Classification development - approach and methods used

Review of classification systems and literature

Before embarking on the development of the MNCR BioMar classification (Connor et al. 1997a, b), a review of existing classification systems was undertaken (Hiscock & Connor 1991). From these, proposals for a classification structure (Connor et al. 1995 a, b) were developed that drew upon the best features of the existing systems, whilst avoiding their weaker aspects. There was subsequent wide consultation on the proposed classification structure, including through two European workshops held during the EC-funded BioMar project (Hiscock ed. 1995; Connor ed. 1997). These workshops helped ensure broad acceptance of the proposed structure and its wide applicability across European seas.

 
In addition to a review of classification schemes, an extensive review of the literature describing marine habitats was also undertaken. This helped formulate the initial lists of types which might form the basis of the classification. For this the scientific literature was of considerable help for sediment habitats (a traditional area for marine studies) but relatively poor for rocky habitats (which, in the subtidal, attracted attention only relatively recently through use of SCUBA diving techniques). These initial lists of types were then refined on the basis of new dedicated field surveys, data analyses and field trials.
 

Consultation and testing

Phases of external consultation and testing of the classification system have been essential to ensure the classification is as robust and usable as possible.
 
The advice of external consultees has been important in two key areas:
 
  • Marine scientists have contributed expertise in their understanding of the marine environment and its communities, both from a generic perspective and with specific knowledge of communities at particular sites around the country. Of particular importance has been advice on the relationships of environmental factors to community structure and the spatial and temporal dynamics of the marine environment.

 

  • Environmental and conservation managers and end-users have helped define their end needs for the classification system. This has been reflected both in terms of the overall structure of the classification, such as the orientation of biotope complexes to mapping and sensitivity needs, the type of information given in the description of each classification type, and the demands of field application.
 

Field surveys and other data acquisition

The Marine Nature Conservation Review (MNCR) undertook a programme of field surveys throughout Britain between 1987 and 1998, collecting data suitable to develop the classification. In addition, data were acquired from the published literature and through collaboration with a wide variety of academic, government and other organisations. Comparable data were collected in Ireland through the BioMar project between 1992 and 1996.
 
The data comprise information on the nature of each site (such as substratum, wave exposure and height or depth surveyed), the type of sampling undertaken, the site's location and the species present (together with an indication of their abundance) within discrete habitats at the site. MNCR field recording techniques are described in Hiscock (1996), with Appendix 8 providing the guidance on how to complete MNCR field recording forms (the forms can be downloaded from here).The terminology relating to field survey methods is described further below, and should help users of the classification interpret the habitat information contained in the biotope descriptions. Procedural Guidelines for a wide range of field sampling techniques are given in the Marine Monitoring Handbook (Davies et al. 2001).
 
In total, data for over 16,000 sites comprising more than 36,000 habitat records from around Britain and Ireland were collated and entered onto the MNCR database (as described by Hiscock, 1996). The database includes a module which holds definitions of each classification type, linked to a national dictionary of marine species and to the field survey data. The field survey data have been made widely accessible via the web-based MERMAID application, and more recently, via the National Biodiversity Network from an MS Access-based 'relational' database, Marine Recorder. The Marine Recorder database application has been specifically developed to accept marine biological data from a wide range of survey techniques, including the data held originally in the MNCR database. The application can be downloaded here, and includes a dictionary of the habitat classification types.
 

Terms used for field recording and habitat definition

For semi-quantitative biological recording, the MNCR SACFOR scale was used.
The following definitions for physical habitat characteristics are taken from guidance notes for MNCR field recording (Appendix 8 in Hiscock ed. 1996). Some terms are modified for use in the classification.
 
Salinity - The categories are defined as follows (the points of separation approximate to critical tolerance limits for marine species):
 
Fully marine
30-40 ‰
Variable
18-40 ‰
Reduced
18-30 ‰
Low
<18 ‰
 
Wave exposure - These categories take account of the aspect of the coast (related to direction of prevailing or strong winds), the fetch (distance to nearest land), its openness (the degree of open water offshore) and its profile (the depth profile of water adjacent to the coast). Estimation of wave exposure requires inspection of charts and maps.
 
