THE EFFECTS OF DESIGN AND OPERATION ON THE EFFICIENCY OF HOSEPIPES AS WATER COLUMN SAMPLERS FOR PHYTOPLANKTON

The efficiency of 5 different hosepipe samplers in providing representative water column samples was tested by comparing the chlorophyll .alpha. concentrations in these samples with those obtained by integrating discrete depth samples collected with a Van Dorn sampler, in two hypertropic South African reservoirs (Hartbeespoort and Reitvlei Dams). The data and observations demonstrated that sampling efficiency increased with: (a) increasing hosepipe diameter from 18 mm to 63 mm; (b) a reduction in the mass of the basal weight to the minimum necessary to straighten the tube plus streamlining the weight to minimize bow-wave effects; and (c) correct deployment and storage of the hosepipe. The discrepancies in chlorophyll .alpha. concentrations between samples collected with poorly-designed hosepipe samplers and integrated discrete-depth samples were larger when cyanobacteria dominated the phytoplankton than when diatoms or green algae were dominant. Therefore, where hosepipe samplers are to be used in eutrophic waters, special care should be taken to select a suitably designed sampler. A design for a hosepipe sampler that provides representative samples is given.