Effects of net, pump and bottle sampling on the abundance estimates of planktonic Rotifera

Eighty-seven freshwater plankton samples collected by means of two different nets, a submersible centrifugal pump and a 5.81 water bottle were compared with respect to their ability to catch the rotifer Keratella cochlearis. Sample size, illumination (day/night), the presence of bridles ahead of the net mouths (versus unbridled nets), and different mouth diameters (0.2 and 0.5 m) did not affect abundance estimates. Slight differences were found between the yields of pushed nets versus towed nets; these are probably due to uneven distribution of the animals in the 0–3 m layer. Both pump and bottle volumes strongly affected abundance estimates (K. cochlearisl−1 in pump samples=164.84 volume of water filtered−0 817; K. cochlearisl−1 in bottle samples=84.74+2336.6 volume of water filtered−1) Net sample results were always considerably higher (4.6–12.3 times) than pump and bottle estimates; these differences are most probably due to evading reactions of the rotifer.