At the air-water interface material, organisms accumulate and form a thin layer of organic and inorganic material called the surface microlayer (SML). In order to investigate the development, composition, and metabolism of SML on lakes, samples were collected using a screen sampler along with subsurface water (SSW) in an eutrophic and a mesotrophic lake from April to September 2007. Wind, solar irradiance, and lake temperature were followed continuously. Samples were analyzed for organic and inorganic compounds as well as for photosynthesis and respiration. Most compounds were enriched in the SML relative to the SSW. Enrichment was small, however, probably because sampling was performed on nonslick areas. Most compounds correlated closely between the SML and the SSW, confirming the hypothesis that most SML material originates from the bulk water. Correlations were strongest in the eutrophic lake, probably because external sources had a greater effect on SML concentrations in the mesotrophic lake. Enrichment of compounds and metabolic rates in the SML had similar seasonality and dependency of climatic conditions in the two lakes, suggesting common regulating mechanisms of enrichment and production. Enrichment factors of several compounds were higher at low bulk water concentrations, suggesting that atmospheric deposition then contributed relatively more to concentrations in the SML. Increasing temperature significantly decreased SML enrichment of TOC (total organic carbon), related to changes in TOC composition and higher heterotrophic activity, while wind and solar irradiance had no pronounced enrichment effect on any compound. Net photosynthesis was significantly lower in the SML, experiencing photoinhibition in one-third of the samples. In contrast, respiration was much elevated in the SML. Nonetheless, respiration in the SML never contributed by more than 0.3% of water column respiration, but the combination of enhanced degradation rates of organic carbon in the SML and strong interaction with water below suggests that the SML, nonetheless, may play an important role in degradation of refractory organic carbon. Combining these results, we found that the SML of nonslicked areas on lakes are enriched in organic and inorganic pools and constitute a strong heterotrophic environment, albeit of minor importance for whole lake pelagic metabolism.