German Federal Agency for Nature Conservation (BfN)

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Survey methods


Video sled and grab sampler for investigating the sea floor and its ecological communities. Photo: IOW
Video sled and grab sampler for investigating the sea floor and its ecological communities. Photo: IOW

Various methods are used as standard to ensure the fullest possible coverage of benthic communities.


The community of organisms living in the sea floor (infauna or endofauna) is typically sampled using a van Veen grab sampler. This can take quantitative samples with a standardised area of approximately 0.1 m2, allowing analysis in terms of the range of species and the abundance and biomass distribution. These parameters are an important part of the basis for subsequent status assessment.


Grab sampler, closed, with sand mason worm (Lanice conchilega). Photo: BfN
Grab sampler, closed, with sand mason worm (Lanice conchilega). Photo: BfN
Grab sampler, open, Photo: BfN
Grab sampler, open, Photo: BfN

Species that live on the sea floor (epifauna) tend to be larger and rarer and many have the ability to take quick flight. These species are therefore sampled with a dredge taking in a much bigger area than the grab sampler. As the exact size of the sample area is usually unknown, the analysis is solely qualitative, i.e. the samples merely supplement the range of species at the station. A dredge can thus also sample the sessile community on hard bottoms (macroalgae, mussels, sponges, etc.). As the method is invasive, however, meaning it damages the habitat under investigation, visual means are increasingly used as a supplement or an alternative.


Dredges used on hard bottoms. Photo: BfN
Dredges used on hard bottoms. Photo: BfN
Towed cameras – a useful aid. Photo: BfN
Towed cameras – a useful aid. Photo: BfN

Visual technologies include vessel-towed cameras pulled along predefined transects just above the sea floor. This allows an impression to be gained of the habitat structure and the prevalence of large species. A significantly smaller range of species is identified in this way, however, than with dredge samples.

Full sampling of the hard bottom community therefore remains a methodological problem. In the Baltic Sea EEZ especially, the full range of species occurring in this zone is not yet known. Initial sampling is therefore also performed by research divers, who can take quantitate samples of small areas in the same way as a grab sampler can, giving a more representative look at the community. However, this method is not suitable for monitoring purposes because when diving to depths of more than 30 m, divers are left too little time to take more than a very small number of samples each day.

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