Vertical Niche Partitioning and the Performance of Mixotrophic Generalists against Autotrophic and Heterotrophic Specialists under Contrasting Light-Nutrient Supply Regimes

AbstractA vertical separation in light and nutrient availability is observed in many terrestrial and aquatic ecosystems. In lakes and oceans, the opposing vertical gradients of light and nutrients typically observed are believed to promote phagomixotrophy, a generalist strategy that combines resource acquisition through photoautotrophic and phagoheterotrophic pathways. While phagomixotrophy is widespread, it is not well understood how this strategy performs against pure specialist strategies in a resource competition context. We simulate the dynamics of three competitors (pure photoautotroph, phagomixotroph, pure phagoheterotroph) and bacterial prey over the vertical dimension of a water column to investigate what conditions of resource availability favor mixotrophy and how the presence of the phagomixotroph alters community dynamics. Since mixotrophs can be more or less photoautotrophic, we incorporated this variability into our model. Under weak vertical mixing, mixotrophs persist under most light and nutrient conditions and negatively affect specialists. Mixotrophs can even be dominant competitors when they display an optimal degree of phototrophy, which is positively related to water transparency and negatively related to nutrient supply. The model indicates that the spatial organization of nanophytoplankton communities in water columns could arise through vertical niche partitioning of multiple resource acquisition strategies and that phagomixotrophy can promote overall community production.