Helmut Fischer

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (rS = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources oforganic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > β-glucosidase > α-glucosidase > exo-1,4-β-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and β-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/β-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). Therelative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2–3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.

Rivers of the Central European Highlands and Plains

The ecoregion of the central European highlands and plains is drained by some of the main rivers that flow into the Baltic and North Seas, including the Weser, Elbe, and Oder Rivers. In addition to these rivers, this chapter describes some smaller but peculiar rivers, such as the Em (Sweden), Skjern (Denmark), Spree (Germany) and Drawa (Poland) rivers. The Weser River exhibits a balanced longitudinal sequence of geomorphologically distinctive river sections typical of the Central European Highlands and Plains. The Weser and its tributaries provide important ecological services to society, including drinking water, sewage removal, water for irrigation, cooling water for power plants and industrial facilities, hydropower, habitat for organisms, and recreation and tourism. With a length of 1094 km, the 8th order River Elbe (Czech: Labe) is the third longest river in central Europe (after the Danube and Rhine). The Elbe is often seen as a river still possessing a natural river bed with active flood-plains. The Oder (Polish and Czech: Odra) is the sixth largest river flowing into to the Baltic Sea, with an annual discharge volume of 17.3 km. Being 854 km long, the Oder is the second longest river in Poland (after the Vistula). It has been used early for navigation both in north-south and east-west directions, as it has been connected early with the Elbe catchment via two canals.

Role of suspended particles for extracellular enzyme activity and biotic control of pelagic bacterial populations in the large lowland river Elbe

Large rivers are often highly loaded with suspended particles, but the microbial metabolism related to these particles has rarely been studied. Hence, we investigated activities of the extracellular enzymes leucine aminopeptidase (LAP), phosphatase, β-D-glucosidase and exo-1,4-β-glucanase as well as microbial communities associated with suspended particles from the 8 th order River Elbe, and their potential controls. Three fractions were investigated separately - rapidly sinking particles (largest particles), intermediately sinking particles (medium- sized particles), and bulk water including slowly sinking particles (smallest particles). Overall, bulk water had the highest abundances of bacteria, heterotrophic fl agellates and ciliates. Extracellular enzyme activities per dry mass were also highest in bulk water, as 83-95 % of total bacteria occurred in this fraction. Medium-sized and large particles had lower enzyme activity per dry mass because of their smaller relative surface area and lower propor- tions of particulate organic carbon (POC) and particulate nitrogen (PN). Bulk water, the medium-sized and the largest particle fractions contributed 40-85 %, 5-20 % and 10-40 % to total enzyme activity per volume of river water, respectively. Leucine aminopeptidase exhibited its highest relative activity in bulk water, which also had the highest chlorophyll-a content. Cell-specifi c enzyme activities were generally correlated with the availability of particulate organic matter (POM) per bacterial cell and were therefore highest on medium and large-sized particles. These results indicate that the abundance and activity of attached bacteria were bottom-up controlled by the size and biochemical composition of the particles. However, bacterivorous fl agellates and ciliates were abundant on the larger particles. Thus, number and feeding types of particle-attached protozoans suggest that top-down control may have occurred during an algal bloom. Our results show that free-living bacteria dominated the overall microbial metabolism of the water column in the River Elbe. However, bacteria attached to medium-sized and large particles were the most active ones, and contributed 15-58 % to total extracellular enzyme activity in the water column, depending on enzyme and season.