Water Stewardship Information Sources
| Citation | St Mary Lake Steering Committee. St Mary Lake Watershed Management Plan. |
|---|---|
| Organization | St Mary Lake Steering Committee |
| URL | http://www.cusheonlakestewardship.com/pdf/ssstmarylkwtrshedmgntmplan-1.pdf |
| Abstract/Description or Keywords | The purpose of this plan is to provide goals, objectives, strategies and proposed actions that will restore and protect the quality of surface waters within the St. Mary Lake watershed. The overall goal of this plan is to restore and protect the sources of water in the St. Mary Lake watershed so that algal blooms will be reduced, St. Mary Lake will provide potable water after reasonable treatment, and fish and wildlife habitat will be improved. 2) The St. Mary watershed is in the north part of Salt Spring Island. While St. Mary is the largest lake on Salt Spring, with a surface area of 182 hectares, the watershed itself is relatively small, with an area of 690 hectares (including the lake). 3) Recent mapping by the Islands Trust has established that there are currently about 210 residences on properties fully within the St. Mary Lake watershed, and about 32 more on properties partially within the watershed. There are also a number of commercial lakeshore resorts and several farms. (Note: These numbers are to be reviewed by Trust staff in light of the ground truthing done for the preparation of this management plan.) 4) With phosphorus concentrations of more than 20 parts per billion (ppb) most of the time, St. Mary Lake is considered eutrophic (highly enriched), and has experienced both algal and cyanobacterial blooms. 5) Phosphorus is the key (limiting) nutrient controlling the amount of algal growth in St. Mary Lake. Recent annual loading of phosphorus to the lake is estimated at 985 kilograms including both external and internal sources. Regeneration from bottom sediments is the largest source of phosphorus, with an input of 63%. Septic fields around the lake contribute another 29%. The various sources are listed below: Regeneration from bottom sediments 63 % 625 kg/yr Domestic sewage (septic fields) around lake 29 % 290 kg/yr Land runoff 4 % 40 kg/yr Rain and dust fall to surface of lake 2 % 20 kg/yr Groundwater entering lake 1 % 10 kg/yr Total loading to the water of the lake 100 % 985 kg/yr 6) To achieve its overall goal, the plan has 8 main objectives: 1.Define and map St. Mary Lake’s watershed, its land uses, creeks, wetlands, the status of riparian vegetation and drainage ditches. 2.Continue to monitor the lake for phosphorus levels and sources as improvements are made. Subject this data to scientific analysis as required. 3.Reduce loading from sediment through aeration. 4.Reduce phosphorus inputs from domestic sewage. 5.Reduce phosphorus runoff from built-up areas. 6.Reduce phosphorus inputs from other land runoff. 7.Continue to assess target levels for phosphorus reduction. 8.Encourage stewardship activities and increased protection by local and provincial government. 7) Specifically, the committee recommends an objective of phosphorus levels in the lower part of the 10-15 parts per billion range in the late autumn lake. Reducing current phosphorus loading to about 285 kilograms a year might put the lake in the range of this objective. If aeration can achieve an 80% removal of the phosphorus entering the lake from internal regeneration, a further 67% reduction of nutrient loading from domestic sewage might be required to meet this target. To reach this target, this management plan recommends an empirical approach for applying remedies to the lake, based on the data on phosphorus sources shown above. As each remedial measure comes into play, the lake’s phosphorus and its algal populations should be measured on a continuing basis. This will show whether the remedy has been effective and whether the expected improvement has taken place. These direct observations on improvements will provide immediate, empirical feedback and guidance on whatever additional actions are needed. 9) In addition to endorsing the recommendations to reduce phosphorus inputs from land management in the Cusheon Watershed Management Plan, the St. Mary Plan endorses the many changes in the revised Official Community Plan that strengthen protection of watersheds and riparian areas. 10) Responsibility for coordinating and evaluating the program should reside with local government, water districts and a strong and proactive St. Mary Lake Stewardship Committee. A simple, defined test program needs to be set up to monitor improvement of the lake over future years. The main components should be yearly measurements of phosphorus just after the fall overturn and of oxygen in deep waters during the summer. |
| Information Type | report |
| Regional Watershed | Vancouver Island South |
| Sub-watershed if known | Salt Spring Island |
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| Project status | complete |
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