The dispersal of 0.13 kg of water-tracer rhodamine WT in the Loosdrecht lakes was surveyed during 19 months. Solid-phase extraction and liquid chromatography were used to measure the very low concentrations of rhodamine WT in the experiment. The detection limit was -2 x lo-l1 kg m-3. The photolysis constant of rhodamine WT was measured in a laboratory experiment. A numerical model for hindcasting the dilution of a photolytic tracer in the lake system was developed. Model results compare well with observations showing that rhodamine WT is well suited for tracer experiments on large time and space scales, provided that photolysis is taken into account.
We describe the use of sulpho rhodamine G (SG) and rhodamine WT (WT) as oceanographic tracers in tidally active waters of the southeastern North Sea. Ten kilograms of SG and 5 kg of WT dissolved in 1,000 liters of sea water were released to produce an initial tracer patch ϳ10 ϫ ϳ0.5 km. Tracer dispersal was monitored for 11 d by high-performance liquid chromatography analysis of samples collected close to the patch center. The patch center was initially identified during surveys for sulfur hexafluoride released simultaneously, and its position was subsequently confirmed by use of a two-dimensional tracer dispersion model. Temporal changes in the SG/WT ratio were undetectable, so photolytic tracer losses were not directly quantified. A simple photolysis model was used to correct to conservative behavior for WT. Results are compared with the dispersal of spores of Bacillus globigii var. niger released simultaneously. Photolytic tracer pairs are ideally suited to tracer experiments in unconstrained water masses.Large-scale experiments involving deliberate tracer releases are becoming increasingly important in limnology and oceanography. Sulfur hexafluoride (SF 6 ) has been used in lakes (Wanninkhof et al. 1985;Upstill-Goddard et al. 1990), 1 Corresponding author (Rob.goddard@ncl.ac.uk).
AcknowledgmentsWe thank the officers and crew, fellow scientists, and RVS/ BODC staff on board RRS Challenger for their invaluable support. We are especially grateful to Wim Helder and Johan den Das for the precise HPLC analyses and to the British Oceanographic Data Centre for data archiving.
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