Environmental Sciences Research Institute

Investigating the relationship between lake morphology and lake trophic state

Richard Douglas, Brian Rippey and Michelle Watson

It is important to use the correct properties when developing a model for the characteristic that describe the trophic state of lakes, the total phosphorus (TP), chlorophyll a (Chl) and suspended solids (SS) concentration. It is widely accepted that the internal shape of a lake, its morphology, affects its trophic state; for example, larger, deeper lakes have lower TP, Chl and SS concentrations than smaller and shallower ones. However, as the morphological characteristics of a lake (e.g. mean depth, maximum depth, volume, dynamic ratio, etc.) are almost always intercorrelated and also correlated with the other characteristics that influence trophic state (e.g. catchment area, hydraulic residence time, alkalinity, etc.), it is very difficult to decide what are the properties that determine the trophic state of lakes, the ones that have a physicochemical basis for controlling the TP, Chl and SS concentrations. Through intensive study of twenty-eight carefully selected lakes, this project identified that maximum water depth is the best way to incorporate lake morphology into models for these three physicochemical properties of lakes, at least for lakes with a mean depth less than 30-50 m. The performance of the model for Chl that uses only lake maximum depth as a predictor variable with an external data set is shown in Fig. X.


Fig. X. The relationship between observed and predicted annual mean chlorophyll a (Chla) concentration in 118 lakes.