Use of fluorescently labeled Lycopodium spores as a tracer for suspended particles in a lowland river

Fluorescently labeled spores of the clubmoss Lycopodium clavatum L. (mean d iameter = 42 mum, rho = 1175 kg/m(3)) were used as a tracer to estimate tra nsport distances and vertical deposition velocities of fine particulate org anic matter (FPOM), dominated by phytoplankton and detritus flocs, in a 6th -order lowland river (River Spree, Germany). Three experiments were conduct ed at discharge levels of 7.9, 8.8, and 14.8 m(3)/s, which were in the midd le of the range of discharge levels in the River Spree. The number of spore s in suspension declined exponentially with distance from the site of trace r release Mean transport distances of the tracer particles ranged from 3000 m to 10,660 m and increased with higher flow velocities. Correspondingly v ertical deposition velocities decreased from 8.4 m/d to 4.7 m/d with higher flow velocities. Also the ratio between measured vertical deposition veloc ities and calculated quiescent-water fall velocities fell from 0.63 to 0.38 . Although the Lycopodium spores cannot represent a wide range of natural s uspended particles because of their uniform size and density, they can be u sed to determine which factors influence the flux of POM from the pelagic t o the benthic zone. When comparing a broad range of stream sizes with regar d to their retention efficiency, discharge seems to be a key factor regulat ing particle retention.