Resource competition between buoyancy-regulating and sinking phytoplankton species along a stratified water column

In temperate lakes plankton dynamics are closely linked to seasonal shifts in water stratification, following air temperature. The onset of summer stratification typically coincides with spring bloom formation of algae in the well mixed surface layer (epilimnion). However, extended stratification periods lead to nutrient depletion and increases the risk of algae to sink out of the sunlit epilimnion. Planktonic primary producers have evolved different traits to counteract sinking, such as specific morphological shapes, but also adaptive mechanisms like active buoyancy regulation. The latter is very common for cyanobacteria and gives them a competitive advantage over sinking taxa specifically during extended stratified periods. Existing conceptual models on plankton phenology neglect the vertical dimension of plankton bloom formation, focusing mainly on epilimnion blooms. This limits projections of how changes in stratification through global warming will affect plankton composition, productivity, and water quality. Here we develop a theoretical framework to investigate resource competition between a passively sinking (S) and a buoyancy-regulating (BR) phytoplankton species along a one-dimensional water column. The BR species adaptively moves towards optimal light and nutrient availability along the water column. Our results indicate that coexistence between BR and S algae is critically dependent on differences in resource-use efficiencies, which can lead to situations of competitive exclusion but also coexistence in overlapping or vertically separated depths. Our results highlight the importance of vertical movement strategies in structuring phytoplankton communities and its consideration for projections on plankton phenology, composition and lake primary production under changing stratification regimes.