Trait-based models of plankton communities: from individuals to ecosystem functions

There is an inherent link between plankton and climate. Climate affects plankton community composition. At its turn, plankton community composition drives ecosystem processes that affect the climate. For instance, plankton play an important role in capturing atmospheric CO2 into the oceans. This process is termed the biological carbon pump and is the result of community composition and complex interactions within the planktonic food-web.

Plankton community composition varies across regions and is being altered by climate change. The aim of this thesis is to investigate the mechanisms driving plankton community composition and associated carbon export. To do so we use trait-based mechanistic plankton models. The trait-based approach characterizes organisms by a combination of traits and trade-offs.

These traits and trade-offs drive community composition and interaction between organisms. For instance, body size drives predator-prey interactions and physiology, and has been suggested to be the primary determinant of community structure. Thus, size-based models have proven useful to model communities.

Most models represent plankton by a box-model, where each population is characterized by the same trait combination. However, multicellular zooplankton grow in size during their life, resulting in different experienced diets and mortality over their life, i.e. they undergo ontogenetic niche shifts. Ontogenetic niche shifts have profound effects on population dynamics.

Given that multicellular zooplankton are the main link between primary producers and fish, and play an important role in driving the biological carbon pump, the second aim of this thesis is to implement zooplankton ontogeny in trait-based plankton models. This thesis includes three manuscripts, addressing one or both objectives.

In the first manuscript we address how temperature affects microbial plankton communities. An interesting aspect in marine microbes is that resource uptakes and metabolism have different temperature dependencies. We thus investigate how temperature affects individual growth, community composition and ecosystem processes given different resource limitations.

We developed a mechanistic trait-based model of unicellular plankton that accounts for the different temperature dependencies in the cellular processes. We show that competition for nutrients increases with temperature without necessarily changing the stratification of the water column. This results in an intensification of the microbial loop and a reduction of carbon export.

By explicitly representing the effects of temperature on traits responsible for growth, we demonstrate how changes on the individual level scale to the ecosystem level. The aim of the second manuscript is to implement zooplankton ontogeny in a generic food-web framework of planktonic communities. We propose a model along the Nutrient-Unicellular-Multicellular axis – a “NUM” framework – as an alternative to the NPZ modelling paradigm.

NUM is a mechanistic size- and trait-based model based on individual-level processes. Here the multicellular component describes the population dynamics of key copepod groups, characterized by their adult size and feeding mode. Community composition is an emergent property of the model. Using this framework, we analyze how competition and predation shape the planktonic community.

We show that intraguild predation is ubiquitous in plankton food-webs and that competition between protists and copepods can emerge. In the third manuscript, we use the NUM framework to investigate the role of zooplankton in driving carbon export efficiency at the global scale. The NUM framework is coupled to a 3D physical model of the oceans.

We show that energy pathways in the planktonic food-web define carbon export efficiency. That is, if copepods process most of the primary production, export efficiency is high. On the other hand, if protists process most of the primary production, export efficiency is low. Light, temperature, nutrients, competition and predation define the food-web configuration.

Finally, we discuss the differences between estimating export efficiency at the seasonal level and at the annual level. The model has emergent food-web configurations and size spectrum of the community and detrital particles. These features render the NUM framework a powerful tool to investigate ecological and biogeochemical processes in an environmental change context.

In conclusion, in this thesis we address several aspects related to climate change, plankton community composition and ecosystem functions. To do so, we developed models that characterize community composition. Perhaps the main product of this thesis is the NUM – Nutrient-Unicellular-Multicellular – framework, which incorporates zooplankton ontogeny in plankton size-spectrum community model.

The NUM framework is generic and can be extended to include higher trait-diversity, functional groups or stoichiometry. With this framework, we improve the representation of multicellular zooplankton in global ecological and biogeochemical models while keeping a relatively low parameter-set and the emergent property of tait-based models.