Benthic sampling techniques

 
Benthic sampling techniques are essential to habitat mapping studies since they provide the 'truth' data on the actual composition of the seafloor.  When they are commonly used in conjunction with either a remote sensing or an acoustic technique, they are said  to 'ground truth' seafloor classifications. If samples are collected in high enough densities over survey areas they can be used to establish distributions and define habitats. 
 
Samples commonly provide two categories of information:
  • a sample of the seafloor material that can be analysed in terms of its structure, is often taken for geological analysis and is known as a physical sample; and
  • a sample providing a collection of the species living on or in the seafloor that can be identified and counted to give detail of the biological assemblage present at a point, is known as a biological sample
 
Biological and geological/physical samples should, in the main, be taken separately from different grabs; however the practice of separating out a portion of a biological sample for geological analysis is been followed by some groups.  This should not be done as quantifiable results  cannot be achieved for either the biological or geological samples. Commonly, for biological sampling to give quantifiable information, replication of the samples is essential, so adding to the number of samples that need to be collected.
 
GMHM3-08_Photograph_of_benthic_grab_sample.jpg
An example photograph of a benthic grab sample from a cobble/ pebble substratum (note the rule giving indication of scale).
 
There is a range of sampling devices commonly in use for benthic sampling, each designed to provide a certain type of sample over a specific ground type.  Grabs and corers are commonly used for both physical and biological sampling in softer, unconsolidated sediments while trawls and dredges provide only biological samples. 
 
Commonly-used grabs include Shipek, Hamon, Van Veen and Day.  There are also a range of corers available for sampling.  Each has advantages and disadvantages depending upon the seafloor material, the type of sample required and its volume.  The size of the sampling device will define the size of vessel needed to deploy it (or vice versa) and the number of people necessary to do so.  For representative particle size analysis, greater volumes are required from gravel rich sediments than for muds.
 
Key to the choice of sampling device will be the targeted type of sample: geological or biological for infauna (animals living within the seabed) or epifauna (animals living on top of the seabed) and a prior knowledge or suspicion of the seafloor material.  Trawls and dredges should not be used on fragile habitats owing to their destructive nature.
 
Grabs disturb the sediment when collecting a sample of the seabed.  To collect an undisturbed sample, a coring device is needed.  Cores give information on the variation of material below the seabed and the depth of biological activity.  Such devices include box corers, megacorers, gravity corers and vibrocorers.  Their depth of penetration depends on the equipment type and the nature of the seabed material.
 

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