Extremely exposed
This category is for the few open coastlines which face into prevailing wind and receive oceanic swell without any offshore breaks (such as islands or shallows) for several thousand km and where deep water is close to the shore (50 m depth contour within about 300 m, e.g. Rockall).
Very exposed
These are open coasts which face into prevailing winds and receive oceanic swell without any offshore breaks (such as islands or shallows) for several hundred km but where deep water is not close (>300 m) to the shore. They can be adjacent to extremely exposed sites but face away from prevailing winds (here swell and wave action will refract towards these shores) or where, although facing away from prevailing winds, strong winds and swell often occur (for instance, the east coast of Fair Isle).
Exposed
At these sites, prevailing wind is onshore although there is a degree of shelter because of extensive shallow areas offshore, offshore obstructions, a restricted (<90o) window to open water. These sites will not generally be exposed to strong or regular swell. This can also include open coasts facing away from prevailing winds but where strong winds with a long fetch are frequent.
Moderately exposed
These sites generally include open coasts facing away from prevailing winds and without a long fetch but where strong winds can be frequent.
Sheltered
At these sites, there is a restricted fetch and/or open water window. Coasts can face prevailing winds but with a short fetch (say <20 km) or extensive shallow areas offshore or may face away from prevailing winds.
Very sheltered
These sites are unlikely to have a fetch greater than 20 km (the exception being through a narrow (<30o) open water window, they face away from prevailing winds or have obstructions, such as reefs, offshore.
Extremely sheltered
These sites are fully enclosed with fetch no greater than about 3 km.
Ultra sheltered
Sites with fetch of a few tens or at most 100s of metres.
 
In the habitat classification exposed (as in exposed littoral rock) encompasses the extremely exposed, very exposed and exposed categories, whilst sheltered (as in sheltered littoral rock) encompasses sheltered to ultra sheltered categories.
 
Tidal currents (or streams) (maximum at surface) - This is maximum tidal current strength which affects the actual area surveyed. Note for shores and inshore areas this may differ considerably from the tidal currents present offshore. In some narrows and sounds the top of the shore may only be covered at slack water, but the lower shore is subject to fast running water.
 
Very strong
>6 knots            (>3 m/sec.)
Strong
3-6 knots           (>1.5-3 m/sec.)
Moderately strong
1-3 knots           (0.5-1.5 m/sec.)
Weak
<1 knot             (<0.5 m/sec.)
Very weak
Negligible
 
In the habitat classification tide-swept habitats typically have moderately strong or stronger tidal currents.
 
Zone - These definitions primarily relate to rocky habitats or those where algae grow (e.g. stable shallow sublittoral sediments). For use of the terms infralittoral and circalittoral in the classification, especially for sediments, refer also to Table 5.
 
Supralittoral
Colonised by yellow and grey lichens, above the Littorina populations but generally below flowering plants.
Upper littoral fringe
This is the splash zone above High Water of Spring Tides with a dense band of the black lichen by Verrucaria maura. Littorina saxatilis and Littorina neritoides often present. May include saltmarsh species on shale/pebbles in shelter.
Lower littoral fringe
The Pelvetia (in shelter) or Porphyra (exposed) belt. With patchy Verrucaria maura, Verrucaria mucosa and Lichina pygmaea present above the main barnacle population. May also include saltmarsh species on shale/pebbles in shelter.
Upper eulittoral
Barnacles and limpets present in quantity or with dense Fucus spiralis in sheltered locations.
Mid eulittoral
Barnacle-limpet dominated, sometimes mussels or dominated by Fucus vesiculosus and Ascophyllum nodosum in sheltered locations. Mastocarpus stellatus and Palmaria palmata patchy in lower part. Usually quite a wide belt.
Lower eulittoral
Fucus serratus, Mastocarpus stellatus, Himanthalia elongata or Palmaria palmata variously dominant; barnacles sparse.
Sublittoral fringe
Dominated by Alaria esculenta (very exposed), Laminaria digitata (exposed to sheltered) or Laminaria saccharina (very sheltered) with encrusting coralline algae; barnacles sparse.
Upper infralittoral
Dense forest of kelp.
Lower infralittoral
Sparse kelp park, dominated by foliose algae except where grazed. May lack kelp.
Upper circalittoral
Dominated by animals, lacking kelp but with sparse foliose algae except where grazed.
Lower circalittoral
Dominated by animals with no foliose algae but encrusting coralline algae.
 
