Analysis of particulate organic matter retention by benthic structural elements in a lowland river (River Spree, Germany)

In a 6th-order section of the lowland River Spree, we analysed the retentio n of particulate organic matter (POM) by the following characteristic benth ic structural elements: stable sands colonised by macrophytes or mussels, s hallow sandy banks, and shifting sands. Our hypothesis was that the retenti on and storage of suspended particles and, therefore, the organic matter bu dget of a specific river reach is determined by the retentive properties of the various types of benthic structural elements in the channel. The relat ive storage capacity of each structural element for POM, chlorophyll-a, and protein was calculated. This storage capacity reflects the dynamics of pro cesses that occur on the time scale of several months. Additionally, the re lative trapping efficiencies of the structural elements were determined by means of tracer particles, which provide information about their retentive properties on shorter time scales. The investigated structural elements dif fered significantly with respect to the amount and composition of stored PO M, and the time scale of retention. Biotic structures were the most effecti ve retention features on longer time scales in the investigated lowland riv er. Macrophyte stands showed the highest relative storage capacity for orga nic matter, followed by stable sands that were colonised by mussels. On a s horter time scale during low discharge conditions, shallow sandy banks were similarly retentive. Shifting sands were least retentive, but the highest nutritional quality of the organic matter was found there. Those spatial pa tterns within a river reach are supposed to be an important factor controll ing the microbial processing of POM in the benthic zone and the distributio n of benthic invertebrates. Based on our results, it can be expected that c hanges in the size and composition of benthic structural elements within th e channel (e.g, due to bank reinforcements, straightening measures, or impo undments) will influence the organic matter budget of a stream ecosystem co nsiderably.