Collection, Enumeration, and Biomass of Zooplankton

Abstract

Zooplankton usually are distributed throughout a lake or reservoir. Most forms are motile, and thus their distribution both vertically and horizontally may be quite variable. Species have different habitat preferences that further accentuate spatial and temporal heterogeneity. Zooplankton are major herbivores as well as important predators in aquatic ecosystems. Therefore, to understand lake metabolism it is necessary to evaluate the biomass and the role of zooplankton in the ecosystem. Quantitative collection of aquatic organisms within their natural habitats usually is difficult. The foremost problem is that only relatively small subsamples can be obtained from a population, which is often mobile, changing in size, and distributed in patches. If zooplankton were distributed unevenly within a lake ecosystem, then discrete samples would grossly overestimate or underestimate the true population size. Thus characterization of the distribution of individuals is a prerequisite for quantitative sampling. Individuals may be dispersed randomly, contagiously, or uniformly (or various combinations of these) throughout the volume of a lake or reservoir (Fig. 11.1). Errors associated with enumeration may be categorized into two types: (1) error associated with counting the individuals in a sample; and (2) errors in obtaining a representative sample of the population from a site within a lake, from the entire lake, or both. The purpose of this exercise is to examine these problems while, at the same time, attempting to characterize a zooplankton population for a lake. Apparently random or uniform distributions are rather uncommon in zooplankton populations as distributions tend to be patchy (Tonolli, 1971; Levin and Segal, 1976). However, various scales of vertical and horizontal distance must be considered. A chi-square test may be used to test for contagion. This test is done by determining the significance of the difference between the observed distribution of individuals in unit volumes and a theoretical Poisson distribution for the same number of individuals and volumes (see Appendix 2). When the distribution is contagious, the degree of contagion can be estimated by the ratio of the variance to the mean. Ratios of 1 indicate randomness, >1 indicate contagion, and <1 indicate evenness. The objectives of this exercise are 1. Evaluate counting procedures. 2. Compare various collection devices. 3. Characterize the population size, distribution, and biomass for a site and for an entire lake.