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Sampling benthic invertebrates

Blue mussels Mytilus edulisBlue mussels - a habitat forming species. Photo: Martin Isaeus.

Benthic invertebrates include many fauna types with different ecological charateristics. Sampling benthic invertebrates therefore requires a range of methods.

The abundance of benthic invertebrates is typically measured in number of individuals (or biomass) per unit area, or, as percent cover. For invertebrate species buried in the sediment and for solitary species living on soft or hard substrata, the number of individuals is an appropriate sampling measurement. Species forming colonies or dense populations covering hard substrates are most conveniently measured as percent cover.

Sampling infauna - standardised methods available
A large portion of the total species diversity live buried beneath the sediment surface as infauna. Because of the great ecological importance, fairly extensive monitoring programs with well standardised sampling methods for benthic infauna have been established in the Baltic Sea. They involve sampling with corers and subsequent handling according to defined procedures (e.g. HELCOM COMBINE methods for soft macrozoobenthos. These methods are also well suited for purposes of predictive modelling because they are discrete and allow independent, representative sampling. The situation is somewhat complicated by the fact that coring devices may not sample equally well in coarser sediments, and because of the costs associated with sampling. It may also be worth considering the desired precision at individual sampling stations. The number of replicates at individual stations will theoretically limit the precision of models and validations, but particularly for abundant species this may not be a serious problem.

Epifauna often sampled over large areas
Epifauna is by definition observed above the sediment or rocky substrates. Sampling is commonly performed through diving, snorkelling and photographic methods. Some of these invertebrates occur individually in relatively sparse populations, and are best measured as numbers per unit area. Such species typically involve mobile crabs, gastropods and star-fish as well as sessile sea-pens and solitary corals. In order to achieve sufficient precision and to avoid excessive errors due to false negatives, fairly large areas have to be sampled. Most of these animals are only present in limited parts of the Baltic Sea. Nevertheless, they can be appropriately sampled with the methods listed above, but special care need to be taken in the design of programs measuring numbers rather than percent cover, which is the most common type of measurement.

Use vegetation methods for habitat-forming epifauna
Some epifauna occur in dense populations which form a dominant or patchy cover of the seafloor. In large parts of the Baltic proper, the blue mussel Mytilus edulis represents one such species. In Kattegatt, there are common examples where soft-corals, sessile polychaetes and brittle-stars completely dominate the habitat. These ”habitat-forming” species are most conveniently measured as percent cover, and can in principle be measured using the same methods as for estimating cover of vegetation. This means that the same considerations need to be taken into account regarding:

New standards needed

Finally, it is worth repeating the need for development of standardised sampling programs suitable for predictive mapping of benthic invertebrates (and vegetation). Geographically explicit infomation on these communities are important components for monitoring according to the Marine Strategy Framework Directive and the Habitats-directive. This is because they harbour important and unique biodiversity as well as beeing important for the function of the Baltic Sea system.

See also the MARMONI project, aiming to improve and develop monitoring methods for the Baltic Sea.


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