Life in the open ocean depends on phytoplankton that grow when nutrient rich water meets sunlight at the ocean surface. Time and place of phytoplankton production depend on the interplay between ocean currents and the seasonal cycle of weather and light, and have been distorted by recent climate change. As a consequence the distribution of phytoplankton and marine life in general has changed during the last decades. Such redistributions have important consequences: they affect fisheries, but they also have an effect back on climate itself, since phytoplankton remove much of the carbon dioxide that mankind releases into the atmosphere. To be able to mitigate negative impacts on mankind and environment, it is crucial to understand how life is distributed in the ocean, how it responds and will respond to climate change, and what the consequences are for ecosystems and climate.
In this thesis I aim to improve this understanding for the important group of marine plankton. Plankton are organisms which cannot swim against the currents and represent the most common form of life in the open ocean. Besides phytoplankton they comprise zooplankton, including single-celled organisms, and tiny animals like copepods. From global field observations I show that the distributions of phytoplankton species are linked to distinct conditions in light intensity, temperature, turbulence, and other factors. Unfortunately, however, these links are not sufficiently strong to be used to make accurate predictions of how the distributions of individual species may change as a consequence of climate change: on the species level, plankton distributions are somewhat chaotic. I therefore developed a novel trait-based approach and found that the distribution of zooplankton is determined by their traits (characteristics), such as body size, feeding and reproductive strategies, and their ability to avoid predation. Individuals with a large body size, for example, mainly occur in cold areas, whereas warm-water areas are inhabited by smaller organisms.
Finally, I demonstrate that the distribution of traits can also be used to investigate the role of zooplankton in ecosystem functioning: for the North Atlantic I estimated the role of zooplankton in carbon sequestering in, i.e., the transport of carbon from the surface to the interior of the ocean, based on their body size and other traits.