180
Exercise 12
productivity and its control in research efforts.
The monographs of Edmondson and Winberg
(1971), Winberg(1971), Brinkhurst(1974),Waters
(1977), Benke (1984), and Rigler and Downing
(1984) are excellent critical compilations on
methods and are highly recommended sources
for further study.
SAMPLING
A number of basic requirements must be met to capture a quantitative sample of the
populations living on and within the sediments. The sampler must penetrate into the
sediments to a sufficient depth to capture all of the organisms inhabiting a defined
area. The device should enclose the same area of sediment each time. As the sampling
device is lowered, care should be taken not to disturb the sediments or give certain
organisms an opportunity to escape prior to collection. The sampler should close
completely so that sediment and organisms are not lost during retrieval. The efficacy
of the many benthic samplers varies greatly; few fulfill all of these requirements
adequately. Fewer samples are needed for a given precision when the population
density is high and when samples are taken with a large sampler (Downing, 1979).
However, the amount of sediment that must be examined increases with increasing
size of the sampler. Hence, small-area samplers are most efficient. A sampler should
not be prone to sediment loss upon retrieval or pressure-wave effects upon entry into
the sediments. The sampler should be sufficiently large to avoid many zero values of
organisms among replicates but should be as small as possible to minimize the time
required to separate the organisms from the sediments.
Many insects emerge from the water as winged, flying adults. Accurate analyses
of population dynamics of these aquatic organisms require a quantitative evaluation
of such emigration. Emergence traps are discussed briefly in Exercise 13 and in
extensive detail by Davies (1984).
Benthic samplers may be grouped into four general categories: (1) metal boxes
closed by jaws at the bottom; (2) hinged grabs in which sediments are scraped toward
the center, each side bucket forming a sector of a horizontal cylinder; (3) coring
devices; and (4) specialized box- and core-type samplers for use in substrata that are
difficult to sample, such as hard, stony sediments and sediments with dense macrovegetation. Several of the more commonly used devices and their operation will be
discussed briefly here; a detailed treatment is found in Edmondson and Winberg
(1971), Holme and McIntyre (1971), Brinkhurst (1974), Elliott and Drake (1981), and
particularly Downing (1984).
Ekman-Type Grab Sampler
The Ekman grab consists of a square or rectangular box (15.2 x 15.2cm), usually
made of brass, with a pair of spring-operated jaws that close tightly when released
to enclose a specific area of the sediments (Fig. 12.1). During operation, a strong
metered line is passed through the tripping mechanism and knotted securely below
the underlying plate. The jaws are cocked in the open position. Caution: Accidental
closure can cause injuries; keep hands clear. Attach the dead end of the line to the
boat. The sampler then is lowered into the water unitil it rests on the sediments.
Usually its own weight is adequate to penetrate soft sediments for all or most of its
height. The messenger is then sent down the line to close the jaws. The sampler is
Exercise 12
productivity and its control in research efforts.
The monographs of Edmondson and Winberg
(1971), Winberg(1971), Brinkhurst(1974),Waters
(1977), Benke (1984), and Rigler and Downing
(1984) are excellent critical compilations on
methods and are highly recommended sources
for further study.
SAMPLING
A number of basic requirements must be met to capture a quantitative sample of the
populations living on and within the sediments. The sampler must penetrate into the
sediments to a sufficient depth to capture all of the organisms inhabiting a defined
area. The device should enclose the same area of sediment each time. As the sampling
device is lowered, care should be taken not to disturb the sediments or give certain
organisms an opportunity to escape prior to collection. The sampler should close
completely so that sediment and organisms are not lost during retrieval. The efficacy
of the many benthic samplers varies greatly; few fulfill all of these requirements
adequately. Fewer samples are needed for a given precision when the population
density is high and when samples are taken with a large sampler (Downing, 1979).
However, the amount of sediment that must be examined increases with increasing
size of the sampler. Hence, small-area samplers are most efficient. A sampler should
not be prone to sediment loss upon retrieval or pressure-wave effects upon entry into
the sediments. The sampler should be sufficiently large to avoid many zero values of
organisms among replicates but should be as small as possible to minimize the time
required to separate the organisms from the sediments.
Many insects emerge from the water as winged, flying adults. Accurate analyses
of population dynamics of these aquatic organisms require a quantitative evaluation
of such emigration. Emergence traps are discussed briefly in Exercise 13 and in
extensive detail by Davies (1984).
Benthic samplers may be grouped into four general categories: (1) metal boxes
closed by jaws at the bottom; (2) hinged grabs in which sediments are scraped toward
the center, each side bucket forming a sector of a horizontal cylinder; (3) coring
devices; and (4) specialized box- and core-type samplers for use in substrata that are
difficult to sample, such as hard, stony sediments and sediments with dense macrovegetation. Several of the more commonly used devices and their operation will be
discussed briefly here; a detailed treatment is found in Edmondson and Winberg
(1971), Holme and McIntyre (1971), Brinkhurst (1974), Elliott and Drake (1981), and
particularly Downing (1984).
Ekman-Type Grab Sampler
The Ekman grab consists of a square or rectangular box (15.2 x 15.2cm), usually
made of brass, with a pair of spring-operated jaws that close tightly when released
to enclose a specific area of the sediments (Fig. 12.1). During operation, a strong
metered line is passed through the tripping mechanism and knotted securely below
the underlying plate. The jaws are cocked in the open position. Caution: Accidental
closure can cause injuries; keep hands clear. Attach the dead end of the line to the
boat. The sampler then is lowered into the water unitil it rests on the sediments.
Usually its own weight is adequate to penetrate soft sediments for all or most of its
height. The messenger is then sent down the line to close the jaws. The sampler is