Sediment trap performance in tidal waters: comparison of cylindrical and conical collectors

A number of different sediment traps have been deployed at two sites which exhibited significantly different tidal streams. Measurements of suspended particle concentration and current velocities at each site during the deployments enabled horizontal particle fluxes to be calculated for periods corresponding to the sediment trap sampling intervals. These data have allowed the performance of the traps to be compared and the effect of the tidal environment on trap performance to be assessed under field conditions. In parallel deployments, different diameter cylindrical traps of similar aspect ratio (height/diameter) collected identical sample masses when corrected for the cross-section of trap aperture but conical traps collected only one-tenth of the material taken by the cylindrical traps. The sample collection rate of cylindrical traps was found to be positively related to the measured horizontal particle flux and this relationship was similar at both sites despite different tidal current strengths and a corresponding variation in the concentrations of suspended particles. Comparisons of the carbon content of ambient suspended particles with that of the samples collected by the traps indicated that the conical traps progressively under trapped carbon-rich particles as current velocity increased. These data confirm that the use of moored sediment traps in tidal waters, where horizontal particle fluxes greatly exceed vertical fluxes, is of limited value in the interpretation of suspended particle dynamics. However, using appropriate geometry for the collectors, sediments traps may provide representative samples of suspended particles for analysis.