Dissolved organic matter (DOM) is an important regulator of metabolism in aquatic ecosystems. The influx of DOM in oligotrophic, high alpine lakes during rain events has been shown to alter lake metabolism and increase net heterotrophy. DOM loading is expected to increase for high-elevation Sierra Nevada lakes as precipitation shifts from snow to rain. To explore the effects of DOM loading on ecosystem metabolism in high-elevation lakes, we characterize ecosystem metabolic response to DOM loading in both in situ incubation experiments and through whole lake measurements of dissolved oxygen made during a summer with unusually high levels precipitation. Experimental in situ incubations show net ecosystem production (NEP) decrease linearly as DOM increases, causing heterotrophy to grow stronger. Gross primary production (GPP) responded in a light-dependent manner, where GPP was stimulated across the DOM loading gradient during sunny, but not cloudy, weeks. ER was also stimulated and exceeded GPP across the entire DOM gradient, causing heterotrophy. ER was coupled to GPP in a light dependent manner, suggesting consumers continue to consume autochthonous carbon even when allochthonous carbon is present. Seasonal metabolism measurements support that DOM shifts a lake towards heterotrophy at the whole lake level, and a large rain event was sufficient to shift lake metabolism from autotrophy to heterotrophy for at least three weeks. Increased DOM loading due to climate induced changes in vegetation cover and the timing and magnitude of precipitation events could alter the amount of carbon cycled in these lakes.