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ABSTRACT

 

The ecology of aquatic insects of Opi Lake was carried out to determine their composition,
abundance and diversity from February to July, 2014. Adult insects of different species were
collected from the water surface using a dip-net with Nytex® netting of 500μm mesh. In
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addition, adult Insects and nymphs were collected from the vegetation around the lake using a
sweep net with mesh size of 250μm, while bottom dwellers were sampled using a scoop net.
The lake was divided into three sampling stations as a result of the nature and amount of the
vegetation, and the type of substratum found in each location. Station 1 had vegetation, shade
and detritus, Station 2 had no shade, very little detritus and vegetation, while Station 3 had shade,
detritus with no vegetation. The physico-chemical parameters and heavy metals concentrations
of the lake were determined while the climatic data of the area was collected from the Center for
Space Science University of Nigeria Nsukka.A total number of 1,042 insects representing 30
species, belonging to 26 families and 8 orders were recorded. Odonata had the highest mean
abundance (44.52%) in all the stations, followed by Hemiptera (23.32%) which was the most
diverse group. Hemiptera had the highest number of families (8 out of the 26 families collected).
Other insect orders collected with their abundance include: Coleoptera (12.28%), Orthoptera
(10.29%), Hymenoptera (5.09%), Diptera (3.36%), Trichoptera (1.06%) and Lepidoptera
(0.01%). Station 1 recorded the maximum number (46.35%) of aquatic insects throughout the
sampling season. However, stations 2 and 3 recorded 28.98% and 24.66% of aquatic insects
respectively. The abundance of insects was maximum in the month of July (20.44%) and
minimum in April (8.16%). The abundance and distribution of insect species varied and were not
constant from one month to another during the period of study, due to biotic and abiotic factors.
There was high species diversity of aquatic insects in the different strata of the lake, indicating
the rich and diverse group of insects in the study area. Dissolved Oxygen had an inverse
relationship with Orthoptera (r = -0.63, p < 0.01) and Hymenoptera (r = -0.54, p < 0.05. Diptera
also had negative relationship with depth (r = -0.48, p < 0.05). There was positive correlation
between Hemiptera and Copper (r = 0.78, p < 0.01), while Iron also correlated positively with
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Coleoptera (r = 0.47, p < 0.05) and Lepidoptera (r=0.59, p < 0.05).Among the insects and
zooplankton, Odonata had positive correlations with Rotifera (r=0.502, p < 0.05), Cyclops (r =
0.541, p < 0.05), Bosmina (r=0.53, p < 0.05) and Daphnia (r = 0.595, p < 0.01). Orthoptera also
showed positive relationship with Fish egg (r = 0.684, p < 0.01). Also, with phytoplankton,
Odonata had positive relationship with Chlorophycaea (r = 0.505, p < 0.05) and Xanthophycaea
(r = 0.499, p < 0.05). Orthoptera correlated positively with Cryptophycaea (r = 0.491, p < 0.05)
and Xanthophycaea (r = 0.487, p < 0.05).This therefore, adds to the fact that undisturbed habitat
quality is more suitable for insects to breed and multiply under the natural ecosystem with
abundant food supply.

 

TABLE OF CONTENTS

 

Title Page…………………………………………………………………………………………..i
Approval Page…………………………………………………………………………………….ii
Dedication…………………………………………………………………………………………iii
6
Acknowledgement………………………………………………………………………………..iv
Table of Contents………………………………………………………………………………….vi
List of Tables ……………………………………………………………………………………ix
List of Figures ……………………………………………………………………………………..x
List of Plates……………………………………………………………………………………..xii
Abstract…………………………………………………………………………………………..xiii
Chapter One: Introduction and Literature Review
1.1 Introduction………………………………………………………………………………..1
1.2 Justification of the Study………………………………………………………………….3
1.3 Objective of the Study…………………………………………………………………….4
1.4 Literature Review………………………………………………………………………….4
1.4.1. Some structural adaptations of insects in the aquatic community………………………….6
1.4.2. Major groups of aquatic insects…………………………………………………………….8
1.4.3. Insect taxonomic diversity ………………………………………………………………..12
1.4.4. Factors affecting the distribution and abundance of insects in aquatic environment……..13
1.4.5. Biological constraints on aquatic insects (role of biotic factors in the distribution and
abundance of aquatic insects)……………………………………………………………………17
1.4.6. Substrate Type…………………………………………………………………………….19
1.4.7. Activities and ecological role of adult aquatic insects…………………………………….19
CHAPTER TWO: MATERIALS AND METHODS
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2.1 Study Area………………………………………………………………………………23
2.2 Meteorological Data………………………………………………………………………25
2.3 Aquatic Insects Sampling………………………………………………..………………25
2.4 Identification of Insects…………………………………………………………………..26
2.5 Determination of Physico-chemical Parameters…………………………………………26
2.6 Macroinvertebrate Sampling……………………………………………………………..29
2.7 Plankton Sampling……………………………………………………………………….30
2.8 Statistical Analysis……………………………………………………………………….30
CHAPTER THREE: RESULTS
3.1 Meteorological Data of Study Area………………………………………………………31
3.2 Aquatic Insects Sampling and Identification…………………………………………….41
3.3 Mean Monthly Abundance of Aquatic Insect Orders in Opi Lake………………………43
3.4 Correlation Matrix of the Relationship between Aquatic Insects Abundance in Opi
Lake………………………………………………………………………………………45
3.5 Mean Monthly Value of the Physico-chemical Parameters of Opi Lake………………..51
3.6 Correlation of physico-chemical parameters/ Heavy Metals and Aquatic insect orders…53
3.7 Mean Monthly Composition and Abundance of Zooplanktons in Opi Lake…………….59
3.8 Mean Monthly Composition and Abundance of Phytoplanktons of Opi Lake………….62
3.9 Mean Monthly Composition and Abundance of Macroinvertebrates in Opi Lake………64
3.10 Correlation of Aquatic Insects and Zooplanktons of Opi Lake………………………….66
3.11 Relationship of Aquatic Insects and Phytoplankton of Opi Lake………………………..70
3.12 Relationship of Aquatic Insects and Macroinvertebrate in Opi Lake……………………74
CHAPTER FOUR: DISCUSSION AND CONCLUSION
4.1 Discussion………………………………………………………………………………..78
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4.2 Conclusion……………………………………………………………………………….84
REFERENCES

 

CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW
1.1 Introduction
Aquatic habitats are known to accommodate a great number of the earth’s arthropods.
