Fluorescence‐based source tracking of organic sediment in restored and unrestored urban streams

Abstract

Fine sediment is a leading cause of impairment of waterways, but its dynamics are generally poorly understood. Most fingerprinting studies focus on the mineral component of fine sediment, yet the organic fraction has distinct effects on ecosystem processes. Here we employed a novel combination of fluorescence spectroscopy and end‐member mixing analysis (EMMA) to distinguish between sources of particulate organic material (POM) in streams with and without restoration structures, situated within urban watersheds of the Chesapeake Bay. Suspended sediment samples and potential source end‐member samples were leached and analyzed for their fluorescence excitation‐emission (EEM) spectra. EEMs were resolved into fluorescent components using parallel factor analysis (PARAFAC). Six conservative components comprised the inputs to EMMA. The final model distinguished seven categories of source contributions: old and recent banks, the streambed, impervious surfaces, forested soils, algae, and a general category of allochthonous contributions from the watershed. The poorly characterizable allochthonous category dominated all samples, particularly at baseflow. During storms, the stream with restoration structures transported larger concentrations of fluorescent POM than the unrestored stream, with greater contributions from banks and soil. However, the bed contribution was greater for the unrestored stream, reflecting greater instream storage of fine sediment during interstorm periods. Relative source contributions in the model were insensitive to variability in the precise spectral characteristics of source end members or the number of end members, and varied in a manner consistent with external measurements, providing confidence in the approach.

Less Fine Particle Retention in a Restored Versus Unrestored Urban Stream: Balance Between Hyporheic Exchange, Resuspension, and Immobilization

“…Although there was increased solute and particle exchange with storage zones in the restored stream (greater V W , V P , and V′ h , Table ), there was not increased net retention of fine particulate matter (lower V P‐NET and similar V′ h,P , Table ). Both local‐scale and reach‐scale results support the conclusion from Larsen et al () that fine particulate matter is more poorly retained in the hyporheic zone of the restored stream than in the unrestored stream. Limited hyporheic retention of fine particles, including particulate organic carbon and microbes, may help to explain why installation of cross vanes does not necessarily lead to increased nutrient uptake (Zimmer & Lautz, ).…”

“…Previous research within the Chesapeake Bay watershed has demonstrated that retention of total suspended sediment within watersheds is greater than export, but restoration efforts have not significantly reduced suspended sediment loads in small‐order streams (Filosofo et al, ). In fact, a restored stream within the Chesapeake Bay watershed was observed to have higher particulate organic matter export during storms compared to an unrestored stream, with contributions from the bed making up a lower proportion of the suspended sediment pool (Larsen et al, ). Detailed quantification of fine particle transport and retention is needed to improve understanding of how restoration structures change fine particle dynamics and overall particle accumulation and efflux in restored streams.…”

Dissolved organic matter transport reflects hillslope to stream connectivity during snowmelt in a montane catchment

“…Evaluating DOM chemistry using fluorescence spectroscopy in headwater catchments during saturation permits us to infer hydrologic connectivity between the terrestrial landscape and the stream by using DOM composition as a tracer of hydrochemistry and streamflow generation sources [ Vidon et al ., ; Miller and McKnight , ; Nguyen et al ., ; Inamdar et al ., ; Larsen et al ., ]. Our data demonstrated qualitative and quantitative evidence of a relatively large contribution of soil water sources to the stream during snowmelt.…”

“…We followed methods similar to Larsen et al . [] to determine which fluorescent components were potentially conservative in our study. We created biariate mixing diagrams with all possible combinations of our measured fluorescent components.…”

Improving Predictions of Fine Particle Immobilization in Streams

“…Fine particles (<100 μm) are important to a broad range of key stream ecosystem processes, including carbon cycling (Battin et al, 2003;Larsen et al, 2015) and reactivity (i.e., metabolic activity and nutrient uptake), but excess particles can clog the streambed sediment matrix and alter streambed oxygen dynamics (Briggs et al, 2015;Hartwig & Borchardt, 2014). Both organic and mineral fine particles provide a colonizable surface area for bacteria (Mendoza-Lera et al, 2016) and, therefore, strongly influence pathogen transmission in streams (Bradford et al, 2013;Drummond et al, 2018).…”