Spatial, temporal, and biotic changes in the natural abundances of stable carbon and nitrogen isotopes in a biological treatment reactor.

The natural abundances of stable isotopes were used to determine prey-predator relationships and material flows in a biological treatment reactor for municipal wastewater. The reactor used in this study was a down-flow hanging sponge (DHS), which is an alternative trickling filter that uses sponge as the packing material. The sponge retained sludge containing a wide variety of organisms, including microfauna. Stable isotope analysis revealed spatial, temporal, and biotic variations in the carbon and nitrogen stable isotope ratios (δ13C and δ15N) of the retained sludge and microfauna (water mites and fly larvae). In addition, adult flies and spiders were present and analyzed. The δ13C and δ15N in sludge were temporally and spatially similar along the reactor. The isotopic signature was associated with treatment characteristics such as a low nitrification efficiency in the DHS reactor. The δ13C and δ15N of sympatric fly larvae differed from those of water mites, which indicated dietary differences between the taxa. Interestingly, the water mites had higher δ13C and δ15N than the retained sludge, which indicated that they were in a higher trophic position in the food web. In addition, the δ13C and δ15N values of spiders were approximately 1 ‰-3‰ higher than those of adult flies. This strongly suggested that a prey-predator relationship existed between adult flies and spiders.