Sampling variability of marine zooplankton in a tidal estuary

Abstract

Zooplankton samples were collected from a semidiurnal tidal estuary. Sampling variability both in the laboratory and in the field were considered. The field sampling variability was estimated by examining (1) the large-scale horizontal drift past a sampling station and (2) a series of repeated tows at a single location. The indices used included the total plankton catch, species diversity, percentage similarity, and dry weight biomass. Among them, species diversity was the least variable for the zooplankton population studied. Subsampling variability was fairly small as compared to field sampling variability. The 95% confidence limits obtained from the repeated (15 min) hauls at a single station were 50–200% of the mean, while for the large-scale horizontal drift (caused by tide), the limits increased to about 25–400%. Tide is, therefore, the most dominant factor in determining the sampling variability in this estuary. The interrelationships between zooplankton abundance and the tidal height were statistically analysed. Species showing significant positive correlation included A. tonsa and A. clausi; negative correlation, T. longicornis; and no correlation, C. hamatus and P. minutus. Increasing the number of plankton hauls and spacing these tows over 24 h tidal cycles is the most efficient way to reduce sampling variability. However, to obtain a representative sample for species composition and abundance, a minimum of two tows should be taken, one at high and the other at low water.