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Sampling vegetation

Diver sampling vegetation                                   Diver sampling vegetation. Photo: Martin Isaeus

The abundance of vegetation is commonly measured as percent cover. Some methods measure biomass, but for the purposes of mapping over large spatial scales, cover is the most important.

As described elsewhere there are numerous methods for sampling vegetation in situations where direct mapping is possible. These include remote sensing methods in shallow areas or areas of limited spatial extent (sonar, satellites, visual inspection from divers, snorkelling, video or aquascope) and assume that more or less the whole area is measured.

Direct mapping – sampling in transects and polygons
Some of the methods for direct mapping, particularly visual inspection by divers, snorkelling, video or aquascope, can potentially also be used to collect data for predictive mapping. However, data for direct mapping are typically collected in the form of transects or polygons and concentrated to specific types of vegetation (e.g. eelgrass). This is true also for many of the existing standards for monitoring of benthic vegetation (e.g. the HELCOM COMBINE Guidelines for monitoring of phytobenthic plant and animal communities in the Baltic Sea). To evaluate the efficiency and suitability of these methods for purposes of direct mapping or monitoring of trends in biodiversity is, however, beyond the scope of PREHAB.

Predictive mapping – point sampling preferable
When predictive mapping is to be applied, i.e when only a very small portion of the area can be sampled, data are most efficiently collected representatively as independent samples (”points”) distributed within the area to be mapped. Therefore, despite the general applicability of several of these sampling methods, issues to do with sampling units, spatial scale and sample size may differ strongly depending on which type of mapping technique that is intended.

Many and discrete samples recommended
The experiences from PREHAB show that successful predictive mapping require:

  1. Large amounts of independently collected data (on the order of 100 < N < 1000)
  2. A spatial resolution capable of resolving important environmental gradients (on the order of ≈5-10 m in coastal areas).

For collection of data on vegetation for preditive mapping we therefore recommend sampling in discrete units using rapid and cost-efficient methods by some sort of visual method (i.e. video or photo) or possibly diving/snorkelling when maximum taxonomic resolution is required.

At present, however, there exist no generally accepted standard method that fulfills these requirements. Several examples of possible approaches exist in national inventories, but the need for development of general standards is urgent.

New directives call for improved standards
One additional, important recommendation in this context is that methodological standards and sampling designs are developed that can be used for monitoring of the distribution and extent of species and habitats according to the Marine Strategy Framework Directive and the Habitats-directive in their own right (i.e. with known uncertainty and statistical power to detect changes). Such coordination of mapping efforts with regular monitoring has the potential to improve both the spatial and temporal representativity of benthic maps!

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


Video mapping

The video below displays four types of vegetation habitats in the Baltic. Transects likt these can be used to collect biological samples for predictive mapping. Video was filmed using a handheld dropvideo camera. Photo: AquaBiota Water Research.

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