Use of fluorescently labeledLycopodiumspores as a tracer for suspended particles in a lowland river

Abstract

Fluorescently labeled spores of the clubmoss Lycopodium clavatum L. (mean diameter = 42 μm, ρ = 1175 kg/m3) were used as a tracer to estimate transport distances and vertical deposition velocities of fine particulate organic matter (FPOM), dominated by phytoplankton and detritus flocs, in a 6th-order lowland river (River Spree, Germany). Three experiments were conducted at discharge levels of 7.9, 8.8, and 14.8 m3/s, which were in the middle of the range of discharge levels in the River Spree. The number of spores in suspension declined exponentially with distance from the site of tracer release. Mean transport distances of the tracer particles ranged from 3000 m to 10,660 m and increased with higher flow velocities. Correspondingly, vertical deposition velocities decreased from 8.4 m/d to 4.7 m/d with higher flow velocities. Also the ratio between measured vertical deposition velocities and calculated quiescent-water fall velocities fell from 0.63 to 0.38. Although the Lycopodium spores cannot represent a wide range of natural suspended particles because of their uniform size and density, they can be used to determine which factors influence the flux of POM from the pelagic to the benthic zone. When comparing a broad range of stream sizes with regard to their retention efficiency, discharge seems to be a key factor regulating particle retention.