Recent literature suggests that for many lakes and rivers, the respiratory breakdown of organic matter (R) exceeds production of organic matter by photosynthesis (gross primary production; GPP) within the water body. This metabolic balance (GPP less than R; heterotrophy ) implies that allochthonous organic matter supports a portion of the aquatic ecosystems respiration. Evidence that many lakes are heterotrophic comes from diverse approaches, and debate remains over the circumstances in which heterotrophy exists. The methods used to estimate GPP and R and the limited extent of lake types studied, especially with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are two reasons for differing conclusions. In this study, O2 and CO2 sondes were deployed during July and August, 2000 to measure diel gas dynamics in the surface waters of 25 lakes in the Northern Highland Lake district of Wisconsin and the Upper Peninsula of Michigan. The lakes were chosen to span wide and orthogonal ranges in DOC and TP concentrations. From these data, we calculated GPP, R and net ecosystem production (NEP=GPP-R). Over the broad range in TP and DOC among the lakes, diel CO2 and O2 changed on a near 1:1 molar ratio. Metabolism estimates from the two gases were comparable, except at high pH. Most lakes in our data set had -NEP, but GPP and R appeared to be controlled by different factors. TP correlated strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were nearly equal, but at higher DOC, GPP and R uncoupled and lakes had -NEP. Strong correlations between lake metabolism and landscape related variables suggest that allochthonous carbon influences lake metabolism. Sampling Frequency: Chemical parameters and physical properties sampled from 1 to 4 times during the summer. Time series data step is 30 minutes. Number of sites: Time series data for 25 lakes. Chemical and physical data from 31 lakes.