How do we map biotopes?

Species are unevenly distributed

The species on the sea floor are not evenly distributed: they appear in clusters associated with various environments and sea floor types. Each combination of certain species with a particular environment is known as a biotope, or “nature type”. It is important to know how species are distributed if you want to manage the sea floor as sustainably as possible. However, as our waters cover a vast area the process of mapping them is time-consuming and expensive. So what is the best way of collecting information about the biodiversity and biotopes in the large areas that need to be mapped? Currently, the only available detailed information about the compositions of sediments and benthic communities covers small areas dotted around the map.

The question is therefore: how do we go from this situation to producing information covering the whole area?

Whole benthic communities are being mapped between “stations”

There are two possible answers to this question: (1) You can map points (“stations”) that are close enough together to provide virtually continuous information about the areas being mapped, or (2) leave larger distances between the stations and then use detailed information about the environment to predict – or to computer model – what biotopes you would expect to find between the stations.
The disadvantage with the first option is that it is expensive to map so many points close together. It is also very time-consuming to analyse either sediment samples taken from the sea floor using a grab, epibenthic sled or trawl, or videos of the sea floor.

The objection to the second option is that we know too little about the relationship between the species and their habitats to allow us to make reliable predictions about the areas between stations. It has proved difficult to predict the distribution of species solely on the basis of information about the surrounding environment. In general, a variety of environmental factors combine in unknown ways, while biological conditions simultaneously “disturb” the known factors. One of key theories of the MAREANO project is that biological communities, which consist of numerous species, are more stable in relation to environmental variation than individual species. Whole communities can therefore be predicted more reliably than any individual species.

“Nature types” – what are they?

“Biotopes” are a key concept in the MAREANO project. A biotope is a combination of species and the physical environment in which they live – a recognisable combination. This definition is close to the definition of the term “nature type”, which is used by The Norwegian Biodiversity Information Centre when defining biotopes in Norway. For consistency, in Norwegian we refer to these biotopes as “nature types”.

Basic bathymetric mapping

The sea floor is large and relatively unexplored, but what is the most efficient way of studying it? What you must do is use the information that you have available to guide your efforts. Although the sea floor in Norwegian waters has been the subject of a number of geological and biological surveys, they are not directly applicable to MAREANO. The most useful source of information for the MAREANO project is the new full-coverage bathymetric maps being produced in the first phase of the project. These maps, which have a resolution of 5x5 metres, will play a key role in helping us to carry out future research as efficiently as possible.

 They allow us to recognise small structures on the sea floor such as coral reefs, pockmarks and iceberg plough marks (which were produced when the ice cap retreated around 10,000 years ago). However, it is hard to spot wrecks and other objects or structures that are shorter than 20 metres long.

When describing biotopes, it makes sense to start by looking at the environmental conditions, and that is exactly what the bathymetric maps allow us to do. Those conditions are what determine which species can live in an area, and thus also what the biotope is. The climatic, hydrological, topographic and geological conditions are the key parameters here. Next, in order to classify a biotope, you need to take into account the species living in the area, which together constitute the benthic community.

Predicting biotopes based on video analyses carried out after research missions

Videos of the sea floor are analysed in order to compare and classify different areas, based on the composition of benthic fauna observed. It turns out that these categories also share many characteristics in terms of water depth, acoustic reflection properties, topography and other indicators obtained from multibeam echo sounding. We have used the correlation between these data sets to predict what biotopes you are likely to find in other similar areas, including places for which we do not have video recordings (i.e. between stations).