Measurement of soluble reactive phosphorus concentration profiles and fluxes in river-bed sediments using DET gel probes
Introduction
Eutrophication of lowland rivers is of global concern in catchments draining intensive agriculture and high population densities, linked to nutrient enrichment (particularly of nitrogen and phosphorus) (Hecky and Kilham, 1988, Grobbelaar and House, 1995, Reynolds and Davies, 2001). The role of river-bed sediments in nutrient cycling is of primary ecological significance because they can act as an internal reservoir, providing a direct nutrient source to rooted plants and benthic algae and, by release from porewaters, a dissolved nutrient source for phytoplankton and periphyton (Reynolds and Davies, 2001). Conversely, uptake of nutrients by bed sediments and algae/bacteria at the sediment–water interface reduces river water concentrations, producing a ‘self-cleansing’ effect (House, 2003, Jarvie et al., 2005, Jarvie et al., 2006a, Jarvie et al., 2006b). Control of nutrient concentrations in rivers is now a major requirement of the European Water Framework Directive (WFD) (Council of European Communities, 2000) in order to establish/maintain Good Ecological Status of rivers. phosphorus (P) is particularly important, as it is often the limiting nutrient for aquatic plants (Reynolds and Davies, 2001, Jarvie et al., 1998, Jarvie et al., 2004). However, there are key gaps in knowledge about (a) the quantitative significance of bed sediment–water interactions on riverine P concentrations and fluxes, (b) the biogeochemical processes controlling P uptake and release from/to bed sediments and (c) the importance of the internal reservoir of P stored in river-bed sediments for river ecology. To date, studies of bed-sediment P exchanges and fluxes have mainly been based on laboratory sorption experiments (e.g. House et al., 1995, Jarvie et al., 2005) and porewater profiling from sediment cores (e.g. House, 2003, Farmer et al., 1994), which disrupt, or fail to resolve, steep gradients in sediment porewater chemistry (Moore and Reddy, 1994, Harper et al., 1997). Indeed, steep diffusion gradients in P concentrations, linked to redox profiles and microbial activity in sediments and biofilm growth (Moore et al., 1998, Jarvie et al., 2002a), may play a crucial role in P-cycling across the benthic interface in rivers.
In this study, we examine the use of DET (diffusive equilibrium in thin films) ‘gel’ probes Mortimer et al., 1998a, Mortimer et al., 1998b, Davison et al., 1991, Davison et al., 1994, Davison and Zhang, 1994) for in situ sampling of soluble reactive P (SRP) concentration gradients and fluxes across the benthic interface. DET probes contain polyacrylamide gel strips mounted within perspex holders (Mortimer et al., 1998a, Mortimer et al., 1998b). The probes are inserted into the sediment and the polyacrylamide gel directly equilibrates with the adjacent sediment porewater or river water. After a period of deployment (to allow equilibration between the gel and adjacent porewater/river water), the gel strips are removed from the probes and sectioned for solute extraction and analysis, in order to measure solute concentration profiles. Solute fluxes can then be calculated from the concentration profiles using Fick’s first law (Mortimer et al., 1998b, Berner, 1980). ‘Peepers’ (dialysis samplers, consisting of a vertical array of cells, covered by a dialysis membrane) have also been used for in situ sampling of sediment porewaters for solute measurement (e.g. Teasdale et al., 1995, Chowdhury and Al Bakri, 2006). However, comparison of the performance of DET and peeper solute samplers (Harper et al., 1997) indicated that DET offers major advantages of (i) greater accuracy at higher resolution than peepers (provided sufficient analyte is present in the DET gel); (ii) much more rapid equilibration responses (experimental observed equilibration times were a few hours for DET, compared with 2–3 weeks for peepers); (iii) easier handling, as the solute is contained in a solid phase gel, facilitating simple probe construction.
While DET has been used extensively for sampling trace metals in freshwaters (Davison et al., 1991, Davison et al., 1994, Davison and Zhang, 1994), there has been very little application of gel probes for in situ measurement of P, with only one published study using another type of gel probe (DGT, diffusive gradient in thin films, incorporating a ferrihydrite impregnated gel) for measuring P profiles in lake water (Zhang et al., 1998). To the best of our knowledge, this work provides the first publication to use DET gel probes for measuring porewater SRP profiles and fluxes in river-bed sediments.
