TROPICAL CYCLONES AS AN EMERGENT CONTRIBUTOR TO DEVELOPMENT OF HARMFUL ALGAL BLOOMS IN NORTHEASTERN NORTH AMERICA

Lake disturbances caused by extreme weather events (e.g. tropical cyclones (TC) can lead to post storm nutrient pulse events; derived from a combination of heavy rainfall related nutrient runoff from the catchment and large wave driven resuspension of lakebed nutrient-rich sediments. This can alter lake ecological conditions to favor cyanobacteria over other phytoplankton, and development of harmful cyanobacterial blooms (CyanoHABs). Although a mechanistic link between TC forcing and lake community shifts has been observed through in-situ monitoring, it has not yet been demonstrated in paleolimnological records.

In this study, we showcase sediment cores dated using ²¹⁰Pb and ¹³⁷Cs from two lakes (Harvey, Wheaton) in SW New Brunswick, Canada, a maritime region that has witnessed a recent rise in lake CyanoHABs. These cores have been analyzed to investigate connections between historical toxic bloom events and influencing factors. Multi-proxy sediment analyses (end member mixing analysis (EMMA) of grain-size distributions, ITRAX-XRF; stable isotopes: δ¹³C; δ¹⁵N) were employed to explore mechanistic links between drivers of lake change (e.g. climate variability, land use change, TC) and past episodes of CyanoHAB development. Cyanobacterial responses were investigated using taxonomically specific sedimentary pigments: aphanizophyll (N²-fixing), echinenone (total cyanobacteria), canthaxanthin and myxoxanthophyll. Microcystin congeners LA, LR, [Dha⁷] LR and RR were quantified using high-resolution tandem mass spectrometry (HRMS/MS).

Although decoupling the compounding drivers of lake cyanobacteria occurrence from sedimentary records is complex, initial EMMA, and ITRAX-XRF derived ratios (Br+Cl/Al) associated with precipitation and air masses displayed co-variance with productivity increases. However, δ¹⁵N, representing fossil nitrogen inputs, exhibited the most significant correlation with the productivity proxies, indicating that catchment mediated processes (e.g. agriculture, urbanization) had a strong influence on CyanoHAB development. A late 20^th century increase in baseline cyanobacteria concentrations is attributed to heightened nutrient inputs. This set the stage for significant CyanoHAB occurrences in 2015, directly attributable to lake fertilization triggered by passage of post-tropical storm Arthur in 2014.