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Chapter I: Zoobenthos of Freshwaters- An Introduction
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Updated: October 09, 1013
Chemical vs Biological monitoring
(Thorp and Covich, 1991; and Williams and Feltmate, 1992)
The most successful terrestrial
phylum and one of the most prominent freshwater taxa is Arthropoda. Its
three subphyla with freshwater members- Uniramia (aquatic insects),
Chelicerata (water mites and aquatic spiders) and Crustacea (crayfish,
fairy shrimp, copepods, etc.)- are all diverse and important components
of lakes and streams. Arthropods occupy every heterotrophic niche in
benthic and pelagic habitats of most permanent and temporary aquatic
systems. These metameric coelomates are characterized by a chitinous
exoskeleton and stiff, jointed appendages modified as legs, mouthparts,
and antennae (except in water mites).
The greatest diversity in form and
habit is exhibited by the insects. They occupy every kind of freshwater
habitat imaginable, including temporary streams and ponds, the
shallowest and deepest areas of lakes, the most pristine and polluted
rivers, roadside ditches, eaves troughs, moss, within and on
macrophytes and all ranges of water chemistry, from acidified to
alkaline bodies of water. They also represent all the functional
feeding groups, including predators, shredders, grazers, (or scrapers),
filter feeders, gatherers, piercers and parasites.
Insects can be separated
immediately from other arthropod classes by the presence of: 1) one
pair of antennae; 2) three pairs of segmented legs in adults and most
larvae (only the larvae of true flies lack segmented legs); and 3) one
to two pair of wings on the adults.
They are conveniently divided
into three taxonomic groups based on the type of wings that develop
from the larval or nymphal stages and on the type of life stages
- Those without wings are apterous
(Greek: a=without; pterous=wings), and they have no change in body form
after hatching from the egg. This type of development is called ametabolous (Greek:a=without; metabolous=change) metamorphosis. Only the springtails of the order, Collembola have ametabolous development in aquatic environments.
- The remaining two groups have winged adults. The wings develop externally in the exopterous (Greek: exo=outside; pterous=wings) forms, and internally in the endopterous (Greek: endo=inside; pterous=wings) forms.
- The development stages of the exopterous forms are called
nymphs or naiads and they closely resemble the adults in appearance.
The nymphs undergo several molts, or instars, before transforming into
an adult. The act of shredding or casting the old skin is called
ecdysis and the cast skin is called an exuvium (plural=exuviae),
commonly referred to as the "shuck". The adult is called the imago and
it differs from the last instar in having fully developed wings and
sexual organs. The entire process, from egg through several nymphal
instars, is called hemimetabolous (Greek: hemi=incomplete or inseparable; metabolous=change) development.
- Only three orders of insects have hemimetabolous development, the Odonata (dragonflies and damselflies), Ephemeroptera (mayflies) and Plecoptera (stoneflies).
- Very similar to the hemimetabolous forms are the paurometabolous
(Greek: pauros=little, small; metabolous=change) forms in which wings
develop externally on the nymphs but the nymphs and adults are often
difficult to distinguish and both live in the same habitat and feed
similarly. Development is gradual and the changes between instars are
subtle and barely noticeable in most instances.
- Only one order, the Hemiptera (true bugs) have paurometabolous development. Aquatic Orthoptera (crickets) also have this type of development but they are rare.
- The endopterous forms have a holometabolous
(Greek: holo=complete, metabolous=change) type of development in which
the egg develops into a worm-like larva. The larvae have no external
evidence of wings. Segmented legs and antennae are usually present but
may be missing in a few orders. Once the larva is fully grown, it
transforms into a resting stage, called the pupa, during which it
metamorphoses into an imago. The wings, legs, antennae and compound
eyes develop in the pupal stage.
- Most species of aquatic insects have holometabolous development, including all the Megaloptera (dobsonflies, alderflies, hellgrammites), Neuroptera (spongeflies), Lepidoptera (aquatic butterflies), Trichoptera (caddisflies), Coleoptera (beetles) and Diptera (true flies).
- Note that "true flies" is two words; only flies of the order
Diptera are spelled with two words, for example, crane flies, black
flies, midge flies, etc.; all other "false flies" are spelled with one
word, e.g. mayflies, caddisflies, etc.
Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera
(caddisflies) and Diptera (true flies) are commonly, or perhaps always,
the four orders used in environmental impact assessments. For this
reason, more emphasis is placed on these orders than on other orders of
insects (Mackie, 1998).
