Defining and Categorising Biogenic reefs

Consideration of biogenic reefs as a group

Conservation importance of biogenic reefs

Representation of biogenic reefs within cSACs

Level of available information

Some important management considerations

There was difficulty in defining and categorising biogenic reef communities at the outset of this study. Much of the existing data and descriptions (including some MNCR biotopes) was not obtained with the category ‘biogenic reefs’ in mind and interpretation is therefore often difficult or ambiguous. This might indicate that further refinement of the definition of ‘biogenic reefs’ is required. During preparation of this work strong consideration was given to the idea of defining communities as reefs if they formed a layer two or more animals thick (so that the top animals were not living on the original substratum). It was felt that this was maybe realistic for mussels but less so for worms, especially Sabellaria spinulosa which can be quite small, and this idea has not been taken any further at this stage.

There are many cases where a community meets the two criteria suggested in chapter I (the unit should be substantial in size and should create a substratum which is reasonably discrete and substantially different to the underlying or surrounding substratum), except that it is not somewhat raised (i.e. it does not ‘rise from the seabed’ as in the JNCC definition of ‘reefs’). Although in many cases it is probably more realistic to refer to these as beds, the ecology, biology, and sensitivity of these areas are nevertheless likely to be very similar to those of protruding reefs, and they have therefore been discussed here alongside true biogenic reef communities. There are also cases, particularly with Mytilus, where non-raised aggregations are formed on hard substrata. In general, these have not been interpreted as biogenic reefs for the purposes of this report.

There is, perhaps surprisingly, little consistency in the biology, ecology and sensitivity within the grouping ‘biogenic reefs’, which is to some degree an artificial conglomerate of biotopes with differing characteristics. This is emphasised in the linked table, which summarises a number of important biological and ecological features of the five biogenic reef types. In addition to these points, the physical structure, size and shape of the reefs varies enormously both between, and to a lesser extent within, each species. It is clear that management issues must be considered separately for each type.

All five biogenic reef types are regarded as of high conservation importance for the reasons outlined in the table below. This may not be apparent to many coastal users, particularly those involved in fishing, since many of these species are common and widespread. Conservation importance of biogenic reefs may need to be the subject of educational campaigns in order to secure support for their conservation. The use of the emotive term ‘reef’ could, of course, be of value in this regard. However, it conjures up images in the general public which are often far removed from the reality of the biotopes under discussion here. There is a danger both with the general public and conservation bodies that overemphasis of the term ‘reef’ could lead to an inflated opinion of the importance of some areas. A realistic approach needs to be adopted.

A summary of information relating to the conservation importance of biogenic reef biotopes according to criteria given in Hiscock (in prep).

Mytilus reef biotopes are well represented in presently designated cSACs and pSACs. Good quality intertidal and subtidal Sabellaria alveolata reef biotopes appear to be included within a number of cSACs, although it is unclear whether the extensive intertidal and subtidal reefs around Dubmill Point are included within the Solway Firth cSAC. Modiolus reef biotopes seem to come in a wider variety than other biogenic reefs, some of which presently fall within cSACs or pSACs. The apparently rarer type, infaunal reefs in areas of very strong tidal currents, are only known presently from nearby non-UK waters but seem likely to exist in UK waters. Sabellaria spinulosa stable reefs, as opposed to thin crusts, are known for certain from only one or two locations, but almost certainly one of these falls partially within the Wash and North Norfolk Coast cSAC. Within the UK, serpulid reefs are known only from Loch Creran (which also has Modiolus beds which probably qualify as reefs) so that they are not covered at all by any cSACs or pSACs.

 General

The level of available knowledge for Mytilus is very high, although much is directed at its ecology and biology in relation to rocky shores which is of little relevance to biogenic reefs (at least as they are considered here). The level of available knowledge of the other biotopes under consideration is moderate, with significant gaps which have been identified in chapter VIII. Some synthesis is presented in the bullet points below:

• There are still significant gaps in our knowledge of distribution of marine benthic biotopes, particularly in the subtidal. The JNCCs MNCR data base, upon which much of our knowledge is based, has significant gaps on the northern and eastern coasts of Scotland and open coasts of eastern England. As an example, around 6 km² of seabed supporting Modiolus reef communities (infaunal gravel bed type) were found to the north of the Isle of Man as recently as 1996, despite the continuous presence of a marine research laboratory within fifty km for over 100 years.

