Ostracoda (Crustacea) as indicators for surface water quality: a case study from the Ledra River basin (NE Italy)

Wastewater discharges associated with urbanisations, farming activities and industry may dramatically reduce the ecological health of river ecosystems. During the reconstruction of the Friuli Venezia Giulia region following the 1976 earthquake, a lot of resources were used to build large numbers of wastewater treatment plants to minimize the impact of human activities on lotic ecosystems. Their efficiency is usually assessed through monitoring of the physical and chemical environment near the discharge point. However, discontinuous monitoring of the abiotic environment may fail to detect periodic malfunctioning and do not recognize indirect effects on the ecosystem. We assessed the potential of an alternative approach to assess the impact of wastewater discharges, based on the monitoring of ostracod density, richness and community composition. We repeatedly measured physical, chemical and microbial parameters and collected ostracod samples at stations up- and downstream from wastewater discharge points scattered over a 21-km stretch of the Ledra River (NE Italy). The results indicate that monitoring ostracods is a potentially valuable approach, for two reasons. Communities appeared to be well differentiated even in the small spatial area of this study, indicating that they can provide sufficient resolution to pick up even minor impacts. Secondly, despite the seasonal succession in species composition, spatial differentiation was consistent over time, suggesting that ostracods provide a time-integrated picture of the water quality. The traditional approach failed to detect any consistent impact of wastewater discharges, apart from an ambiguous increase in nutrient levels. The density and/or richness of the ostracod communities was altered by some wastewater discharges, but not by others. We identified a general trend for wastewater discharges to systematically replace regionally rare ostracod species with common species. In particular, the species Ilyocypris inermis was very sensitive to discharges, and may be used as an indicator species for good ecosystem health.