Substratum
 
Bedrock
Includes very soft rock-types such as chalk, peat and clay.
Boulders
Very large (>1024 mm), large (512-1024 mm), small (256-512 mm)
Cobbles
64-256 mm
Pebbles
16-64 mm
Gravel
4-16 mm
Coarse sand
1-4 mm
Medium sand
0.25-1 mm
Fine sand
0.063 - 0.25 mm
Mud
<0.063 mm (the silt/clay fraction)
 
Each division of sediment type above represents two divisions on the Wentworth scale (Wentworth 1922).
 
In the habitat classification, bedrock, stable boulders, cobbles or pebbles and habitats of mixed boulder, cobble, pebble and sediment (mixed substrata) as well as artificial substrata (concrete, wood, metal) are collectively referred to as rock. Highly mobile cobbles and pebbles (shingle), together with gravel and coarse sand are collectively referred to as coarse sediments. Mixed sediment consists of heterogeneous mixtures of gravel, sand and mud and may often have shells and stones also.
 

Data analysis

For the 1997 classification, data analyses
using the TWINSPAN and DECORANA clustering and ordination techniques were employed to help define the types. The analytical processes adopted are described in Mills (1994).
 
The 1997 version was revised and refined to develop the present version. Extensive re-analyses of the data were carried out using  the analytical techniques available in PRIMER (Clarke & Warwick, 2001). The data were initially divided into the five broad habitat types shown in the primary habitat matrix, i.e. Littoral Rock, Littoral Sediment, Infralittoral Rock, Circalittoral Rock and Sublittoral Sediment. Due to the large size of the datasets within each broad habitat, some further a priori divisions of the data within broad habitats were necessary before analysis was possible. Additional analyses were carried out on data from "borderline" habitats to ensure these a priori splits did not force artificial divisions into the classification where this was not supported by differences in the survey data. Analysis within each broad habitat was led by a specialist for that habitat type. Figure 2 shows the data analysis process for the littoral sediment section. The following paragraphs describe the analyses
carried out within each broad habitat:

Littoral rock

As the biotopes defined in version 97.06
(Connor et al., 1997 a, b) were generally considered satisfactory, analysis focused on clarifying the boundaries between closely related types and confirming the validity of certain less-well defined types. This included attention to the inter-relationship of fucoid-dominated types regarding the bedrock/boulder/mixed substrata and fully marine/variable salinity transitions and examination of the various red algal-dominated types. Additionally new data from intertidal caves enabled substantial development of the classification here. On the basis of these analyses, some restructuring at biotope complex level was necessary.

Littoral sediment

Due to the size of the Littoral Sediment dataset
(>4000 records), some a priori division was necessary to provide datasets that could be managed within PRIMER (Clarke & Warwick, 2001). Data were divided based on the sediment type categories at habitat complex level in the 97.06 classification (Connor et al., 1997a, b): gravels and sands, muddy sands, sandy muds, muds and mixed sediments. Semi-quantitative epifaunal data were considered to be of less value than quantitative infaunal data for the purposes of the analysis and were thus excluded. Epifaunal data were however used to define types where a significant proportion of species would be sampled in epibiota sampling techniques, and/or where few infaunal samples were available, e.g. for mussel beds.
 
Cluster analysis was carried out based on species matrices listing individual counts per m2 in each sample, using the PRIMER software package (Clarke & Warwick, 2001). The data were divided into small clusters of biologically similar records, based on the resulting dendrograms. Comparative tables were produced to compare the species data and physical data between each of the small clusters. Where there were no notable differences between the physical and biological characteristics of the small clusters, they were amalgamated into larger groups which would form the preliminary basis for biotopes and sub-biotopes. Where similar biological and physical profiles appeared from clusters derived from different datasets, those data were joined and re-analysed. In particular, there was some overlap between the 'gravels and sands' and the 'muddy sands', and between the 'muddy sands' and 'mud' datasets. This re-analysis was carried out to ensure that the a priori divisions of the data did not artificially force divisions of otherwise coherent clusters. The resulting preliminary biotope and sub-biotope groups of records were then checked to ensure cohesion of both the environmental and species data. Individual records which differed significantly from the average profile for the group (in terms of biology or physical habitat characteristics) were removed, resulting in a group of records which formed the basis of the biotope descriptions (core biotope records). The physical and biological profiles from the core biotope records were then used to group biotopes of similar character into biotope complexes, and these in turn were assigned to habitat complexes and broad habitats. Note that, in addition to the habitat complexes defined on sediment character, two additional categories were created based on epifaunal characteristics (littoral sediments dominated by macrophytes, and littoral biogenic reefs).

