The role of positive abiotic-biotic feedbacks in increasing resilience of alternative ecosystem states in a planktonic system
Interactions between organisms and feedbacks with their environment can lead to alternative ecosystem states in a variety of habitats. There is a need to identify the interactions that lead to the persistence of one state over another so trajectories of these systems can be predicted. We investigate how a positive feedback between herbivores and the physical environment increases the resilience of an alternative ecosystem state in a planktonic system. Alternative ecosystem states emerged unexpectedly during a mesocosm experiment designed to explore freshwater zooplankton interactions. The two alternative ecosystem states observed were the classic clear water with lower pH and lower phytoplankton versus turbid water with higher pH and higher phytoplankton abundance. Previous laboratory experiments have demonstrated that the pH levels we observed in the turbid state (9.5-11) can negatively affect Daphnia species. We hypothesized that high pH, elevated by increased photosynthetic activity at increased phytoplankton density, would have strong negative effects on Daphnia pulex, and that reduced grazing by D. pulex would maintain the turbid state of higher pH and higher phytoplankton.
High pH (>9.5) in mesocosms had a strong negative relationship with D. pulex, the major grazer in the system. Phytoplankton abundance and pH remained high for the six-week experiment. Moreover, phytoplankton abundance increased over time in mesocosms with high pH while D. pulex densities decreased. A laboratory experiment confirmed the phytoplankton in the tanks was a suitable resource for D. pulex, leaving only the effect of water chemistry to explain the reduction in D. pulex abundance. These results suggest the positive feedback of photosynthetically-elevated pH led to a reduction in top-down control of phytoplankton and increased the resilience of the alternative ecosystem state. Abiotic-biotic interactions are routinely considered in ecology, and this work contributes to the growing understanding of the role of positive feedback mechanisms in the maintenance of ecosystem states. The alternative ecosystem states in our experiment are analogous to the alternative equilibria in shallow lakes observed by Scheffer et al. (1993), warranting further investigation into the pH-phytoplankton-zooplankton interaction as a potentially important mechanism for regulating plankton populations in ponds and small lakes where higher pH levels can be common. Future work should investigate the stability of the alternative states to perturbations, and investigate this mechanism in natural settings.