These include insects, many of which are known to utilize the aquatic ecosystem in various
ways, and sometimes only at certain stages of their life cycle (Pennak, 1978; Voshell, 2002).
Insects are very successful in aquatic ecosystem, especially the freshwater environment. This is
demonstrated by their diversity, abundance, broad distribution and their ability to exploit most
types of aquatic habitats. The importance of these organisms range from their provision of
support to terrestrial lives through aquatic maintenance of food chains to serving as indicators of
water quality due to their varying tolerance limits to organic and inorganic substances (Bass,
1994; Mason, 2002).
There are about 751,000 known species of insects, which is about three-fourths of all
species of animals on the planet. Most insects live on land; their diversity also includes many
species that are aquatic in habit (Westfall and Tennessen, 1996). Freshwater makes up only about
0.01% of world total water body and contains about 100000 species (8%) out of 1.3 million
scientifically described species (Dudgeon, 1999). Aquatic insects are extremely important in
ecological systems for many reasons (Merritt et al., 2008) and are the primary bio-indicators of
freshwater bodies such as lakes, ponds, wetland, streams and rivers. They serve various purposes
such as food of fishes and other invertebrates, as vectors of pathogens to both humans and
animals (Foil, 1998; Chae et al., 2000). Bio-monitoring pertains to the use of insects and/or their
differential responses to stimuli in their aquatic habitat to determine the quality of that
16
environment (Merritt et al., 2008). Aquatic insects are very good indicators of water qualities
since they have various environmental disturbances tolerant levels (Arimoro and Ikomi, 2008).
Insects generally dominate freshwaters in terms of species number, biomass and
productivity. They have a variety of morphological adaptations for aquatic life. For breathing,
some diving beetles (Coleoptera) and bugs (Hemiptera) entrap an air bubble beneath the
elytra (beetles) and hemelytra (bugs) within the hydrofuge. The bubble can last for hours or days.
Some adult beetles and bugs have an expanse of hydrofuge to form a layer of air around them.
This oxygen layer is known as a plastron and is replenished by diffusion from the surrounding
water allowing these insects to stay under water permanently. Many other nymphal insects tend
to have gills- abdominal, rectal or around the mouthparts, to enable under water breathing. Some
fly larvae (Diptera) and damselfly nymphs (Odonata) swim by serpentine action. Dragonfly
larvae (Odonata) are capable of jet propulsion by forcing air from the rectum. Most bugs
(Hemiptera) have modified legs – paddle-like, fringed with hair. Fast swimmers have bodies that
are flattened to be aqua dynamically streamlined. Surface dwelling bugs have non-wettable hairs
(hydrofuge) that allow them to rest upon and move across the surface of the water.
Adult insects typically move about by walking, flying, or sometimes swimming. As it
allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with
their legs touching the ground in alternating triangles. Insects are the only invertebrates to have
evolved flight. Many insects spend at least part of their lives under water, with larval adaptations
that include gills, and some adult insects are aquatic and have adaptations for swimming. Some
species, such as water striders, are capable of walking on the surface of water. Insects are mostly
solitary, but some, such as certain bees, ants and termites, are social and live in large, wellorganized
colonies. Some insects, such as earwigs, show maternal care, guarding their eggs and
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young. Insects can communicate with each other in a variety of ways. Male moths can sense the
pheromones of female moths over great distances. Other species communicate with sounds:
crickets stridulate, or rub their wings together, to attract a mate and repel other males.
Lampyridae in the beetle order Coleoptera communicate with light.
