Linking population dynamics and nutrient cycling: The effect of the nuisance cyanobacterium Gloeotrichia echinulata on lake ecosystem resilience

Background/Question/Methods

Ecologists have long been interested in understanding how, when and where single species have the potential to modulate ecosystem functioning. We hypothesize that the physiology and life history of the colonial cyanobacterium Gloeotrichia echinulata make this species a biotic key that unlocks otherwise inaccessible stores of atmospheric nitrogen (by N fixation) and sediment phosphorus (during the process of germination and recruitment from sediment resting stages).  Because G. echinulata has the potential to alleviate both N and P limitation, we predict that the factors that drive G. echinulata blooms may also indirectly accelerate lake eutrophication by eroding the resilience of the oligotrophic state.

We are examining the drivers of G. echinulata abundance and the impact of this species on eutrophication and resilience using field observations of key biotic and abiotic variables in combination with modeling and targeted field experiments.  Specifically, we are using empirical models to understand the abiotic and biotic factors that drive fluctuations in G. echinulata density and simple differential equations models to explore how the changes in N and P cycling caused by G. echinulata might affect lake ecosystem stability and resilience.

Results/Conclusions

Modeling and empirical work to date suggest that water column densities of G. echinulata result from a complex interplay of biotic and abiotic factors affecting recruitment from sediment resting stages and subsequent growth in the water column.  Light and temperature appear to predict recruitment from resting stages, while water column stability, grazing, and density-dependent growth influence water column populations.

In our simulation models, changes in nutrient cycling due to G. echinulata affect stability and resilience.  Simple one-box P cycling models suggest that internal loading by G. echinulata can create a second, eutrophic equilibrium and decrease the resilience of the oligotrophic equilibrium, particularly in lakes with oxic hypolimnia and low external inputs.  Moreover, our coupled model of N and P cycling shows equilibria whose exact formulas are quite complicated, but whose close approximations also suggest that internal recycling raises equilibrium nutrient concentrations.  Thus, the activities of G. echinulata are likely to be important in affecting the ability of oligotrophic lakes to maintain low-nutrient conditions.  We look forward to exploring these findings further in a coupled population-nutrient model.