(Excerpts from Allan, 1995)
Email from Prof. Dr. Noel Hynes, University of Waterloo, Ontario:
Prof. Dr. Noel Hynes
received the highest award in Limnology in the world, the coveted
Naumann-Thienemann Medal for 1998 for establishing lotic limnology
Date: Fri, 20 Mar 1998 10:05:30
From: Noel Hynes
sorry, but I have only my own copy of The Ecology of Running Waters.
Many thousands were printed, so it must be available on the second-hand
market, but it is very out of date. I think that it was responsible for
an enormous advance in our knowledge, during the nearly 30 years since
its publication, because people became confident that the field had
been reviewed. I worked very hard in order to make that review
complete, and I always refused to do a second edition because I knew
that it would not be possible to make a complete review and I did not
wish to get into the "recent advances" loop. The best current text on
running water is J.D. Allen, 1995, Stream ecology. Chapman and Hall
ISBN 0 412 29430.
............................................................................... Noel Hynes
Substrate is a complex aspect of the
physical environment. What comes to mind first are the cobbles and
boulders in the bed of a mountain stream, and silts and sands that are
more typical of lowland rivers. Organic detritus is found in
conjunction with mineral material, and can strongly influence the
organism's response to substrate. Determination of the role of
substrate is further complicated by its tendency to interact with other
environmental factors. Fro example, slower currents, finer substrate
particle size and (possibly) lower oxygen are often correlated. In
addition, the size and amount of organic matter, which affect algal and
microbial growth, vary with substrate. This natural covariation of
environmental factors makes it very difficult to ascribe causality from
Table 1: The classification of mineral substrates by particle size, according to the Wentworth Scale
|Size Category||Particle Diameter|
(range in mm)
| Very coarse||1-2
| Very fine||0.063-0.125
Substrate ofcourse depends on the
parent material available, but there is a general tendency for particle
size to decrease as one proceeds downstream.
Very small organic particles (less than 1
mm) usually serve as food rather than as substrate, except perhaps for
the smallest invertebrates and micro-organisms. Larger organic
material, from plant stems to submerged logs, generally functions as
substrate rather than food. However, autumn-shed leaves on the
streambed are a substrate to insects that graze algae from their
surfaces, and food to insects that eat the leaves themselves.
Aggregations of leaves on the stream bottom usually support the
greatest diversity and abundance of invertebrates, and the addition of
leaves to mineral substrates results in higher densities of animals.
Even logs meet the nutritional needs of some invertebrates. More
commonly, however, large organic substrates serve as perches from which
to capture food items transported in the water column, as sites where
fine detrital material accumulates, and as surfaces for algal growth.
Fine-scale heterogeneity in
current and mineral substrate affects the distribution of organic
detrital particles, and the availability of detritus influences the
distribution of organisms within the substrate.
Characteristic fauna of major substrate categories
The great majority of stream-dwelling
macroinvertebrates live in close association with the substrate, and so
they have been the main focus of organism-substrate studies. When one
compares broad categories such as sand, stones, and moss, many taxa
show some degree of substrate specialization. When one examines
preferences among stones of various sizes, substrate specialization is
less apparent, and preference is often exhibited as statistical
patterns of abundance across the particle size spectrum. However, some
stream-dwelling organisms are quite restricted in the conditions they
are those found in association with stony substrates. Streambeds of
gravel, cobble and boulders occur in a great many areas around the
world, harbouring a diverse fauna that Hynes (1970) remarks is broadly
similar almost everywhere. Many species are equally common on stones of
all sizes, some are demonstrably more likely to be found associated
with a particular size class, and a few are highly restricted in their
- Larvae of the water penny (Psephenidae)
occur mainly on the undersides of rocks, and often under boulders in
torrential flow. Pyralid moth larvae live underneath silken shelters
constructed within depressions on rock surfaces.
- Attached and encrusting growth forms require a substrate that
is not easily overturned by current, and large stones are ofcourse more
stable. The longer the life-span, the more critical this is.
