A study on the fine-scale patchiness of zooplankton in the coastal area off Louisiana

Abstract

To examine the fine-scale patchiness of zooplankton and related sample variability in the coastal area off Louisiana, zooplankton samples were collected for one night and two days in October 1985 using a 1 m² Multiple Opening/Closing Nets and Environmental Sensing System fitted with 0.333 mm mesh nets. Data obtained from 21 tows of three different tow-lengths were analyzed. Although 133 taxonomic groups were identified and counted from the total of 168 samples, only 5 groups comprised more than 92% of total abundance in terms of number of individuals. The nine most abundant taxonomic groups with mean abundances greater than 30 individuals/m³ were included in the analyses since they were relatively free from errors associated with subsampling and counting. Species groupings related to vertical migration patterns were observed, and these patterns supported the result that the major source of the abundance variation was the sampling time. The relationship between mean abundances and tow distances was weak, but the variances of the abundance estimates showed an exponentially decreasing trend with the increase of tow distance when the population was increased at sampled depth due to the vertical migration. From regression analyses the minimum tow distances that would provide stabilized variances of abundance estimates were shown to vary among species from 43 m to 140 m with an average of 80 m. Patch parameters were obtained by the Skellam model. There were different scales of patches in the same area with smaller patches of 100-400 m in diameter and larger ones of 460-1280 m. Patches themselves seemed to be made up of smaller patches. The numbers of animals per patch and the numbers of patch centers per quadrat (sample) ranged from 10⁴ to 10⁹ and from 0.0003 to 0.0269, respectively. Only the number of animals per patch was shown to increase with an increase in total population density. Patch centers were considered to be randomly distributed because of the wide range of the distances between two adjacent patch centers. Biological interactions rather than physical factors are shown to be responsible for the observed patchiness of zooplankton. Vertically stratified sampling with the minimum tow distance of 80-100 m is suggested as a proper sampling scheme for the unbiased estimation of abundances in an environment like the sampling site of this study.