Added value of the NemaSPEAR[%]-index to routinely used macrofauna-based indices for assessing the quality of freshwater sediments

Abstract

Assessments of the ecological status of freshwater ecosystems, such as mandated by the EU Water Framework Directive, are routinely conducted by monitoring macroinvertebrates. However, for the quality assessment of fine sediments, macroinvertebrates are of limited suitability. In such habitats they show a low species diversity and often low densities, whereas a more diverse meiofauna can be found. Among the meiofaunal groups in benthic habitats, nematodes are one of the most abundant and species-rich. Fine, cohesive sediments considerably contribute to many ecosystem services, but they are often hotspots of chemical contamination as well. In the present study, the added value of the recently developed and validated NemaSPEAR[%]-index was evaluated by directly comparing it to routinely used macrofauna-based indices. Macrofaunal and nematode communities were synchronously monitored at seven sites in six different streams. The results of a chemical analysis of sediment pollutants combined with sediment quality guidelines revealed widely diverging toxic potentials at the seven investigated locations. The seasonal robustness of the NemaSPEAR[%]-index compared with macrofauna-based indices was also determined, by additionally obtaining synchronous samples of macrofauna and nematodes over the course of one year at one of the seven sites, a reference stream with very low toxic potential.The NemaSPEAR[%] performed robustly despite seasonal variations in the nematode community in the sediment of the unpolluted stream. At the seven sampling sites, representing a pollution gradient, the NemaSPEAR[%]-index correlated well with the toxic potential of the sediments. By contrast, the macrofauna-based indices did not correlate significantly with either the toxic potential of the sediments or with the results of NemaSPEAR[%] at the seven sites. For many non– endobenthic macroinvertebrates, chemical exposure is mostly through the water phase, such that the toxic potential of the sediments will not necessarily be reflected directly by macrofaunal indices. Accordingly, identifying the stressors that contribute to degrading the ecological status of a water body requires the inclusion of methods that examine different types of stressors, targets, and exposure pathways. Our study shows that the NemaSPEAR[%]-index provides added value to routinely used macrofaunal-based indices.