Infralittoral rock

As the biotopes defined in version 97.06 were generally considered satisfactory, analysis focused on clarifying the boundaries between closely related types and confirming the validity of certain less-well defined types. This included particular attention to the tide-swept kelp types and the inter-relationship of highly grazed and poorly grazed kelp habitats. On the basis of these analyses, some restructuring at biotope complex level was necessary.  Attention was also paid to the vertical rock section of the infralittoral rock classification, and examining how these additional biotopes could be fitted into the existing biotope complexes, reflecting the subtle differences in their biological character.
 

Circalittoral rock

Due to the complexities of this part of the classification, especially the more subtle differences between types on the open coast, a full re-analysis of the data were undertaken.  The large size of the circalittoral rock dataset meant that some a priori division was necessary to provide datasets that could be managed within PRIMER (Clarke & Warwick, 2001). Data were divided on the basis of three previously determined energy levels; high, moderate and low energy.  Cluster analysis was carried out using epifaunal species matrices exported from the AREV database, using the PRIMER software package (Clarke & Warwick, 2001). The data were divided into small clusters of biologically similar records, based on the resulting dendrograms. Comparative tables were produced to compare the species data and physical data between each of the small clusters. Where there were no notable differences between the physical and biological characteristics of the small clusters, they were amalgamated into larger groups which would form the preliminary basis for biotopes and sub-biotopes. Where similar biological and physical profiles appeared from clusters derived from different datasets, those data were joined and re-analysed.  This re-analysis was carried out to ensure that the a priori divisions of the data did not artificially force divisions of otherwise coherent clusters. The resulting preliminary biotope and sub-biotope groups of records were then checked to ensure cohesion of both the environmental and species data. Individual records which differed significantly from the average profile for the group (in terms of biology or physical habitat characteristics) were removed, resulting in a group of records which formed the basis of the biotope descriptions (core biotope records). The physical and biological profiles from the core biotope records were then used to group biotopes of similar character into biotope complexes, and these in turn were assigned to habitat complexes and broad habitats.  As in the infralittoral rock section, further analysis was also carried out on the vertical rock section of the circalittoral rock classification.
 

Sublittoral sediment

A full re-analysis of the existing data on the MNCR database in addition to data supplied by the sublittoral specialist was carried out (approximately 10,000 records in total). This followed a similar approach to that described for littoral sediment and as outlined in Figure 2. Data were split according to sediment type, data type (infaunal or epibiota) and sampling technique (where appropriate). Poor quality data was also removed prior to analysis for later manual assessment. Cluster analysis was undertaken using either PRIMER (as described for the littoral sediments) or TWINSPAN (following the guidelines in Mills, 1994). Clusters of biologically similar records were produced and assessed using comparative tables. Clusters with poor species definition or highly variable physical characteristics were further sub-divided until more homogenous groups were derived. Where similar biological and physical profiles appeared from clusters derived from different main habitat datasets those data were combined and re-analysed using the same clustering methods as described above in order ensure that the a priori divisions of the data did not bias the results of the analysis.
 
Where similar biological and physical profiles were found in clusters from datasets of differing sampling method or those with different types of data (e.g. epibiota or infauna) the groups were re-analysed where possible at a lower level of resolution (either presence-absence or on the MNCR SACFOR scale) using PRIMER or TWINSPAN such that the differences in data type were reduced. As for the littoral sediments the resulting groups were then checked for cohesion with regard the physical and biological data, and individual records assigned to the groups were checked against the profiles of the groups as a whole and re-assigned if necessary.
 
The physical and biological profiles from the core records for each type were then used to group types of similar character into the broader biotope complexes and these in turn were assigned to one of the six main habitats for sublittoral sediment, derived from the EUNIS classification. The relationship between the sublittoral sediment biotopes is shown for separate depth bands in a series of habitat matrices, available to download as images from the classification website.