Synthesis of Water Quality Data and Modeling Non-Point Loading in Four Coastal B.C. Watersheds: Implications for Lake and Watershed Health and Management

Date

2015-12-24

Authors

Rodgers, Lisa

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Abstract

I compared and contrasted nitrogen and phosphorus concentrations and land use differences in two oligotrophic lakes (Sooke and Shawnigan) and two meso-eutrophic lakes (St. Mary and Elk) in order to evaluate nutrient concentrations over time, and evaluate the relationship between in-lake nutrients and land use in the surrounding watershed. I used MapShed© nutrient transport modeling software to estimate the mass load of phosphorus and nitrogen to each lake, and evaluated the feasibility of land use modifications for reducing in-lake nutrients. In comparing nitrogen and phosphorus data in Sooke and Shawnigan Lakes, I determined that natural watershed characteristics (i.e., precipitation, topography, and soils) did not account for the elevated nutrient concentrations in Shawnigan verses Sooke Lake. Natural watershed characteristics indicated that external loads into Shawnigan Lake would be lesser-than or equal to those into Sooke Lake if both watersheds were completely forested. I evaluated trends of in-lake nutrient concentrations for Sooke and Shawnigan Lakes, as well as two eutrophic lakes, St. Mary and Elk. Ten to 30-year trends indicate that nitrogen and phosphorus levels in these lakes have not changed significantly over time. Time-segmented data showed that nutrient trends are mostly in decline or are maintaining a steady-state. Most nutrient concentration data are not precipitation-dependent, and this, coupled with significant correlations to water temperature and dissolved oxygen, indicate that in-lake processes are the primary influence on lake nutrient concentrations -- not external loading. External loading was estimated using, MapShed©, a GIS-based watershed loading software program. Model validation results indicate that MapShed© could be used to determine the effect of external loading on lake water quality if accurate outflow volumes are available. Based on various land-cover scenarios, some reduction in external loading may be achieved through land-based restoration (e.g., reforestation), but the feasibility of restoration activities are limited by private property. Given that most of the causal loads were determined to be due to in-lake processes, land-based restoration may not be the most effective solution for reducing in-lake nitrogen and phosphorus concentrations.

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Keywords

nitrogen, phosphorus, lake, watershed, nutrients, water quality, nutrient transport modeling, British Columbia, algae bloom, eutrophic, oligotrophic, Geographic Information Systems

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