Spatial and temporal variability in the recruitment of the cyanobacterium Gloeotrichia echinulata in an oligotrophic lake

Background/Question/Methods

Cyanobacteria have become more abundant in both high- and low-nutrient freshwater lakes worldwide, with adverse consequences for ecosystem functioning. Planktonic populations of cyanobacteria increase through recruitment from dormant stages in the benthos and division in the water column. In particular, recruitment from the benthos can provide a critically important inoculum for surface populations, including blooms. We investigated whether recruitment might account for recent increases in the large (1-3 mm) colonial cyanobacterium Gloeotrichia echinulata in low-nutrient lakes in northeastern North America. To do this, we quantified surface abundances and recruitment from littoral sediments at multiple near-shore sampling sites in oligotrophic Lake Sunapee, New Hampshire, USA, during the summers of 2005 through 2012. We used this dataset – the longest known continuous record of cyanobacterial recruitment – to test the hypothesis that climatic variables might be particularly important to inter-annual variability. Specifically, we investigated associations with ice-out date, which affects the length of the growing season; regional precipitation, which affects nutrient inputs and turbidity; air temperature, which drives water temperature; water temperature, which affects biological process rates, including recruitment; and lake stratification, which affects mixing and nutrient availability

Results/Conclusions

We found extensive variability in Gloeotrichia recruitment over space and time across the eight-year dataset. Spatially, recruitment was higher at some sites than others; within seasons, recruitment into replicate traps at the same site was generally more similar than recruitment at different sites. This suggests that local factors such as substrate quality or the size of the seed bank may be important controls on recruitment. Benthic recruitment generally contributed <4%, but up to ~8% of pelagic populations, within the range observed in previous studies but suggestive of a key role for growth within the water column in dictating bloom formation. Across years, higher seasonal recruitment rates were associated with greater lake mixing during August, including deeper thermoclines, lower Schmidt stability, lower minimum air temperatures, and greater variability in water temperature. Over the long term, a comprehensive understanding of both in-lake, microsite characteristics and regional-to-local influences on watersheds will be fundamentally important for predicting recruitment dynamics. Moreover, if we hope to anticipate and possibly even manage cyanobacterial populations, we will need to understand not only the key drivers of recruitment, but also the drivers of pelagic division.