Quantitative sampling of demersal zooplankton: reentry and airlift dredge sample comparisons

Abstract

The distributions and abundances of demersal gammarid amphipods from a shallow sandy benthos were examined using core, reentry and airlift dredge sampling methods. Approximately 90% of the gammarid amphipods sampled with cores were found in the uppermost 2.5 cm of natural sediments. Reentry samplers, designed to contain sediments similar to natural sediments, were used in situ to sample demersal gammarids returning to the benthos after nocturnal vertical migrations. Airlift dredge samplers were used to estimate the absolute abundance of demersal gammarids within the benthos. Comparisons of reentry and airlift dredge samples indicated that only ≈11% of the gammarid fauna was actively migrating and recolonizing sediments during the night sampled. There were no differences between the relative orders of abundance of gammarid species sampled by reentry and airlift methods. Although reentry samplers may not be used to estimate absolute abundances of demersal organisms, they do estimate accurately the relative orders of abundance of demersal gammarid species. Therefore, reentry samplers may be used to test hypotheses regarding the effects of environmental variables upon the relative abundance, distribution, and sediment colonization behavior of demersal organisms.