Biological field and laboratory methods for measuring the quality of ...

predetermined depth. Grabs with springactivated
closing devices include the Ekman,
Shipek, and Smith-McIntyre; gravity-closing
grabs include the Petersen,* Ponar, and Orange
Peel. Excellent descriptions of these devices are
given in Standard Methods (2) Welch (57). Grabs
are useful for sampling at all depths in lakes,
estuaries, and rivers in substrates ranging from
soft muds through gravel.
In addit ion to the previously discussed
problems related to the patchy distribution of
organisms in nature, the number and kinds of
organisms collected by a particular grab may be
affected by:
• depth of penetration
• angle of closure
• completeness of closure of the jaws and loss
of sample material during retrieval
• creation of a "shock" wave and consequent
"wash-out" of near-surface organisms
• stability of sampler at the high-flow
velocities often encountered in rivers.
Depth of penetration is a very serious problem
and depends on the weight of sampler as
opposed to the particle size and degree of
compaction of the bottom sediments. The
Ekman grab is light in weight and most useful
for sampling soft, finely divided substrates
composed of varying proportions of fine sand,
clay, slit, pulpy peat, and muck. For clay
hardpan and coarse substrates, such as coarse
sands and gravels, the heavier grabs such as the
orange peel or clam shell types (Ponar, Petersen,
Smith-McIntyre) are more satisfactory.
Auxiliary weights may be added to aid penetration
of the substrate and to add stability in
heavy currents and rough waters.
Because of differences in the depth of penetration
and the angle of "bite" upon closure,
data from the different grabs are not comparable.
The Ekman essentially encloses a square,
which is equal in area from the surface to
*Forest Modification of the Petersen grab described in Welch
(57).
7
MACROINVERTEBRATE GRABS
maximum depth of penetration before closure.
In soft substrates, for which this grab is best
suited, the penetration is quite deep and the
angular closure of the spring-loaded jaws has
very little effect on the volume of sample
collected. In essence this means that if the depth
of penetration is 15 em, the organisms lying at
that depth have the same opportunity to be
sampled as those lying near the surface.
In clam-shell type grabs, such as the Petersen,
Ponar, Shipek, and Smith-McIntyre, the original
penetration is often quite shallow: because of
the sharp angle of "bite" upon closure, the area
enclosed by the jaws decreases at increasing
depths of substrate penetration. Therefore,
within the enclosed area, organisms found at
greater depths do not have an equal opportunity
to be sampled as in the case of the Ekman grab
and other sampling methods described in the
next section. This problem is particularly true of
the Shipek sampler - the jaws do not penetrate
the substrate before closure and, in profile, the
sample is essentially one-half of a cylinder.
Probably one of the most frustrating aspects
of sampling macroinvertebrates with various
types of grabs relates to the problem of incomplete
closure of the jaws. Any object - such as
clumps of vegetation, woody debris, and gravel
- that cannot be sheared by the closing action
of the jaws often prevents complete closure. In
the order of their decreasing ability to shear
obstructing materials, the common grabs may be
ranked: Shipek, Smith-McIntyre, Orange Peel,
Ponar, Petersen, and Ekman. If the Ekman is
filled to within more than 5 em of the top,
there may be loss of substrate material on
retrieval (16). An advantage of the Ekman grab
is that the surface of the sediment can be
examined upon retrieval, and only those samples
in which the sediment surface is undisturbed
should be retained.
All grabs and corers produce a "shock" wave
as they descend. This disturbance can affect the
efficiency of a sampler by causing an outward
wash (blow-out) of flocculent materials near the
mud-water interface that may result in