10 - Zooplankton biomass and body size

Summary

Introduction

Zooplankton affect lake ecosystem processes by grazing on phytoplankton, recycling nutrients through excretion, and serving as prey for both vertebrate and invertebrate planktivores (Brooks & Dodson, 1965; Peters, 1975; Neill, 1981). Consequently, the zooplankton can be analyzed from two points of view: as a dependent variable with respect to planktivores, and as an independent variable with respect to algal community dynamics and nutrient recycling. In this chapter, we focus primarily on the responses of zooplankton biomass and body size to manipulations of fish populations. Responses of the phytoplankton to changes in the zooplankton community are addressed in Chapters 11 and 13.

Planktivorous fish feed selectively on larger and more conspicuous zooplankton (Zaret, 1980; Neill, 1984; Vanni, 1986). Zooplankton community changes associated with fish manipulations were discussed in Chapter 8. Here, we examine the implications for the total biomass and the size structure of the zooplankton, which are associated with rates of key ecosystem processes. Rates of grazing or nutrient excretion per unit biomass are proportional to the body mass of individual zooplankters (Peters, 1983). The range of particle sizes consumed by filter-feeding cladocerans is also proportional to body size (Burns, 1968).

Total rates of grazing or nutrient excretion scale directly with zooplankton biomass. Therefore, zooplankton body size and biomass indicate the potential rates of ecosystem processes, such as grazing and nutrient excretion, that are performed by the herbivorous zooplankton.