MEPS 208:93-106 (2000)  -  doi:10.3354/meps208093

ABSTRACT: The particle accumulation model for cylindrical traps of Gust et al. (1996; Aquat Sci 58:297-326) was verified in 2 limnic field experiments for 70 and 45 mm diameter traps under inclusion of new plate sediment traps in which the particles settle onto a flat surface under bottom stresses generated by the approaching flow. During 2 measuring campaigns in the River Spree of the total sinking-particle mass, only the 2 fastest-settling groups, representing a mass percentage <30%, contributed significantly to the material accrued in the cylindrical traps. Of the total accumulation rate of cylinders, the amount obtained by gravitational settling (5 to 10%) was very small for the heaviest particle groups and dropped beneath the experimental noise level for slower-sinking groups. Particles settling at speeds w_s < 5 m d^-1 under approach velocities >12 cm s^-1 were hardly retained by the 70 mm cylinder. The plate sediment trap collected material for which the depositional stress τ_d of the particles was not exceeded by the bottom stress. Based on the accumulation model and the data, a new and robust trapping protocol was developed which enables the determination of the in situ sinking-particle flux (c₀w_s)_i of particle subgroups (running index i) retained by cylinders. The in situ particle concentrations c_0i are obtained by solving a system of accumulation equations for a set of simultaneously deployed calibrated cylinders of different geometry, while the particle settling speeds w_si are determined optically in the quiescent settling zone of one of the cylinder traps used. The accumulation equation also provides a method to calibrate cylinder traps on-site. Independent experiments are now needed to give statistical relevance to the new trapping protocol, which operates under the premise that the collected mass is comprised of stable particles (aggregates).

KEY WORDS: Sinking-particle flux · Trap protocol · Sediment trap technology · Sample evaluation · Mass accumulation equation · Trap flux under flow