Use of the Morphoedaphic Index to Predict Nutrient Status and Algal Biomass in Some Canadian Lakes

The underlying assumptions of the morphoedaphic index (MEI; total dissolved solids (TDS)/mean depth), which has been used to predict fish yields in lakes, and its power for predicting biomass at lower trophic levels were examined using published data. The assumptions included the relationship between mean depth of lakes and various hydrologic characteristics (flushing rate and stratification regime), water transparency characteristics (water color and turbidity), and the stoichiometric relationship among ions (expressed as a proportion between TDS and the concentration of primary nutrients, total phosphorus (TP) and total nitrogen (TN)). Although these basic assumptions could be supported empirically, the predictive power of the MEI became progressively weakened with increasing trophic level. MEI accounted for up to 85% of the variation in TP and TN, less than 50% of the variation in [Chl a], and none of the variation in the biomass of herbivorous zooplankton. The functions relating TDS to both TP and TN were fundamentally different: as lakes increased in salinity, the TN:TP ratio decreased dramatically so that TP almost exceeded TN concentrations in extremely saline lakes. This necessitated the development of separate MEI–nutrient relationships for saline (TDS > 1000 mg/L) and nonsaline lakes.