Sediment & Benthos Station - Inland Seas Education Association

X. SEDIMENT & BENTHOS STATION
INTRODUCTION
Lake sediment is composed of dead organic material (detritus) and
inorganic material like clay, silt, and sand particles. Sediment also
makes up a record of activity in the surrounding watershed. For
instance, wood chips from past logging operations, coal, and slag
become a part of this historical record.
Lake sediment also harbors organisms such as bacteria, fungi,
insects, crustaceans, clams, and many others. These organisms
that live on or near the bottom of a body of water are known as
benthos.
In this station, students will identify benthos and discuss their role
in the food web of the lake. Students will also learn the role that
sediment and its associated organisms play in recycling organic
material and releasing essential nutrients into the ecosystem.
LEARNING OBJECTIVES
Students will be able to:
1. Identify the collection device commonly used to sample sediment.
2. Classify sediment collected in terms of color and texture.
3. Identify and count benthos from the Ponar dredge samples and record their findings.
4. Identify and determine the relative abundance of plants from the otter trawl and record their
findings.
5. Identify and determine the relative abundance of benthos collected in the otter trawl and record
their findings.
6. Describe key habitat conditions found in the deep water habitat (Ponar sample) and the shallow
water habitat (otter trawl sample) in terms of light availability, temperature, and food supply.
7. Recognize benthos as important food for fish and as “recyclers” of organic material and their
place in the food web.
8. Discuss the source and role of excessive nutrients in promoting benthic algae growth.
TOOLS
5-gallon bucket
Animal key
Draining tub
Food web diagram
Forceps
Magnifying glasses
Petri dishes
Plant key
Plastic washtubs
Rubber mat
Sediment & Benthos
Station manual
Sediment chart
Screen, large
Sieve, small
Towel
White tray
Wash bottle
TEACHING THE STATION
After the otter trawl, the lead instructor will collect the plant material and other benthos and will place it
in a 5-gallon bucket with water. During the large group sampling, the lead instructor should deposit all of
the sediment from the 2 Ponar dredge samples in plastic washtubs. On Manitou it is important to have
wash bottles and 5-gallon buckets filled with water to rinse the sediment sample. On Inland Seas, there is
an electric pump for washing the samples.
Before discussing the samples, it is important to define benthos to the students. Most people have not
heard this term before, but understanding the definition of benthos is crucial to the student’s
comprehension of the activities during this station. Be sure to use the food web to give students a visual.
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During this station are four primary activities:
(1) Classify sediment in terms of color and texture.
(2) Compare characteristics of the shallow water sample (trawl) and the deep water sample (dredge).
(3) Identify and count benthos from the Ponar dredge samples.
(4) Identify plants and benthos from the otter trawl and determine their relative abundance.
(1) Pass around the tub of sediment from the Ponar dredge for students to feel (texture) and see (color).
Students should use the sediment key to classify sediment in terms of color and texture using the
geotechnical gauge.
(2) After the sediment is classified, have students take about 1/3 of a tub sample and wash this through
the large screen. Using forceps, have students pick organisms from the screen and place them in a petri
dish containing water. Using the Schoolship animal key, have students identify and count each organism.
Students may use magnifying glasses to help identify organisms. Make sure the students record the
benthos found in their student log books.
(3) Have students reach into the 5-gallon bucket containing plants and other benthos from the trawl, pull
a handful of plant material out, and place it on a white tray containing some water. Students should sort
through the plant sample and pull out different plant species. Have students identify these plants using the
Schoolship plant key. As students sort through the plants they will notice organisms begin to emerge from
the plant material. Students should identify these organisms and determine their relative abundance. If
zebra or quagga mussels are found in either the dredge or trawl sample, discuss the impact of these exotic
species on water clarity. More information about zebra and quagga mussels can be found on pages 103
and 104 of this manual.
Start a discussion on the role of benthos in the food web and their role as recyclers of organic material.
Discuss differences in habitat conditions between the deep water Ponar sample and the more shallow otter
trawl sample in terms of the species of benthos found, the amount of sunlight available for
photosynthesis, and the availability of food.
