Okanagan Deep Lake Biology Study

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District of Lake Country

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The primary goal of this research was to help water suppliers determine the optimal depth for an intake and to identify biological factors that would affect water treatment. Deeper intakes avoid suspended sediments, detritus and larger bacteria counts seen in intakes shallower than 16 m, but they must be located at least 3 meters above the sediments to realize that benefit. Based on cyanobacteria numbers, the best depths for an intake are 30-40 m in the south sub-basin, 40-45 m in the central sub-basin and 45-50 m in the north sub-basin. These results follow the nutrient and turbulence gradients typical in the three sub-basins. The optimal depth for an intake will also depend on site-specific variables such as stormwater and creek outfall plumes. The second goal of this research was to assess the risk of exposure to chronic low doses of cyanotoxins in drinking water drawn from Okanagan Lake. While there were three hazardous surface blooms on Okanagan Lake with dangerously high cell counts during the last 40 years, these events are rare and do not affect the entire lake. However, cell counts near the 2000 cells/mL threshold were encountered regularly at many intakes. Thirteen cyanotoxin-producing species were identified in this study. Different species inhabit the deep water (e.g. Planktothrix / Lyngbya) than the ones that are capable of forming a surface bloom (e.g. Anabaena / Microcystis / Aphanizomenon). Each species produces a range of toxins, but not every strain of that species tests positive for toxins and the toxins are not produced all the time by toxic strains. The deep Okanagan Lake samples were dominated by Planktothrix, a cyanobacteria that always produces microcystin and therefore always presents the possibility of chronic low dose exposure to microcystin. The risk these cyanotoxins present to Okanagan Lake drinking water is dependent on dose and toxin mixture. Chronic low dose exposure to cyanotoxins that can be produced by cyanobacteria found in Okanagan Lake can cause a range of diverse effects including skin irritation, inflammatory response, liver and kidney damage, DNA damage, tumor promotion and cancer, nervous system disruption and promotion of neurological degenerative diseases. The most vulnerable people are the immune-compromised, children and the elderly. The cyanobacterial numbers found in this study correspond to a very low risk of acute toxicity and a low risk of chronic low dose toxicity.

The final goal of this study was to review the state of knowledge on, analysis of, and water treatment for cyanotoxins. Analysis and monitoring of cyanotoxins using a decision tree prepared by the World Health Organization was used in this study. It relies on visual screening and algae counts, progressing through to field screening tests and laboratory analyses. Cyanotoxins confined in cells can be treated using a combination of filtration, coagulation and dissolved air floatation. However, any effect that breaks cells open, such as copper sulphate treatment of water source, hydraulic pressure during filtration or boiling the water, makes effective treatment more difficult and increases potential toxicity. Chlorination can degrade microcystin under specific pH conditions. Natural biodegradation of the toxins within a lake or on a filter is a relatively slow process. The risk of cyanotoxicity in Okanagan Lake water can be minimized by strategically locating intakes.

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