Assessing the Impact of Effluents from a Modern Wastewater Treatment Plant on Breakdown of Coarse Particulate Organic Matter and Benthic Macroinvertebrates in a Lowland River

Wastewater treatment plants (WWTP) with insufficient technologies for wastewater purification often cause a distinct nutrient pollution in the receiving streams. The increased concentrations of dissolved nutrients can severely disturb the ecological integrity of streams, which has been recently shown for basic ecosystem processes like mineralization of coarse particulate organic matter (CPOM). The present study investigated the impact of a modern WWTP (Zentralkläranlage Jena) on breakdown rates of CPOM exposed in net bags (1 mm mesh size) to the effluent of a large municipal WWTP and an upstream control site in the Saale River (Thuringia, Germany) from April to October 2005. Control and effluent site differed significantly in water chemistry with increased concentrations of dissolved organic carbon (DOC), ammonium, sulfate, and chloride at the effluent site, while the control site displayed higher concentrations of nitrate. However, breakdown rates of toothpickers and small twigs were not significantly different between the sites, whereas breakdown rate of leaf litter was significantly higher at the effluent site (k = 0.0124 day⁻¹) than at the control site (k = 0.0095 day⁻¹). Benthic invertebrate assemblages inhabiting the sandy stream bed at both sites were dominated by Chironomidae and Oligochaeta, typical inhabitants of fine sediments. Although the Shannon diversity of the benthic invertebrates was slightly higher at the effluent site (0.85) than at the control site (0.63), no significant difference could be detected. Bacterial numbers in water samples and surface biofilms on glass slides also displayed no significant differences between the two sites. This study showed that the effluents of a WWTP with modern technologies for wastewater purification did not directly affect breakdown rates of CPOM, bacteria numbers in epibenthic biofilms and the water column, and the community composition of sediment inhabiting aquatic macroinvertebrates in an effluent-receiving river with already increased concentrations of dissolved nutrients.