Feeding Rates by Protists and Larger Zooplankton

Abstract

Direct consumption by zoolankton can have appreciable effects upon phytoplankton and bacterioplankton populations. By means of selective grazing, zooplankton can influence the seasonal succession of the phytoplankton [cf., Porter (1977)].

Most cladocerans and copepods remove particulate organic matter from the water by filtration and concentration of particles by water movements toward the mouth area. The size of the particles that can be cleared from the water is a function of the morphology of the setae on the moving appendages or of entrapment as the movement of the animal brings particle-laden water to the setae. The maximum rate at which energy is gained from food is a function of the combined rates of filtering and ingestion, and of the abundance, size, and digestibility of food [cf., Lehman (1976)].

Clearance rate is generally defined as the volume of water cleared of suspended particles per unit time (hour or day). This term, usually synonymous with filtration capacity, should not imply, however, that the volume of water passed over the filtration appendages is known, that all particles of a given type have been removed from the water, or that all particles retained by the filtration apparatus have been consumed (Rigler, 1971). The contemporary view of zooplanktonic feeding behavior is that the animals do not filter the water in the sense of sieving. Instead, high speed photography has shown that particles are captured as parcels of water are moved within the feeding structures (Peters, 1984; Omori and Ikeda, 1984). Although the term filtering rate is still used widely, the terms clearance or clearance rate are more representative of the actual process being measured.

In contrast, feeding rate is a measure of the quantity of food ingested by an animal in a given time (measured as number, volume, dry weight, or chemical content of cells, or other components of the ingested food). Measurements of feeding rate are made by observing changes in the number of particles removed over time by grazing, or by measuring the rate of removal of food particles labeled radioac-tively or fluorescently. These methods measure food actually taken into the gut. Loss of filtered particles may occur either through active rejection or during the maceration process. It should be kept in mind that ingestion rates do not equal assimilation rates, which can be highly variable depending on the type and concentrations of food particles and their chemical content.

This exercise evaluates several methods of measuring clearance and grazing rates in pro-tists and in larger zooplankton. Quantification of the rates of clearance and feeding rates allows an evaluation of the effects of various environmental factors on ingestion.