Predictive Model

The Nutrient Limitation model is based on the results from the Nutrient Limitation Component of the Maryland Long-term Biological Monitoring Program. Bioassay results (n = 322) from 1990-1996 from Maryland (Tom Fisher and Anne Gustafson, Horn Point Environmental Laboratory) and Virginia (Larry Haas, Virginia Institute for Marine Science) were used for design and optimization of the model; bioassay results from 1997-1999 (n = 130) were used to validate the model. The model uses the CATMOD procedure in SAS to distinguish between nitrogen limitation, phosphorus limitation and resource saturation simultaneously (Tom Fisher and Elgin Perry, in preparation).

The Nutrient Limitation Model uses five water quality parameters: log(DIN), log(PO4), log(TN/TP), water temperature and salinity. Prediction is based on which category (nitrogen limited, phosphorus limitation, or nutrient saturation) is determined to have the highest probability. Overall accuracy of predictions (when compared to actual bioassays) is 89%; by category, accuracy for nitrogen limitation is
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94%, phosphorus limitation is 73% and nutrient saturation is 54%. More extensive documentation of the model development and application will be available in Fisher & Perry, in preparation.

Managers can use the nutrient limitation model to predict which nutrient is limiting at a given time and location and use the information to assess what management approach might be the most effective for controlling excess phytoplankton growth. Interpreting the results can be a little counter-intuitive, however. Remember that nitrogen limited means that phosphorus is in excess. Initially, it would seem that the best management strategy would be to reduce phosphorus inputs. However, it may actually be more cost effective to further reduce nitrogen inputs to increase the amount of ‘unbalance’ in the relative proportions of nutrients so that phytoplankton growth is even more limited. The nutrient limitation predictions, then, must be used along with other information available from the water quality and watershed management programs to fully assess and evaluate the best management approach. The nutrient limitation predictions are an extremely valuable tool, however, for making these assessments.

For more information, please contact Renee Karrh
at (410) 260-8628.

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