Some observations on the effect of sampling technique under different conditions on numbers of some freshwater planktonic Entomostraca and Rotifera caught by a water-bottle

Abstract

Summary 1. The effect of variations in sampling technique on numbers of freshwater planktonic Entomostraca taken in a 51. water-bottle has been investigated. 2. All samples were taken at 4 m below the surface in deep water of a lake. The tests were aimed to find out more about the effect on the larger planktonic animals of (1) leaving the sampling device open for different intervals of time, in light and in dark (2) lowering the sampling device at different rates and (3) the opacity and the direction of approach of the sampling device. 3. Species differed in their response to lengthening intervals of time before closure of the water-sampler. In daytime, at intervals of more than 5 sec, numbers of the cladoceran Daphnia hyalina fell by up to 50% but the response of other species was less pronounced. In the dark, there was no loss of D. hyalina: in moonlight, there was some loss but not as great as in daylight. Loss of Daphnia thus appears to be associated with a visual response. 4. Rate of lowering of the ...