Depth of the biomass maximum affects the rules of resource competition in a water column.

The theory of resource competition in spatially extended systems with resources and biomass fluxes is far from trivial. Here, we analyze the competition between two phytoplankton species for light and a nutrient in a weakly mixed water column. We develop a general framework for such an analysis and show that the competition outcome can be largely understood from a single parameter, the slope of the invasion threshold in the plane of resources. Using this approach, we show that the competition outcome crucially depends on the depth of the biomass maximum. Under eutrophic conditions, when the phytoplankton production peaks on the surface, species composition depends on the ratio of resource supplies, and the competition outcome follows the “classic” rule: coexistence is possible if each competitor has the greatest effect on its most limiting resource. By contrast, in oligotrophic systems, characterized by deep biomass maxima, the absolute level of resource supplies drives species composition, and coexistence becomes more feasible if each competitor mostly consumes its least limiting resource. Finally, when the production peaks in the subsurface, good nutrient competitors are favored. Our findings are supported by empirical data.