1.2 Justification of the Study
Aquatic insects are of great importance to not only the aquatic ecosystem but also to
the terrestrial environment. They serve as important sources of food for fishes and other
invertebrates; others act as vectors through which disease pathogens are transmitted to both
humans and animals (Foil, 1998; Chae et al., 2000). These insects are therefore known to play
significant roles in the public and veterinary health and thus, needs to be scientifically explored
more extensively. Most importantly, aquatic insects are very good indicators of water quality,
since they have various environmental disturbance tolerant levels (Arimoro and Ikomi, 2008).
The presence or absence of certain families of aquatic insects can indicate whether a particular
water body is healthy or polluted. Worldwide, due to over human explosions, most of the fresh
water bodies are being subjected to increasing pollution loads. Consequently, changes in the
physico-chemical properties (temperature, dissolved oxygen, carbonates, alkalinity, phosphates,
nitrates and metal concentrations) can adversely affect the diversity, distribution and composition
of aquatic insects (Foil, 1998; Chae et al., 2000; Bauernfeind and Moog, 2000).
Presently, to the best of our knowledge, there is dearth of literature on the bio-diversity
of aquatic insects communities in Opi Lake, Nsukka, Enugu State, Nigeria. This research is
therefore necessary to provide an inventory on the aquatic insects found of Opi Lake and also
baseline information on their performance among other biological and physic-chemical factors in
the ecosystem.
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1.3 Objectives of the Study
The objectives of this study were to;
1. provide an inventory of the aquatic insects present in Opi Lake.
2. determine the species composition of aquatic insects in Opi Lake.
3. ascertain the abundance and distribution of aquatic insects in Opi Lake.
4. determine the monthly and seasonal abundance and distribution of aquatic insects in OpiLake
5. determine the effects of physico-chemical parameters and heavy metals in the abundance and
distribution of aquatic insects in Opi Lake.
6. determine the role of plankton, macrophytes and macroinvertebrates in the abundance and
distribution of aquatic insects in Opi Lake.
1.4 Literature Review
Insects constitute the largest class of the largest phylum of the animal kingdom and
include about 80% of the total described species of the entire animals (Richards and Davies,
1977). Insects occupy a dominant position in the animal world, remembering all other
inhabitants they are very successful animals of the estimated 1.35 million living species of
animal more than 900,000 are insects (Nayar et al., 1979). They are widely distributed in every
conceivable habitat with the exception of the marine ecosystems, and are particularly important
in fresh water food chains (Foil, 1998; Chae et al., 2000). Also, certain aquatic insect species
including, members of the Orders Ephemeroptera, Plecoptera, Trichoptera and Diptera because
they live relatively long and are capable of integrating temporal environmental conditions, may
serve as good indicators of aquatic pollution; and have been used over the decades in fresh water
bio-monitoring programs and assessment of environmental impacts (Arimoro and Ikomi, 2008).
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Some are very vulnerable and sensitive to pollution, while others can live and
proliferate in disturbed and extremely polluted waters (Merritt and et al., 2008). Most water
bodies have been subjected to increasing pollution loads consequently, affecting greatly their
quality and health status. These results in changes in the physico-chemical properties of water
e.g., temperature, dissolved oxygen, alkalinity, phosphates, nitrates and metal concentrations.
Variations in these water properties greatly influence the distribution patterns of aquatic insects
in the water, since some of them are highly sensitive to pollution while others are somewhat
tolerant or completely tolerant to pollution and environmental disturbances (Bauernfeind and
Moog, 2000).The responses of aquatic insects to pollution are usually manifested as nil species
occurrence, reduced relative abundance of individuals or species relative predominance,
depending on the tolerance limits of the species. To this end, significantly spatial variations in
species composition and densities of entomo-indicators of fresh water pollution within localities
have been reported. According to Popoola and Otalekor (2011), such variations in distribution of
aquatic insects may be attributed to the degree of anthropogenic interference in the ecological
balance of water bodies. Poor anthropogenic practices have resulted in the discharge of untreated
wastes and chemical-laden agricultural run-offs into fresh water ecosystems, with the consequent
effects of reducing their water quality (Arienzo et al., 2001; Azrina et al., 2005).
This development results in unhealthy changes in physico-chemical properties of
the water bodies thus, influencing the species composition and relative abundance of the inherent
entomo-fauna. In Nigeria, significant spatial variation in aquatic entomo-diversity have been
reported (Ugbogu and Akinya, 2001; Tyokumbur et al., 2002; Zabbey and Hart, 2006), probably,
indicative of differential pollution status of the water bodies assessed. With increasing rates of
fresh water pollution in Nigeria, coupled with the high costs of the use of physico-chemical
20
analysis in detecting and monitoring fresh water pollution, there is an urgent need to promote the
use of entomo-indicators as integral tools for the management of fresh water pollution in the
country. However, the success of this strategy depends largely on a good understanding of the
composition and distribution of aquatic insect species in different eco-geographic zones of the
country.
