Drift and emergence patterns of nonbiting midges (Chironomidae, Diptera, Insecta) in a subtropical river

Abstract

The passive dispersal of macroinvertebrates suspended in the water column is known as drift. In tropical and subtropical rivers, which show a distinct hydrological cycle with defined flood and dry periods, chironomids may use flood periods for effective downstream dispersal, particularly if drift coincides with emergence, due to lower fish predator efficiency because of increased turbidity and turbulence. If this is true, chironomids should display a seasonal drift and emergence pattern. The abundances in drift of pupae, their exuviae and imagines (PEI) should be higher during the flood periods. Additionally, in response to predator pressure, it should be advantageous to drift, emerge, or both, to take place at night. We investigated drift and emergence patterns of chironomids in the subtropical Sinos River in southern Brazil during two annual cycles in monthly collections with sampling intervals of 3 hr during 24 hr. About 70% percent of all drifting macroinvertebrates were chironomids. Larvae showed a tendency to increase drift with higher stream flow. No relation of larval drift with the day or night period was detected. However, PEI drifted and emerged at significantly higher abundances during increased discharges. This pattern was not related to the period. During high flow, turbidity and turbulence increase. Hence, emergence efficiency may increase by higher survival rates under these conditions, due to reduced predation by visually oriented fish.

Diel variation of aquatic insect drift in streams of Southern Brazil

“…For example, Brittain & Eikeland (1988) found that Ephemeroptera, Simuliidae, Plecoptera and Trichoptera were the most abundant aquatic insects in a diel drift evaluation. Likewise, Schulz, Rosa, & Klein-Stolz (2019) found that Chrironomidae comprised 70% of the total macroinvertebrates drifting in a Neotropical River in Southern Brazil.…”