Seasonal succession in phytoplankton cell size contrasted with an adaptive dynamic food-web model in a periodic system: How zooplankton regulate seasonal turnover in phytoplankton traits

Background/Question/Methods

The importance of food-webs in explaining the functional structure of communities is often overlooked in favor of single trophic level studies. Leibold’s 1996 keystone predator/diamond food-web module provides a mechanism for the maintenance of two species on the same resource if they share a common predator and have a tradeoff between resource uptake and predator resistance. This is an especially apt model when viewed in terms of planktonic systems comprised of an inorganic resource nutrient, two primary producer phytoplankton species, and a generalist zooplankton grazer. Larger phytoplankton species may escape predation by being too big for the filter feeding zooplankton, but would be less efficient at nutrient uptake because of a decreased surface area to volume ratio, for example. Klausmeier and Litchman (2011) have taken this analogy farther by embedding the model in a periodic system resembling the seasonal dynamics of many freshwater lakes; mixing in the fall and spring with stratification or ice cover in the summer and winter.  This model captures the bones of seasonal succession observed in freshwater communities, but because it is restricted to only 2 producer species, it doesn’t begin to capture the diversity. Our objective was to see if this model could be expanded to be directly comparable with the actual seasonal changes in the distribution of cell sizes in a reference lake.

Results/Conclusions

We expanded the middle trophic level of the diamond model to be a continuous axis on which species are differentiated by a single trait value, analogous to cell size. We then used adaptive dynamics techniques to simulate the process of seasonal succession that takes place in periodic environments like a temperate lake. We also used a dataset comprised of bi-weekly phytoplankton and zooplankton counts as well as bio-volume estimates to define an observed pattern of seasonal turnover in cell-size in response to predation. Our results show that this kind of tradeoff in a periodic, tri-trophic, adaptive model yields repeatable, periodic changes in trait value that closely match those seen in natural systems. This shows that simple trophic interactions can go a long way in explaining the seasonal dynamics of freshwater phytoplankton communities, and shows that food web structure is an important feature in maintaining community diversity.