Sediment traps provide a powerful tool for monitoring sediment deposition in lacustrine environments. Sedimenting material can be collected from a lake over a discrete time interval, thus providing a valuable method for physical and biological monitoring. Sediment traps provide data which are complimentary to both annual monitoring of epilithic diatom communities and to the sediment core studies performed earlier in the Network´s history.
Two arrays of cylindrical sediment traps with high aspect ratios (height : diameter) are anchored and buoyed about one metre above the sediment surface at locations close to the deepest point at each lake site. Each trap array contains three individual sediment traps, each having a height of 33 cm and an aspect ratio of 6.47. The traps are serviced and the accumulated sediment recovered on an annual basis in August. Since 1999 twin thermistors have been attached to the trap arrays, measuring two-hourly temperature 1m below the surface and at the trap itself.
Diatoms
Sub-samples of the trap sediment are prepared for diatom analysis and 500 diatom valves per site are counted according to the procedures described in the page on epilithic diatoms.
All diatom count data are stored on the central Amphora biological database. A diagram of trap diatom taxa comprising > 1% of the total count is presented in the the UK UWMN annual data report for each lake site.
Spheroidal Carbonaceous Particles
Spheroidal carbonaceous fly-ash particles (SCPs) are produced by the high temperature combustion of fossil-fuels such as coal and oil. They are emitted to the atmosphere with flue gases and dispersed over wide geographical areas with the diffusing plume. They are not produced by any natural processes and therefore are unambiguous indicators of atmospheric deposition from power generation and other industrial sources (Rose 2001).
Following the analysis of SCPs over the full industrial period in sediment cores from each of the UK UWMN lake sites (Patrick et al. 1995), annual sediment trap samples have been analysed since 1990 in order to provide a continuous record of these atmospherically deposited contaminants. Sediment trap samples typically provide a better temporal resolution than sediment cores and are not subject to any post-depositional alteration resulting from, for example, bioturbation. For monitoring of atmospherically deposited pollutants they are therefore a very useful tool and complement the retrospective trends obtainable from the lake sediment record.
Laboratory techniques for the analysis of SCPs from lake sediment or sediment trap material follow the method of Rose (1994) whereby unwanted sediment fractions are removed by sequential mineral acid attack. This process leaves carbonaceous material and a few persistent minerals as a suspension in water. A known fraction of this suspension is evaporated onto a coverslip and mounted on a microscope slide using 'Naphrax' diatom mountant, and the SCPs on the whole of each coverslip counted at x400 using a light microscope.
Concentrations of SCPs are expressed in terms of numbers per gram dry mass of sediment (gDM-1). For sediment trap samples these data can then be converted into flux values (no. cm-2yr-1).
Metals
Sediment trap metals samples are collected by transferring sub-samples from the traps to rigorously cleaned Teflon bottles, allowing them to settle then pipetting off the supernatant liquid. The sediments are carefully transferred to weighed plastic bags and freeze-dried. Trace elements (Hg, Pb, Cd, Zn and Ni) are measured using atomic absorption spectroscopy (AAS) and major elements (Fe, Mn, Al, Si, Ti and Ca) are measured using a Metorex Xmet920 X-ray fluorescence (XRF) spectrometer. Prior to analysis for trace elements the samples are digested by heating them for 1 hour at 100 C with 8 ml concentrated Aristar HNO3.
Certified standard reference materials (Buffalo River sediment SRM2704, Stream sediment GBW07305) are digested and analysed at the same time as the samples. For AAS reference material and sample blanks are analysed every 20 samples and for XRF reference material is measured every 5 samples.
Metals analysis ceased in 2007.
References
Rose, N. L. (1994) A note on further refinements to a procedure for the extraction of carbonaceous fly-ash particles from sediments. Journal of Paleolimnology, 11, 201-204.
Rose, N. L. (2001) Fly-ash particles. In: Tracking Environmental Change Using Lake Sediments: Volume 2. Physical and Chemical Techniques, 319-349, Kluwer Academic Publishers, Dordrecht, The Netherlands.
Patrick, S. T., Monteith, D. T. & Jenkins, A. (1995) UK Acid Waters Monitoring Network: The first five years. Analysis and interpretation of results, April 1988 - March 1993. 1-320. ENSIS Publishing, London.