10500

10500 BENTHIC MACROINVERTEBRATES*
10500 A. Introduction
1. Definition
Benthic macroinvertebrates are animals that inhabit the sedi-
ment or other substrates on the bottom of freshwater, estuarine,
and marine ecosystems. During their life cycles, these organisms
may construct attached cases, tubes, or nets that they live on or
in; roam freely over rocks, organic debris, and other substrates;
or burrow freely in substrates. According to historical defini-
tions, macroinvertebrates are visible to the unaided eye and
retained on a U.S. Standard No. 30 sieve (0.595- or 0.600-mm
openings).
1
In actuality, however, some are difficult to see with-
out magnification.
The standard sieve for collecting freshwater, estuarine, and
marine benthic macroinvertebrates is the U.S. Standard No. 30
sieve; however, some estuarine and marine programs use the
U.S. Standard No. 50 sieve (0.300-mm openings) or the U.S.
Standard No. 35 sieve (0.500-mm openings). For all aquatic
assessment programs, the No. 30 sieve is recommended for
collecting benthic fauna from freshwater, estuarine, and marine
habitats or from any water transport system. To accommodate
some old historical databases (if the study’s data-quality objec-
tives permit), investigators might use a U.S. Standard No. 28
sieve (1.0-mm openings). To obtain a more representative sam-
ple of the benthos (one that includes smaller forms, early life-
stages, and other taxa of macroinvertebrates), they may use a
U.S. Standard No. 60 sieve (0.250-mm openings).
The standardization of bioassessment for species composition,
taxa richness, diversity, evenness, trophic levels, and major
taxonomic spatial and temporal patterns may be enhanced sig-
nificantly by the conventional use of a U.S. Standard No. 30
sieve.
The major macroinvertebrates found in freshwater are flat-
worms, annelids, mollusks, crustaceans, and insects. The major
macroinvertebrates found in estuarine and marine waters are
bryozoans, sponges, annelids, mollusks, roundworms, cnidarians
(coelenterates), crustaceans, insects, and echinoderms.
2. Response to Environment
The species composition and density (numbers of individuals
per unit area) of macroinvertebrate communities in streams,
lakes, estuaries, and marine waters can be uniform from year to
year in unperturbed environments. However, life-cycle dynamics
produce variations in species composition and abundance either
temporally or spatially.
Most aquatic habitats—particularly free-flowing streams and
waters with acceptable water quality and substrate conditions—
support diverse macroinvertebrate communities in which the
total number of individuals is divided into a reasonably balanced
distribution of species. Such communities respond to changing
habitats and water quality by altering community structure (in-
vertebrate abundance and composition). However, many habi-
tats— especially disturbed ones—may be dominated by a few
species.
The responses of macroinvertebrate communities to environ-
mental changes are useful in assessing the effects of municipal,
industrial, oil, and agricultural wastes, and of other land uses on
surface waters. Investigators have documented macroinverte-
brate community responses to increased inorganic nutrients,
increased organic loading, substrate alteration, and toxic chem-
ical pollution. Inorganic nutrients and severe organic pollution
usually reduce the variety of macroinvertebrates to only the most
tolerant ones [usually associated with low dissolved oxygen
(DO) concentration] and increase their density. Sometimes se-
vere organic pollution, siltation, or toxic chemical pollution may
reduce or even eliminate the entire macroinvertebrate commu-
nity from an affected area. However, macroinvertebrate re-
sponses may be mediated by other environmental (biological,
chemical, and physical) conditions.
Assessing the effects of a pollution source generally involves
comparing macroinvertebrate communities and their physical
habitats in polluted areas with those in adjacent unpolluted sites
(e.g., a gradient away from point sources of contamination). The
procedure includes sampling and analyzing communities from
different sites and subsequently determining whether and how
they differ. The basic information required in most structure
analyses cases is a count of individuals per taxon. Then, inves-
tigators can characterize and compare communities according to
community structure, density, diversity, metrics, pollution indi-
cators, or other analyses,
1
including various statistical methods
(see 10500D). They also can estimate biomass and productiv-
ity.
1– 4
Equally important is a characterization of DO concentra-
tion, substrate, water depth, sediment type and grain size, total
organic carbon (TOC), and other site- and situation-specific
characteristics.
