Benthic Sampling Flashcards
What is benthic sampling?
Methods for sampling aquatic benthic fauna (macrofauna techniques)
What is the benthos?
The community of organisms that live on or near the seabed or on the bottom of freshwater bodies
What size are macrobenthos?
> 1mm
eg. Polychaetes, corals, crabs, lobsters
What size is meiobenthos?
0.1 - 1mm
eg. Nematodes, copepods, ostracods
What size is microbenthos?
<0.1mm
eg. Ciliates, diatoms, bacteria, Amoeba
What are the largest invertebrates called?
Not strictly defined in terms of size, often referred to as megabenthos
How can megabenthos and others be defined?
Usually defined operationally e.g. benthos that can be sampled using towed gear of a certain mesh size or recorded on seabed photographs e.g. Holothuroidea, Asteroidea, some Crustacea
What is the hyperbenthos?
Organisms that live just above the sediment
What is the epibenthos/epifauana?
Organisms that live on the substrate
What is the endobenthos/infauna?
Organisms that live in the substrate
What are two types of benthos sampling methods?
-Ex-situ sampling methods
-In-situ sampling methods
Describe ex-situ sampling methods
-A portion of the sediment and fauna are removed for latter processing and examination
-Trawls, dredges, cores, grabs, remotely operated vehicles (ROVs)
Describe in-situ sampling methods
-Benthos is directly observed
-Onshore observations, imaging, scuba observations
What is quantitative sampling?
-Sampling that produces data that can be measured numerically and transformed into usable statistics
eg. quadrats, cores can provide density estimates
What is qualitative sampling?
-Descriptive sampling, data can be observed but not measured explicitly
eg. some trawls/dredges
What is semi-quantitative sampling?
Eg. CPUE, some measure of numerical data
Describe intertidal sampling
-Relatively easy to access
-Must consider impact of tides on benthic fauna
-Often intertidal is sampled at a series of regular intervals from high to low water mark with replicates taken at each location eg. quadrats, small hand-hild cores (quantitative)
Describe the Riley Pushnet (Qualitative)
-Designed to sample benthic macrofauna (shrimp, juvenile flatfish) in shallow intertidal areas
-Light metal frame seated on two metal skies
-Net of 5-10mm mesh with finer mesh cod end
-Weighted footrope
-Tickler chain in front of mouth disturb organisms and drive them into net
What are some disadvantages of subtidal sampling?
-More expensive
-Requires shiptime
-Weather dependent
Describe trawls/dredges
-Skim over surface of the substrate
-Large swept area- suitable for rarer epifauna
-Selective sampling method- some organisms will escape
-Qualitative but can be used in a semi-quantitative fashion (CPUE)
eg. Beam, Agassiz, Otter Trawls
Describe beam trawls
-Used commercially to catch fish
-Mouth of net (12.5 mm mesh) held open by beam 1- 12m length
-Weighted chain ground rope lies behind upper mouth of net to prevent disturbed animals escaping
-Used on smaller vessesls
Describe dredges
-Heavy metal frame designed for breaking off rock, scraping organisms off hard surfaces or collecting top layer of sediment
-Rectangular dredge/rock dredge
Describe an anchor dredge
-Semi-quantitative sampling of firmly packed soft sediments
-Fitted with metal box that digs into sediment to depth of up to 25 cm
-Not towed behind boat- deployed from drifting vessel
-Takes a wedge shaped sample theoretically of equal area to the digging plate
-Easier to quantify than trawls and other dredges
-Several versions of this
Describe the epibenthic sledge
-Metal frame with fine mesh net attached. Scrapes sediment collecting organisms from the surface
-May include open/closing devices and flowmeter and camera attachments
-Several nets may be used
-Often used to collect samples in the Deep-sea
What effects and improves efficiency?
-Efficiency of trawls and dredges low
-Varies on nature of substrate, the ships system (length of warp, type of warp, speed of towing)
-Fitting of depressors, plates, tickler chains can improve efficiency
-Modern trawls employ depth and net sensors, fitted with cameras and ctd- semi-quantitative
Describe grabs
-Used to take quantitative samples of infauna from top 15cm of the sediment
-Depth of bite can vary on how packed is sediment
-Cover a surface area of 0.1-0.2 m2, several replicate samples taken per station
-Suitable for quantitative determination of more common species- not good for rarer species
-Fast moving species can escape grab
-Can be supplemented with sledge/trawl samples, underwater video or diving
Describe a Petersen grab
-Gauze-covered window allows water to escape when grab closes
-Lowering rope slackens when gear hits substrate, and release hook operates and buckets close together before grab leaves the bottom
What disadvantages are there to Petersen grab?
