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Climate change and differing lake characteristic's impact on cyanobacteria bloom formation in oligotrophic systems

Climate change and differing lake characteristic's impact on cyanobacteria bloom formation in oligotrophic systems

Cyanobacteria bloom observed in Mingo Lake, Nova Scotia

Cyanobacteria, commonly referred to as Blue-Green Algae, are a ubiquitous photosynthetic bacteria that under the right conditions – high light, warm temperatures, high nutrient concentrations, high thermal stability, low surficial sediment redox– cyanobacteria are able to rapidly proliferate, forming dense populations or blooms (often creating dense mats on the water surface). Freshwater blooms are detrimental to environmental and human health due to the production of liver and nerve toxins and unpleasant taste and odour compounds by some strains, the build-up of surface scum, and their contribution to sediment anoxia which is harmful to multicellular life. Cyanobacteria blooms, a global concern, are increasing in duration, frequency, and extent which lowers the recreational, aesthetic, industrial, residential, ecological value, and overall health of waterbodies and increases the urgency to improve existing management tools.

The majority of cyanobacteria research has focused on eutrophic (nutrient rich) and hypereutrophic (excessively enriched) waterbodies because the risk of bloom formation is strongly correlated with high concentrations of phosphorus (P). As a result, cyanobacteria management has focused on reducing point (PS) and non-point source (NPS) P loading into aquatic systems. Reducing PS P loading has been effective in systems with relatively large PS loads but has been less effective in systems with relatively large NPS loads, for example, Lake Erie. Controlling NPS P loading has been problematic, but efforts are underway in many jurisdictions.

Cyanobacteria blooms are also increasing in oligotrophic (nutrient poor) systems where lowering nutrient inputs is not a feasible management strategy. It is clear that in oligotrophic systems with blooms, the driver is not anthropogenic P loading. It has been hypothesized that increasing incidence of blooms in oligotrophic and eutrophic waters in recent decades is related to climate change through increased thermal stability and longer growing seasons. Understanding the cause of blooms in oligotrophic waters is an emerging area of research which should complement existing research on eutrophic waters: the current P paradigm, while having achieved much success, has been unable to predict the timing and frequency of blooms and needs modification. Recent work by Canadian scientists has identified a driver for blooms across a P concentration gradient that has been overlooked: the development of anaerobic sediments with very low redox potential caused by depletion of dissolved oxygen and nitrate. Anaerobic conditions in sediments releases critical nutrients, especially orthophosphate and ferrous iron (Fe2+) into the sediment-water interface (boundary layer between sediment and water column). Buoyant species of cyanobacteria are able to vertically migrate into, or adjacent to, these anoxic layers securing the nutrients needed to support population growth. There is some evidence, albeit limited, that the procurement of Fe2+ is critical for cyanobacteria bloom onset, allowing a shift in the dominant phytoplankton species in the system.

Thermal stratification of waterbodies is believed to be a response to climate change. Rising temperatures and decreasing windspeeds (atmospheric stilling) – the result of global warming – are increasing surface water temperatures faster than bottom waters, allowing for more frequent episodes of high thermal stability in shallow waters and prolonged thermal stratification of deeper lakes. In turn, this increases the risk of anaerobic sediments and internal nutrient loading occurring. However, the migratory ability of cyanobacteria is variable, with long travel distances to secure Fe2+ likely to hinder the benefits.

Rebecca Gasman

Rebecca Gasman’s proposed PhD research will focus on three topics. First, paleo-limnological techniques will be used to determine how environmental change has affected long-term cyanobacteria bloom formation in oligotrophic systems in Ontario study sites. Most lakes in Ontario do not have consistent long-term data available, so established paleolimnological techniques will be used to reconstruct cyanobacteria abundance, climate, dissolved oxygen, and P data for the lakes chosen. Reconstruction of nitrate is not possible, but most lakes do not have oxidizing levels of nitrate so dissolved oxygen can be used as a proxy for sediment redox. Sediment cores will be analyzed for chlorophyll-a, a pigment produced by cyanobacteria and other algae, and phycocyanin, a pigment specific to cyanobacteria, to infer primary productivity and cyanobacteria abundance in oligotrophic lakes over time. Two groups of insects will be used to infer dissolved oxygen levels and thermal stability will be inferred by changes in the relative abundance of diatom species. Second, permission to use traditional ecological knowledge (TEK)/Indigenous Knowledge will be sought to complement the scientific knowledge in the study sites. Local and Indigenous communities will be interviewed. To these will be added observational data from local health units, provincial and federal governments where possible. Third, shallow near-shore bays and deep offshore waters in oligotrophic systems will be assessed for differences in thermal stability, anoxia and bloom formation using high frequency data loggers during the ice-free period. Anoxia is longer lasting in deep waters, but these waters may be too deep for migratory cyanobacteria to reach, hence, shallow protected bays experiencing anoxic episodes during warm periods with low winds are more likely to provide the necessary conditions for bloom formation. Knowing the location of ‘nurseries will allow control strategies to be more focused, less costly and more effective.

This PhD will use a combination of methods – paleolimnological, community interviews, and modern-day sampling techniques – to better understand the formation of cyanobacteria blooms in oligotrophic systems. This combination of methods will position the research in a novel interdisciplinary framework, whereby the range of factors that contribute to cyanobacteria blooms can be better understood. The research area will be the Kawartha and Muskoka regions in Ontario, as the southern portion of the province has recorded an increase in cyanobacteria blooms in oligotrophic systems over the last several decades. As previously mentioned, current cyanobacteria management strategies are not applicable to blooms in nutrient poor systems due to their focus being on nutrient reductions. This research will therefore provide a more holistic understanding of bloom formation to aid in the development of in-situ management strategies for blooms, which is currently lacking in Canadian bloom management.

Rebecca Gasman is a PhD student in Physical Geography and a recent MA graduate in Human Geography from the Faculty of Environmental and Urban Change. Rebecca’s MA research “A pan-Canadian comparison of cyanobacteria bloom management policies, programs, and practices” compared provincial cyanobacteria (blue-green algae) bloom management strategies across five provinces – Alberta, Manitoba, Nova Scotia, Ontario, Saskatchewan – in Canada. Her master’s thesis work was co-supervised by Profs. Lewis Molot and Daniel Walters and her PhD dissertation is supervised by Prof. Jennifer Korosi.