While the following macroinvertebrate methods are tradition-
ally used to sample and quantify benthic invertebrate communi-
ties, other methods are also being evaluated in an effort to
develop and implement narrative biological criteria for surface
waters.
5
Not discussed here are U.S. Environmental Protection
Agency (EPA)-developed rapid bioassessment techniques
6,7
and
Environmental Monitoring and Assessment Program (EMAP)
protocols for field operations, as well as methods for sampling
macroinvertebrates and assessing the ecological conditions of
wadeable streams.
8,9
3. References
1. KLEMM, D.J., P.A. LEWIS,F.FULK & J.M. LAZORCHAK. 1990. Mac-
roinvertebrate Field and Laboratory Methods for Evaluating the
Biological Integrity of Surface Waters, EPA-600/4-90-030. Envi-
* Approved by Standard Methods Committee, 2010.
Joint Task Group: Donald J. Klemm (chair), Peter M. Chapman, Philip A. Lewis,
Don W. Schloesser, William T. Thoeny.
1

ronmental Monitoring Systems Laboratory, U.S. Environmental
Protection Agency, Cincinnati, Ohio.
2. WETZEL, R.G. & G.E. LIKENS. 1991. Limnological Analyses, 2nd ed.
Springer-Verlag, New York, N.Y.
3. PLANTE, C. & J.A. DOWNING. 1989. Production of freshwater inver-
tebrate populations in lakes. Can. J. Fish. Aquat. Sci. 46:1489.
4. PLANTE, C. & J.A. DOWNING. 1990. Empirical evidence for differ-
ences among methods for calculating secondary production. J. N.
Amer. Benthol. Soc. 9(1):9.
5. GIBSON, G.R., ed. 1996. Biological criteria. In Technical Guidance
for Streams and Small Rivers, EPA-822-B-96-001. Off. Science &
Technology, U.S. Environmental Protection Agency, Washington,
D.C.
6. PLAFKIN, J.L., M.T. BARBOUR, K.D. PORTER, S.K. GROSS & R.M.
HUGHES. 1989. Rapid bioassessment protocols for use in streams and
rivers. In Benthic Macroinvertebrates and Fish, EPA-440/4-89-001.
Off. Water, U.S. Environmental Protection Agency, Washington,
D.C.
7. BARBOUR, M.T., J. GERRITSEN, B.D. SNYDER & J.B. STRIBLING. 1999.
Rapid Bioassessment Protocols for Use in Streams and Wadeable
Rivers: Periphyton, Benthic Macroinvertebrates, and Fish, EPA
841-B-99-002. Off. Water, U.S. Environmental Protection Agency,
Washington, D.C.
8. LAZORCHAK, J.M., D.J. KLEMM & D.V. PECK. 1998. Environmental
Monitoring and Assessment Program-Surface Waters: Field Oper-
ations and Methods for Measuring the Ecological Conditions of
Wadeable Streams, EPA 620-R-94-004F. National Exposure Re-
search Lab., U.S. Environmental Protection Agency, Cincinnati,
Ohio, and National Health & Environmental Effects Research Lab.,
U.S. Environmental Protection Agency, Corvallis, Ore.
9. BARBOUR, M.T. 1997. Perspectives from North America on the
programmatic and scientific elements of an effective bioassessment.
Human Ecol. Risk Assess. 3:929.
4. Bibliography
HYNES, H.B.N. 1963. The Biology of Polluted Waters. Liverpool Univ.
Press, England.
MACAN, T.T. 1963. Freshwater Ecology. John Wiley & Sons, New York,
N.Y.
RUTTNER, F. 1966. Fundamentals of Limnology. Univ. Toronto Press,
Toronto, Ont.
MACKENTHUN, K.M. & W.M. INGRAM. 1967. Biological Associated Prob-
lems in Freshwater Environments. Federal Water Pollution Control
Admin., Washington, D.C.
HYNES, H.B.N. 1970. The Ecology of Running Waters. Univ. Toronto
Press, Toronto, Ont.
ODUM, E.P. 1971. Fundamentals of Ecology, 3rd ed. Saunders Publish-
ing Co., Philadelphia, Pa.
COULL, B.C., ed. 1977. Ecology of Marine Benthos. University of South
Carolina Press, Columbia.
