Phytoplankton functional traits under the impact of environmental change: from single cells to communities

Abstract

Phytoplankton are responsible for half of the total oxygen production on Earth and play an important role in ocean productivity as the base of most aquatic food webs. However, human activities are leading to changes in marine abiotic parameters, and phytoplankton are facing simultaneous changes in temperature, partial pressure of carbon dioxide (pCO2), and dissolved nutrient concentrations, as well as more frequent and intense extreme weather events. Trait-based approaches were used to investigate these effects in several multiple driver experiments at different ecological scales, from individual phytoplankton cells to populations to entire phytoplankton communities. This thesis indicates that temperature was the most important driver affecting phytoplankton growth and other functional traits. For phytoplankton species currently growing below their thermal limits, growth rates will likely increase with projected temperature rises but the magnitude of this growth enhancement may be dampened by simultaneous increases in pCO2 and N:P ratios. Furthermore, increasing growth rates within cultures of the ubiquitous diatom species Thalassiosira weissflogii, as well as increasing temperatures and pCO2, caused a decline in cell-to-cell variability, which could reduce the potential for phytoplankton to cope with changes in environmental conditions. The impact of abrupt changes in temperature due to marine heatwaves was investigated using the diatom Phaeodactylum tricornutum. Alternative oxidase (AOX) activity was examined as a stress marker but no increase in AOX activity due to the abrupt increase in temperature was observed. Most changes in cellular traits were rather observed after acclimation to higher temperatures. These results indicate that phytoplankton cells might have the potential to buffer environmental fluctuations such as marine heatwaves yet undergo significant changes in their traits. The potential impact of increased river discharge was investigated on natural North Sea phytoplankton communities. Pulses of nutrient-rich freshwater caused an increase in community growth rates but also decreased phytoplankton cell size within the different phytoplankton groups and a decrease in cellular N:P ratio. At the same time, no strong negative effect of decreasing salinity was observed, indicating a large salinity tolerance of North Sea phytoplankton communities. Overall, responses of phytoplankton communities depended more on the initial community structure at each sampling site than on the origin of the communities. Still, an abrupt increase in river discharge might have the potential to restructure phytoplankton communities and modulate short-term responses in higher trophic levels. This thesis provides a comprehensive picture of the effects of different types of environmental changes on the functional traits of phytoplankton, from single cells to phytoplankton communities. Long-term environmental changes, as well as short-term extreme weather events, can lead to substantial changes in growth rates, cell size, and the biochemical composition of phytoplankton, which has the potential to alter marine food webs and biogeochemical cycling.