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Abstract

Summary 1. Over the last 30 years, many investigations have been performed on the dynamics of bacteria and organic matter in the Breitenbach, a first-order stream in central Germany. The data now available allow a synthesis of the role of bacteria in the carbon budget, as an example of the general importance of bacteria in stream ecosystems. 2. Comparing measured and estimated inputs and outputs to the ecosystem, the organic matter budget of the Breitenbach is fairly balanced: 1.84 kg C m⁻² year⁻¹ (sum of inputs) versus 1.88 kg C m⁻² year⁻¹ (sum of outputs). No major missing link remains. 3. The basis of the food web in the Breitenbach is mainly allochthonous organic matter (dissolved and particulate 1.02 and 0.42 kg C m⁻² year⁻¹, respectively). Autochthonous gross primary production is 0.4 kg C m⁻² year⁻¹. Most of the organic matter leaves the stream via transport to the River Fulda (dissolved and particulate 0.74 and 0.34 kg C m⁻² year⁻¹, respectively), the rest by respiration (0.80 kg C m⁻² year⁻¹ or 43% of total outputs). 4. Bacteria constitute an important part (36%) of heterotrophic biomass (average: 0.004 kg m⁻² bacterial C of 0.011 kg m⁻² total heterotrophic C). Bacteria also account for the major fraction (71%) of heterotrophic production: 0.20 of 0.28 kg C m⁻² year⁻¹ total heterotrophic production. Bacterial production in the Breitenbach is similar in magnitude to the estimate of photoautotrophic net primary production: both approximately 0.20 kg C m⁻² year⁻¹. 5. Protozoa, the main consumers of bacteria in the Breitenbach, consume approximately one-third of bacterial production (0.07 kg C m⁻² year⁻¹). Small metazoa (meiofauna, <0.5 mm) play a lesser role in the consumption of bacteria, consuming <0.01 kg bacterial C m⁻² year⁻¹. Larger metazoa (macrofauna, >0.5 mm) consume approximately 10% of bacterial production. Although this is a considerable amount of the carbon resources needed by the macrofauna (0.02 kg C m⁻² year⁻¹ of bacterial production versus 0.06 kg C m⁻² year⁻¹ macrofauna production plus respiration), the carbon demand of the macrofaunal community is met to a larger extent by particulate organic matter than by bacteria. 6. Bacteria are the main decomposers in the Breitenbach. They account for 78% of heterotrophic respiration (0.47 of 0.60 kg C m⁻² year⁻¹) and 59% of total respiration (0.47 of 0.80 kg C m⁻² year⁻¹).