The monotetrazolium redox dye 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) has been used as a vital stain of actively respiring bacteria for several years. In this study, inhibitory effects on bacterial metabolism of this redox dye have been examined in a brackish water environment (Kiel Fjord, Germany) and a freshwater environment (Elbe River, Germany). As the results from time series experiments (1 to 10 h) show, bacterial growth and respiration of the investigated natural communities were clearly reduced by CTC supply. Compared with untreated controls (100%), CTC-treated samples showed distinctly lower heterotrophic bacterial plate counts (0 to 24 and 11.8 to 23.7%, respectively), bacterial production (0.9 to 14.1 and 1.1 to 9.6%, respectively), bacterial respiration (4.1 to 9.4 and 6.8 to 43.8% for several concentrations of 14 C-labeled glucose), and [ 14 C]glucose incorporation (0.2 to 4.2%). Additionally, toxicity of CTC was demonstrated by luminescence in a Microtox bioassay. CTC concentrations of 0.1 and 5.0 M required only 15 min for decreases of approximately 50 and 100%, respectively. The suppression of CTC on several bacterial metabolic processes suggests that determination by the CTC technique underestimates the actual number of active cells distinctly. This conclusion is confirmed by the comparison of generation times calculated on the basis of thymidine uptake data and active bacterial counts determined by the CTC assay and microautoradiography. While unrealistic short generation times (0.5 to 5 h) resulted from the CTC assay, the generation times calculated according to microautoradiography ranged within values (7 to 21 h) reported elsewhere for comparable aquatic environments. The inhibitory effect of CTC demonstrated in our experiments is an aspect with regard to the application of this tetrazolium dye for the estimation of active bacteria in natural aquatic environments which hitherto has not been considered.

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A high abundance of particles and aggregates is a characteristic of estuaries and may represent important loci of increased bioproductivity and microbial activity. From a survey performed in the Elbe estuary, particle concentration (using a particle counter), bacterial biomass and abundance (AODC method), bacterial production rates ([3H] thymidine method) and the microbial extracellular enzyme activity of 11 enzymes (MUF and MCA method) were determined with particular respect to their relationship to particles and aggregates (sizefractionated). Although higher particle abundance was found compared to other sections of the river Elbe, neither relative nor absolute bacterial activity associated with particles and aggregates was enhanced. The activity of free dissolved microbial enzymes was found to dominate the breakdown of organic material. The productivity of free dispersed bacteria ≥0.2…<2 μm (2…69%) and bacteria attached to small particles ≥2…<10 μm (27…93%) considerably surpassed that of bacteria associated with particles >10 μm (0…40%). This indicates that the larger particles and aggregates do not always contribute substantially to the overall microbial activity in rivers and estuaries.

The extreme flooding of the Elbe in August 2002 led to resuspension of large amounts of sediment and thus to elevated loads of organic substance in the river. The higher organic carbon concentration was only slightly balanced by increased microbiological self‐purification capability, and this led to regional organic pollution along the river Elbe, especially in lentic water bodies. The remobilisation of sediments also led to increases in the concentrations of some elements by a factor of more than three times. In October 2002, the concentrations of most elements decreased to their 1998 values with the exception of As, Cd, W, Bi and U. One year after the flood an algae bloom and biogenous decalcification led to increases in organic matter and calcite in the suspended matter and sediments and therefore to dilution of most of the elements. However, the concentrations of Cd, W and Bi in the 2002 and 2003 sediments were higher than in 1998.