PEARSON,T.H.&R.ROSENBERG. 1978. Macrobenthic succession in
relation to organic enrichment and pollution of the marine environ-
ment. Oceanogr. Mar. Biol. Annu. Rev. 16:229.
BROWER, J.R. & J.H. ZAR. 1984. Field and Laboratory Methods for
General Ecology. Wm. C. Brown Publ., Dubuque, Iowa.
RESH, V.H. & D.M. ROSENBERG. 1984. The Ecology of Aquatic Insects.
Praeger Publ., New York, N.Y.
WARD, J.V. 1984. Ecological perspectives in the management of aquatic
insects. In V.H. Resh & D.M. Rosenberg, eds. The Ecology of
Aquatic Insects, p. 558. Praeger Scientific, New York, N.Y.
WIEDERHOLM, T. 1984. Responses of aquatic insects to environmental
pollution. In V.H. Resh & D.M. Rosenberg, eds. The Ecology of
Aquatic Insects, p. 508. Praeger Scientific, New York, N.Y.
CUMMINS, K.W. & M.A. WILZBACH. 1985. Field Procedures for Analysis
of Functional Feeding Groups of Stream Macroinvertebrates, Con-
tribution 1611. Appalachian Environ. Lab., Univ. Maryland, Frost-
burg.
LEONARD, P.M. & D.J. ORTH. 1986. Application and testing of an index
of biotic integrity in small, coolwater streams. Trans. Amer. Fish.
Soc. 115:401.
WARWICK, R.M. 1986. A new method for determining pollution effects
on marine macrobenthic communities. Mar. Biol. 92:557.
HILSENHOFF, W.L. 1987. An improved biotic index of organic stream
pollution. Great Lakes Entomol. 20:31.
ROHM, C.M., J.W. GIESE & C.C. BENNETT. 1987. Evaluation of aquatic
ecoregion classification of streams in Arkansas. Freshwater Ecol.
4:127.
SHACKELFORD, B. 1988. Rapid Bioassessments of Lotic Macroinverte-
brate Communities: Biocriteria Development. Arkansas Dept. Pol-
lution Control & Ecology, Little Rock.
STEEDMAN, R.J. 1988. Modification and assessment of an index of biotic
integrity to quantify stream quality in southern Ontario. Can. J.
Fish. Aquat. Sci. 45:492.
U.S. ENVIRONMENTAL PROTECTION AGENCY. 1988. Proc. 1st National
Workshop on Biological Criteria, Lincolnwood, Ill., Dec. 2– 4,
1987, Rep. No. 905/9-89/003. U.S. Environmental Protection
Agency, Chicago, Ill.
WHITTIER, T.R., R.M. HUGHES & D.P. LARSEN. 1988. Correspondence
between ecoregions and spatial patterns in stream ecosystems in
Oregon. Can. J. Fish. Aquat. Sci. 45:1264.
OHIO ENVIRONMENTAL PROTECTION AGENCY. 1989. Biological Criteria for
the Protection of Aquatic Life: Volume III. Standardized Biological
Field Sampling and Laboratory Methods for Assessing Fish and
Macroinvertebrate Communities. Ohio Environmental Protection
Agency, Div. Water Quality Monitoring & Assessment, Surface
Water Section, Columbus.
REYNOLDSON, T.B., D.W. SCHLOESSER & B.A. MANNY. 1989. Develop-
ment of a benthic invertebrate objective for mesotrophic Great
Lakes waters. J. Great Lakes Res. 15:669.
BURD, B.J., A. NEMEC & R.O. BRINKHURST. 1990. The development and
application of analytical methods in benthic marine infaunal stud-
ies. Advan. Mar. Biol. 26:169.
NATIONAL RESEARCH COUNCIL. 1990. Managing Troubled Waters: The
Role of Marine Environmental Monitoring. National Academy
Press, Washington, D.C.
WESTON, D. 1990. Quantitative examination of macrobenthic changes
along an organic enrichment gradient. Mar. Ecol. Prog. Ser. 61:
233.
YOUNT, J.D. & G.J. NIEMI. 1990. Recovery of lotic communities and
ecosystems from disturbance—a narrative review of case studies.
Environ. Mgmt. 14:547.
