Table of Contents:
The aim of the present work was to examine the impact of introduced wastewater on the distribution and diversity of denitrifying bacteria in the surface water and in the hyporheic interstitial of the River Lahn and to detect possible changes as well as their causes. In the years 1999 to 2001 water samples from wastewater, surface water and interstitial water by multilevel probes simultaneously from different sediment depths (5 cm, 15 cm, 25 cm and 45 cm) were taken.
The electric conductivity serving as conservative tracer refer a strong exchange between surface and interstitial water down to 45 cm sediment depth and a low input of groundwater on the examined sediment layers.
In the examined interstitial region the availability of nitrate was not a limiting factor for bacterial denitrification. A determination of the distribution of culturable denitrifying bacteria in the interstitial by nitrate or by the sediment depth was not determined in this study. Also a temporary increased input of nitrate by the wastewater led neither to a clearly increased nitrate concentration nor to an increased denitratation potential in the interstitial region downstream of the wastewater treatment plant.
Nitrite enrichments already described from previous studies could be confirmed in this investigation for the interstitial region downstream of the plant in the summer months. Due to low ammonium concentrations and denitratation potentials within this area the nitrite enrichments could not be attributed to incomplete denitrification, but to incomplete nitrification.
Approx. 250 m downstream of the wastewater treatment plant no nitrite accumulation could be determined, however an increased denitratation potential. Due to the reduced ammonium concentration of the wastewater from the treatment plant the denitratation potential was degraded, this refers to an apparently effective coupling of nitrification and denitrification in the interstitial during increased ammonium input through the wastewater.
The morphology of the research area exhibited a riffle-pool-riffle sequence and allow to analyse the impact of different hydraulic conditions of upwelling and downwelling area on the distribution of denitrifying bacteria. In the examined upwelling area an increased number of denitrifying bacteria were isolated, so that a distribution preference of culturable denitrifying bacteria within the upwelling area due to low oxygen concentrations is assumed.
Due to day-night fluctuations of the oxygen concentration in the surface water and the measured difference between the oxygen concentration in surface and interstitial water, alternating oxygen concentrations and the formation of anoxic sites in the interstitial of the Lahn leading to an enhanced denitrification.
The isolated denitrifying bacteria were divided by molecular ARDRA-method into 24 phylogenetic different groups. This genotypic diversity is similar to culturable denitrifying bacteria in soils, determined in earlier studies with the same method.
Pseudomonas, Enterobacter, Bacillus, Aeromonas, Shewanella, Stenotrophomonas and Klebsiella species showed a distribution independent of the wastewater impact belonging to the permanent denitrifying bacterial population in the surface and interstitial water of the Lahn.
Isolated denitrifying Pseudomonas and Bacillus species were determined to persist in the hyporheic interstitial of the Lahn for at least five years and showed a wide distribution in the surface and interstitial water of the Lahn particular Bacillus cereus and Pseudomonas fluorescens, whereas Ps. fluorescens dominates the culturable denitrifying bacterial population.
In contrast Ochrobactrum anthropi and Serratia spec. were only detected in the surface water upstream the wastewater treatment plant, which refers to an impact of the upper Lahn through point and non-point sources.
While the distribution of Alcaligenes species and Acinetobacter spec. was dependent on the conditions in the wastewater, Comamonadaceae were determined not only in the wastewater, but also persisting in surface and interstitial water. The change within the culturable denitrifying bacteria population of the Lahn caused by wastewater particularly shows up by the restrictive isolation of Comamonadaceae, which were detected only in and downstream of the wastewater treatment plant.
A direct input of these denitrifying bacteria by wastewater in the interstitial region and a persistence in the interstitial could be determined.