Stewardship Component
Incorporating water clarity into the benthos station is an important piece of stewardship because zebra and
quagga mussels are benthic organisms that are responsible for the significant water clarity changes in the
Great Lakes. This water clarity increase means that sunlight is able to penetrate to greater depths and
when combined with increased nutrients, the result are large algal blooms that foul beaches after big
storms and create prime habitat for botulism type E, a naturally existing bacterium in the Great Lakes.
Note#1, zebra and quagga mussels bring nutrients to the lake bottom by consuming plankton and then
excreting on the lake bottom. Note #2, botulism type E has seen significant increases in recent years and
this has been linked to increased anoxic conditions along the lake bottom due to decomposing algae mats.
Teaching Tip
Teaching Tip
There is a lot of material to cover in the Sediment & Benthos Station. It is most efficient to divide
students into 2 groups: half with the dredge sample and half with the trawl sample. Once the groups
have found several organisms, have them come together to begin identifying the plants and animals.
DATA COLLECTION
Students record the number of organisms found in each Ponar dredge sample since it is a quantitative
sample (the area of the sample is known). Each dredge sample collects 0.25 ft2 of sediment from the
bottom of the lake. Counting the number of benthic organisms provides the density (number of organisms
per unit area) of organisms sampled.
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The otter trawl is a qualitative sample (the area of the sample is unknown) so it is not valid to draw
conclusions about density from a count. Therefore, the relative abundance of benthos sampled using the
otter trawl is recorded.
Students will record the number of benthic organisms found per Ponar dredge sample in their log books,
as well as a relative abundance of the benthos found in the fish trawl. As a Sediment & Benthos Station
instructor, be sure to keep a running tally of the benthos from these two sampling methods separate so
that the proper data can be collected by the lead instructor at the end of the program.
VOCABLUARY
Benthos: organisms that live on or near the bottom of a body of water.
Consumer: organisms that do not produce their own food (must consume other organisms to produce
energy).
Decomposer: organism that breaks down organic material, such as dead plants and organisms, to produce
energy.
Dichotomous key: series of questions that lead to the identification of an item.
Ecosystem: group of living and non-living things that interact with each other.
Exotic species: species introduced into an area beyond its native range.
Food web: complex network of feeding relationships between organisms in an ecosystem.
Forage fish: small fish that feed on benthic organisms and provide food for larger fish and birds.
Invasive species: exotic species that causes or is likely to cause economic or environmental harm or harm
to human health.
Organism: individual living thing, such as bacteria, fungi, plants, or animals.
Ponar dredge: device used to collect sediment from the lake bottom.
Producer: plant that produces its own energy through photosynthesis using sunlight and nutrients.
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Trophic level: feeding position in a food web. Organisms that feed on the same prey are categorized on
the same trophic level in a food web.
BACKGROUND INFORMATION
Benthos are an integral part of the aquatic food web. Located near the base of the food web, benthos feed
on plankton and decomposing matter and are prey (food) for forage fish. Some benthos (i.e. bacteria) are
called decomposers because they derive their energy from dead material produced at all trophic levels. It
is important students recognize benthos as food for fish and “recyclers” of organic material. More
information on food webs and feeding relationships is on page 139-142 of this manual.
Students will identify the benthos in this station using the Schoolship plant and animal keys. The plants
and animals described below are found in these keys. The plant and animal keys can be found in the
Sediment & Benthos Station section of this manual (on pages 108 and 109).
PLANTS
Stonewart or the genus Chara is a green alga that lies along the lake bottom in nearshore areas. Its stems
have whorls of stiff, short branches that are gray-green and often encrusted with lime. This plant thrives
in alkaline water. Chara is commonly found in Grand Traverse Bay and is our most common plant
collected aboard the Schoolship. Other common plants found in our trawl samples are Eurasian Milfoil
(Myriophyllum spicatum) and Canadian Waterweed (Elodea canadensis).
Chara sp.
