Opening Pandora’s box: The potential role of Gloeotrichia echinulata in lake phosphorus cycling

Background/Question/Methods

Gloeotrichia echinulata is a nuisance cyanobacterium that is common in eutrophic systems, but has recently begun to bloom in economically important oligotrophic and mesotrophic lakes in northern New England. G. echinulata has a meroplanktonic life history that combines a sediment resting stage with a summer pelagic stage. In eutrophic lakes, recruitment from the sediments typically occurs synchronously across multiple sites and can transport large amounts of P into the water column (up to 66% of internal loading). We hypothesize that translocation of nutrients by recruiting colonies could be even more important in oligotrophic lakes, since there is typically little to no recycling of sediment P back to the water column in well-oxygenated systems. To test this hypothesis, we added biological recycling of sediment P to the simple one-box P cycling model created by Carpenter et al. (Ecological Applications 9:751-771) and examined the potential effects of this change on oligotrophic lake ecosystems. In addition, we have been collecting data to estimate the magnitude of P transport by G. echinulata in Lake Sunapee (NH) since 2005. Finally, in 2007 we conducted a microcosm experiment to estimate whether the transported P might be released into the water column by zooplankton grazing.
Results/Conclusions

Equilibrium analyses of the modified model indicate that if G. echinulata recycles a significant amount of sediment P in the absence of hypolimnetic anoxia, it could have a considerable impact on oligotrophic lakes. Most importantly, adding sediment P recycling at a low water column P concentration reduced the threshold P concentration at which a lake ecosystem had both oligotrophic and eutrophic states; this has considerable implications for lake management because it creates the potential for hysteretic dynamics much sooner than usual. To determine if these results may apply to real lakes, we need to know how much P is translocated by G. echinulata in oligotrophic systems, and whether this P is retained in the epilimnion. Our measurements of recruitment rates and G. echinulata P concentrations indicate that G. echinulata could have moved up to 0.01 μg P L^-1 d^-1 into the epilimnion during peak recruitment in Lake Sunapee in 2007. Moreover, at least some of this P likely stayed in the water column due to release by zooplankton grazers. If bloom duration and extent continue to increase exponentially, net P recycling could reach ecologically significant levels within the next few years, with potentially serious consequences for lake water quality.