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Chapter I: Zoobenthos of Freshwaters- An Introduction

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: October 09, 1013           Freshwater Benthic Ecology and Aquatic Entomology Homepage

Chemical vs Biological monitoring


Important links

Superphylum Arthropoda

(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).

Subphylum Uniramia

Class Insecta

(Mackie, 1998)

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 present

The 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).

Substrate influence

(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
Subject: book
I'm 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 field surveys.

Inorganic Substrates

Table 1: The classification of mineral substrates by particle size, according to the Wentworth Scale
Size CategoryParticle Diameter
(range in mm)
    Very coarse1-2
    Very fine0.063-0.125

Substrate ofcourse depends on the parent material available, but there is a general tendency for particle size to decrease as one proceeds downstream.

Organic substrates

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 occupy.

Lithophilous taxa

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 occurrence.


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.

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 seasonally.


are the invertebrate taxa that live in association with aquatic plants. Many species utilize moss, and a few are found primarily in moss.

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 speciesDiversity
Cobbles and pebbles2,130762.02
(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 matter.


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.

Substrate texture

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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