The vertical distribution of phytoplankton: Observations, theory, experiments

Extrinsic and intrinsic factors have been shown to determine spatial pattern formation of populations and we show how both of these factors interact to determine spatial patterns in phytoplankton communities. Determinants of the spatial (vertical) distribution of phytoplankton remain under-investigated and untested. One of the leading hypotheses to explain phytoplankton vertical distribution patterns is that competition for essential resources, nutrients and light, in opposing gradients determines vertical distribution. We used a combination of mathematical modeling, experiments in plankton towers, and lake surveys across major environmental gradients to study what determines the vertical distribution of phytoplankton, focusing on competition for nutrients and light.;In chapter 2, we combined two previous bodies of theory on well-mixed and poorly- mixed water columns to develop a model for a stratified water column. In addition, we considered multiple nutrient sources- one source to the bottom of the water column and one source to the surface mixed layer. Employing this combination of stratification and multiple nutrient sources in our model, we found eight distinct evolutionarily stable strategy (ESS) distributions. The predicted ESS distributions from our model accurately reflected phytoplankton distributions we have observed in lakes. Algae can exist in the mixed layer, deep layer, or in the mixed layer and deep layer together. Algae in either of these locations can be limited by nutrients or colimited by nutrients and light, but counterintuitively, only the mixed layer can be light limited. We predict how the ESS distribution should change as a function of relevant major environmental parameters known to vary across bodies of water.;In chapter 3, we conducted experiments to examine how spatial patterns in algae form as the result of interacting processes: active movement and habitat selection, growth, and depletion of resources. These experiments were conducted to test prior theory that predicted that algae will aggregate into a thin layer at a depth and that that depth depends on resource supply rates. In all experimental plankton towers, algae formed thin layers at the surface, which disappeared at low nutrient supply by the end of the experiment. Higher nutrient supply rates created more biomass and made the algae relatively more light limited. Thus, results qualitatively supported many of our theoretical predictions. In addition, algae exhibited spatial dynamics consistent with niche construction processes, i.e. modification of their environment and responses to these modifications through behavioral movement. These experiments explain how intraspecific competition can drive vertical distributions.;In chapter 4, we surveyed stratified lakes to measure the variation in vertical distributions of phytoplankton and ascribe environmental conditions to the vertical distributions. A primary purpose was to evaluate model predictions of the vertical distribution of phytoplankton in stratified lakes with multiple nutrient sources, while also considering that the phytoplankton communities are made up of many different species. Lakes with single pronounced peaks of algal biomass mostly conformed with model predictions of how environmental variables affect vertical distribution. Phytoplankton also frequently formed multiple peaks in biomass within a water column. The number of peaks that formed depended on the physical space available and the amount of resource heterogeneity in that space. Peaks were typically dominated by a single species and each peak in a water column existed at different nutrient and light conditions. Taxonomic groups sorted across lakes and with depth within lakes and surface and deep vertical niches could be filled by different taxonomic groups and species. Competition for nutrients and light appears to be a primary driver of phytoplankton community structure and vertical distribution.