Problems concerning quantification of maerl biota

Methodology for quantification of maerl biota

Much of the early work on the fauna of maerl beds was based on benthic grab samples. More recently, faunal recording has used divers extensively but this is only suited to a few of the species present as many are difficult to see. For the flora, there are similar problems with some additional ones. The development of an adequate quantitative sampling technique for maerl epiflora presents several problems (Maggs, 1983a), as follows:

• Fixed quadrats cannot be used due to the mobility of the substratum. (However, fixed rods may be helpful as they can withstand substratum mobility.)
• The same sample cannot be examined twice.
• Maerl branches in three dimensions, so the sampling is not planar. This means that the total surface area sampled using a quadrat based system will vary depending on the size and shape of the maerl thalli.
• Possible methods of assessment of abundance of maerl epiflora species are severely limited by the size and form of the substratum and the epiflora species themselves.
• Abundance scales would be almost meaningless due to the complex configuration of the substratum and the size and growth habit of many of the algae, which do allow reasonable estimates of relative cover.
• Biomass of individual species could not be used for the maerl community with its high proportion of crustose, shell-boring and minute species. For erect species, some estimates of biomass could be made, however.
• Most of the species do not grow as individuals, and thus counts of numbers per sample cannot be made.

In addition, as with all benthic habitats, there is the issue of the patchy nature inherent to the distribution of flora and fauna. This raises the additional questions of :

• How large a surface area should form each sample?
• How deep a sample should be collected?
• How many samples are needed in order to obtain an adequate representation of the species diversity and biomass of the site?

The minimal sample size is one in which the species composition of the community must be Aadequately represented@ (Mueller-Dombois & Ellenberg, 1974). There is no exact definition of minimal area, and in some studies the sample size has been chosen to be practical and manageable despite containing only a fraction of the species present within the biotope. Maggs (1983a, b) suggests the minimal subsample size to be taken from a maerl sample should be one where a 10% increase in the number of species in the subsample is derived from a 10% increase in the area. Sears & Wilce (1975) used individual shells of Crepidula as a sample unit, recording presence, absence and frequency of species occurrence. Lieberman et al. (1979) treated each cobble as a separate sample with a known surface area while investigating a seasonally devastated cobble based community in Ghana, recording the algal flora by the weight of each species per unit area.

onspicuous species can be recorded semi-quantitatively using divers and abundance scales as for hard substrata. Species richness can be determined only by removal of samples of known size and subsequent laboratory examination. Maggs (1983a) used 300 cm³ maerl samples, each of which took several hours to work through, recorded the presence of all species. The only practicable approach to the assessment of abundances of small maerl epifloral species is to use a point quadrat method.

Divers can use hand-held circular cores which are driven into the maerl bed, sealed top and bottom and returned to the surface. Box corers can be remotely operated from the ship. Samples are taken to a depth of about 20 cm of sediment. The sediment sample is then washed through a sieve (e.g. 0.5 mm mesh) and the sample treated with Rose Bengal to stain living material before being preserved. The sample is then later picked through to remove any visible plants and animals which can then be identified and counted or weighed as needed (Hall-Spencer, 1995a).

A pair of divers positions a quadrat boundary of known size and either collects all the material within the quadrat to a specified depth, records all visible species without disturbing the surface, or collects all living material from the quadrat as a sample for detailed analysis in the laboratory. Only the larger species would be collected by this method.

Suction dredges can be either diver-operated or can be used from the surface. They are not accurately quantitative. The samples of algae and soft-body invertebrate fauna are frequently severely damaged by this collection method. Large quantities of waste sediment and silt may be produced which could prove deleterious to the maerl bed being sampled. The range of species collected and the population structure of the collection are dependent on the mesh size used for sieving. Keegan & Könnecker (1973) designed a suction sampler for use on substrata including maerl beds. Hall-Spencer (1998) provides data on the mollusc species most suitably sampled by suction sampling, as opposed to diver-recorded quadrat or diver-operated corer.

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