Shifting limitation of primary production: experimental support for a new model in lake ecosystems

The limits on primary production vary in complex ways across space and time. Strong tests of clear conceptual models have been instrumental in understanding these patterns in both terrestrial and aquatic ecosystems. Here we present the first experimental test of a new model describing how shifts from nutrient to light limitation control primary productivity in lake ecosystems as hydrologic inputs of nutrients and organic matter vary. We found support for two key predictions of the model: that gross primary production (GPP) follows a hump-shaped relationship with increasing dissolved organic carbon (DOC) concentrations; and that the maximum GPP, and the critical DOC concentration at which the hump occurs, are determined by the stoichiometry and chromophoricity of the hydrologic inputs. Our results advance fundamental understanding of the limits on aquatic primary production, and have important applications given ongoing anthropogenic alterations of the nutrient and organic matter inputs to surface waters.

Data and R code have been made available to conduct statistical analyses and generate Figures 2-4. Figures 2 & 4 use the same data file titled 'compiledNutrientLightData.csv'. R code titled 'NutrientLightData_Analyses.R' and 'nutrientLimitationAnalysis.R' generate Figure 2 and Figure 4, respectively. Files for Figure 3 are titled 'gppData.csv' and 'gppAnalyses.R'. Save files and set working directory in R code to where data files are saved.