Bioenergetic cost of living in polluted freshwater bodies: respiration rates of the cyclopoid Eucyclops serrulatus under ammonia-N exposures

Ammonia-N (NH3+ NH4+) is considered one of the most important pollutants of freshwater bodies worldwide due to its high toxicity and ubiquity in freshwater ecosystems. The aim of this study was to assess if short-term exposures to sublethal ammonia-N concentrations affect the metabolic rates of the cyclopoid Eucyclops serrulatus. This is a target species for routine biomonitoring due to its world-wide distribution, wide ecological niche, short life-cycle and suitability to be easily reared in the laboratory. We measured the oxygen consumption as a proxy of possible metabolic reaction to stress under a 3-day exposure to two different ammonia-N concentrations, dosed as NH4+, at 15 degrees C. We also measured the respiration rates of juveniles (C1-C5 copepodids) separately from those of adults in order to assess whether the metabolic cost differed between the two stages. The respiration rates of both adults and copepodids increased with increasing NH4+ concentrations. However, the adults of E. serrulatus experienced a significant energetic stress under exposure to 12 mg L-1 NH4+. On the contrary, the copepodid respiration rates significantly varied under exposure to 1 mg L-1 NH4+, just one order of magnitude greater than the current European threshold value for freshwater bodies. According to these outcomes, it is prudent to consider river monitoring sites not contaminated by ammonia-N under the current legislation to represent a risk for the juveniles of this species.