Seston sedimentation in a lowland river (River Spree,Germany): their spatial and temporal variations and controlling factors

Sedimentation of organic particles plays a decisive role in streams in relation to pelagic loss as well as retention of nutrients and other substances. The plate sediment trap allows for the direct measurement of these net fluxes. Biweekly measurements were undertaken in the eutrophic lowland River Spree (MQ 14 m³ s^–1) 10 km upstream of Berlin in 1999 and 2000. Trapping rates between 0.5 and 25 g DW m^–2 day^–1 were found near the bank. The variance of seston sedimentation is controlled by the seston concentration, the settling velocity of the particles and the flow velocity. The sinking velocity exhibits significant seasonal fluctuations with highest values in summer. It is shown that the critical flow velocity for sedimentation is another important parameter. This controls the distribution of sedimentation over the width of the river and thus the effective average sedimentation rate for the entire river segment. This average rate ranged between 0.9 and 6.6 g DW m^–2 day^–1.     

The influence of particle size distribution and composition on seasonal sedimentation rates in a temperate lake

To evaluate the effect of particle size and composition on seasonal changes in the sedimentation rate, suspended and sedimenting particles were collected from Trout Lake, Wisconsin, USA during the 2002 ice-free season. Particles were characterized with regard to particulate biogenic silicon (PBSi) concentration and distribution between four size classes. The concentration of particulate chlorophyll and chlorophyll degradation products was also measured in water column particles and sediment trap material collected during the first half of the sampling period. The highest rates of mass sedimentation during the stratified period were measured in spring and early summer. Seasonal increases in sedimentation are related to the presence of large colonial diatoms in the water column as indicated by elevated PBSi concentrations. The majority of particulate matter in the water column was found in the smallest (<20 m) size fraction while most of the mass of sediment trap material was in larger size fractions (>20 m). Smaller cells appear to carry out most of the photosynthesis in Trout Lake but larger cells are responsible for seasonal trends in sedimentation. These results may explain how pelagic photosynthesis and sedimentation rates can be quantitatively decoupled across a range of trophic conditions but appear correlated when based on mid-summer measurements.