Fucus vesiculosus on mid eulittoral mixed substrata
Ecological and functional relationships
The luxuriance of the Fucaceae is normally a striking feature of sheltered shores in the eulittoral zone. Areas of bedrock or large boulders may be festooned with masses of long-fronded Ascophyllum nodosum. In such areas, Fucus vesiculosus may be the only other seaweed of note and is as often attached to the bladders of Ascophyllum nodosum, as to the rock beneath. Ascophyllum nodosum will germinate beneath a canopy of Fucus vesiculosus and will eventually outgrow and displace it, although Ascophyllum nodosum does benefit from the protection offered against desiccation by fronds of Fucus vesiculosus. However, sheltered and very sheltered mid eulittoral pebbles and cobbles lying on sediment are typically characterized by Fucus vesiculosus rather than Ascophyllum nodosum. The low abundance of Ascophyllum nodosum in SLR.FvesX is related to the unsuitable anchorage which the small pebbles and cobbles provide for plants whose fronds may grow up to 2 meters in length, and it is noticeable that any Ascophyllum nodosum on stony beaches is typically small (Lewis, 1964).
The pebble and cobble beaches of SLR.FvesX have a poor fauna in comparison to open shore locations on bedrock, presumably as a result of siltation, variable salinity and the instability of the substratum. Where Fucus vesiculosus is absent there is a scattering of the barnacle Semibalanus balanoides and occasional Patella vulgata whose abundance is limited by the availability of larger rocks. Littorina littorea which is tolerant of both muddy/silty and brackish conditions tends to cluster on the tops of small stones. Although Mytilus edulis is less common on cobbles and pebbles than on larger boulders or bedrock, its beds serve to enhance the stability of the substratum. Patchiness is a fundamental feature of rocky shore communities and although probably modified to some extent on mixed substrata, would probably still be observable in the SLR.FvesX biotope. For instance, Patella vulgata can play a role as a structuring agent owing to its grazing activity. Reductions in limpet density allows the settlement of Fucus vesiculosus whose cover encourages aggregations of mobile fauna. Semibalanus balanoides is often excluded from the larger, most stable boulders in the biotope by Fucus vesiculosus, owing to the 'sweeping' effect that the fronds have upon the rock. However, in extremely sheltered locations, even the smallest stones are relatively stable but remain unoccupied by algal sporelings so barnacles settle (Lewis, 1964; Raffaelli & Hawkins, 1996).
The characterizing species of the sediment beneath the pebbles and cobbles are infaunal. Hediste diversicolor displays plasticity in its feeding methods. Hediste diversicolor is primarily a deposit feeder but is able to switch to suspension feeding when conditions allow. Obligate deposit feeders such as Arenicola marina are also numerous in the sediment (McLusky & Elliott, 1981; Nielsen et al., 1995).
Seasonal and longer term change
The biotope occurs in extremely sheltered conditions so temporal changes associated with winter storms are not likely. However, seasonal changes in growth and recruitment would be expected in this biotope. In summer months, a covering of the ephemeral seaweed Ulva tends to develop on the surface of smaller mobile pebbles.
Habitat structure and complexity
The mixed nature of pebbles and cobbles on sand/mud creates a habitat of considerable complexity. Larger cobbles covered in Fucus vesiculosus
provide substratum and shelter for a very wide variety of species, including the tube worm Spirorbis spirorbis
, herbivorous isopods, such as Idotea
, gammarid amphipods, surface grazing snails, and provides considerable substratum for epiphytic species. Beneath the largest cobbles and pebbles (if free of sediment) underboulder communities may be found. The size range of the pebbles and cobbles adds extra complexity as some will be too small and too unstable (e.g. subject to overturn) for some species to persist. The sediment beneath is likely to be poorly sorted as currents are modified locally by the uneven surface topography, silt content is likely to be high owing to the weak tidal flow and wave sheltered locations where the biotope occurs, that allows particulate matter to fall out of suspension.
Rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al
., 1998). However in the SLR.FvesX biotope, floral and faunal species do not attain the same biomass that may be found on stable rocky substrata on the open coast, so in comparison productivity in this biotope is likely to be considerably less. Macroalgae, such as Fucus vesiculosus
and Ascophyllum nodosum
in the SLR.AscX biotope, exude considerable amounts of dissolved organic carbon which are taken-up readily by bacteria and may even be taken-up directly by some larger invertebrates. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.
