Biodiversity & Conservation

The principal feature of the SIR.PolFur biotope is an algal turf dominated by perennial species tolerant of low salinity, high turbidity and low water flow and therefore released from the competition of less tolerant algae which typically limits them in less specialized conditions. The characterizing species include Furcellaria lumbricalis, Polyides rotundus and to a lesser extent, Chondrus crispus.

Fast growing ephemeral algae, e.g. filamentous brown Ectocarpus sp. and filamentous green Cladophora sp., grow epiphytically and colonize gaps in the perennial turf as and when they occur (Barnes & Hughes, 1992).

Encrusting coralline algae grow epiphytically on the turf forming species.

The density of the algal turf discourages a rock-attached fauna but many grow epifaunally on the algae (Lewis, 1964). The fauna are dominated by encrusting suspension feeders including the ascidians, Clavelina lepadiformis, Ciona intestinalis and Ascidiella aspersa, the sponge, Halichondria panicea, spirorbid tubeworms and the bivalve, Mytilus edulis.

Grazing pressure by the gastropod, Littorina littorea, on the ephemeral algae and on germlings of the perennial species, probably promotes algal diversity (Barnes & Hughes, 1992; Raffaelli & Hawkins, 1996).

Mobile epibenthic predators include the shore crab, Carcinus maenas, and the starfish, Asterias rubens. Predation of Mytilus edulis by Asterias rubens may prevent the mussels dominating and therefore promote a mixed algal and faunal turf (Lubchenco & Menge, 1978).

Gobies, particularly the two-spotted goby, Gobiusculus flavescens, predate small crustaceans, including mysid shrimps, in the water column and sheltering in the algae (Fish & Fish, 1996).

The dominant algal species in the biotope are perennial and therefore present throughout the year. However, they do exhibit seasonality in terms of growth and reproduction. For example, maximum growth of Furcellaria lumbricalis occurs in March/April (Austin, 1960b) and release of carpospores and tetraspores occurs in December/January (Bird et al., 1991). The annual algal species, for example the filamentous greens and browns, are likely to proliferate in spring and summer in conjunction with increased irradiance and temperatures, and then die back in autumn and winter.
Recruitment processes and recolonization by macroalgae are very dependent on time of year as spores are only available for limited periods. For instance, fucoids are limited to recruitment when spores are available as there is no free-living alternate generation to persist on the substratum while conditions are unsuitable for development (Kain, 1975). Recruitment may occur out of season in species (e.g. Laminarians and Desmarestia viridis) where the propagules are able to lie dormant or develop slowly and then establish themselves when conditions become suitable, for example, following canopy removal (Kain, 1975). The advantage of being fertile through the winter, as in the case of Furcellaria lumbricalis, is the availability of substrata for colonization as other annual species die back (Kain, 1975).
The fauna in the biotope also exhibit seasonal variations. Clavelina lepadiformis, for example, reproduces sexually and recruits in late summer and autumn. The colony then dies back over the winter as the adult zooids regress and the colony survives as winter buds from which new zooids develop in spring (Fish & Fish, 1996).
Storms and increased wave action are more likely to occur in the winter months and may cause physical damage to the community. Austin (1960b) reported damage to Furcellaria lumbricalis plants during storms and Sharp et al. (1993) reported that plants may be cast ashore by increased wave action. Physical disruption of the algal turf is likely to promote diversity as spaces become available for colonization.

The dense algal turf provides shelter for a variety of fauna and sites for attachment of both epifauna (e.g. ascidians such as Ciona intestinalis on Halidrys siliquosa) and epiphytes (e.g. Ectocarpus sp. on Chorda filum and Saccharina latissima (studied as Laminaria saccharina) (Lewis, 1964).
Where the substratum is not continuous bedrock, the Furcellaria lumbricalis/Polyides rotundus turf is restricted to the upper surfaces of the larger stones and boulders, while the intervening areas of smaller stones, cobbles and sediment support a new algal community of short-lived annuals including Ulva sp., Asperococcus fistulosus, Dictyota dichotoma, Stilophora tenella, Ectocarpus sp. and Polysiphonia sp. (Lewis, 1964).