1.4.1. Some structural adaptations of insects in the aquatic community
Most insects that land on water are trapped by the water surface tension, and tiny
ones can even drown inside a water droplet, unable to break out of the bubble surface. Aquatic
insects cope by having waterproofed skin so large amounts of fresh water do not diffuse into the
body. Many are covered with a water-repellent waxy layer. They also usually have hairy or waxy
legs, which repel water so they don’t get trapped by the water surface tension. Many of these
insects are strong swimmers or crawlers as nymphs or larvae and as adults can also fly, although
the degree to which they use their ability to fly varies quite a bit. Water Boatmen are the only
aquatic insects that can take off from the water – without having to crawl out of the water first.
Aquatic insects have some other useful adaptations to help them live in aquatic environments
(Voshell, 2002):
a. Breathing under water
Water is much heavier than air and there is much more oxygen in air (20%) than in the water.
So, in order to extract oxygen from water, an insect will have to process a lot of water to get a
sufficient amount of oxygen. That is probably one reason why adult aquatic insects continue to
breathe air instead of developing gills. Aquatic insects have some fascinating adaptations for
breathing under water.
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Snorkel with a breathing tube: Mosquito larva and water scorpions use breathing tubes. The
end of the tube usually has bristles to break the water surface tension and keep the tube open.
This method, however, does not allow the insect to travel far from the water surface.
Scuba tank: Some aquatic insects create an “air tank” for greater freedom of movement
underwater. There are two types of air tanks: one type, used by a water beetle, uses a skin of air
that is trapped by hairs on the body or under the wing covers. The insect breathes the air in the
bubble through the holes in its abdomen (spiracles) just like other insects. A second type, called a
diving bell is used by the water spider. The water spider (Argyroneta aquatica) is not an insect,
but is adapted very well to aquatic conditions. It lives underwater by creating an underwater air
chamber. It gathers a small bubble of air from the surface on its hairy hind legs, and then releases
it into a silken web woven among water weeds. The bubble allows the insect to absorb oxygen
directly from the water. As the insect uses up the oxygen in the bubble, dissolved oxygen in the
water diffuses into the bubble so the insect can actually get more oxygen than was originally in
the bubble. However, nitrogen must be present for this to happen. The nitrogen provides stability
to the bubble (it diffuses more slowly into water than other gases). The spider goes back to the
surface to replenish nitrogen rather than to get fresh oxygen. The spider mates and lays eggs
inside this chamber.
b. Walking on water (Water Walking)
Skates: Some aquatic insects skate on the water surface by distributing their body weight over
long, thin, waterproof legs. They paddle with the middle pair of legs, steer with the hind legs and
use the short front legs to attack and hold prey.
22
Jet skis: The Camphor Beetle (Stenus) also skates on the water surface. When alarmed, it
releases a chemical from its back legs that reduces the water surface tension. In this way, the
water surface tension on the front pulls it forwards. It shoots forwards on its front feet which are
held out like skis, and steers itself by flexing its abdomen. This tiny beetle is the size of a rice
grain but can travel nearly 1m a second this way! It does not hunt on water, but at the edge of the
water, and saves this trick to escape predators.
c. Other aquatic adaptations
Ripple effect: Most aquatic insects are sensitive to water ripples to detect predators or prey.
Some even create their own ripples on the water surface and process the returning “echoes” to
detect prey. Many, such as the whirligig beetle, also create ripples to find mates and
communicate with each other.
Double vision: The Whirligig Beetle has eyes divided horizontally to see both under and above
water.
Oars: Many aquatic insects paddle underwater with oar-like legs. These legs are long, flattened
and fringed. The hairy fringes spread out on the power stroke increasing the surface area, and
bend in on the return stroke to reduce water resistance. Examples would include the water beetle
and the water boatman. These insects usually have flattened streamlined bodies or are torpedoshaped
1.4.2. Major groups of aquatic insects
There are so many different kinds of aquatic insects; it is difficult to appreciate their
biological diversity without considering some of the individual kinds. The following section
provides a brief summary of the eight major groups as reported by Voshell (2002).
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Mayflies (Ephemeroptera)
Larvae of mayflies live in a wide variety of flowing and standing waters. Most of them
eat plant material, either by scraping algae or collecting small pieces of detritus from the bottom.
Larvae breathe dissolved oxygen by means of gills on the abdomen. They have incomplete
metamorphosis. Most mayflies are sensitive to pollution, although there are a couple of
exceptions. The most unusual feature of mayflies is that the adults only live a few hours and
never eat.
Dragonflies and Damselflies (Odonata)
Larvae of dragonflies and damselflies are most common in standing or slow-moving
waters. All of them are predators. Larvae breathe dissolved oxygen with gills, which are located
either inside the rear portion of the abdomen (dragonflies) or on the end of the abdomen
(damselflies). They have incomplete metamorphosis. Many kinds are fairly tolerant of pollution,
but some kinds only live in unique habitats, such as bogs high in the mountains. The most
unusual feature of this group is the way the larvae catch their food with an elbowed lower lip,
which they can shoot out in front of the head.
