The effects of sampling gear and environmental conditions on the abundance estimates of zooplankton.

BOLTOVSKOY DEMETRIO; MAZZONI HORACIO

REVUE D4 HYDROBIOLOGIE TROPICALE

Office de la recherche scientifique et technique outre-mer

Lugar: Paris; Año: 1988 vol. 21 p. 21 - 34

0240-8783

Freshwater zooplankters (Rotifera, Cladocera and Copepoda) were counted in 96 plankton samples collected mainly from a small eutrophic reservoir in Co'rdoba, Argentina. Sampling gear used were two different nets (pushed ahead of the boat and towed behind it, with and without bridles), a submersible centrifugal pump, and a water bottle. The results suggest that neither net size nor bridles have measurable effects on the net yields. Disturbance of the upper layer during the daytime prior to collection engenders avoiding reactions which are responsible for conspicuously lower net, pump and bottle catches, while at night disturbance-related biases were not present. The zooplankters surveyed are more efficiently collected by nets in overcast weather and at night than under full sun; this is tentatively attributed to optomotor dodging reactions. As compared with net samples, both the stationary pump and the bottle underestimate densities; disturbance-related dispersion of the animals is partly responsible for this bias. In addition, all three groups studied have very strong positive rheotactic reactions and actively avoid the intake of the pump. The efficiency differences observed span, in some cases, several orders of magnitude.