Benthic Fauna of Lakes
183
most severe disadvantage of this sampler is that its closed or nearly closed design
creates pressure waves that disturb the sediments and benthic fauna as the sampler
approaches the sediments. A modified grab, known as the Ponar grab, has been
altered from the Petersen construction so that the upper portions are covered with
screen to reduce the pressure waves (Powers and Robertson, 1967). It has been
demonstrated that a portion of the sediment within the area sampled is lost and that
sampling is deeper at the edges of the bite than in the center (Gallardo, 1965). The
Ponar and Ekman grabs have been found to sample with acceptable efficiency in
soft sediments up to a particle size of fine gravel (Elliott and Drake, 1981). The Ponar
grab was found to be adequate for sampling a substratum of small stones; neither
were adequate for sampling among larger stones (> 16-mm diameter). Efficiencies of
sampling always should be evaluated; for example, benthic biomass from box core
samplers obtained with SCUBA were 1.7 times greater than in samples obtained with
the Ponar grab (Nalepa et al., 1988).
Coring Samplers
A large number of core-type samplers have been devised. All consist of a tube that
is pushed into the sediment, either by free fall, by force exerted from the surface, or
by means of a piston system within the tube. A valve that closes upon retrieval usually
is included at the top of the tube to prevent the sediments from washing out. It is
most important that the closing device does not impede water flow through the tubes
during descent. When the water outflow is impeded, even slightly, the tube and water
pressure will push aside loose sediment at the sediment-water interface. Even with a
closing device at the top of the tube, very soft and flocculent sediments can fall out
during retrieval. Some core-type samplers have closing devices at the bottom to
prevent loss of sediments.
In shallow sediments, plastic or metal tubes simply may be pushed into the sediment
and then stoppered or sealed with a manually operated valve [e.g., Gillespie et al.
(1985)]. Such cores also have been used at greater depths with SCUBA diving
apparatus. In most cases, however, sampling from the surface is necessary. Coring
devices, such as the Kajak-Brinkhurst corer (Brinkhurst, 1974), are allowed to fall
freely to the sediments. The valve closes upon retrieval.
The relatively small area (10 to 50cm 2 ) that corers sample is a drawback when
organisms are distributed sparsely. It is mandatory to take a number of replicates at
each depth. A multiple corer, such as that illustrated in Fig. 12.3, is excellent and
permits several cores to be taken simultaneously. In this device, a messenger dropped
along the line from the surface releases the soft half-balls that seal the top ends of
the tubes (Hamilton et ai., 1970). In a comparison with diver-operated cores and
several other samplers, the abundance of organisms taken with this multiple corer
showed no significant difference from the diver-operated cores, although it did slightly
underestimate chironomid density. In addition, the multiple corer has the following
advantages over the Ekman grab:
1. It is designed to create a minimal pressure wave.
2. Surface sediment is not lost during retrieval.
3. Variance within a sampling station can be determined since four cores are taken
at once over a very small area.
4. Sieving and sorting time is reduced since the four cores combined produce a
smaller volume than one Ekman grab sample.
183
most severe disadvantage of this sampler is that its closed or nearly closed design
creates pressure waves that disturb the sediments and benthic fauna as the sampler
approaches the sediments. A modified grab, known as the Ponar grab, has been
altered from the Petersen construction so that the upper portions are covered with
screen to reduce the pressure waves (Powers and Robertson, 1967). It has been
demonstrated that a portion of the sediment within the area sampled is lost and that
sampling is deeper at the edges of the bite than in the center (Gallardo, 1965). The
Ponar and Ekman grabs have been found to sample with acceptable efficiency in
soft sediments up to a particle size of fine gravel (Elliott and Drake, 1981). The Ponar
grab was found to be adequate for sampling a substratum of small stones; neither
were adequate for sampling among larger stones (> 16-mm diameter). Efficiencies of
sampling always should be evaluated; for example, benthic biomass from box core
samplers obtained with SCUBA were 1.7 times greater than in samples obtained with
the Ponar grab (Nalepa et al., 1988).
Coring Samplers
A large number of core-type samplers have been devised. All consist of a tube that
is pushed into the sediment, either by free fall, by force exerted from the surface, or
by means of a piston system within the tube. A valve that closes upon retrieval usually
is included at the top of the tube to prevent the sediments from washing out. It is
most important that the closing device does not impede water flow through the tubes
during descent. When the water outflow is impeded, even slightly, the tube and water
pressure will push aside loose sediment at the sediment-water interface. Even with a
closing device at the top of the tube, very soft and flocculent sediments can fall out
during retrieval. Some core-type samplers have closing devices at the bottom to
prevent loss of sediments.
In shallow sediments, plastic or metal tubes simply may be pushed into the sediment
and then stoppered or sealed with a manually operated valve [e.g., Gillespie et al.
(1985)]. Such cores also have been used at greater depths with SCUBA diving
apparatus. In most cases, however, sampling from the surface is necessary. Coring
devices, such as the Kajak-Brinkhurst corer (Brinkhurst, 1974), are allowed to fall
freely to the sediments. The valve closes upon retrieval.
The relatively small area (10 to 50cm 2 ) that corers sample is a drawback when
organisms are distributed sparsely. It is mandatory to take a number of replicates at
each depth. A multiple corer, such as that illustrated in Fig. 12.3, is excellent and
permits several cores to be taken simultaneously. In this device, a messenger dropped
along the line from the surface releases the soft half-balls that seal the top ends of
the tubes (Hamilton et ai., 1970). In a comparison with diver-operated cores and
several other samplers, the abundance of organisms taken with this multiple corer
showed no significant difference from the diver-operated cores, although it did slightly
underestimate chironomid density. In addition, the multiple corer has the following
advantages over the Ekman grab:
1. It is designed to create a minimal pressure wave.
2. Surface sediment is not lost during retrieval.
3. Variance within a sampling station can be determined since four cores are taken
at once over a very small area.
4. Sieving and sorting time is reduced since the four cores combined produce a
smaller volume than one Ekman grab sample.