Whorls of stiff, short branches
Myriophyllum sp.
Feathery looking leaves;
whorls of 4
Elodea sp.
Thick leaves; whorls of 3
ANIMALS
Non-Segmented Worms
Phylum Platyhelminthes – Free-living Flatworms
Class Turbellaria – Flatworms & Planarians
- Flat with a distinct head, eyespots very prominent
- Can be confused with leeches, but do not have suckers
- Gastrovascular cavity noticeable, mouth usually on underside
- Found moving along bottom material
- Feeds upon dead animals
Phylum Nematoda – Roundworms
- Length usually less than 1 cm
- Characteristic whip-like motion
- Eats detritus and small plants and animals
Class Nematomorpha – Gordion Worms
- Rounded, often appear tangled
- Length 10 cm. to 70 cm.
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- Slow, clumsy swimmers
Segmented Worms
Phylum Annelida – Worms & Leeches
Class Oligochaeta – Aquatic Earthworms
- Segmented body
- No legs
- No suckers
- Look similar to terrestrial earthworms
Class Hirudinea – Leeches
- Suckers fore and aft
- Feed upon snails, insect larvae, worms and crustaceans
- Scavengers (only a few kinds suck blood)
- Slow, shape-changing movements
Clams & Snails
Phylum Mollusca – Clams & Snails
Class Gastropoda – Snails & Limpets
- Single shell
- Crawls with a fleshy foot
- Host to parasites
- Feeds upon attached algae and dead organisms
- Eaten by fish and ducks
Gilled Snails
- Have gills
- Have an operculum
- Example – Pleurocera sp.
Pulmonate Snails
- Have no gills (have “lung” for respiration)
- Have no operculum
- Example – Physa sp. or Helisoma sp.
Class Bivalvia – Mussels & Clams
- 2 shells (bivalves)
- 2 siphons (water in and water out), used to bring in food and oxygen
and force out waste
- Most bury themselves in the sediment
- Larvae are parasitic on fish, except for the zebra mussel larva, which
is planktonic
- Common small mussels: Sphaerium sp. and Psidium sp.
- Common large mussel: Anodonta grandis
Zebra Mussels (Dreissena polymorpha)
- mussel shaped
- striped shell
- often seen clumped together
- exotic species (native to the Caspian Sea)
- triangular-shaped shell
- cover native clams and spawning beds
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Quagga Mussels (Dreissena bugensis)
- mussel shaped
- dark concentric rings around shell
- rounder shells than zebra mussels
- exotic species (native to the Ukraine)
- inhabit deeper, colder waters than zebra mussels
It can be difficult to distinguish between zebra and quagga mussels. In terms of habitat, quagga mussels
prefer deeper, colder waters than zebra mussels, although there is a lot of overlap. Both zebra and quagga
mussels are found in the samples taken on the Schoolship. The main differences in physical features are
described below.
Zebra Mussel
Quagga Mussel
Shell color
Dark, concentric circles
Dark, concentric circles (white edge near hinge)
Shell shape
Triangular with ridges
Round without ridges
Ventral side
Flattened
Convex
Byssal groove
Large; middle of ventral side Small, near hinge of ventral side
Depth preference
Shallow
Deep, cool water
Habitat preference
Attach to hard substrate
Attach to hard or soft (sandy) substrate)
More information on zebra and quagga mussels can be found on pages 183-188 of this manual.
