Benthic Fauna of Lakes
181
Figure 12.1. The Ekman grab. Left: In open form, as during descent through the water; upper doors would
be open to permit water to flow through the sampler. Right: After the messenger has tripped the release
mechanism and the jaws have closed.
retrieved, and a container is placed beneath the sampler just as it breaks the surface
of the water. The jaws then are opened and all of the sediments are washed into the
container. If the jaws were not closed completely, the sample should be discarded.
The Ekman grab sampler is best suited for use in soft, finely divided sediments.
The grab will not function well on sand substrates or at all where hard objects (sticks,
leaves, stones, or mollusc shells) are common. In extremely flocculent, loosely dispersed
organic sediments, the grab may penetrate below the surface of the sediments and a
portion of the surface sediments may be excluded. Use of a high-form Ekman grab
[cf., Kajak (1971)J will minimize this problem. This modified Ekman sampler . is
similar in every respect to that shown in Fig. 12.1 except that the box portion is
about twice as high.
The Ekman grab may be modified by attaching a pole for controlled sampling in
shallow waters ( < 2 m) where the sediments are very soft or where the grab must be
forced into compacted sediments. The closure system may be replaced by levers for
closing the jaws in compacted, sandy sediments.
The Ekman grab departs from the ideal sampling criteria in that it can create a
pressure wave before entering the sediment, thus "blowing" away lightbodied
organisms on the sediment surface. Also, mud and organisms may be lost out of the
181
Figure 12.1. The Ekman grab. Left: In open form, as during descent through the water; upper doors would
be open to permit water to flow through the sampler. Right: After the messenger has tripped the release
mechanism and the jaws have closed.
retrieved, and a container is placed beneath the sampler just as it breaks the surface
of the water. The jaws then are opened and all of the sediments are washed into the
container. If the jaws were not closed completely, the sample should be discarded.
The Ekman grab sampler is best suited for use in soft, finely divided sediments.
The grab will not function well on sand substrates or at all where hard objects (sticks,
leaves, stones, or mollusc shells) are common. In extremely flocculent, loosely dispersed
organic sediments, the grab may penetrate below the surface of the sediments and a
portion of the surface sediments may be excluded. Use of a high-form Ekman grab
[cf., Kajak (1971)J will minimize this problem. This modified Ekman sampler . is
similar in every respect to that shown in Fig. 12.1 except that the box portion is
about twice as high.
The Ekman grab may be modified by attaching a pole for controlled sampling in
shallow waters ( < 2 m) where the sediments are very soft or where the grab must be
forced into compacted sediments. The closure system may be replaced by levers for
closing the jaws in compacted, sandy sediments.
The Ekman grab departs from the ideal sampling criteria in that it can create a
pressure wave before entering the sediment, thus "blowing" away lightbodied
organisms on the sediment surface. Also, mud and organisms may be lost out of the