Plankton Trait Ecology in a Changing Marine Environment

The marine habitat is a major component of the Earth System with the oceans covering 70% of earth’s surface, and for physical as well a biological reasons the oceans play a major role in global climate. Biologic activities in the oceans draw down 48.5 Gt C from the atmosphere every year, whereof up to 25% of this carbon is transported out of the photic zone via the biological carbon pump. Even though it is a small fraction of this that is sequestered at depth (1-3%), it underlines the huge potential of the marine biota to impact and shape global climate, both in the past, present and future ocean. The current anthropogenic climate change is, among other things, predicted to alter ocean circulation, stratification and nutrient availability in the marine environment, all three aspects that play a crucial role for marine biota with feedback on global climate. For this reason, it is crucial to improve our understanding of the functioning of marine ecosystems in relation to environmental (abiotic and biotic) variability gradients. As primary producers, planktonic organisms are a key compartment and energetic pathway that link resource availability to marine food webs. As such, the planktonic community play a central role in biogeochemical cycles in the ocean in general and production and export in particular. The trait based approach forms the base of the studies in my thesis, as it is ideal to provide simple models of the individual organisms and their interaction with each other and the environment. Using this approach we create a link between performance traits of organism and the actual ecological niches in space and time in which they dominate, and find a general resemblance of our predictions with observations. The success of the trait based approach in relation to my studies is taking the step from predicting under which conditions a certain trait combination is optimal, to exploring how biomass of organisms showing these specific traits evolve in a dynamic seasonal cycle. In this thesis I quantify and systematize two important aspects of marine planktonic life that influence the export and sequestration of carbon to the sea floor: the phenomena of diel vertical migration (DVM) in zooplankton and the general success of diatoms in seasonal environments. DVM is a strategy among zooplankton such as calanoid copepods to increase the chance of survival and hence their fitness. Their presence in the sunlit ocean surface attracts predators, and to decrease this predation risk many zooplankton individuals migrate to darker depths during the day. Consequently these migrators feed predominantly during night, when they reside in the surface layers. Besides the immediate decrease in predation risk, the migrating zooplankton faces a lost feeding opportunity, by abandoning the productive surface layer for a large part of the day. The question of DVM hence becomes a question of a trade-off between costs and benefits; zooplankton have to balance costs of lost feeding opportunity and swimming expenditure to the benefit of reduced predation risk, factors that vary with size of the zooplankter, availability of food and degree of illumination of the water column. Along with respiration, some of the food that is consumed in the surface is later excreted at depth, which has been proposed as an enhancing effect on the carbon export, and hence adds an active component to the biological carbon pump. Using a model estimate of food abundance in the surface layers we predict that 16-30 % of the carbon exported in the North Atlantic might be mediated by diel vertically migrating zooplankton. Indeed this number might also be influenced by the type of diet the zooplankton consumes; an increase in fecal pellet production and density is observed in copepods feeding on diatoms. In the modern ocean, diatoms are responsible for as much as 50 % of carbon exported via the biological carbon pump, and hence they have a crucial role in ecosystem functioning with feedback to global climate. Their special physiology is characterized by a silica exo-skeleton and a large water-filled, central vacuole, and their dominance in marine ecosystems are widespread in nutrient rich, upwelling areas. The lifestyle of diatoms is in several ways filled with dichotomies; they rely on a nutrient that they themselves deplete, they build a heavy shell, that they have to counter act with buoyancy and the protection that the shell affords seems to be at odds with the conspicuous success of diatoms in grazer -poor environments. We take a mechanistic approach to illuminating the success of diatoms and find that their special physiology leads to several advantages strongest in nutrient-rich springlike conditions. Our results underscore the grazing pressure as the main driver for vacuolation. The succession of diatoms and mixotrophs cover most of the phytoplankton community’s strategies with regards to acquiring nutrients, from the strictly autotroph diatoms to heterotrophic dinoflagellates. The last part of my thesis sets up a competition experiment modelling the strategies of vacuolation as opposed to mixotrophy, and examines how the optimal strategies evolve in terms of abundance in the seasonal cycle. From the unification of these two types in a dynamic seasonal cycle there emerges a realistic succession pattern of large, highly vacuolated diatoms in spring superseded by a community of small diatoms and mixotrophs coexisting with large heterotrophs in the nutrient depleted summer. This emphasizes the propagation of nutrient-rich, well mixed ocean environment as a prerequisite for the continued success of diatoms in the future ocean.