Streams and rivers metabolize dissolved organic matter (DOM). Although most DOM compounds originate from natural sources, recreational use of rivers increasingly introduces chemically distinct anthropogenic DOM. So far, the ecological impact of this DOM source is not well understood. Here, we show that a large music festival held adjacent to the Traisen River in Austria increased the river's dissolved organic carbon (DOC) concentration from 1.6 to 2.1 mg L-1 and stream ecosystem respiration from -3.2 to -4.5 mg L-1. The DOC increase was not detected by sensors continuously logging absorbance spectra, thereby challenging their applicability for monitoring. However, the fluorescence intensity doubled during the festival. Using parallel factor analysis, we were able to assign the increase in fluorescence intensity to the chemically stable UV-B filter phenylbenzimidazole sulfonic acid, indicating organic compounds in sunscreen and other personal care products as sources of elevated DOC. This observation was confirmed by liquid chromatography coupled with mass spectrometry. The elevated respiration is probably fueled by anthropogenic DOM contained in beer and/or urine. We conclude that intense recreational use of running waters transiently increases the anthropogenic DOM load into stream ecosystems and alters the fluvial metabolism. We further propose that chemically distinct, manmade DOM extends the natural range of DOM decomposition rates in fluvial ecosystems.
