Biodiversity & Conservation

The principal feature of the MIR.PolAhn biotope is an algal turf dominated by perennial species tolerant of soft or friable rock and overlying sand. They are therefore released from the competition of less tolerant algae, which typically limits them in less specialized conditions. The characterizing species include Ahnfeltia plicata, Furcellaria lumbricalis, Polyides rotundus and Chondrus crispus. (Lewis, 1964.)

Other algae which contribute to the mixed turf include Cryptopleura ramosa, Dilsea carnosa, Halidrys siliquosa, Phyllophora crispa, Phyllophora pseudoceranoides and Dictyota dichotoma.

The density of the algal turf discourages a rock-attached fauna (Lewis, 1964). Chondrus crispus for example is highly resistant to intense physical and herbivore induced disturbance ensuring a dense canopy which largely excludes recruitment of fauna and other algae (Chopin & Wagey, 1999). Fast growing ephemeral algae, e.g. 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.

Epiphytes and understorey algae are grazed by a variety of amphipods, isopods and gastropods (Birkett et al., 1998b).

Fauna include the sand tolerant anemone, Urticina felina, the detritivorous hermit crab, Pagurus bernhardus and the opportunistic tube forming annelid, Pomatoceros triqueter.

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). Reproductive bodies are present on the gametophytes of Ahnfeltia plicata between July and January and mature carposporophytes occur between October and July (Maggs & Pueschel, 1989). The annual algal species, for example the filamentous greens, 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. The advantage of being fertile through the winter, as in the case of Ahnfeltia plicata, Furcellaria lumbricalis and Chondrus crispus, is the availability of substrata for colonization as other annual species die back (Kain, 1975). Dickinson (1963) reported that Chondrus crispus was fertile in the UK from autumn to spring, but that the exact timing varied according to local environmental conditions. Similarly, Pybus (1977) reported that although carposporic plants were present throughout the year in Galway Bay, Ireland, maximum reproduction occurred in the winter and estimated that settling of spores occurred between January and May.
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. Dudgeon & Johnson (1992) noted wave induced disturbance of intertidal Chondrus crispus on shores of the Gulf of Maine, USA, during winter. 25-30% of cover of large Chondrus crispus thalli was lost in one winter. Physical disruption of the algal turf is likely to promote diversity as spaces become available for colonization.

Habitat complexity is provided by the mixed substratum of bedrock, cobbles, pebbles and mobile sand. It is this complexity which determines the species of algae which characterize the biotope. Only species tolerant of sand cover and sand scour, e.g. Ahnfeltia plicata, Polyides rotundus and Furcellaria lumbricalis, are able to persist in the community.
The dense algal turf provides shelter for a variety of fauna and sites for attachment of both epifauna (e.g. ascidians, bryozoans and hydroids) and epiphytes (Lewis, 1964).

Primary production by the slow growing, perennial red algae which dominate the biotope is probably 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 mg C/g dry wt/hour. These figures suggest that the contribution made by the perennial algal turf to macroalgal production in the biotope is likely to be very small. 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 Ahnfeltia plicata, Chondrus crispus, 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 Ahnfeltia plicata, Chondrus crispus, 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. Scrosati et al. (1994) commented that viability of spores of Chondrus crispus was low (<30%) and suggested that reproduction by spores probably does not contribute much to maintenance of the intertidal population in Nova Scotia, compared to vegetative growth of gametophytes.
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, 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 Ahnfeltia plicata, 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).
The anemone, Urticina felina, disperses via a large pelagic larvae (Chia & Spaulding, 1972) or may be able to brood its offspring until they are well developed (Spaulding, 1974). Either way the species has poor dispersive powers (Sole-Cava et al., 1994) and therefore is most likely to recruit from local populations.

Maturity of the community is likely to be limited by the time it takes the climax, perennial algae to settle, grow and reach reproductive viability. Minchinton et al. (1997) documented the recovery of Chondrus crispus after a rocky shore in Nova Scotia, Canada, was totally denuded by an ice scouring event. Initial recolonization was dominated by diatoms and ephemeral macroalgae, followed by fucoids and then perennial red seaweeds. After 2 years, Chondrus crispus had re-established approximately 50% cover on the lower shore and after 5 years it was the dominant macroalga at this height, with approximately 100% cover. The authors pointed out that although Chondrus crispus was a poor colonizer, it was the best competitor. Furcellaria lumbricalis grows even more slowly than Chondrus crispus (Bird et al., 1979) and may take 5 years to reach fertility (Austin, 1960b).
It is expected that, although the species which characterize the biotope would probably establish themselves after 2-3 years, a climax reproductive community may not be achieved until 5 years or more.