Diel patterns in nitrate concentration suggest importance of microbial pathways for in-stream processing

Abstract. Diel cycles in stream nitrate concentration represent the sum of all processes affecting nitrate concentration along the flow path. Being able to partition diel nitrate signals into portions related to different biochemical processes would allow to calculate daily rates of such processes that are urgently needed for water quality predictions. In this study we analyzed diel nitrate patterns at three locations in a 5.1 km long stream reach draining a 430 km², mainly forested but anthropogenically influenced catchment during one growing season. We tested if the observed diel variability in nitrate concentration resulted from upstream sources and subsequent downstream transport or emerged simultaneously along the stream. We determined time lags between monitoring sites by cross-correlation. We found that time lags were closer to zero than travel time estimation assuming plug-flow suggested and concluded that ubiquitous in-stream processes prevailed in the creation of diel variability. To further analyze the diel nitrate signals we used k-means clustering to identify patterns in the diel portion of nitrate concentrations and interpreted the resulting clusters with regard to potential drivers and the calculated nitrate balance of sub-reaches. We found that 70 % of all diel patterns were attributed to clusters negatively related to the diel course of insolation with highest nitrate amplitudes on warm and sunny days and low water levels. We argue that temporal shifts towards the remaining clusters are rather due to shifts in microbial nitrate processing than in photosynthesis-driven plant uptake. These results suggest that the magnitude of microbial nitrate processing may be large compared to plant uptake.