Litter and sulfur controls on population dynamics of a freshwater aquatic plant (Zizania palustris)

Background/Question/Methods

Vegetated sediments are full of complex and sometimes competing processes involving organic matter, nitrogen, iron, and sulfur. Slow anaerobic decomposition of litter can lead to asynchrony between plant N demand and nitrogen availability in subsequent generations. This asynchrony gives rise to biomass oscillations as highly productive growing seasons are followed by several growing seasons of low productivity as nitrogen is immobilized in the productive year’s litter cohort for several years. Freshwater aquatic plant populations are increasingly exposed to elevated sulfate through seawater intrusion or increased anthropogenic sulfate loading. In anaerobic sediment, sulfate stimulates mineralization of litter, but produces sulfide, which limits plant nitrogen uptake. How will population cycles driven by nitrogen availability respond to increased sulfate loading? We studied this question using wild rice (Zizania palustris), an annual aquatic plant with well-described oscillations and sensitivity to sulfide. Using self-sustaining mesocosms, we investigate the relative importance of litter-controlled nitrogen availability and sulfide-inhibited uptake of nitrogen in population stability. We added iron to the sediment of some mesocosms (n = 20) to investigate whether iron sulfide precipitation might mitigate sulfide’s phytotoxic effects. The populations were grown in surface water with elevated sulfate (300 mg/L, n=20) or ambient sulfate (8 mg/L, n=20).

Results/Conclusions

Populations of wild rice exposed to our elevated sulfate concentrations declined to extinction in six years or fewer. Populations exposed to ambient sulfate concentrations oscillated with peaks in biomass three years apart, corresponding to patterns observed in local wild rice stands. Sulfate exerted a stronger control over the population trajectory than litter-driven oscillations in nitrogen availability did. Iron did not strongly affect population trajectories. Given that a geochemistry plays an important role in determining population dynamics, how can we know whether a population is cycling stably or being driven towards extinction by unsuitable water or sediment chemistry? We evaluated the slope of change in biomass over biomass which is analogous to an eigenvalue (of an undefined linear model describing our data) and can be interpreted for stability of population oscillations after just a few generations. As expected, this analysis indicates that our elevated sulfate conditions were unstable while our low ambient sulfate conditions had stable population oscillations. This type of analysis may be especially useful for predicting population stability in ecosystems where controls are poorly understood or that are undergoing gradual changes in sulfate or other environmental stressors.