The vertical distribution of phytoplankton in stratified water columns

Background/Question/Methods Spatially heterogeneous patterns of populations may be explained by underlying spatial heterogeneity in resources and/or other biotic variables as well as intrinsic growth and movement. We develop a model to explore how phytoplankton respond through growth and movement to opposing resource gradients and different mixing conditions. We find that if nutrients are supplied both from the sediments and into the surface mixed layer, the four possible vertical distributions for phytoplankton are: persist in the surface mixed layer only, persist in the deep poorly mixed layer only, persist in the surface mixed layer and deep poorly mixed layer, or not persist in the water column. In the cases where phytoplankton persist in the deep poorly mixed layer, they may move to alleviate resource limitation. However once in the surface mixed layer, we assume their movement cannot overcome mixing. In addition, we conducted a lake survey to evaluate model predictions across environmental gradients. Results/Conclusions Examining how model results depend on environmental parameters we see that increasing nutrient input from the sediments or mixed layer results in phytoplankton becoming relatively more light limited and increases total biomass. Increasing background light attenuation results in phytoplankton becoming relatively more light limited and generally decreases total biomass. Increasing mixed layer depth may have no effect on phytoplankton resource limitation or may result in phytoplankton becoming relatively more light limited. Increasing mixed layer depth may have positive or negative effects on amount of total biomass. Thus environmental conditions alter the spatial distribution, relative resource limitation, and amount of biomass of the phytoplankton. Lake survey results show that vertical distribution, relative resource limitation, and amount of biomass qualitatively follow model predictions for most environmental parameters. This study should serve to direct avenues of future research in aquatic ecosystems subject to anthropogenic stressors such as nutrient loading, turbidity, and climate change.