Phytoplankton competition for nutrients and light in a stratified water column.

Phytoplankton compete for nutrients and light in a vertically heterogeneous environment determined by turbulent mixing. We analyzed a model of competition between two phytoplankton species in a stratified water column. We assume that the surface layer is uniformly mixed and that the deep layer is poorly mixed, as is commonly observed in lakes and oceans. We employed two analytical techniques, I(out) - (R)theory in the mixed surface layer and a game theoretical approach in the deep layer. Under our assumptions, at equilibrium, each species is either absent or resides in the benthic layer, the deep layer, or the surface layer. Assuming a trade-off between nutrient- and light-competitive abilities, we obtained five spatial configurations of coexistence and the corresponding parameter regions where they occur. Good light competitors show two distinct ecological niches: in mesotrophic conditions, they live as understory species below a layer of good nutrient competitors; in eutrophic conditions, they live as competitive dominants in the surface layer. Multiple regions of alternative stable states are possible in parameter space. This work extends previous phytoplankton competition theory to stratified water columns, as commonly found in lakes and oceans.