Stoneflies (Plecoptera)
Larvae of stoneflies live only in flowing waters, often cool, swift streams with high
dissolved oxygen. Some feed on plant material, either by shredding dead leaves and other large
pieces of detritus, while others are predators. Larvae breathe dissolved oxygen. Some have gills
on their thorax, but others just obtain dissolved oxygen all over their body. They have
incomplete metamorphosis. Almost all of the stoneflies are sensitive to pollution. The most
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unusual feature of this group is that some kinds are programmed to emerge only during the
coldest months; hence, they are called the winter stoneflies.
True Bugs (Hemiptera)
Most of the true bugs live on land, but the aquatic kinds are most common in the
shallow areas around the edge of standing waters. Both the adults and the larvae of the aquatic
kinds live in the water. Both stages are usually found on submerged aquatic plants. Almost all of
them are predators. They breathe oxygen from the air, either by taking a bubble underwater or by
sticking a breathing tube up into the air. They have incomplete metamorphosis. Most kinds are
tolerant of pollution. The most unusual feature of this group is the way they kill and eat their
prey. True bugs have a sharp beak that they stick into the body of their prey, and then they pump
in poison to kill their prey, after which they suck out the body fluids. Some of the larger kinds
feed on small fish and tadpoles.
Dobsonflies and Alderflies (Megaloptera)
Larvae of different kinds live in flowing or standing waters. They are all predators.
They breathe dissolved oxygen by means of gills and their overall body surface. They have
complete metamorphosis. Mature larvae leave the water and dig out a protected space under a
rock or log for the pupa stage. Different kinds are either sensitive or tolerant to pollution. Larvae
of some of the larger kinds are called hellgrammites, which are popular as live bait for
smallmouth bass and other warm-water fish species.
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Water Beetles (Coleoptera)
There are more species of beetles than any other insects, but most of them live on land.
Most of the water beetles are more common in standing or slow-moving waters, but a few kinds
are only found in swiftly flowing waters. Both the adults and the larvae of the aquatic kinds live
in the water. Water beetles feed in different ways, primarily by preying on other animals,
scraping algae, or collecting small particles of detritus from the bottom. All of the adults breathe
air by taking a bubble underwater, while most of the larvae breathe dissolved oxygen by a
combination of gills and their overall body surface. They have complete metamorphosis and
leave the water for the pupa stage. Water beetles range from sensitive to somewhat tolerant of
pollution. The most unusual feature of water beetles is that some of the adults live for several
years.
Caddisflies (Trichoptera)
Larvae of different caddisflies live in a wide variety of flowing and standing waters.
They also have a wide range of feeding habits, including scraping algae, collecting fine particles
of detritus from the bottom or from the water, shredding dead leaves, and preying on other
invertebrates. They breathe dissolved oxygen by means of gills and their overall body surface.
Caddisflies have complete metamorphosis and remain in the water for the pupa stage. Most kinds
are sensitive to pollution, but a few kinds are somewhat tolerant of moderate levels of pollution.
The most distinctive feature of caddisflies is their ability to spin silk out of their lower lip. They
use this material to glue together stones or pieces of vegetation into a small house for their
protection during the larva and pupa stages. Some also use strands of silk to make a net for
filtering particles of food from the water.
26
True Flies (Dipterans)
This group has more kinds on land, but there are also many aquatic kinds. They have a
wide range of feeding habits, including scraping algae, collecting fine particles of detritus from
the bottom or from the water, shredding dead leaves, and preying on other invertebrates. They
breathe dissolved oxygen by means of gills and their overall body surface. True flies have
complete metamorphosis and remain in the water for the pupa stage. The most distinctive feature
of this group is their ecological diversity. Some kinds live in the cleanest habitats (e.g., swift,
cool, mountain streams), while others live in some of the harshest natural habitats on the earth
(e.g., arctic tundra ponds, geothermal springs, alkaline lakes, mucky swamps). They have equally
diverse responses to pollution, with some kinds being exceptionally sensitive, while other kinds
endure the worst imaginable water quality (e.g., raw sewage or acid mine drainage).
1.4.3. Insect taxonomic diversity
The diversity of insects can only be described as amazing. More than half of all known
species of living things (microbes, plants, and animals) are insects. There are about 751,000
known species of insects, which is about three-fourths of all species of animals on the planet.
While most insects live on land, their diversity also includes many species that are aquatic in
habit (Westfall and Tennessen, 1996). In North America, there are more than 8,600 species of
insects associated with freshwater environments during some part of their lives. Ponds, lakes,
and wetlands therefore exhibit very high diversity, depending on water quality conditions, littoral
development, and substrate characteristics. Due to taxonomic problems associated with this high
diversity, the role of aquatic insects in aquatic ecosystems is often difficult to study.
27
Just about anywhere you go on the planet; there is some kind of insect that will live in
almost any place that stays wet for a week or so. Aquatic insects are important food for fish and
waterfowl. They also play important roles in keeping freshwater ecosystems functioning
properly.
