EXERCISE 15
Zooplankton Feeding
Direct consumption by zooplankton 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. 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 offood (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 act ural
process being measured.
In contrast, feeding rate is a measure of the
quantity offood 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 radioactively. 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 may 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
zooplankton. With the knowledge of the rate of
clearance, effects of various environmental
factors on ingestion can be evaluated.
227
Zooplankton Feeding
Direct consumption by zooplankton 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. 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 offood (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 act ural
process being measured.
In contrast, feeding rate is a measure of the
quantity offood 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 radioactively. 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 may 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
zooplankton. With the knowledge of the rate of
clearance, effects of various environmental
factors on ingestion can be evaluated.
227