Fish health in the Nidda as an indicator for ecosystem integrity: a case study for Central European small streams in densely populated areas

BACKGROUND: In Germany and the EU, most headwaters are still far from reaching a good chemical and ecological status as it is required by the European Water Framework Directive (WFD), until 2027 the latest. Particularly, in densely populated areas, impacts from municipal and industrial wastewater discharges or diffuse agricultural emissions are still a matter of concern. This also applies to the Nidda River which is considered to be in a moderate to rather poor condition. In our study, we investigated short-term and long-term consequences of anthropogenic pollution on fish health via one monitoring with caged fish (CF) and two field sampling campaigns (FF). In the CF monitoring, rainbow trout (Oncorhynchus mykiss) were caged for seven weeks at four selected sites along the Nidda, whereas in the FF monitoring approach, feral fish, including brown trout (Salmo trutta f. fario), European chub (Leuciscus cephalus) and stone loach (Barbatula barbatula) were caught in June and September 2016. RESULTS: Histopathological analyses of liver and gills were conducted, accompanied by measurements of hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity to assess the cytochrome P450 (CYP1A1) function, and genotoxicity via the micronucleus assay. Caged as well as field-captured fish exhibited impaired health conditions showing lesions particularly in the liver, and a presumably overwhelmed CYP1A1 system, whereas genotoxicity was not induced. The variation between sampling sites and seasons was rather low, but two trends were recognisable: (a) liver condition was poorest around spawning season and (b) tissue integrity and EROD activity were most affected downstream of industrial dischargers. Furthermore, effects were species dependent: the generally highly sensitive S. trutta f. fario proved to be impacted most, whereas L. cephalus with its pelagic lifestyle was affected less than the benthic B. barbatula, indicating a relevant contamination of sediments. CONCLUSION: Our results confirm the impaired ecological state of the Nidda and emphasise that a sustainable improvement of aquatic ecosystem health needs to include both water quality and sediment contamination to approach the ambitious WFD goal.