• Lack of knowledge on rates of recovery after damage is a recurring theme. In Mytilus the pressing need is only for information on recovery after very large scale damage, while recovery from more modest scales of damage should be studied for the other biogenic reef communities. Modiolus communities in particular are thought to be likely to have extremely slow rates of recovery, potentially decades or more. Sabellaria spinulosa reefs have apparently been lost from several large areas due to fishing for prawns using bottom gear, and in at least some cases deliberate breaking up of the reefs using heavy fishing gear to improve the fishing To our knowledge none of these areas have recovered. It seems likely that this is largely because bottom fishing activities have continued, but we do not know whether, or how fast, reefs might return if bottom fishing ceased or reduced. Lack of recruitment may be a problem given the wide areas involved. This could be aggravated where fishing has resulted in the development of Mytilus dominated communities which might continue to dominate in the absence of heavy Sabellaria recruitment. Long-term fishing is thought to be able to alter sediment structure which conceivably might further influence the likelihood of recolonisation.
• Sensitivity to direct physical impacts in general is another recurring theme. Sensitivity is linked to some extent to structure of the reefs, to vulnerability in terms of position, and strongly to recoverability, which in general we know little about.
• Lack of knowledge of associated communities is a further recurring theme. This is less important for the intertidal (Mytilus, S. alveolata) than for the subtidal where communities are likely to be both richer and more diverse, and less well known.
• We know almost nothing about predators and their effects on the worm reefs (Sabellaria, Serpula) whereas there is a great deal of literature on the effects of predators on Mytilus and to a lesser degree Modiolus.
• There are a number of potentially important species interactions which need to be studied, including Mytilus and S. alveolata on scar grounds; S. alveolata and algae on rocky and shallow sublittoral shores. The possibility that Ophiothrix communities might spread over subtidal biogenic reef communities and prevent them from feeding and perhaps settling has been noted several times. This is rather speculative but is based on a number of known facts; dense beds of Ophiothrix are thought to have greatly reduced recruitment to at least one year class of Sabellaria spinulosa, and to have reduced feeding and fecundity, on reefs in the Bristol Channel; Ophiothrix populations have been known to fluctuate greatly in some situations; Ophiothrix beds can be present on and near to Modiolus reef areas; Ophiothrix is a very efficient and relatively un-selective filter feeder which can take a wide range of food particles, and can feed well on Artemia larvae in laboratory conditions, for example (J. Allen, pers. comm.).

It is important to try to prioritise the gaps in knowledge. The following are thought to be of particularly high priority:

• Serpulid reefs are of national importance and are not found in any presently proposed cSACs. It would be extremely useful to know the true status in Loch Sween. This probably requires the preparation of as detailed a report as possible on the recent diving surveys (not presently available) followed if necessary by further surveys.
• Information on the potential for recovery of reefs of Sabellaria spinulosa and Modiolus modiolus damaged by physical impacts and especially by fishing is of high importance, but at present, given the political sensitivities of closure of fishing grounds, it is not likely that a realistic programme could be devised to investigate recovery from fishing impacts. In the absence of such a programme, studies on recruitment processes aimed at improving our knowledge of the major influences on them should take a high priority.
• Further information on distribution of S. spinulosa reefs and certain types of Modiolus communities is of high importance. Allied to this is the need for development of better, and standardised, survey and monitoring methods for subtidal biogenic reefs.
• Recruitment range and sources for S. alveolata, S. spinulosa, Mytilus edulis and Modiolus modiolus reefs need to be identified.
• A better understanding of the ‘Added Value’ of the biodiversity of reef areas versus adjacent areas, and in particular a better understanding of the role of reef builders as ecosystem engineers, would help in promoting the conservation of biogenic reefs.
• A better knowledge of the natural variation in extent, density and population structure of reefs, especially serpulid and Sabellaria spinulosa reefs, is required.

Only the most important potentially damaging activities or other management considerations are mentioned here and the reader should refer to the relevant chapter for more details.

Limits of acceptable change for monitoring and management purposes are going to be very difficult to determine, except for Modiolus which appear to be relatively stable communities in which any detectable changes over periods of a few years are likely to be regarded as unacceptable. Intertidal Sabellaria alveolata populations are generally highly variable and it seems likely that large scale losses over wide areas can be attributable to natural causes such as cold winters or lack of recruitment. Even in areas where S. alveolata is always found, there may be very large scale fluctuations in populations over periods of years due to variations in recruitment. There is some evidence that recruitment cells are moderately localised, as with Mytilus, but considerably more information on the geographical scale over which recruitment occurs is essential with all species to be able to decide whether local action or more widespread action would be required to prevent, or mitigate, loss of communities in an SAC. For example, would the maintenance of the extensive populations at the Dubmill Point area of the Solway Firth SAC ensure future recruitment elsewhere in that SAC, or elsewhere along the Cumbrian Coast north of St Bees Head? It is presently not possible to answer such questions.

Some impacts are too widespread for local management decisions to be effective, but need to be recognised so that changes resulting from them may be distinguished from those where local action may be effective. These include general eutrophication; global warming; and diffuse pollutants. Apart from the obvious potential problems from anoxia and clogging of gills caused by dense phytoplankton blooms, one possible problem associated with eutrophication is that enrichment often appears to be associated with changes in the species composition of phytoplankton, often favouring smaller groups at the expense of diatoms (Smayda, 1990) and this could have unknown consequences for all filter feeding organisms including biogenic reef species.