-Rope may slacken momentarily as ship rolls- grab closes prematurely
-Grab may not penetrate sufficiently into sediment
-Closure may be incomplete leading to loss of sediment from grab
Describe the van veen grab
-Long arms give better leverage for closing
-Lowering rope slackens when gear hits substrate, release hook operates and buckets close together before grab leaves the bottom
Describe the day grab
-Pressure plates are pushed upwards on hitting substrates, releasing transverse beam so that hooks holding the buckets are released
Describe shipek sediment sampler
-Single semi-circular scoop activated by powerful springs
-Small sampling area: 0.04 m2; mainly used to take sediment samples
Describe corers
-Quantitative sampling method
-Length of plastic or metal tube pushed into sediment
-Often deployed by divers
-Bung inserted in tube to retain contents
Describe box samplers and corers
-Deploys corer into sample
-Cutting arm released to close bottom of tube
-Samples area 20 x 30 cm down to 45cm depth
-Weighs 750kg
-Modified designs allow multiple cores to be taken
-Provide deep and relatively undisturbed sediment sample sample- suitable for deep sea use
-Requires large vessel and calm conditions
What can the box corer be used to do?
-Suitable for deep-sea sampling
-Can be used to profile the sediment
Describe suction samplers
-Pump attached to wide coring tube (36cm diameter, 30cm long)
-Coring tube is inverted once it is lifted off the bottom retaining sample
-Some designs are operated underwater by divers
-Suction samplers often attached to ROVs for sampling individual organisms (targeted)
What effects the choice of sampler
-Requirements of survey
target organisms, spatial coverage etc.
-Quantitative/qualitative sampling
-Working conditions – weather, vessel size power etc.
-Availability of suitable gear
What are some alternative sampling methods
-Some species are not readily taken by conventional sampling methods (trawls, dredges, grabs, corers)
-Too sparsely distributed to be recorded in limited sampling area
-Live in habitats inaccessible to conventional gear eg. reefs
Describe various imaging techniques
-Video - hand operated and remote options
Go-pros attached to sleds/ trawls
Non - invasive, used to assess Zebra mussels in lake Michigan
-Collected continuous footage from 500 m long quadrats
Describe Remotely Operated Vehicles (ROVs)
-Tethered/ Untethered
-Video/ still footage
-Sampling arms/tubes
-Auto Hold
Describe the imaging technique Sediment profile imagery (SPI)
-Inverted periscope penetrates sediment and photographs profiles of sediments
-Can be interfaced with a computerised measuring system
- Cost-effective survey method
-Used for monitoring environmental status
What is an example of indirect detection?
Fish stomach contents useful for detecting organisms not available to trawls and dredges
-Must consider selective feeding of fish and differential rates of digestion
Describe artificial substrates
-Synthetic structures used to collect settling organisms
-Advantages
-Can be used to sample difficult areas
-Passive collection method
-Confounding effects of habitat differences are minimized by providing a standardized microhabitat
-Sampling variability is decreased due to a reduction in microhabitat patchiness
What are disadvantages of artificial substrates?
-Must be deployed and recovered - two sampling trips
-Prone to loss, natural damage or vandalism
-Substrate material will influence the composition and structure of the community
-Orientation and length of exposure of the substrate will influence the composition and structure of the community
How can hidden organisms be collected?
-Organisms that hide in rock crevices, weeds etc
-Can be removed using
-Explosives, anesthetics
-Electrofishing
-Baited/ light traps
Summary of trawls
-Qualitative / semi-quantitative. Suitable for epifauna on mud/sand substrates. Can be used from small vessel
Summary of dredges
General use on all bottom types. Qualitative (anchor dredge design semi-quantitative)
Summary of grabs
-Suitable for muds/sands
-Quantitative (although bite depth and shape can very with substrate)
-For shallow infauna
-Suitable for small vessels
Summary of box corer
Heavy, needs large vessel
-Good depth penetrations
-Quantitative
-Mud/sand
Summary of remote suction sampler (Knudsen)
-Can collect known area by great depth within core
-Large amount to sort, to diving depth only, very good for finding what other methods miss and to calibrate other techniques
Summary of cameras: remote and in-situ
See only surface species or marks, need to be able to identify from film
Summary of sediment profile imagery
see cross section of bottom, can get much data, but not many species
