Video survey

Grab and dredge samples

Photographic monitoring of fixed quadrats

The towing of a remote camera or the use of divers on sledges to record video images enables a large area to be examined. It is an excellent method for providing basic information on the extent of a maerl bed and gross features such as patchiness. The disadvantages are the inability to record any information about the biotope below the surface of the maerl bed; few species can be identified using this method.

These are useful methods to employ if large but non-quantitative samples are sought. Much of the early work on the extent and species diversity of maerl beds was based on benthic samples using these methods. The major disadvantage, other than the sampling of an uncertain surface area, is that the depth to which the devices penetrate the substratum cannot be controlled. Dredge sampling has been employed in the most extensive recent survey of sediment fauna, which covered 849 stations (O'Connor et al., 1993). Smaller grab samples were also taken for quantitative analysis.

More recently, the BIOMAERL project has found that grabs sample very effectively in the top 10 cm. However, Keegan & Könnecker (1973) have shown that many large animals can penetrate to depths of 40-60 cm into maerl or maerl debris.

For determining change in biotopes at a particular spot, for example with regard to a localised impact, a fixed locating point can be established by drilling a positioning pole through the maerl and underlying sediments to a depth at which it remains stable, without disturbing the surface of the maerl bed. The mobile habitat that is the maerl bed can then move past the fixed point and photographs be used to record any changes. This method requres skilled divers to locate sites and take the photographs and has the disadvantage of recording only the surface species. Few species within a maerl biotope can be identified by this method, but it can record broad changes in biotopes.

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