Nature and Importance of Circalittoral Faunal Turfs (CFTs)
What are CFTs?
The importance of CFTs
Classification strategies for CFTs
What are CFTs?
'Faunal turfs' are basically assemblages of attached animals growing on
hard substrata. These organisms can vary substantially in growth form, 'turf' being used
in a highly generic sense. So they will range from low encrusting forms less than a
centimetre high, such as many ectoprocts (sea mats) and sponges, to tall erect forms such
as alcyonarians (soft corals) and gorgonians (sea fans) which may exceed 25 cm in height.
The CFT community will also include prominent mobile organisms associated with the
attached fauna such as decapod crustaceans, echinoderms, molluscs and fish, which may play
important structuring roles in the community. Also included are a diverse but cryptic
assemblage of small mobile organisms such as isopods, amphipods, nemerteans and
polychaetes. Although by definition CFT communities are animal dominated, there will be
algae present. In the upper regions of the circalittoral there will be foliose red algae.
Deeper these will disappear, but crustose reds such as the calcified lithothamnia and
non-calcified Cruoria will persist for some distance. Some general features of
circalittoral communities have been pointed out by Sebens (1985a) which contrast them with
intertidal ones. Zonation is very much broadened, with zones extending for ten
metres or more in depth. Space is less frequently monopolised by single dominant species.
Physical disturbance, creating large areas of bare space, is much less common,
but nevertheless species diversity is generally high.
Figure - Profile of rocky
shore and seabed to show the biological zones; depths are typical values for south-west
'Circalittoral' is generally taken as those depths where the light
intensity is reduced to a level where it will no longer support substantial algal growth.
Thus the normal sublittoral progression with increasing depth will be from the
algal-dominated infralittoral biotopes to the animal-dominated circalittoral biotopes, and
there will be a zone where the two merge (see figure linked above). However, as will be
elaborated in section II, depth is not the sole factor determining light availability -
the slope of the substratum is also important. So on overhangs, and especially in caves,
'circalittoral' communities occur in depths where algae would normally be expected to
dominate. Grazing activities can also mediate the outcome of animal-plant interactions and
limit plant dominance. There is no formal lower limit to the circalittoral, and it will
extend well below the depths encountered in any of the candidate SACs.
The importance of CFTs
The brief outline above may leave the impression that CFTs are a
complex of biotopes, not easily accessible, and of limited interest other than in a
scientific context. Fifty years ago this impression might have been true, but not any
longer. The major factor in changing our perception of CFT communities has been the
development of scientific and sports diving over the intervening period. Amateur diving is
still growing in popularity, and its capabilities are being continually extended by
technological developments. The proliferation of RIBs (rigid inflatable boats) has
provided greater mobility and seagoing capability, bringing more inaccessible areas within
easier reach. Advances in suit technology, the introduction of nitrox and mixed gas diving
to the amateur and scientific field, and the development of rebreathing systems, have all
enabled longer and deeper dives to be safely accomplished. The circalittoral is
increasingly being brought to the divers' doorstep.
Consequently increasing numbers of divers are attracted to the CFT
biotopes, because this is the most scenic and spectacular of the underwater environments.
The rock faces covered with a diversity of multicoloured animal life are in sharp contrast
to the relatively drab algal cover of the infralittoral, and the temperate circalittoral
can indeed rival the tropical reefs in spectacle, if not in diversity. So concern for CFTs
is an important component of the SAC project, because although CFTs are quite widespread,
high quality accessible CFT areas are not. Such areas will be subject to very heavy diving
(and other) pressure, and can degrade under such pressure. Management strategies will be
required. In addition to this aesthetic appeal, CFTs have a high scientific interest
because of their biodiversity, and the complex biological interactions within their
communities (currently far from fully understood). They also support shellfisheries of
considerable economic importance.
Classification strategies for CFTs
The highly diverse nature of CFTs has made definitive classification of
CFTs into various schemes a challenge - there are two approaches to the classification of
CFT communities. The commonest approach is based upon a heirarchical division according to
environmental variables, the approach which has culminated in the MNCR biotope
classification. The highest ranked variable under this scheme is degree of exposure to
water movement, and lower rank variables include slope and scour. This approach will of
necessity be somewhat subjective, though in compiling the classification the MNCR did
include multivariate techniques. However, the outcome is readily understood, and is usable
in the field.
