The Invertebrate Drift in the River Duddon, English Lake District

The Duddon drainage is divided into two distinct areas: an upper basin, in which Plecoptera are predominant; and a lower basin, in which Ephemeroptera are predominant, with Plecoptera common and Gammaruspulex common over most of the area. Drift samples were taken simultaneously at four stations: two in the upper basin and two in the lower basin. Nearly all species taken in the bottom samples were also taken in the drift. It was concluded that the predominance of Plecoptera in the upper basin is not due to an ability to withstand detachment better than other groups. The total drift samples from December 1965 to August 1966 gave a fairly accurate list of the species present at each station, and also gave a rough estimate of the percentage contribution of each species to the total benthos. The drift samples were generally poor indices of the relative size of the standing crops at the four stations, and it was concluded that drift rate could not be used as an index of the production rate of the benthos. Nocturnal drift rates were greater than diurnal drift rates for all groups, except the Hydrachnellae. Six species of Plecoptera and Ephemeroptera were recorded in the drift for the first time. There was a strong inverse correlation between light intensity and drift rate. More emerging Trichoptera and Chironomidae were taken at night than during the day, whereas the reverse was true for emerging Ephemeroptera. The greatest numbers of emerging Plecoptera were taken at night in the lower basin and during the day in the upper basin. Most species of Plecoptera emerged later in the upper basin than in the lower basin. Pe3loMe