Elucidating the role of light and carbon dioxide in Antarctic phytoplankton succession

Saisonal and climatic changes in CO2 and light in shelf regions. The effects of ocean acidification on coastal phytoplankton, especially the composition of communities as well as carbon fixation and its export is largely unknown. Besides the ongoing rise in atmospheric carbon dioxide (CO2) concentrations, light availability is an important factor influencing growth and productivity of phytoplankton in coastal and shelf areas on a seasonal basis. While in spring highly variable irradiances are associated with a deep mixed layer, in summer due to higher temperatures the mixed layer is shoaled, leading to high and less variable irradiances. Next to changes in irradiance, also large seasonal changes in pCO2 are frequently observed. To date, information on how changes in pCO2 influence coastal phytoplankton communities is scarce, but they may have a strong impact on Southern Ocean phytoplankton succession.

Seasonal changes of pCO2 and mixed layer depth (MLD) in Antarctic coastal waters. As gas exchange between water and atmosphere was hindered by ice cover, pCO2 is increased after winter. In early spring winds induce an increase of the upper MLD. With the rise in temperature and the subsequent freshwater input from ice melting the MLD gets shallower during summer. Enhanced vertical stratification and reduced MLD will elevate mean irradiances phytoplankton cells encounter. In this now highly stratified upper water layer pCO2 is drawn down during intense phytoplankton blooms, but increases again when winds get stronger and more frequent in autumn.

Anthropgenic-induced changes in pCO2 and light in the future Southern Ocean. Besides the seasonal changes in pCO2 it is predicted that the ongoing anthropogenic carbon dioxide CO2 emissions will result in an overall increase in atmospheric and consequently seawater pCO2. Further, the projected ocean warming and freshening of Antarctic surface waters resulting from ice melt could lead to decreased upper MLDs and thereby to higher integrated daily irradiances. There are, however, also diverging projections postulating more storms could in turn deepen the MLDs thereby weakening or even offsetting the aforementioned increased stratification.

Diverging scenarios for future predicted changes in pCO2 and MLDs in Antarctic coastal waters compared to present day conditions. Seawater CO2 concentrations as well as surface water temperature are predicted to rise. This could lead on the one hand to (A) enhanced stratification and subsequently a shallower mixed-layer-depth (MLD), which on the other hand could be counteracted by (B) increased wind and storm events.

Approach. In order to shed more light onto this topic the combined effects of CO2 and light will be examined in several ecologically relevant coastal phytoplankton species and a natural phytoplankton community of the Western Antarctic Peninsula, which is particularly prone to climate change. Their CO2 sensitivity will be tested under altered light conditions using different levels of constant and variable irradiances. This research aims to identify physiological traits that aid explaining seasonal patterns of phytoplankton occurrence and potential changes in community composition caused by altered light and pCO2 conditions in a future ocean.


Jasmin Heiden