1.4.4. Factors affecting the distribution and abundance of insects in aquatic environment
a. Physico-chemical constraints on aquatic insects
The physical environment of aquatic ecosystems exerts substantial control over the
population abundances and hence community composition of insects. Variation in the physicochemical
properties greatly influence the distribution pattern of aquatic insects in water, since
some of them are highly sensitive to pollution while others are somewhat tolerant or completely
tolerant to pollution and environmental disturbances (Bauernfeind and Moog, 2000). Within
these habitats, the physic-chemical factors of particular importance to aquatic insects include:
dissolved oxygen concentration, water temperature, water chemistry, type of substrate, and
hydrodynamics.
1. Dissolved oxygen
As aerobic organisms, all insects must obtain sufficient oxygen to drive their
metabolic machinery (Thorp and Covich, 2009). This presents a particular challenge for aquatic
insects because water, even when saturated, contains much less oxygen than terrestrial
environments (a maximum of about 15 ppm oxygen in water compared to over 200,000 ppm in
the air).
Dissolved oxygen is one of the most important abiotic factors affecting aquatic
invertebrate abundance and diversity (Thorp and Covich, 1991). Many forms of lentic larvae can
28
supplement atmospheric oxygen with dissolved oxygen. Oxygen levels below 2 mg/L may
decrease the fitness and chances of survival for many aquatic invertebrates. For example,
Caddisfly larvae are especially vulnerable to decreased oxygen levels because their locomotion is
restricted (Thorp and Covich, 1991).
The level of aquatic oxygen can determine habitat suitability for benthic insects, and
differences in hypoxia tolerance can therefore play a role in explaining distributions in the field.
The oxygen concentration of the water and the upper sediment layer is of considerable
importance to benthic communities (Ward, 1992; Chapman et al., 2004). Fluctuating oxygen
levels are often observed in inland waters, as a result of complex diurnal and annual variations
depending on both abiotic and biotic variables such as light intensity, current velocity or
disintegration processes, as well as human activities like hydrological and geomorphological
modifications or additional input of organic matter (Jacob and Walther, 1981; Paerl et al., 1998).
Minimal content of oxygen is an important factor limiting the distribution of benthic organisms
and the ecological recovery of aquatic ecosystems. For example, Neumann (1994) and Becker
(1987) demonstrated that re-colonization of the caddisfly Hydropsyche contubernalisin the River
Rhine coincided with increasing oxygen levels.
It has been clearly demonstrated that exposure to lowered oxygen concentrations
changed the behavioral patterns of Hydropsyche angustipennis larvae. In general, however, the
specific tolerance towards lowered oxygen concentrations differs greatly between aquatic
insects. The sensitivity distribution based on effect concentrations show that the effect of
concentration vary even among species, ranging from almost anoxic to concentrations little less
than saturated. Also, it can be seen that no specific sensitive or tolerant insect orders can be
distinguished. Furthermore, Philipson and Moorhouse (1974) demonstrated that even between
29
closely related hydropsychid species, differences could be found in their sensitivity towards low
oxygen.
2. Temperature
Temperature is the cue most responsible for orchestrating the lifecycle of
invertebrates. Both the actual temperature (measured in degree-days) and its rate of change are
important in determining the time period from oviposition to hatching (Ward and Stanford,
1982). Given unlimited food supplies, juveniles grow faster in warmer water since metabolic
rates (and hence food assimilation and tissue building) are directly proportional to temperature
(Ward and Stanford, 1982). While photoperiod plays a role in cueing emergence, it is nearly
always coupled with and overshadowed by temperature (Allan, 1995)
Water temperature strongly influences the geographic distribution of a species (Hart,
1985; Bell, 2006) and changes in water temperature may lead to structural changes in the
abundance, density, biomass, diversity and composition of aquatic communities. In the Northern
Hemisphere, water temperature variability has been positively correlated with species diversity
limit the occurrence of certain species (Vannote and Sweeney, 1980). Hawkins et al. (1997)
consider summer to be a critical season for many aquatic insect populations in that much of the
biological production occurs when temperatures are highest.
3. Water chemistry
Many aspects of water chemistry can restrict the occurrence or abundance of aquatic
insects, including pH, salinity, and concentrations of specific ions or elements. Generally, it is
the extremes in any of these parameters that result in change to aquatic insect communities,
while levels around the mean have less direct impact. Low pH, as is found in acidified lakes and
streams of pH less than 5 (due to acid deposition, mine drainage, organic acids, or poor buffer30
ing), can alter community composition such that only acid-tolerant taxa are found (Thorp and
Covich, 2009). Several taxa of Ephemeroptera declined at pH below 5.5 in Ontario lakes, but
odonates and chironomids increased, possibly due to the absence of fish predators (Thorp and
Covich, 2009). Many orders of aquatic insects show tolerance to low pH except for Plecoptera.
Studies have shown that no Plecoptera species can survive in a pH below 4 (Resh and
Rosenberg, 1984). Other studies show that aquatic invertebrate species diversity and abundance
will significantly decrease at and below a pH of 5.5 (Friday, 1987), and that the breaking point is
a pH of 5 (Resh and Rosenberg, 1984). In a natural and unpolluted state, the relationship
between pH and macroinvertebrate composition may be minimal.
