[Diel rhythm of benthic macroinvertebrate's drift in streams in Hengshishui River watershed of Guangdong, China]

An investigation was conducted on the species composition and diel rhythm of benthic macroinvertebrate's drift in two adjacent 3rd order streams in the Hengshishui River watershed of Guangdong, China. One of the streams was seriously polluted by acid mine drainage (AMD) discharged from the Guangdong Dabaoshan Mine, with a very low pH (3.45) and high contents of heavy metals that exceeded the Chinese National Standards for surface water, and the another adjacent was a clean stream. The results showed that the individuals and species of benthic macroinvertebrates were much more in clean stream than in AMD-polluted stream. In the drift samples from clean stream, a total of 6871 macroinvertebrate individuals belonging to 52 taxa of 10 orders were collected, among which, 99.5% were aquatic insects. However, the total drift density was dominated by a few taxa, of which, the dominant drift taxa with relative abundance in excess of 5% included Pseudocloeon morum (28.5%), Cinygmina yixingensis (13.8%), Cheumatopsyche sp. (13.2%), Serratella albostriata (7.5%), Chironomidae (6.5%), and Psephenoides sp. (5.0%). The most abundant drifter both in species and in individuals was Ephemeroptera, constituting 65% of the entire caught (dominated by Baetidiae, which represented 63% of total mayfly individuals), followed by Trichoptera (18%). The macroinvertebrate's drift showed significant diel rhythm, i. e., the drift was strongly nocturnal, with the peaks at 21:00 and 2:00, and the average drift densities being (70.3 +/- 10.8) and (289.0 +/- 124.6) ( ind x 100 m(-3)), respectively. No diurnal drifter was observed. Most dominant drift taxa showed slightly differences in their drift peaks, but some dominant taxa, e. g., chironomids and Psephenoides sp., showed no clear diel rhythm of drift. Polypedilum sp., an acidophilic species of Chironomidae, was the only drift animal found in AMD-polluted stream, which had three distinct drift peaks, i. e., at 19:00, 0:00 and 4:00, with the highest drift density being only (6.7 +/- 5.2) (ind x 100 m(-3)). It was indicated that AMD not only reduced the numbers and species diversity of drift benthic macroinvertebrates, but also altered their drift patterns.