Effects of wastewater treatment plant discharge on ecosystem structure and function of lowland streams

Abstract

Secondary and tertiary wastewater treatment is common in developed countries, but little is known about the responses of lotic ecosystems to contemporary wastewater treatment plant (WWTP) discharge. We examined the effects of WWTP discharge on various ecosystem components and functions of 2 morphologically and chemically impacted lowland streams near Berlin, Germany. We sampled one reach upstream and one reach downstream of a WWTP in each stream during each of 5 sampling campaigns. Discharge of treated wastewater resulted in increased concentrations of total organic C, total N, and total P in the sediments and in elevated macrophyte and benthic invertebrate biomasses. However, adverse effects of the WWTPs on the benthic invertebrate communities were small compared to effects reported in previous studies. This difference was a result of the higher purification efficiency of modern WWTPs, but also of significant structural degradation and eutrophication of the streams that already had impoverished the invertebrate community upstream of the WWTPs. Whole-stream community respiration (CR24) and gross primary production (GPP) were both enhanced by WWTP discharge. WWTP discharge generally caused diminished NH4- and PO4-uptake efficiencies, but did not necessarily lead to diminished NO3- uptake efficiencies of streams. Increases in areal NO3-uptake rates caused by the discharge of a large municipal WWTP were high enough to result in increased load-specific NO3-uptake efficiencies. Our study shows that the effects of present-day WWTPs on stream ecosystem functioning clearly differ from the former impacts of poorly treated wastewater. Present-day WWTP discharges mainly cause eutrophication and subsequent side effects and low nutrient-retention efficiencies relative to the high nutrient concentrations and loads of impacted streams. Our results highlight the need for efficient tertiary treatment of wastewater and for the refinement of agricultural practices to reduce diffuse nutrient loadings. We found evidence that even efficiently treated wastewater can have extensive effects on stream ecosystem structure and function. Therefore, adequate dilution rates always should be considered when routing treated wastewater through lotic networks. Our findings on the response of key ecosystem variables to present-day WWTP loading underline the importance of scientifically based stream management.