) grew best at the highest light intensity applied in this study. Only one strain of E. huxleyi showed light inhibition in its photosynthetic activity that was well above the detection limit in P-I curves up to 2000 μmol photons m-2 s-1. Apart from a well-known decrease in Chl a/C ratio with increasing light intensity, little variation in the concentration of cellular components (POM, PIC) was observed. Nutrient experiments were carried out in a chemostat with two strains of E. huxleyi and one G. oceanica. Phosphorus limitation led to an increase in cell volume (112- 157%) and particulate organic carbon (21-54%) in E. huxleyi and G. oceanica, relative to cultures grown under nitrogen limitation. Comparison of uptake rates for phosphate and nitrate with other phytoplankton groups showed that both species are very good competitors for phosphate and relatively poor competitors for nitrate. The initial PlankTOM10 model simulation overestimated biomass compared with a new observational database, and underestimated surface calcium carbonate compared with satellite data. Changing the coccolithophore parameterisation in PlankTOM10, based on the laboratory results, did not lead to significant improvements relative to the observations. However, the response of the model to the parameter changes could be explained either directly from the changed parameters, or indirectly from changes in the model ecosystem. |
