Filstrup, C.T., T. Wagner, P.A. Soranno, E.H. Stanley, C.A. Stow, K.E. Webster, and J.A. Downing. 2014. Regional variability among nonlinear chlorophyll-phosphorus relationships in lakes. Limnology and Oceanography 59:1691-1703.
The relationship between chlorophyll a and total phosphorus (TP) is a fundamental relationship in lakes that reflects several aspects of ecosystem function and is also used in the regulation and management of inland waters. The exact form of this relationship has substantial impacts on its meaning and its use. We assembled a spatially extensive dataset to examine whether nonlinear models are a better fit for chlorophyll a-TP relationships than traditional log-linear models, whether there were regional differences in the form of the relationships, and if so, which regional factors were related to these differences. We analyzed a dataset from 2,105 temperate lakes across 35 ecoregions by fitting and comparing three different nonlinear models and one log-linear model. The three nonlinear models fit the data better than the log-linear model. In addition, the parameters for the best fitting model varied among regions: the maximum and lower chlorophyll asymptotes were positively and negatively related to percent regional pasture agricultural land use, respectively, and the rate at which chlorophyll increases with TP was negatively related to percent regional wetland cover. Lakes in regions with more pasture agricultural land use had higher maximum chlorophyll concentrations at high TP concentrations, but lower minimum chlorophyll concentrations at low TP concentrations. Lakes in regions with less wetland cover showed a steeper chlorophyll-TP relationship compared to wetland-rich regions. Our findings suggest that interpretation of chlorophyll-TP relationships depends upon regional differences and that theory and management based on a monolithic relationship would be inaccurate.