Understanding natural controls on N and C biogeochemical cycles is important to estimate human impacts on these cycles. This study examined the spatiotemporal relationships between time series of weekly monitored stream and groundwater N and C (assessed by NO 3 − and dissolved organic C [DOC]) in the forested Wüstebach catchment (Germany). In addition to traditional correlation analysis, we applied wavelet transform coherence (WTC) analysis to study variations in the correlation and lag time between the N and C time series for different time scales. Median transit times were used to connect hydrologic and water chemistry data. We defined three stream-water groups: (i) subsurface runoff dominated locations with strong seasonal fluctuations in concentrations, short transit times, and strong negative C/N correlations with short time lags, (ii) groundwater dominated locations, with weaker seasonal fluctuations, longer transit times, and weaker C/N correlations with lags of several months, and (iii) intermediate locations, with moderate seasonal fluctuations, moderate transit times, and strong C/N correlations with short time lags. Water transit times could be identified as key drivers for the C/N relationship and we conclude that C and N transport in stream water can be explained by mixing of groundwater and subsurface runoff. Complemented by transit times and the hydrochemical time series, WTC analysis allowed us to discriminate between different water sources (groundwater vs. subsurface runoff). In conclusion, we found that in time series studies of hydrochemical data, e.g., DOC and NO 3 − , WTC analysis can be a viable tool to identify spatiotemporally dependent relationships in catchments.Abbreviations: COI, cone of influence; CWT, continuous wavelet transform; DOC, dissolved organic carbon; MedTT, median transit time; WTC, wavelet transform coherence; XWT, cross wavelet transform.The cycles of nutrients such as N and C are closely linked. For example, N saturation is linked to C limitation in the soil for microbial processes (Kopáček et al., 2013). In aqueous ecosystems, increasing NO 3 − concentrations (Galloway et al., 2008) and widespread significant increases in the concentrations of dissolved organic C (DOC) in streams during the last two decades have become a major global issue (e.g., Evans et al., 2005;Galloway et al., 2008;Vitousek et al., 1997). Newly created reactive N (biologically or photochemically active) contributes to environmental problems such as air pollution, eutrophication, soil acidification, and climate change (Vitousek et al., 1997;Galloway et al., 2008). The environmental and therefore also social consequences require detailed spatiotemporal models to predict the fate of reactive N across environments (e.g., Mulholland et al., 2008;Taylor and Townsend, 2010;Barnes and Raymond, 2010). For this purpose, it might be important to understand causal links to the spatiotemporal patterns of DOC.
Core Ideas• WTC analysis was used to elucidate the non-stationary C/N relationship at different time...
