COS 29-5 - A spatial competition model of mixotrophic and specialist trophic strategies in a stratified water column

Background/Question/Methods

Mixotrophy is recognized as an important plankton process, but we are still far from a complete understanding of the advantages it provides for resource competition. Along the opposing vertical gradients of light and nutrients usually present in stratified lakes, mixotrophy provides a generalist trophic strategy; an alternative to autotrophic or heterotrophic specialist strategies in the water column. Yet it is still not entirely clear how mixotrophy, with its associated physiological trade-offs, can be viable against specialist strategies. Given the difficulty of measuring mixotrophy rates in the field, modeling allows us to test hypotheses pertaining to its role in the nanophytoplankton community and along typical environmental gradients. We developed a model that simulates the dynamics of three competitors (pure autotroph, mixotroph and pure heterotroph) and bacterial prey populations over the vertical dimension of a stratified water column to investigate the viability of mixotrophy. We used this model to simulate the dynamics of a nanoplankton community over a range of light and dissolved nutrient availabilities to assess under which conditions the mixotroph can coexist with specialists. We also investigated the spatial structure of the competitors over the dual opposite vertical gradients of light and nutrient.

Results/Conclusions

In a pure resource competition framework, our model shows that phago-mixotrophy is a viable nutritional mode when the availability of resources is spatially heterogeneous. This arises because the mixotroph is able to coexist with specialized autotrophic and heterotrophic competitors across a variety of resource availability conditions. The mixotroph dominated the community when nutrients were scarce and light relatively abundant, especially when the generalist was mainly autotrophic. Mixotrophy was always viable and sometimes the best strategy at a range of autotrophic affinities, indicating that a degree of plasticity in this functional trait could help explain the pervasiveness of mixotrophs in a variety of natural systems. Our results also indicate that the success of mixotrophs stem from their ability to grow over a large portion of the water column relative to specialists. Finally, our model predicts that different competitors with distinct trophic modes maximize their biomass at different depths over the vertical light and nutrient gradients, indicating that a diversity of trophic strategies could contribute to the vertical structuring of nanophytoplankton communities observed in stratified lakes.