- Because they grow more slowly, mosses, bryozoans and sponges
are found mainly on larger stones or in locations where scouring is
is generally considered to be a poor substrate, especially for
macroinvertebrates, due to its instability, and because tight packing
of sand grains reduces the trapping of detritus and can limit the
availability of oxygen. Nevertheless, a variety of taxa, termed psammophilous,
are specialists of this habitat. The meiofauna, defined as
invertebrates passing through a 0.5 mm sieve, can be very abundant,
dwelling interstitially to considerable depth. The psammophilous fauna
includes some macroinvertebrates as well, and they can exhibit
distinctive adaptations, often associated with respiration.
- In a sandy bottom stream in Virginia,
meiofaunal densities (rotifers, oligochaetes, early instar chironomids,
nematodes and copepods) averaged over 2,000 per 10 sq.cm. and at times
reached nearly 6,000 per 10 sq.cm.
- Densities of very small midge larvae (Chironomidae) as high
as 85,000/sq.m. were recorded from shifting-sand regions within a large
river of Northern Alberta.
- The dragonfly nymph Lestinogomphus africanus, found burrowing deep in sandy-bottom pools in India, has elongated respiratory siphons that reach above the sand surface.
- Several mayflies, including Dolania in the southeastern USA, have dense hairs that apparently serve to keep their bodies free of sand.
- The larva of a South American species of Macronema, a
caddisfly in the family Hydrophychidae, builds a chimney-like intake
structure into its feeding chamber in order to exclude sand grains from
its food-capturing net.
Burrowing taxa can be quite specific in the particle size of substrate they inhabit. The mayflies Ephemera danica and E. simulans burrow effectively in gravel. Hexagenia limbata cannot, but does well in fine sediments. Substrates composed of finer sediments generally are low in oxygen, and H. limbata meets this challenge by beating its gills to create a current through their U-shaped burrows.
or wood-dwelling taxa illustrate that woody debris constitutes yet
another substrate category of lotic environments. Wood appears to be
substrate more often than it is food, although some taxa, such as the
beetle Lara avara, feed mainly on wood and many taxa obtain
some nourishment from a mix of algae, microbes and decomposing wood
fibre found on wood surfaces. Woody material is an important substrate
in the headwater streams of forested areas, where 25-50% of the
streambed is wood and wood-created habitat. It is also very important
in lowland rivers where 70% or more of the bed is composed of sand, and
wood provides the only stable substrate. In lowland streams that flood
nearby forests, wood is a significant component of habitat available
are the invertebrate taxa that live in association with aquatic plants.
Many species utilize moss, and a few are found primarily in moss.
- Examples include the free-living caddis larva Rhyacophila verrula, and a number of mayflies with backward-directed dorsal spines, evidently to prevent entanglement.
- A substantial number of invertebrates are also found on the surface of submersed macrophytes.
The influence of substrate on organism abundance and diversity
In general, diversity and abundance increase
with substrate stability and the presence of organic detritus. Other
factors which appear to play a role include the mean particle size of
mineral substrates, the variety of sizes, and surface texture, although
it is difficult to generalise about their effects.
Table 2: Abundance and species diversity of aquatic insects
found in five habitats (characterised mainly by their substrates) in a
Quebec stream. Values are annual averages.
|No. of species||Diversity|
|Cobbles and pebbles||2,130||76||2.02
(finely divided leaf material in pools and along stream margins)
In general, diversity and
abundance of benthic invertebrates increase with median particle size
(MPS), and some evidence suggests that diversity declines with stones
at or above the size of cobbles. The amount of detritus trapped within
the crevices is also likely to be important, and substrates of
intermediate size are superior in this regard. A variable mix of
substrates ought to accommodate more taxa and individuals, and particle
size variance usually increases with MPS. Evidently the amount and type
of detritus contained within the sediments is sufficiently dependent on
the size and mix of the mineral substrates that it is unwise to measure
substrate preference without concurrent study of trapped organic
in small amounts, benefits at least some taxa. When silt was added to
larger mineral substrates in laboratory preference tests, silt enhanced
the preference for coarse substrates in the mayfly Caenis latipennis and the stonefly Perlesta placida.
In large amounts, silt generally is detrimental to macroinvertebrates.
It causes scour during high flow, fills interstices thus reducing
habitat space and the exchange of gases and water, and reduces the
algal and microbial food supply.
refers to surface properties such as hardness, roughness, and perhaps
ease of burrowing, along with other aspects. Researchers have found
that more invertebrates colonized granite and sandstone, which have
comparatively rough surfaces, than the smoother quartzite. Other
experiments also found diversity and abundance to be greater on
irregular than on smooth substrates of the same overall size.
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