However, it is unclear whether it is the low pH itself that adversely affects insects or
some related factor. For example, iron can precipitate under acidic conditions with an associated
reduction in oxygen. Acidic waters can also leach metals from soils and rock, especially
Aluminum, which can increase drift and possibly have toxic effects on aquatic insects (Thorp
and Covich, 1991). Salinity gradients that form in coastal estuaries, along saline lakes, and even
from runoff after road salting, can affect insects, most of which are salt-intolerant. However,
some insects such as brine flies (Diptera: Ephydridae) thrive in warm, saline water, where they
have few competitors. Many other chemical features, such as calcium concentration and total
ionic strength, have potential importance to aquatic insects, but little is known about the
requirements of specific taxa.
4. Hydroperiod
Hydroperiod is defined as the number of days/ length of time a vernal pool contains
water. Many factors combine to affect the hydroperiod including geology and precipitation
which are the most important. The hydroperiod of vernal pools has been shown to have the
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greatest effect on the composition of aquatic invertebrates (Brooks, 2000). The biotic factors
affected by hydroperiod include species richness, invertebrate abundance, predator size, predator
diversity and abundance, and invertebrate and amphibian reproductive success (Brooks, 2000).
The ephemeral nature of vernal pools is an obstacle for many groups of organisms. Many
inhabitants have specialized adaptations in their physiology or life cycle. Several groups of
habitat generalists colonize vernal pools but habitat specialists are the most successful (Brooks,
2000). Studies revealed a wide array of highly specialized organisms that depend upon the
abiotic and biotic characteristics of vernal pools for development and survival (Zedler, 2003).
Adaptations fall into three categories: escape of the habitat prior to desiccation, resistance to
drying during a stage of the life cycle, or an actual life cycle specifically adapted to resist
desiccation (Graham, 1998). Studies have demonstrated hydroperiod to be strongly correlated
with maximum volume (Brooks and Hayashi, 2002). Even though area is related to hydroperiod
volume, it is valuable to determine which factors are most important. Vernal pools have been
related to the island biogeography theory because they are isolate ecosystems (Brooks, 2000).
Island biogeography theory states that as area increases, the diversity of organisms increases
(Molles, 2002). Area is strongly related to the diversity of microhabitats within a vernal pool
(Brooks, 2000). A greater diversity of habitats should support a greater diversity of
macroinvertebrates. For example, species richness of aquatic beetles has been shown to increase
with increasing pool area (Nilsson et al., 1994; Brooks, 2000).
1.4.5. Biological constraints on aquatic insects (role of biotic factors in the distribution and
abundance of aquatic insects)
Biotic factors are more difficult to measure, as they are mainly interaction between
organisms. These interactions include predation, competition (intraspecific or interspecific), and
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parasitism. Numerous potential effects of vertebrate predators on aquatic macroinvertebrates
have been described, such as salamander predation on macroinvertebrates (Taylor et al., 1988).
However, other studies showed that salamander/macroinvertebrate predation relationship is not
well determined (Taylor et al., 1988; Petranka, 1989). Other predator such as fish can affect and
alter development (Diehl, 1992), species composition (Healey, 1984), and species abundance
(Macan, 1977; Healey, 1984) of benthic macroinvertebrate communities. The attenuation and
elimination of specific taxa (e.g., Chaoborus) has also been reported (Stenson, 1978). Predation
interaction have been studied by Murdoch and Bence (1987) who concluded that predators were
sources of instability in freshwater environments, while Thorp (1986) proposed that predators
contributed directly and indirectly to community regulation and population stability.
Aquatic insects are an important part of the food chain, especially for fish. Many feed
on algae and bacteria, which are on the lower end of the food chain. Some shred and eat leaves
and other organic matter that enters the water. Because of their abundance and position in the
aquatic food chain, benthos plays a critical role in the natural flow of energy and nutrients. As
benthos die, they decay, leaving behind nutrients that are reused by aquatic plants and other
animals in the food chain.
Also, Riparian vegetation is vital for the health of lotic communities, and its effects are
both indirect and direct. Indirectly, abundant riparian vegetation stabilizes stream banks and thus
acts as a sediment sink, leading to increased grain sizes (Allan, 1995, Mount, 1995);
consequently, these streams also tend to be deeper. Shading provided by riparian vegetation
often results in a decrease in water temperatures and periphyton density (Ebersole et al., 2003).
Directly, the leaves and branches of riparian vegetation serves as a food resource created outside
the stream (i.e., allocthonous). This input and how it is processed is a major factor controlling the
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longitudinal distribution of aquatic invertebrates according to the RCC (Vannote and Sweeney,
1980). Thus, riparian vegetation’s effects on the aquatic community are widespread and
significant.