KARR, J.R. & B.L. KERANS. 1992. Components of biological integrity:
their definition and use in development of an invertebrate IBI. In
T.P. Simon & W.S. David, eds. Environmental Indicators: Mea-
surement and Assessment Endpoints, Chapter 1, EPA 905-R-92-
003. U.S. Environmental Protection Agency, Chicago, Ill.
BLAKE,J.A.&A.LISSNER. 1993. Taxonomic atlas of the benthic fauna
of the Santa Maria Basin and Western Santa Barbara Channel, Vol.
1. Santa Barbara Mus. Natural History, Santa Barbara, Calif.
CAIRNS, J., JR. 1993. A proposed framework for developing indicators of
ecosystem health. Hydrobiologia 263:1.
ROSENBERG, D.M. & V.H. RESH, eds. 1993. Freshwater Biomonitoring
and Benthic Macroinvertebrates. Chapman-Hall, New York, N.Y.
GURTZ, M.E. 1994. Design of biological components of the National
Water-Quality Assessment (NAWQA) Program. In S.L. Loeb & A.
Spacie, eds. Biological Monitoring of Aquatic Systems. Lewis
Publishers, Ann Arbor, Mich.
DAVIS, W.S. & T.P. SIMON, eds. 1995. Biological Assessment and
Criteria: Tools for Water Resource Planning and Decision Making.
Lewis Publishers, Boca Raton, Fla.
BENTHIC MACROINVERTEBRATES (10500)/Introduction
2
BENTHIC MACROINVERTEBRATES (10500)/Introduction

HAUER, F.R. & G.A. LAMBERTI, eds. 1996. Methods in Stream Ecology.
Academic Press, New York, N.Y.
KARR, J.R. 1996. Aquatic invertebrates: sentinels of watershed condi-
tion. Wings 19(2):22.
PATRICK, R. 1996. Rivers of the United States: Vol. III: The Eastern
and Southeastern States. John Wiley & Sons, New York, N.Y.
PEPPER, I.L., C.P. GERBA & M.L. BRUSSEAU. 1996. Pollution Science.
Academic Press, San Diego, Calif.
U.S. ENVIRONMENTAL PROTECTION AGENCY. 1996. Biological Criteria:
Technical Guidance for Streams and Small Rivers, EPA 822-B-96-
001. Off. Science & Technology, U.S. Environmental Protection
Agency, Washington, D.C.
BARBOUR, M.T. 1997. The re-invention of biological assessment in the
U.S. Human Ecol. Risk Assess. 3:933.
U.S. ENVIRONMENTAL PROTECTION AGENCY. 1997. Priorities for Ecological
Protection: An Initial List and Discussion Document for EPA, EPA
600-S-97-002. Off. Research & Development, U.S. Environmental
Protection Agency, Washington, D.C.
MACBROOM, J.G. 1998. The River Book. Connecticut Dept. Environ-
mental Protection, Hartford.
PATRICK, R. 1998. The Mississippi River and Its Tributaries North of St.
Louis, Vol. IV, Part A. John Wiley & Sons, New York, N.Y.
PATRICK, R. 1998. The Mississippi River and Its Tributaries South of St.
Louis, Vol. IV, Part B. John Wiley & Sons, New York, N.Y.
U.S. ENVIRONMENTAL PROTECTION AGENCY. 1998. Lake and Reservoir
Bioassessment and Biocriteria, Technical Guidance Document,
EPA 841-B-98-007. Off. Water, U.S. Environmental Protection
Agency, Washington, D.C.
BATZER, D.P., R.B. RADER & S.A. WISSINGER. 1999. Invertebrates in
Freshwater Wetlands of North America: Ecology and Management.
John Wiley & Sons, New York, N.Y.
GRIFFITHS, R.W. 1999. BioMAP: Bioassessment of Water Quality. Cen-
tre for Environmental Training, Niagara College, Niagara-on-the-
Lake, Ontario, Canada.
KARR, J.R. & E.W. CHU. 1999. Restoring Life in Running Waters. Island
Press, Washington, D.C.
GERRITSEN, J., B. JESSUP, E.W. LEPPO &J.WHITE. 2000. Development of
lake condition indexes (LCI) for Florida. Tetra Tech, Inc., Owings
Mills, Md., Appendices A-1 to B-13. Florida Dept. Environmental
Protection, Stormwater & Nonpoint Source Management Section,
Tallahassee.
