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Mapping Techniques: Sediment Sampling

Grab Sampling

Example of Retrieval

Retrieval of a grab sampler filled with sediment.
Courtesy: Science Applications International Corporation

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.

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. 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 analyzed by taxonomists in the laboratory.

Example of Sediment

Sediment scooped out of a grab sampler will be sent to a laboratory for chemical testing.
Courtesy: Science Applications International Corporation

Uses

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 Benthic Habitats of New York/New Jersey Harbor mapping project used benthic grabs in conjunction with sediment profile imaging and plan-view photography to classify and map the benthic habitats of the harbor. Two grab samples were collected at one third of all stations sampled with the sediment profile imaging camera. One of these samples was used for analysis of sediment texture and organic content, while the other was used for identification and enumeration of benthic organisms. Collection of grab data in this study served as verification for the imagery interpretation, and it allowed for community comparisons with other benthic community datasets.

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