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

and 25 percent, respectively, of the sample mean
at a 95 percent probability level, if 10 replicates
were collected. (See Biometrics Section.)
Precision would, of course, be increased if
additional samples were collected, or if the
sampling method were more precise.
Si nce the assumptions necessary for the
statistical calculations shown in Tables 3 and 4
are not likely met in the data of different
investigators collected from different habitats,
the above calculations only provide a gross
approximation of the precision to be expected.
They do, however, serve to emphasize the very
imprecise nature of grab sample data and the
resultant need for careful stratification of the
type of the habitat sampled and sample replication.
TABLE 4. MEAN COEFFICIENTS OF
VARIAnON (EXPRESSED AS PERCENTAGE)
FOR NUMBERS OF INDIVIDUALS AND
NUMBERS OF TAXA OF MACROINVERTE­
BRATES COLLECTED IN DIFFERENT
SUBSTRATES BY GRAB-TYPE DEVICES
AND A CORER DEVICE*
Sampling
device
Mud
Ind. Taxa
Substrate
Sand
Ind. Taxa
Gravel
Ind. Taxa
Ekman 49 40 41 21 106 74
Petersen 41 29 50 41 49 20
Ponar 46 25 38 33 48 19
Corert 50
*Calculated from data in references (8,16,31, 45, 53, 54).
tOligochaetes only.
3.3.2 Sieving devices
For quantitative sampling, the well-known
Surber square-foot sampler (2, 57) is the most
commonly used sieving device. This device can
be used only in flowing water having depths not
greater than 18 inches and preferably less than
12 inches. It is commonly used for sampling the
rubble and gravel riffles of small streams and
may be used in pools where the water depth is
not too great.
MACROINVERIEBRAIE SIEVING AND CORING DEVICES
9
When using a sieving-type device for quantitative
estimates, reliability may be affected by:
• adequacy of seating of the frame on the
substrate
• backwash resulting from resistance of the
net to water flow - at high velocity of flow
this may be significant
• care used in recovering the organisms from
the substrate materials
• depth to which the substrate is worked
• drift of organisms from areas upstream of
the sample site
To reduce the possibility of bias resulting
from upstream disturbance of the substrate,
always stand on the downstream side of a sieving
device and take replicates in an upstream or
lateral direction. Never start in the upstream
portion of a pool or riffle and work in a downstream
direction.
The precision of estimates of standing crops
of macrobenthos obtained with Surber-type
sieving devices varies widely and depends on a
number of factors including the uniformity of
substrate and distribution of organisms therein,
the care used in collecting samples, and level of
sample replication.
For a large series of Surber samples from
southeastern U. S. trout streams, the coefficient
of variation (C) ranged from 11 percent to
greater than 100 percent (Table 3). The mean
value of C was near 50 percent, and more than
one-half of the C values fell between 30 and 50
percent. These values are similar to the 20 to 50
percent reported by Allen (1) and for those
discussed above for grab sample data.
3.3.3 Coring devices
Included in this category are single- and
multiple-head coring devices, tubular inverting
devices, and open-ended stovepipe-type devices.
Coring devices are described in Standard
Methods (2) and Welch (57). Corers can be used
at various depths in any substrate that is
sufficiently compacted so that the sample is
retained; however, they are best suited for
sampling the relatively homogeneous soft
sediments of the deeper portions of lakes.