In early guidance from the Scottish Office on proposed SACs (Anon., 1996) it is stated that "By their nature, reefs are protected from trawling and dredging and so this is not at present an issue for this habitat." While this is clearly true for the vast majority of rocky reefs it certainly does not apply to biogenic reefs. Extensive areas of Modiolus reef have been lost or damaged by trawling or dredging for scallops and queen scallops, including notably in Strangford Lough cSAC. Potential exists for similar fishing damage elsewhere, and there is a possibility that widespread damage has already occurred, for example, to Modiolus reefs in the Shetland Voes. The experience gained during the designation of Strangford Lough as an MNR in 1993 will be invaluable (Leekley, pers. comm.; Weyl, pers. comm.): Fisheries regulations preventing the use of mobile fishing gear in areas which include some relatively undisturbed beds of Modiolus were introduced in 1993. This was prior to designation but as a direct consequence of the MNR consultation procedure. There was some conflict with fishermen during this process and lessons may be learned from the experience. The opportunity now exists to study the recovery process in damaged areas (Service, pers. comm.).

Similarly, Sabellaria spinulosa reefs are widely been reported to have been lost in areas subject to fishing for pink shrimp, Pandalus montagui. Recovery after widespread loss appears to be extremely poor even in the absence of pink shrimp fishing, reasons for which are unknown. Serpula vermicularis reefs are not known to be heavily impacted by dredges and trawls, but would undoubtedly be very badly damaged by them.

In the case of all three of these species (Modiolus modiolus, Sabellaria spinulosa and Serpula vermicularis), where important reefs occur within cSACs in areas where trawling or dredging can occur they can only be protected from damage by prohibiting such bottom fishing.

Serpulid reefs are susceptible to physical damage and it seems likely that they would also be damaged by potting, and are known to be susceptible to damage by mooring systems for salmons cages. Likewise, Modiolus reefs and Sabellaria spinulosa reefs would doubtless be badly impacted by physical activities such as cable trenching and pipelaying. The latter is particularly likely to be directly affected by sand and gravel extraction since it tends to inhabit areas that are suitable for commercial exploitation of aggregates. Such activities should be prevented over areas of good quality reefs both within and, preferably, outside cSACs.

Mytilus alone of the biogenic reef species is of importance as a fishery. This may lead to conflict with fishermen and re-layers and those, such as SFCs, charged with managing the fishery. It must be taken into account that legislation frequently requires that the fishery is managed so as to actively develop the fishery. However, Mytilus is a resilient species, even in reef communities, which tends to regenerate quickly from natural losses except where these are on the scale of entire large embayments such as the Wash, when recruitment failure can occur. In the most productive and exploitable biogenic reef communities the associated fauna and flora is relatively unremarkable, and ecologically speaking its importance as a food source for birds (particularly oystercatchers and eiders, but also others) is likely to be of over-riding importance in many areas. It should therefore be possible to find compromises which allow active developments of fisheries without detriment to bird populations, which are likely mainly when mussels have been overexploited (which is of course also undesirable from the fishery point of view). Guidance will need to be sought from those responsible for management of SPAs, who will have a greater knowledge of the requirements of birds in such instances. There should be opportunities for sharing resources between conservation managers and fisheries scientists or managers in monitoring certain important aspects of biogenic reef communities such as recruitment, growth rates and size/age structure of populations, and this may in some cases include access to detailed unpublished historical data.

Where more stable Mytilus reefs exist within cSACs, greater importance should be attached to the associated fauna and flora (which are likely to be richer and more diverse) by carefully limiting, and if necessary preventing, exploitation.

Caution is required regarding Modiolus since there may be potential for a fishery. It is large, edible, and may turn out to be more accessible than has hitherto been known. Potential for cultivation has been commented on in the past (Comely, 1978) but seems unlikely. It certainly has a rapid growth rate in early years but obtaining seed mussels would be very difficult. Direct effects of aquaculture as a commercial venture would probably not be great but if it created a demand fishing on some grounds could conceivably become an issue. It is worth noting that fisheries legislation often refers only to shellfish in general terms, so that theoretically the requirement for fisheries managers to actively promote fisheries within an area could also include Modiolus. The authors are not aware of any significant areas where this is presently likely to be a problem, however.

There are a number of localised factors in enclosed sea lochs, such as organic enrichment from salmon farms, which could potentially be a problem for biogenic reefs found there. There is no evidence of any special sensitivity of any of the biogenic reefs under study to such enrichment, and sensible location of farms should be able to prevent widespread damage.

References