10500 B. Sample Collection
1. General Considerations
Before conducting a benthic survey, determine specific data
quality objectives and clearly define the information sought.
Data quality objectives are qualitative and quantitative state-
ments developed to specify the quality of data needed to support
specific decisions and conclusions about the information sought.
Discussion with water chemists, hydrologists, limnologists, and
individuals from other disciplines will be helpful. Ultimately, the
choice of methodology will depend on whether the habitat to be
studied is a stream, lake, reservoir, or marine area. For example,
investigators only need a few sampling stations upstream and
downstream of a discharge to determine whether the macroin-
vertebrate community downstream is damaged. However, if the
objective is to delimit the extent of damage from a discharge or
series of discharges, they will need reference stations upstream
of, downstream of, and bracketing all discharges. In marine
waters, they may need to sample a nearby estuary. In open ocean
waters, they may need to sample some distance from the dis-
charge point.
Characterize the physicochemical properties of faunal sam-
pling-station substrate and overlying water. Measure such prop-
erties as sediment size class distribution (sand, silt, and clay);
organic content and toxic pollutant concentrations; temperature,
salinity, hardness, alkalinity, DO, total organic carbon (TOC),
ammonia, sulfides, and nutrient (total and dissolved) concentra-
tions; biochemical oxygen demand; water depth; and velocity of
flowing streams.
After gaining a thorough understanding of the water body’s
characteristics, select specific areas to be sampled. There is no
predetermined number of sampling stations that will be appro-
priate for all situations. No water quality survey is routine, nor
can one be conducted totally on a “cookbook” basis. However, if
investigators adhere to the following basic rules, they can design
a sound survey.
a. Always establish a reference station(s) outside the influ-
ence of all wastewater discharges of concern (but in the same
water body). Because most surveys are made to determine the
damage that pollution causes aquatic life, this will be the basis
for comparing biota in polluted and unpolluted areas. Preferably
establish at least two reference stations: one far from the effluent
discharge and the other near the discharge, but not subject to its
influence. (For example, if the discharge were in a river, one
station would be far upstream of the discharge and the other
would be immediately downstream.) Whenever feasible, use
reference stations with physicochemical characteristics similar to
the receiving area’s substrate and overlying water.
b. Locate a station immediately downstream of each dis-
charge or in the affected area in its immediate vicinity, as
appropriate.
c. If the discharge does not mix completely on entering the
body of water, but instead channels along one side or disperses
in a specific direction, then locate stations in the left-bank
(looking upstream), midchannel, and right-bank sections of the
stream; in concentric arcs in lakes and oceanic waters; or in any
other configuration that will meet study objectives.
d. Establish stations at various distances downstream from the
last discharge of concern to determine the linear extent of dam-
age. In a marine environment, a nearby estuary may be sampled;
in open ocean waters, samples may be taken in a nearby area
with comparable currents, depth, sediment characteristics, and
salinity.
e. To permit comparison of macroinvertebrate communities,
be sure that all sampling stations are ecologically similar. For
example, select stations with similar bottom substrate (e.g., sand,
gravel, rock, mud, organic content), depth, presence of riffles and
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection
3
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection

pools, stream width, gradient, flow velocity, bank or shore cover,
salinity, hardness, TOC, nutrient and DO concentrations, and
wave exposure.
f. Collect samples for physical, toxicological (if applicable),
and chemical analyses as close to biological sampling stations as
possible to ensure correlation of findings; take such samples at
the same time and from the same grab when possible. Collect
substrate samples for physicochemical analyses from the upper
few centimeters, where most organisms live.
g. Locate macroinvertebrate sampling stations in the best
physical habitat [areas not influenced by atypical conditions
(bridges, dams, etc.)].
h. Discharges in coastal areas may be subject to various
degrees of salt water intrusion (salt water wedge). Macroinver-
tebrate populations may change drastically in such areas; docu-
ment and/or allow for this effect.
i. When sampling in small, wadeable, first- to third-order
streams, begin at the station farthest downstream and proceed
upstream to minimize disruptions induced by the sampling itself.
This is unnecessary for non-wadeable streams and rivers.
