Methods

About the Data

Data for this project was obtained from STRATOGEM (CTD and ferry data) and the UBC Geography department (BC Map).  CTD (Conductivity, Temperature, and Depth Sensors) and ferry data gave nitrogen, phosphorus, and silica concentrations, with latitude and longitude coordinates, and have an instrumental accuracy of about 10-15%.  The data analyzed was from June 2003, because the Strait of Georgia is likely nutrient limited, and because June had the greatest number of sampling times throughout the month.  Ferry data was only available at one depth, while the CTD data from 0, 5 and 10m for each location were averaged into to one measurement.  Latitude and longitude coordinates from CTD and ferry data and were added as an excel file into ArcMap, displayed using NAD 1983 Geographic Coordinate System, and then projected into the Albers Projected Coordinate System.  Albers was chosen as the projection for this data because it is an equal area projection, and because BC data is readily available in this format.

From the nitrogen, phosphorus, silica, and chlorophyll a (Chl-a) data, a linear Kriging interpolation was used to generate maps of these concentrations in the Strait of Georgia. Nitrogen, phosphorus, and silica concentrations were then normalized to a common scale of 0 to 1 using a linear Fuzzy Membership function.  The point of maximum concentration for each nutrient was given a value of 1, while point the minimum concentration was given a value of 0. All intermediate values were scaled linearly in between.  A linear relation was assumed because under biomass limiting conditions, increasing the amount of a limiting nutrient would increase the primary production by a proportional amount.

These normalized surfaces were used in the multi-criteria evaluation.  A weighted sum was used to predict the relative magnitudes of Chl-a throughout the Strait of Georgia.  To simulate nitrogen limited conditions, weights of 0.8, 0.1, and 0.1 were chosen for nitrogen, phosphorus and silica, respectively. For phosphorus limited, phosphorus had a weight of 0.8, and for silica limited, silica had a weight of 0.8.  Since a nutrient co-limited regime, all three nutrients would be equally important, a map using equal weights (0.333) for all nutrients was generated.