Grab Sampling

Brief Description

The grab sampler is a device that simply grabs a sample of the topmost layers of the seabed by bringing two steel clamshells together and cutting a bite from the soil. Grab sampling is a common technique used to examine the surface sediment (from about 10-15 cm deep). Different grab samplers are used depending upon the type of substrate being sampled (soft or hard) and the size of the sample required.

See NOAA's summary table at summary view of grab sampling survey technique (180 KB PDF).

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Deployment of the Smith-Mackintyre Grab for sand and muddy sand sediments

(Coastal Geomorphology and Classification Subproject, Sydney Harbour Sediment Sampling, August 2003)

There are several different models of grab samplers; however, each one has a similar mechanism for collecting the sample. The open sampler is lowered to the seafloor, where it penetrates the sediment-water interface and then closes. The device is then raised to the surface with the intact sediment sample.

The shipeck grab in the loaded or closed position prior to deployment example of Muddy sand sediment collected using the Smith-Mackintyre Grab Sampler

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Advantages and Limitations

Sediment samples can be tested for a variety of chemical (that is, toxicity, organic content, oxygen concentration) and physical (that is, sediment type, grain size) parameters in the laboratory. If knowledge of the biological composition of the sampled area is desired, grab samples may be washed and sieved to separate organisms from the sediment. The organisms are then identified and enumerated.

Grab samplers are relatively light and easy to operate; however, the sediments are disturbed during sampling, so grab samples will not yield information about sediment structure.

Sediment sampling and underwater video equipment used in the Cockburn study. A) Small (0.5 litre) stainless steel Van Veen grab; B) 
Submersible vibracorer; C) Towed underwater video camera used by the CWHM team from University of Western Australia.

Grab samplers are designed for soft sediments, so hard bottom or reef structures cannot be sampled effectively using grab samplers. Additionally, grab sample analysis can be time-consuming and relatively expensive since samples must be analysed by taxonomists in the laboratory.

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These data can be used for stock assessments of commercially important species. Biological samples can be tested further for chemical and toxicity levels in the tissues as an indicator of bioaccumulation and habitat health.

The case studies have reports of Benthic Habitat mapping projects using benthic grabs in conjunction with sediment profile imaging and plan-view photography to classify and map the benthic habitats of the seabed. Collection of grab, box and vibrocoring data in the case studies serve as verification for the imagery interpretation, and allow for community comparisons with other benthic community datasets.

Larger Van Veen grab loaded and ready for deployment, with examples of sample composition

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