For a long-term biological monitoring program, consider col-
lecting macroinvertebrates at each station at least once during
each of the annual seasons, though this may not always be
necessary and would depend on the study design.
1
More frequent
sampling may be necessary if effluents’ characteristics change or
spills occur. Make allowance for collections at night where
“drift” or night feeding organisms are of special concern. In
general, the most critical period for macroinvertebrates in
streams is during periods of high temperature and low flow,
whereas in estuarine and marine environments it is the period of
maximum stratification and poor vertical mixing. If available
time and funds limit sampling frequency, make at least one
survey during the critical time.
2. Sampling Design
Some terms have multiple meanings. In biology, for example,
apopulation is a group of individuals that are all members of the
same species or taxonomic group. In statistics, a population is
the entire set of values for the characteristic of interest in a whole
sampling universe. For example, researchers interested in deter-
mining the mean density of worms on a lake bottom might take
ten grabs from lake sediments. The number of worms in each
grab would be an observation, the density of worms would be the
characteristic of interest, and the contents of each grab would be
an experimental unit or sampling unit. The entire lake bottom
would be the sampling universe and enough grabs to equate with
the area of the entire lake bottom would be the population (of
units).
Similarly, the term sample has two often contradictory uses. In
typical studies, observations usually are not made of all possible
sampling units; instead, observations are only made of a small
fraction of the total. Statistically, this set of observations is called
asample. In the example given above, the ten grabs collectively
would be a sample. However, in everyday language (and as used
in this book and most scientific publications), a sample is a
portion of the real world that has been selected for measurement
(e.g., a water sample, plankton haul, or bottom grab). Therefore,
in the example above, each individual grab would be a sample
(i.e., “ten samples were taken”).
Collecting a representative statistical sample is difficult be-
cause of variation in successive scientific samples. Without
knowing the sampling variability, investigators cannot know the
degree to which the data truly represent the population. Make
replicate observations of a population if definitive statistical
inferences about the population will be made.
2–11
Standardize sampling design to consider the following re-
quirements:
a. Approximate the set of all samples that can be selected
(i.e., separate the sampling universe into all possible samples).
For example, if the location (site) containing the population has
an area of 1000 m
2
and the sampling device samples an area of
1m
2
, then 1000 samples could be collected in the sampling
universe.
b. Assign each sample an equal probability of being selected.
Using the situation above, divide the area to be sampled into
1000 discrete units.
c. Use a table of random numbers to select sites for sampling
(i.e., sample randomly, not haphazardly).
d. The sampling design outlined above is known as simple
random sampling. When using this design, it is often advanta-
geous to determine the number of samples necessary for a certain
level of precision:
N⫽
冉
t⫻s
D⫻x¯
冊
2
where:
N⫽number of samples,
t⫽tabulated tvalue at 0.05 level with the degrees of
freedom of preliminary survey (generally t⬇2.0 at
larger sampling sizes),
s⫽sampling standard deviation of samples, known from a
preliminary survey,
D⫽required level of precision expressed as a decimal (0.30
to 0.35 usually yields a statistically reliable estimate),
and
x¯ ⫽sample mean density of preliminary survey.
To estimate the number of samples needed, analysts first need
specific information (the mean and standard deviation) about the
population to be sampled. Because this information is unknown
(because sampling has yet to occur), estimate the population’s
mean and standard deviation by one of three ways: conduct a
pilot study, use results from an earlier or similar study, or make
educated estimates.
3,12
For example, if investigators want to
determine the mean chironomid density in relatively homoge-
neous lake sediments during summer, and they know that six
grabs taken the previous summer produced a mean density of
4230 chironomids/m
2
and a standard deviation of 1628 chirono-
mids/m
2
, then they can use these data to estimate their study’s
mean chironomid density [⫾30%, with a 5% probability of error
(
␣ ⫽0.05)]. Using the formula given above,
N⫽
冉
2.5706 ⫻1628
0.30 ⫻4230
冊
2
(t⫽2.5706 at a 5% probability of error and 5 degrees of
freedom)
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection
4
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection

N⫽10.88 ⬇11
Thus, it is estimated that 11 grabs will be necessary.
e. A simple random sampling design is useful when sampling
relatively homogeneous areas. However, most taxa are not dis-
tributed uniformly over water bottoms. Different habitats (sand,
mud, gravel, or organic material) support different densities and
species of organisms. In which case, a stratified random design
is more useful.
