Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 225
Stream or river
5. Sand (and other fi ne sediment)—Usually the least productive macroinvertebrate habitat
in streams, this habitat may be the most prevalent in some streams. Sample banks
of unvegetated or soft soil by bumping the net along the surface of the substrate rather
than dragging the net through the soft substrate; this reduces the amount of debris in
the sample.
In a biological sampling program (i.e., based on our experience), the most common sampling
methods are the transect and the grid. Transect sampling involves taking samples along a straight
line either at uniform or at random intervals (see Figure 8.1). The transect involves the cross section
of a lake or stream or the longitudinal section of a river or stream. The transect sampling method
allows for a more complete analysis by including variations in habitat.
In grid sampling, an imaginary grid system is placed over the study area. The grids may
be numbered, and random numbers are generated to determine which grids should be sampled
(see Figure 8.2). This type of sampling method allows for quantitative analysis because the grids
are all of a certain size. For example, to sample a stream for benthic macroinvertebrates, grids that
are 0.25 m (squared) may be used. Then, the weight or number of benthic macroinvertebrates per
square meter can be determined.
Random sampling requires that each possible sampling location have an equal chance of being
selected. After numbering all sampling locations, a computer, calculator, or a random numbers table
is used to collect a series of random numbers. An illustration of how to put the random numbers to
work is provided in the following example. Given a pond that has 300 grid units, fi nd eight random
sampling locations using the following sequence of random numbers taken from a standard random
numbers table: 101, 209, 007, 018, 099, 100, 017, 069, 096, 033, 041, 011. The fi rst eight numbers of
the sequence for sample collection will depend on the type of sample.
SAMPLE COLLECTION *
Longitudinal transect
FIGURE 8.1 Transect sampling.
Cross-sectional transects
After establishing the sampling methodology and the sampling locations, the frequency of sampling
must be determined. The more samples collected, the more reliable the data will be. A frequency of
once a week or once a month will be adequate for most aquatic studies. Usually, the sampling period
covers an entire year so that yearly variations may be included. The details of sample collection will
∗ The following procedures are suggested by USEPA, 2000b.
Lake or reservoir
Cross-sectional transects