The games algae play: Competition for nutrients and light in poorly-mixed water columns

Background/Question/Methods Organisms not only respond to heterogeneity in their environment, they also create it by determining the resource environment for their competitors and themselves. These acts may modify behavior and habitat selection, causing further modification of the resource environment. Population dynamics create additional feedbacks in the resource environment. We closely monitored phytoplankton competing in one dimensional spatial habitats through time. These habitats contained opposing resource gradients of nutrients and light, essential resources for phytoplankton. We used motile phytoplankton species growing in poorly-mixed water columns to examine how intraspecific competition determines the vertical distribution of phytoplankton. Under these conditions, theory we developed predicts a single evolutionary stable strategy depth in the water column where individuals should aggregate. This depth of aggregation, or biomass maximum, should change through time due to modification of the resource environment. In addition, we predict the depth of the ESS to change across broad environmental gradients. In particular, it should be deeper under low amounts of nutrient loading and shallower under higher amounts of nutrient loading. Results/Conclusions In our experimental water columns, we routinely observed a single biomass maximum, where most of the population was located. The location changed through time due to behavioral movement, self-shading, nutrient uptake, and growth. The location of the biomass maximum also changed along an environmental gradient of nutrient loading. Supporting predictions, the depth of the biomass maximum was negatively related to nutrient input at the bottom, i.e. deeper under low nutrient loading and shallower under higher nutrient loading. In addition, the heterogeneous resource environment was strongly modified by the activities of the phytoplankton. The light level leaving the bottom of the water column, Iout, declined through time as the phytoplankton grew and was negatively related to nutrient loading. Nutrients were strongly depleted where biomass was present by the end of the experiment. Despite the importance of the vertical distribution of phytoplankton for primary production and energy transfer to higher trophic levels, our mechanistic understanding of the phenomenon is poor. This study is the first to show that the vertical distribution of phytoplankton may be driven by intraspecific resource competition in space.