Phylum Arthropoda – Insects, Arachnids, & Crustaceans
Class Arachnida – Water Mites
- 8 legs like a spider (larval form has 6 legs)
- No antennae
- Round body, often red or reddish-orange
- Crawls along the bottom or cruises above the bottom
- Feeds upon insects and worms
Class Malacostraca
Order Decapoda – Crayfishes
- Hard exoskeleton, carapace covers thorax
- Large pincer claws
- Omnivorous/scavenger
- Food for fish, birds and muskrats
- Generally captured in fish trawl rather than in Ponar dredge
Order Isopoda – Isopods (aquatic sowbugs)
- Flattened top-to-bottom
- 7 pairs of legs
- Usually found on vegetation or under rocks
and sticks
- Scavengers
- Similar to terrestrial sow bugs or pill bugs
- Asellus is a common genus in the Great
Lakes
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Order Amphipoda – Amphipods (side-swimmers)
- Flattened side-to-side
- Lives on the bottom
- Omnivorous, feeds at night
- Food of fish and aquatic insects
- Diporeia sp. Common in Great Lakes
Class Hexapoda – Insects
- Segmented body and jointed legs
- Chitinous exoskeleton, shed as the animal grows
- Metamorphosis
o Complete: egg  larva  pupa  adult
o Incomplete: egg  nymph  adult
- Larva is the longest lived stage in most aquatic insects
Order Ephemoroptera – Mayflies (nymphs)
- Nymphs have gills along sides of abdomen
- Three long filamentous/hairlike tails
- Only order of aquatic insects that molts after the
nymph stage
- Adults don’t eat
- Most lake forms burrow into the sediment
- Presence indicates good water quality
Order Odonata – Damselflies & Dragonflies (nymphs)
- Dragonfly nymphs are spider-like, with heavy bodies
- Damselfly nymphs are more slender
- Damselflies have gills at the end of abdomen (three plate-like tails)
- Dragonflies have internal gills in abdomen
- Both damselflies and dragonflies are predators (feed on aquatic insects, worms, crustaceans)
- Dragonfly adults are larger than damselfly adults and hold their wings horizontally outward
- Damselfly adults are more delicate and hold their wings parallel to the body, or tilted upward
Damselfly nymph
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Order Trichoptera – Caddisfly (larvae)
- Terminal hooks present
- Many larvae make cases from sticks, shells and sand
- Larval stage lives longer, overwinters
- Pupal stage lasts about two weeks
- Adults live 30 days or less
- 1-2 generations/year
Order Diptera – True Flies
Family Chironomidae – Midges (larvae)
Adults look like mosquitoes, but do not bite, have fuzzy antennae and are common in late
spring/early summer. An example of a midge life cycle can be found in the Benthos Station
manual (also shown on page 109 of this manual).
Blood midge larvae
- Wormlike and red
- Prolegs present
- Most common benthic organism found on the Schoolship
Phantom midge larvae
- Transparent
- Diurnal migration to feed on
zooplankton
Family Tipudlidae – Crane Flies (larvae)
- Grub-like in appearance
- Larvae have disk (spiracular disk) and fleshy
arms (spiracles) on posterior end
Note – all illustrations by:
Remy Champt (ISEA).
Shelley Downes (from Mitchell, M.K. and William B. Stapp. Copyright 1999. Field Manual for Water
Quality Monitoring)
Joan Thomson (from Caduto, Michael J. 1990 Pond and Brook).
REFERENCES
Merrit, R.W., and K.W. Cummins. 1978. An Introduction to the Aquatic Insects of North America.
Kendall/Hunt Publishing Co., Dubuque.
Mitchell, M.K., and W.B. Stapp. 1999. Field Manual for Water Quality Monitoring, 11th Edition.
Kendall/Hunt Publishing Co, Dubuque.
Pennak, R.W. 1989. Freshwater Invertebrates of the United States. John Wiley & Sons, Inc., New York,
NY.
Internet Sites of Interest
http://www.benthos.org (North American Benthological Society)
http://www.epa.gov/glnpo/glindicators/biology/benthica.html (results of EPA's benthos sampling in Lake
Michigan)
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http://sun.science.wayne.edu/~jram/zmussel.htm (Wayne State University)
http://www.great-lakes.net/envt/flora-fauna/invasive/zebra.html (Great Lakes Information Network)
http://www.greatlakes.net/search/indexglin.html?slxtn=quagga%20mussel&topic_slxtn=link_info&n_sel=500 (Additional
links to Zebra and Quagga Mussel Resources)
DIAGRAMS & RELEVANT DATA
Plant Key (page 108)
Midge Life Cycle (page 109)
Animal Key (page 110)
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