Influence of carbonate chemistry and light intensity on natural phytoplankton assemblages with emphasis on species composition

The influence of two components of climate change - CO2 concentration and light availability - was tested on natural phytoplankton assemblages and on the coccolithophore species Gephyrocapsa oceanica. An ecologically relevant range of CO2 mixing ratios of approximately 180, 360 and 780 ppm were applied to simulate the CO2 conditions prevailed during glacial times, encountered in todays oceans and expected for the end of this century. Main emphasis was placed on the response of different natural phytoplankton communities to varying CO2 with respect to species composition, primary production and/or bulk phytoplankton dynamics. In a monoculture experiment Gephyrocapsa oceanica showed an increase in the cellular POC content and a decrease in calcification with rising CO2 concentrations. This CO2 effect further depended on light availability. Growth rate of G. oceanica declined under low and enriched CO2 conditions, suggesting that this coccolithophore species might not profit from future changes in seawater carbonate chemistry. Incubation experiments with mixed phytoplankton assemblages indicated that moderately growing phytoplankton might respond more sensitive to CO2 induced environmental changes as fast-growing diatom-dominated phytoplankton. Thus diatoms characteristic for spring-bloom phytoplankton as Thalassiosira sp. and Lauderia borealis were not affected by changes in CO2 availability. Growth of Emiliania huxleyi and the diatom Cerataulina pelagica becoming more dominant in summer was negatively correlated to CO2 concentration. Other summer diatoms responded positively to enriched CO2 conditions. Significant changes in productivity and taxonomic composition within diatom-dominated communities - irrespective of growth rate - seem rather improbable as a response to rising seawater CO2 concentrations. However, light intensity had a strong impact on community structure and productivity patterns and was able to modulate the CO2 effect on the autotrophic assemblages.