Many rocky shore species possess a planktonic stage: gametes, spores or larvae which float in the plankton before settling and metamorphosing into adult form. This strategy allows species to rapidly colonize new areas that become available, such as gaps created by grazers or abrasion. Recruitment processes for the characteristic and abundant species in this biotope are described below.
- Fucus vesiculosus is a fast growing species (young plants have shown variation from 0.25 to 0.7 cm per week in linear growth), able to colonize patches of clear substratum rapidly. Development of the receptacles takes three months from initiation until when gametes are released. On British shores, receptacles are initiated around December and may be present on the plant till late summer. Gametes may be produced from mid winter until late summer with a peak of fertility in May and June. Plants are dioecious. Eggs and sperm are released into the seawater and fertilised externally. Zygotes settle to the seabed and begin development wherever they fall. The egg becomes attached to the rock within a few hours of settlement and may adhere firmly enough to resist removal by the next returning tide (Knight & Parke, 1950).
- In contrast, Ascophyllum nodosum is a slow growing species that can live between 10-15 years, but recruitment of Ascophyllum nodosum is very poor with few germlings found on the shore. The reason for poor recruitment is unclear, because the species invests the same high level of energy in reproduction as other fucoids and is extremely fertile every year (Printz, 1959). However, the reproductive period lasts about two months, much shorter than for other fucoids.
- Ulva is a rapidly growing opportunistic species which can colonize bare substrate soon after it is created. The haploid gametophytes of Ulva produce enormous numbers of motile gametes which cluster, fuse and produce zygotes. The resulting sporophytes also produce large numbers of motile spores. Together, zygotes and spores are termed 'swarmers' which are released as the incoming tide wets the thallus. The degree of release peaks just before the highest tide of each neap-spring cycle (Christie & Evans, 1962)
- Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Also, estuaries provide a more nutritious environment than the open coast (Fish, 1972). Sexes are separate, and fertilisation is internal. Littorina littorea sheds egg capsules directly into the sea. Egg release is synchronized with spring tides and occurs on several separate occasions. In estuaries the population matures earlier in the year and maximum spawning occurs in January (Fish, 1972). Fecundity value is up to 100,000 for a large female (27mm shell height) per year. Female fecundity increases with size. Larval settling time or pelagic phase can be up to six weeks. Males prefer to breed with larger, more fecund females (Erlandsson & Johannesson, 1992). Parasitism by trematodes may cause sterility in Littorina littorea.
- Recruitment of Patella vulgata fluctuates from year to year and from place to place (Bowman, 1981). Fertilization is external and the larvae are pelagic for up to two weeks before settling on rock at a shell length of about 0.2mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.
- Barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as wind direction and success depends on settlement being followed by a period of favourable weather. Long term surveys have produced clear evidence of barnacle populations responding to climatic changes. During warm periods Chthamalus spp. predominate, whilst Semibalanus balanoides does better during colder spells (Hawkins et al., 1994). Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June.
- The infaunal polychaetes Hediste diversicolor and Arenicola marina have high fecundity and the eggs develop lecithotrophically within the sediment or at the sediment surface. There is no pelagic larval phase and the juveniles disperse by burrowing. Recruitment must occur from local populations or by longer distance dispersal of postlarvae in water currents or during periods of bedload transport. For example, Davey & George (1986), found evidence that larvae of Hediste diversicolor were tidally dispersed within the Tamar Estuary over a distance of 3 km, as larvae were found on an intertidal mudflat which previously lacked a resident population of adults. Recruitment is therefore likely to be predictable if local populations exist but patchy and sporadic otherwise.
Time for community to reach maturity
No specific information was found concerning time taken for the community to reach maturity. However, the characterizing species of the SLR.FvesX biotope are widespread, highly fecund and quick to grow and mature and so the community would be expected to reach maturity within 5 years.
The time for the SLR.AscX biotope (also represented by this review) to reach maturity is likely to be many years because Ascophyllum nodosum
has very poor recruitment and is very slow growing. The species does not reach sexual maturity until about 5 years of age and individual fronds can live to be up to 15 years old and whole plants for several decades. In their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum
population. There is a long-recognised shortage of sporelings (David, 1943) and the failure of the species to recolonize denuded areas for decades. However, the species is extremely fertile every year and Printz (1959) suggested that it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization. If plants are not removed completely Ascophyllum nodosum
plants cut within 10-15cm of the base recover fully in 4-5 years (Printz, 1959).
This review can be cited as follows:
Fucus vesiculosus on mid eulittoral mixed substrata.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=329&code=1997>