Cyanobacteria are increasing in freshwater lakes worldwide, with adverse consequences for ecosystem function. These increases are expected to continue with ongoing warming, since cyanobacteria typically dominate pelagic phytoplankton communities in deep temperate lakes under warmer, more strongly stratified conditions. However, these forecasts ignore the fact that most bloom-forming taxa overwinter on or near the sediments, and thus must recruit from the sediments into the water column to initiate new blooms. Moreover, many climate change scenarios for mesic north temperate regions predict an increase in high-intensity storms, which will increase episodic water column mixing during otherwise thermally-stable summers. We seek to address these gaps by building a robust understanding of how stratification and mixing affect cyanobacterial populations across the complete life cycle using field observations and modeling. Our long-term data on recruitment of benthic life stages of Gloeotrichia echinulata into the pelagic zone in Lake Sunapee, NH, provide information about intra- and inter-annual responses to stratification, mixing, and other environmental factors. Using these data and literature values, we are developing a simulation model that tracks population dynamics in three key life history stages - dormant benthic, germinated benthic, and active pelagic – in response to environmental conditions, including stratification and mixing.
Contrary to previously published findings for pelagic life stages, we found that mixing is positively associated with recruitment of benthic life stages of Gloeotrichia into the water column of Lake Sunapee; light and water temperature are also important determinants of the timing of recruitment events within a summer. Increased recruitment with greater mixing has also been observed in laboratory experiments and other field studies with Gloeotrichia several other common cyanobacterial taxa, including Dolichospermum and Microcystis. Thus, while episodic mixing adversely affects short-term pelagic survival and population density by causing premature loss of active colonies from the water column, mixing can also trigger the return of benthic cyanobacteria to the water column. To evaluate the net effects of these contrasting effects on pelagic vs. benthic life stages, we are conducting simulation studies in which pelagic survival and benthic recruitment are linked to the occurrence of episodic mixing events, across a gradient of increasing importance of benthic recruitment to pelagic population sizes. Modeling results to date suggest that both benthic life stages and episodic mixing events need to be considered when forecasting how cyanobacteria will respond to global climate change, especially when recruitment is key to initiating blooms.