Macrophytes Increase Spatial Patchiness of Fluvial Sedimentary Records and Effect Temporal Particulate Nutrient Storage

Previous studies on fluvial substrates often lack high spatial resolutions for sedimentary characteristics in relation to biotic structures, such as the aquatic vegetation. A sedimentological survey was therefore performed in a section of the Lower River Spree, in order to analyse the riverbed substrates for small-scale changes in grain size and nutrient parameters, indicative of the impact of submerged macrophytes on sediment composition and particulate nutrient retention. We cored the riverine substrate using a systematic grid of 3m width and 10m length, and mapped the aquatic vegetation. Sedimentary records were characterised according to facies, grain size and carbon, nitrogen and phosphorus analyses. Facies description and statistics for the sedimentary parameters provided evidence for a significant riverbed patchiness, which was related to the occurrence of macrophytes. In vegetated areas, stable fine sands were covered by an organic layer, a facies significantly different from that of non-vegetated areas, which comprised coarse sandy to gravely sediments. Distinct facies alternated in short distances across the river, partly due to the effects of macrophytes on the flow regime. Mean particulate nutrient concentrations were two orders of magnitude higher in vegetated (TOC 5.16, TN: 0.50 and TP: 0.24 of dry matter (DM)) than in non-vegetated sediments (TOC: 0.27, TN: 0.03 and TP: 0.06 DM). Therefore, the organic layer contained nearly the entire nutrient pool of the surface sediments, and thus significantly contributed to a temporary retention of particulate nutrients (at maximum 80gnitrogenm^sup -2^month^sup -1^ and 38gphosphorusm^sup -2^month^sup -1^) during vegetation period. Submerged macrophytes were found to effect a considerable spatial heterogeneity in riverbeds, and to cause high seasonality in particulate nutrient retention, a result amending previous geological facies analyses.[PUBLICATION ABSTRACT]