As phytoplankton photosynthesis is dependent on light, one might assume that all the phytoplankton activity occurs in the surface of our oceans. This assumption was, however, challenged early in the history of biological oceanography when chlorophyll sampling and fluorescence profiling showed deep chlorophyll maxima (DCM) to be a general feature in the ocean. Today, it is generally accepted that DCMs occur in most of our oceans still, despite this empirical knowledge, subsurface primary production is still largely ignored in marine science.

The work included in this PhD examines the vertical distribution of phytoplankton activity and its importance to several major themes of societal interest; carbon transport, fisheries ecology and eutrophication effects in the North Atlantic, the Sargasso Sea and the Baltic Sea transition zone respectively. The results presented are based on process studies from each of the three regions combined with 15 years of survey data for the Baltic Sea transition zone. Overall, the results of this PhD work show that the vertical distribution of phytoplankton and their activity is important for the understanding, dynamics and functioning of pelagic ecosystems. It, thus, emphasizes that future research and modelling exercises aimed at improving understanding of pelagic ecosystems and their role in the global ocean should include a consideration of the vertical heterogeneity in phytoplankton distributions and activity.