In a stratified random sampling design, a heterogeneous uni-
verse (different bottom substrates, current velocities, depths,
temperatures, etc.) is divided into more homogeneous strata.
Once the strata are defined, use simple random sampling within
each stratum. Stratified random sampling has two important
advantages: it provides data on various subsets of a population
(e.g., density of benthic invertebrates in each sediment type), and
it reduces variability because it deals with more homogeneous
subpopulations, allowing for more accurate and precise popula-
tion estimates.
The data needed to divide the population into various strata
usually is acquired via pre-study reconnaissance (a pilot study).
A systematic sampling design often is used in such pilot studies.
In a systematic-transect design, investigators conduct sampling
at equal intervals along a number of transects in a habitat to
identify and locate existent strata.
3,12
f. In descriptive studies, investigators should take at least
three replicate sampling units per station.
3,13
If statistical testing
is planned, more replicates probably will be needed.
g. Standardize data acquisition and recording when practical.
Use metric units.
3. Sampling Devices, Quantitative
Quantitative and qualitative samplers have been designed to
collect organisms from the bottom of different water bodies. The
most common quantitative sampling devices are the Petersen,
Ponar
®
,* and Ekman grabs and the Surber or square-foot stream
bottom sampler, all described below.
a. Grab samplers:
Before using each grab sampler, calibrate it for actual surface
area sampled.
1) The Petersen grab (Figure 10500:1) is used for sampling
hard bottoms (e.g., sand, gravel, marl, and clay) in swift currents
and deep water.
3
It is an iron, clam-type grab manufactured in
various sizes that will sample an area between 0.06 and 0.09 m
2
.
It weighs approximately 13.7 kg, but may weigh as much as
31.8 kg when auxiliary weights are bolted to its sides. The extra
weights make the grab stable in swift currents and provide more
cutting force in fibrous or firm bottom materials. Modify the
sampler by adding end plates, by cutting large strips out of the
top of each side, and by adding a hinged 30-mesh screen (as in
the Ponar grab).
14
To use the Petersen grab, set the hinged jaws and lower to the
bottom slowly to avoid disturbing lighter bottom materials. Ease
rope tension to release the catch. As the grab is raised, the lever
system closes the jaws.
2) The Ponar grab (Figure 10500:2) is used increasingly in
medium to deep rivers, lakes, reservoirs, and estuaries.
15
It is
similar to the Petersen grab in size, weight, lever system, and
sample compartment, but has side plates and a screen on top of
the sample compartment to prevent sample loss during closure.
With one set of weights, the standard 23- ⫻23-cm sampler
weighs 20 kg. A 15- ⫻15-cm petite Ponar may be used. The
large surface disturbance associated with a Ponar grab can be
reduced by installing hinged (rather than fixed) screen tops,
thereby reducing the pressure wave associated with the sam-
pler’s descent. This sampler is best used for mud, sand, gravel,
or small rocks with mud, but it can be used in all substrates
except bedrock.
3) The Van Veen grab (Figure 10500:3) is used to sample in
open marine waters and in large lakes. The sampler’s long arms
tend to act as stabilizers without disturbing water at the water–
substrate interface. It is basically an improved version of the
Petersen grab for mud, gravel, pebble, and sand substrates. The
sampler is heavy; lower it from a boat or ship platform via
mechanical or hydraulic lifts.
4) The Smith-McIntyre grab (Figure 10500:4) has the heavy
steel construction of the Petersen, but its jaws are closed by
strong coil springs.
16
Its chief advantages are stability and easier
control in rough water. Its bulk and heavy weight require oper-
ation from a large boat equipped with a winch. The 45.4-kg grab
can sample an area of 0.2 m
2
,
17–19
but smaller models (0.1 m
2
or
0.05 to 0.06 m
2
) are available.
* Registered trademark of Morris & Lee, Inc. d/b/a Wildlife Supply Co., Buffalo,
NY.
Figure 10500:1. Petersen grab.
Figure 10500:2. Ponar
®
grab.
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection
5
BENTHIC MACROINVERTEBRATES (10500)/Sample Collection