January 30, 2017

Lake Winnipeg Nutrient Loading Model

Lake Winnipeg Nutrient Loading Model

This nutrient loading model was developed at the Freshwater Institute, Department of Fisheries and Oceans Canada, to estimate the impacts of various nutrient management strategies on Lake Winnipeg.

Authors: S.J. Page, R. Hesslein, G. McCullough, M. Stainton

Model Description

Various agencies have been collecting hydrometric data on or around Lake Winnipeg since the early 1910s. Sampling for water chemistry did not commence until 1969, and only in a select few tributaries. Since 1999, a more thorough sampling program has been initiated, providing current data for model calibration.

We employ a STELLA based nutrient loading model which simulates lake nutrient concentrations (carbon, nitrogen and phosphorus) from 1913 to 2006 based on historical river inputs. In addition, we then use the same model to predict future lake concentrations based on various flow regimes as well as to estimate the impact of various proposed nutrient management strategies.

The model estimates all sources of nutrient inputs based on calculated loading from nutrient concentration versus discharge versus time analysis. Because we know the discharge data from each of the inputs (km3/month) and have generated modeled concentrations of the nutrients in each input (ug/L), we are able to ascertain an estimate of the monthly loading (tonnes) to the Lake.

Users can run the model using three different scenarios.

  1. Run under historical flows and concentrations.
  2. Run simulation as if all of the loading came in as 20ug/L water (Precipitation scenario)
  3. Run simulation as if all of the loading came in as 400ug/L water (Red River only scenario)

Annual phosphorous sedimentation rate (%) can also be adjusted.

As each of the mitigation strategies deals with a reduction in loading of nutrients to Lake Winnipeg, the model provides insights into the effectiveness of each proposed approach.

Conclusions

While there is no doubt that there has been an increase anthropogenic impacts on Lake Winnipeg in recent decades, we conclude that the dominant factor driving recent increases in frequency and size of cyanophyte blooms has been the increase in flow of the phosphorus rich waters of the Red River relative to other Lake Winnipeg tributaries.

Lowering Lake Winnipeg phosphorus concentration may be difficult. As we implement our management strategies, we must take into consideration that some areas of the watershed have a larger impact than others, and focus our efforts accordingly. But if these recently observed climatic and human-induced changes do in fact continue, adaptation may become just as important as mitigation.