Lake eutrophication continues to be a major concern in many lake regions, but long-term monitoring data are often lacking. Therefore, indirect proxy methods must be used to infer these missing data sets. Two methods were applied to infer pre-industrial and present-day lakewater total phosphorus concentrations (TP) in a suite of 50 hardwater lakes in southern Ontario (Canada). One method inferred TP from the diatom species composition in the tops (present-day inferences) and bottoms (pre-1850 inferences) of sediment cores. The other method applied the Lakeshore Capacity Model (LCM), which is a mass-balance model based on phosphorus export coefficients that relate lakes and their watershed characteristics to epilimnetic nutrient concentrations. Diatom-based estimates of preindustrial to present-day change show that 78% of the lakes increased in TP (29% significantly) and 8% decreased. According to model error, 63% of the lakes have not significantly changed. LCM estimates show that 56% of the lakes have increased in TP, and the remainder (44%) have not changed. The average inferred increase in TP was similar for both models, but a lake-by-lake comparison indicated marked differences in model output. In particular, a paired comparison of diatom-based and LCM-based inferences of preindustrial TP shows no correlation. It is suggested that lake managers be thorough when collecting data for either model, and model selection should be carefully considered. The LCM and diatom-based models perform better in regions that are geologically similar to where the respective models were calibrated. Advantages and disadvantages of each model are further discussed.
Bibliographical noteFunding Information:
We thank Joanne Little, Saloni Clerk and Rideau Lakes Authority personnel for help with field work. This work was funded by a Natural Sciences and Engineering Research Council strategic grant to J.P.S. Andy Gemza provided some total phosphorus and Secchi depth data as part of the Ministry of Environment’s Southern Ontario Lake Partner Program (Dip-In project). Don Galloway (Ontario Ministry of Environment and Energy, Kingston office) helped with collection of background data for our study lakes. Personnel at the Ontario Ministry of Environment (Etobicoke, Ontario) performed some of the water chemistry analyses. Martyn Futter and Bev Clark of the Ministry of Environment and Energy helped with the development and application of the Lakeshore Capacity Model software. An anonymous referee provided helpful comments on the manuscript. We dedicate this paper to the memory of Don Galloway.
- Lakeshore capacity