An alternative approach is to be strictly objective, and to analyse a
range of populations by multivariate methods, without preconceptions, to see what
groupings emerge. This approach has been adopted for Scottish hard-substratum communities
(Kluiver, 1993). This resulted in a clear separation of communities between the photic and
aphotic zones - as might be expected. The communities of the aphotic (circalittoral) zone
grouped into eight clusters, which generally correlated with different depths and angles
of slope. This analysis used a quite restricted dataset of 60 stations.
It must be remembered that any attempt to classify communities within
the circalittoral (or indeed any other such environmental division) means attempting to
divide up what is essentially a continuum. For convenience and descriptive purposes it
must be done, but it involves describing rather arbitrary 'typical' communities.
Intermediate stages between all of the described communities will invariably be found as
well. There are three classification schemes which relate to CFT biotopes.
EU Habitats directive
The UK Marine SACs project has been developed largely in response to
the EC Habitats Directive, so how relevant are CFT biotopes to the Annex I Features
(Habitats) of the Directive? Two of the listed habitat features are 'reefs' and 'sea
caves', and both of these are potential habitats for CFTs..
UK Biodiversity Action Plan
CFTs additionally have importance in relation to the UK Biodiversity
Action Plan. In the 'Broad Habitat' listing they fall within the very broad category of
'Offshore Sea Beds'. A number of the species listed under the Biodiversity Action Plan are
prominent members of the CFT community - e.g. Strongylocentrotus droebachiensis,
the northern sea urchin; Alcyonium glomeratum, the red sea finger; Leptosammia
pruvoti, the sunset star coral; Eunicella verrucosa -the pink sea fan. These,
and other species on the Biodiversity Action Plan list will be discussed in detail elsewhere.
MNCR biotope classification
The MNCR biotopes classification provides a framework within which CFT
communities can be placed. Circalittoral Faunal Turfs can be directly equated with the
majority of the 'circalittoral rock' biotopes listed in the MNCR biotopes classification
(Connor et al., 1997). This classification divides the biotopes into those characteristic
of exposed, moderately exposed and sheltered circalittoral rock, which comprise 12, 26 and
13 biotopes respectively. These are listed in Appendix 1. 'Faunal turf' is
interpreted broadly in the present context, and both faunal turf and faunal crust biotopes
in the MNCR classification are considered to fall within the remit - they share the same
environments, and it would be pointless to separate them. The only MNCR circalittoral rock
biotopes which are not discussed here as CFT biotopes are those which are dealt with
elsewhere under the SAC Project. These are biogenic reefs, such as dense Modiolus (horse
mussels) growing on mixed substrata, and brittlestar beds. For management purposes
the reviews on Biogenic Reefs and Brittlestar Beds must be considered in
conjunction with this review for any SACs containing circalittoral rock, since they may
occur in a mosaic pattern with the typical CFT communities. Following major perturbations,
such as toxic algal blooms, CFT communities may be temporarily replaced by
mussel-dominated communities (Lundälv, 1990). Excluded Circalittoral Rock biotopes (8 in
all) are indicated by asterisks in the appendix. Within the
biotope classification there is a Circalittoral Offshore Rock division which contains only
one biotope - Lophelia reefs. These deep sea corals will not be found within the
limits of any proposed Marine SACs, and in any case are biogenic reefs rather than faunal
turfs. Figure 2 provides a quick reference as to which CFT biotopes are most likely to be
found under various environmental conditions.
This classification into biotopes is temptingly definitive, but the
following cautionary comment in Connor et al. (1997) should be kept in mind. "It is
typical for the community not to be dominated by single species, as is common in shore and
infralittoral habitats, but rather comprise a mosaic of species. This, coupled with the
range of influencing factors, makes circalittoral rock a difficult area to satisfactorily
classify; particular care should therefore be taken in matching species and habitat data
to the classification." However, since most survey data have been collected by
reference to the MNCR biotope scheme, and the JNCC MNCR database hold records in this
form, the scheme must constitute an essential component of SAC management protocols.
Figure - Matrix of
circalittoral rock biotopes, ordered according to current speed on the vertical axis and
wave exposure on the horizontal
There are also a number of faunal-dominated communities in shallower
water described under the Infralittoral Rock category, generally on verticals, in gullies,
or in conditions of strong exposure. These have a basic affinity with CFTs, and are also
listed in the appendix.