Metabolism in the surface waters of north temperate lakes

Metabolism is a fundamental process of lake ecosystems. The balance between gross primary production (GPP) and respiration (R) is an index of lake trophic status and relates to the role lakes play in processing landscape carbon. I examined metabolism in north temperate lakes to determine its response to drivers, such as phosphorus (P) and dissolved organic carbon (DOC). I combined comparative studies, modeling, and analysis of long-term trends to address the following ecosystem questions. (1) How do P and DOC drive GPP and R in lakes? (2) What role do lakes play in processing landscape carbon? (3) What do differences between bottle and sonde estimates of metabolism tell us about littoral versus pelagic metabolism? (4) Over decadal time periods, what drives lake dissolved inorganic carbon (DIC), and what are the implications for long-term atmospheric flux of carbon from lakes to the atmosphere?; Both TP and DOC were important drivers of lake metabolism. At low DOC concentrations, both GPP and R were nearly in balance and directly related with TP. At moderate to high TP concentrations, GPP exceeded R. When lake DOC concentration rose above 10 mg L-1, R generally exceeded GPP. Most lakes had negative net ecosystem production (NEP = GPP-R). Lakes were conduits for terrigenous inorganic carbon and mineralization sites for terrigenous organic carbon. Flux of terrigenous carbon to the atmosphere through lakes accounted for a significant proportion of terrestrial net ecosystem production. Allochthonous carbon loading resulted in most lakes being net heterotrophic and net sources of carbon to the atmosphere. The method used for measuring lake metabolism was important for the interpretation of results. Bottle estimates of metabolism were consistently lower than sonde estimates, possibly due to the exclusion of littoral benthic metabolic signals from bottles. Reductions in acid deposition explained decadal increases in lake acid neutralizing capacity and DIC. The concentrations of carbon dioxide (CO2) dissolved in surface waters during summers were consistent with expectations based on lake productivity. However, interannual variations in CO2 concentrations and flux to the atmosphere were explained best by changes in acid deposition.