Section snippets
Study sites
This study was undertaken at three locations: at Stretford Brook (a tributary of the River Lugg, within the Wye catchment), which drains a predominantly agricultural subcatchment in Herefordshire, western England (Fig. 1; Jarvie et al., 2003); and at two sites at Loddington, draining headwater tributaries of the Eye Brook which drains into the River Welland in Leicestershire, eastern England (Fig. 1). These three sampling sites were chosen because they are representative of rural stream
Description and preparation of DET gel probes
DET gel probes consist of a polyacrylamide hydrogel held within a perspex frame, with a window (2 cm × 15 cm) to allow water to diffuse into the gel. The window was covered with a 0.45 μm Whatman cellulose nitrate filter membrane to exclude particulates (for further details of DET probe construction, see Davison et al., 1991, Davison et al., 1994). Probes were inserted directly into the sediment by hand. Solutes diffuse through the membrane and into the gel, allowing it to equilibrate with the
Comparison of stream water, bulk sediment and porewater concentrations at Stretford Brook, Belton Bridge and Digby Farm
Average river-water SRP concentrations from the 2-year weekly water quality monitoring were over an order of magnitude higher at Stretford Brook (489 μg-P l−1) than at Belton Bridge (31 μg-P l−1) and Digby Farm (8 μg-P l−1) (Table 1). Comparison of these 2-year average SRP stream water concentrations with SRP concentrations measured from grab samples at the time of probe deployment (Table 2), revealed that, at the time of probe deployment, stream water SRP concentrations were close to the long-term
Discussion
The three sampling sites in this study reflect a wide range of P sources and stream water P concentrations: a relatively pristine lowland stream site at Digby Farm (mean 8 μg-SRP l−1), an arable impacted stream at Belton Bridge (mean 31 μg-SRP l−1) and a stream subject to sewage effluent and intensive arable farming at Stretford Brook (mean 489 μg-SRP l−1). Stream water SRP concentrations at Stretford Brook therefore significantly exceeded the 100 μg-P l−1 threshold concentration, which is used to
Wider comments
This work provides the first study of DET probes for in situ measurement of SRP profiles in river-bed sediments. The results provide new insights into the impact of different land use practices on the SRP concentrations in bed sediment porewaters and the implications for sediment–water exchanges of SRP at times of eutrophication risk (steady-state low flows in spring/summer). This work has implications for assessing the role of agricultural sediments (derived from field erosion) in
Acknowledgements
The authors wish to thank Dr. Chris Stoate at the Allerton Research and Educational Trust for providing access to the sampling sites at Loddington.
References (54)
- et al.
Biological phosphate removal in a prototype recirculating aquaculture treatment system
Aquacul. Eng.
(2000) - et al.
Phosphorus removal in a marine prototype, recirculating aquaculture system
Aquaculture
(2003) - et al.
Comparison of the uptake of inorganic phosphorus to suspended and stream bed sediment
Water Res.
(1995) - et al.
Nitrogen and phosphorus in east coast UK rivers: speciation, sources and biological significance
Sci. Total Environ.
(1998) - et al.
Phosphorus uptake into algal biofilms in a lowland chalk river
Sci. Total Environ.
(2002) - et al.
Role of river bed sediments as sources and sinks of phosphorus across two major eutrophic UK river basins: the Hampshire Avon and Herefordshire Wye
J. Hydrol.
(2005) - et al.
Sewage effluent phosphorus: a greater risk to river eutrophication than agricultural phosphorus
Sci. Total Environ.
(2006) - et al.
Within-river nutrient processing in Chalk streams: the Pang and Lambourn, UK
J. Hydrol.
(2006) - et al.
Use of gel probes for the determination of high resolution solute distributions in marine and estuarine porewaters
Mar. Chem.
(1998) - et al.
The effect of macrofauna on porewater profiles and nutrient fluxes in the intertidal zone of the Humber estuary
Estuar. Coast. Shelf Sci.
(1999)