Temporal Drivers of Tryptophan-Like Fluorescent Dissolved Organic Matter Along a River Continuum

Abstract

Tryptophan-like fluorescence (TLF) has been used as a marker to indicate anthropogenic inputs of Dissolved Organic Matter (DOM), notably from wastewater, in rivers. We hypothesised that other sources of DOM, such as baseflow and planktonic microbial biomass can also be drivers of riverine TLF dynamics. This was investigated by sampling 19 contrasting sub-catchments of the River Thames, UK. Multivariate mixed linear models were developed for each site using 15 months of weekly water quality observations and with predictor variables selected according to a stepwise procedure. The variables that could potentially be included in the models were potassium (wastewater indicator), nitrate (baseflow indicator), chlorophyll-a (phytoplankton biomass), and Total bacterial Cells Counts (TCC) by flow cytometry. The wastewater indicator was a significant predictor of TLF in all sub-catchments (and hence was included in every model).  Baseflow was included in 68% of models, particularly those with higher baseflow indices (0.50-0.86). At these sites, baseflow acted as a negative control on TLF, diluting other potential sources. Additionally, TCC was included positively in the models of nine (47%) sub-catchments. The models on the Thames itself using TCC were more rural sites with lower sewage inputs. Phytoplankton biomass was only used in four sub-catchment models, despite the seasonal phytoplankton blooms. It is also notable that, in terms of the strength of evidence for inclusion in the model and the proportion of TLF variance explained, the wastewater indicator did not always have the strongest evidence of inclusion in the model. For example, there was stronger evidence for the inclusion of baseflow and TCC than wastewater in 26% and 16% of catchments, respectively. Our study underscores the complex interplay of wastewater, baseflow, and  planktonic microbes, driving TLF dynamics in riverine environments, with their influence determined by sub-catchment characteristics.

Keywords: Fluorescent organic matter, Tryptophan-Like, Microbial Processing, Insitu Microbial Production, Catchment characteristics