Effects of single and repeated experimental acid pulses on invertebrates in a high altitude Sierra Nevada stream

1. We examined responses of aquatic macroinvertebrates to pulsed acidification experiments in twelve streamside channels located in the Sierra Nevada, California. Experiment 1 consisted of a single 8 h acid addition, and Experiment 2 consisted of two 8 h acid additions administered 2 weeks apart. Replicated treatments (four reps/ treatment) consisted of a control (pH 6.5–6.7) and pH levels of 5.1–5.2 and 4.4–4.6. Invertebrate drift was monitored continuously and benthic densities were determined before and after acid addition. 2. Drift responses to pH reduction were: (i) increased drift during acidification in pH 5.2 and pH 4.6 treatment channels, often with depressed post-acidification drift in treatment channels relative to controls (exhibited by Baetis only). Depressed post-acidification drift in treatment channels appeared to be due to low benthic densities because a positive relationship between benthic and drift densities was noted for most common taxa; (ii) increased drift rates during acidification only at pH 4.6 (Epeorus, Drunella, Paraleptophlebia, Zapada, and Simulium); (iii) decreased drift at pH 5.2 and/or pH 4.6 relative to control channels (Rhyacaphila and chironomid larvae); (iv) no significant response to acidification (Ameletus, Amiocentrus, Dixa and Hydroporus). 3. A high proportion (45–100%) of acid-induced drift in Baetis, Epeorus, and chironomid larvae could be attributed to dead, drifting individuals. 4. Except for chironomids, most common invertebrates (i.e. Baetis and Paraleptophlebia) showed reduced benthic densities in treatment relative to control channels after acidification. 5. For sensitive taxa, drift was enhanced and benthic densities reduced by single (Experiment 1) and initial Experiment 2(a)] acid pulses. Drift responses to a second acid pulse Experiment 2(b)] were not as pronounced as those to the single or initial acid pulses Experiments 1 and 2(a)], and the second acid pulse had no additional effect on benthic density.