Primary production by the slow growing, perennial red algae which dominate the biotope is low. Wallentinus (1978) measured in situ primary production by macroalgae in the northern Baltic Sea. Productivity of Furcellaria lumbricalis was 0.36-0.54 mg C/g dry wt/hour. The comparative figure for Cladophora glomerata, a filamentous green alga was 1.47-11.38 and for Dictyosiphon foeniculaceous, a filamentous brown alga, was 4.21-8.76. These figures suggest that the contribution made by the perennial algal turf to macroalgal production in the biotope is likely to be very small. The fast growing, ephemeral, annual species with rapid turnover probably account for the majority of macroalgal primary production.
However, the contribution to primary production of all macroalgae in the biotope is likely to be small in comparison with the phytoplankton. Jansson & Kautsky (1976), for example, recorded annual macroscopic plant production of hard bottoms in the Baltic shallow subtidal to be approximately 4% of the total primary production, suggesting that phytoplankton are by far the most important carbon fixers. Additionally, they noted that fast growing species with rapid turnover, for example the filamentous brown algae, contributed approximately one third of macroalgal production and that there was a relatively small contribution made by the slow growing perennials.

Vadas et al. (1992) reviewed recruitment and mortality of early post settlement stages of benthic algae. They identified 6 intrinsic and 17 extrinsic factors affecting recruitment and mortality. They concluded that grazing, canopy and turf effects were the most important but that desiccation and water movement may be as important for the early stages. The review indicated that recruitment is highly variable and episodic and that mortality of algae at this period is high. Chance events during the early post settlement stages are therefore likely to play a large part in survival.
As with all red algae, the spores of Furcellaria lumbricalis and Polyides rotundus are non-flagellate and therefore dispersal is a wholly passive process (Fletcher & Callow, 1992). In general, due to the difficulties of re-entering the benthic boundary layer, it is likely that successful colonization is achieved under conditions of limited dispersal and/or minimum water current activity. Norton (1992) reported that although spores may travel long distances (e.g. Ulva sp. 35 km, Phycodrys rubens 5 km), the reach of the furthest propagule does not equal useful dispersal range, and most successful recruitment occurs within 10 m of the parent plants. It is expected, therefore, that recruitment of Furcellaria lumbricalis, Polyides rotundus and the majority of other macroalgae in the biotope would occur from local populations and that establishment and recovery of isolated populations would be patchy and sporadic.
As and when bare substratum becomes available for colonization, for instance following storm events, it is expected that algal recruitment and succession would follow a predictable sequence (Hawkins & Harkin, 1985). Initial colonizers on bare rock are often epiphytic species, such as the brown filamentous Ectocarpus sp. and the green Ulva sp., suggesting that it is competition from canopy forming algae that usually restricts them to their epiphytic habit (Hawkins & Harkin, 1985). Gradually, the original canopy or turf forming species, in this case Furcellaria lumbricalis, Polyides rotundus and Chondrus crispus, then become established. These findings suggest that interactions between macrophytes are often more important than grazing in structuring algal communities (Hawkins & Harkin, 1985).
Ascidians, the main faunal component of the biotope, have a very brief larval stage with limited dispersal and therefore are also likely to recruit locally. However, long distance dispersal may occur by adult attachment to drifting structures.

Ascidians, the dominant epifauna in the biotope, develop quickly and reach maturity within a year, while many of the algae are ephemeral, annual species. Maturity of the community is therefore limited by the time taken by the climax algal species, specifically Furcellaria lumbricalis and Polyides rotundus to settle and grow. Bird et al. (1979) reported growth rates of Furcellaria lumbricalis in the laboratory as a doubling in weight in 25-50 days or a 3.3% increase in fresh weight per day. For comparison, the corresponding rates for Chondrus crispus are 10 days and 7.3%, and for Fucus serratus are 12.5 days and 6.2%. These figures suggest that Furcellaria lumbricalis grows slowly in comparison to other red and brown seaweeds. The community would be expected to reach a qualitative climax within a few years. However, Austin (1960b) reported that in Wales, the species typically takes 5 years to attain fertility.