1.4.6. Substrate Type
Substrate type, size, and stability are important for protection, food, and its effects on
current velocity. Many taxa are adapted for living within or on top of a substrate of a certain
grain size: Hexagenia mayflies burrow into silts, the flattened water pennies (Psephenidae) are
most common on the undersides of boulders, and stoneflies, as their name suggests, are often
found clinging to cobbles (McCafferty, 1981; Allan, 1995). A stable substrate provides an
immediate retreat and one likely to persist in times of peak discharge, thus affording greater
security than more labile zones to cover-seeking invertebrates. For instance, plecopteran
densities were highest in immobile gravel and cobble substrates of an Ozark stream (Philips and
Kilambi, 1994). Substrates that are composed of a significant amount of organic material (e.g.
leaf litter, woody debris) offer a diversity of food resources and shelters; as a result, the density
of invertebrate individuals in such areas is often very high (Allan, 1995). Some species (e.g.
Agapetus caddisflies) are adapted for exploiting eddies created by cobble-sized substrates that
provide food and a refuge from the current, and are thus restricted to such areas (Wellnitz et al.,
2001).
1.4.7. Activities and ecological role of adult aquatic insects
The adult stages of most aquatic insects inhabit the riparian zone (Anderson and
Wallace, 1984; Erman, 1984). Although adults are generally short-lived (from one day to a few
weeks), they often exhibit physical characteristics and life history traits that facilitate their
survival in the terrestrial environment and their reproductive success (Butler, 1984; Jackson,
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1988). Such adaptations suggest that the brief interaction between adult aquatic insects and the
riparian zone has been, and presumably continues to be important to their survival. At the same
time, aquatic insects can influence the distribution and abundance of riparian insectivores
because the adults of aquatic insects often represent an important food resource (Jackson and
Fisher, 1986).
Activities of adult aquatic insect
Adult aquatic insects engage in several activities, the most common of which are
reproduction, dispersal, and feeding. Reproduction i.e., mate location, followed by oviposition in
water, or on overhanging vegetation by females (Anderson and Wallace, 1984), is essential for
the initiation of the next generation in the aquatic environment.
Reproductive activities are often well defined with respect to time of the day and
location in the riparian zone. Some species form mating swarms e.g., mayflies (Ephemeroptera),
caddisflies (Trichoptera), and chironomid midges (Diptera) at specific times of the day, distances
from the stream, and heights above the ground (Edmunds et al., 1976; LeSage and Harrison,
1979).
Recent studies in the Coast Range of northern California demonstrated that males of
some species of caddisflies locate conspecific females by following sex pheromones (i.e.,
chemicals that mediate reproductive activities between males and females) which are released by
females (Wood and Resh, 1984; Resh and Wood, 1985).
Dispersal is also a key activity for most adult aquatic insects. The short-lived adult
stage in many species (e.g., most mayflies, stoneflies and caddisflies) would appear to limit the
distance that they can potentially disperse, either passively or actively. However, long distance
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(greater than 1 kilometer) movements both within stream corridors (Coutant, 1982) and across
land (Edmunds et al., 1976) have been recorded for some species in these groups. Not surprising,
long distance movements have been commonly observed in species that have relatively longlived
adults e.g., some blackflies, mosquitoes, dragonflies. Long-lived adults often feed (e.g.,
they are predators or require a blood meal for egg development) and prey or host availability
may contribute to the distance traveled.
The above activities contribute to the distribution and abundance of adults in the
riparian zone. In a recent study of the distribution of adult aquatic insects in the mixed evergreen
forest adjacent to a third-order California stream (Big Sulphur Creek, Sonoma Co.), it was found
that the abundance of adult aquatic insects was greatest near the stream and decreased as distance
from the stream increased. The rate of decrease in abundance varied among species. For
example, two species of caddisflies, Gumaga nigricula and Helicopsyche borealis (Hagen)
(Trichoptera: Helicopsychidae), were very abundant in the trees next to the stream. However, H.
borealis was almost absent 40 meters from the stream whereas G. nigricula was still common
150 meters from the stream. By flying more than 50 meters from the stream, adult aquatic insects
would be outside of most riparian buffer zones that are recommended to protect water quality
and aquatic life in streams (Brinson et al., 1981).
Ecological role of adult aquatic insects
Various studies have shown that between 1 percent and 57 percent of the biomass
produced by immature aquatic insects (i.e., secondary production of aquatic insects) emerges
from the aquatic system in the form of adult insects (Jackson and Fisher, 1986). Because many of
these adults die in the riparian zone, much of this biomass does not return to the aquatic habitat
(Jackson and Fisher, 1986). This export of biomass reduces the organic matter and nutrients that
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are available to aquatic insectivores (e.g. fish, amphibians, other macroinvertebrates) and
increases organic matter and nutrients that are available to riparian insectivores (e.g., birds, bats).
For riparian insectivores, the importance of this export of aquatic biomass depends on the
abundance of adult aquatic insects as prey relative to the abundance of terrestrial insects as prey.
Adult aquatic insects represented 37 percent of total arthropod numbers and 25 percent
of total arthropod biomass captured by sticky traps placed in trees 5 meters from Big Sulphur
Creek; 150 meters from the stream, adult aquatic insects still represented 15 percent of numbers
and 11 percent of biomass.
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