Biodiversity & Conservation

Due to the moderately strong currents associated with this biotope, suspension feeders are the dominant trophic group, indicating the importance of a planktonic input to the benthic community. Suspension feeders frequently associated with this biotope represent four phyla: the sponges Halichondria panicea and Hymeniacidon perleve, ascidians such as Ascidiella aspersa, molluscs including the slipper limpet Crepidula fornicata and common mussel Mytilus edulis and crustaceans such as barnacle Elminius modestus.

The sand mason Lanice conchilega (a polychaete worm) is primarily a deposit feeder, feeding preferentially on detritus. However, when Lanice conchilega occurs in high densities, competition at the sediment surface forces it to suspension feed (Buhr & Winter, 1977).

The common periwinkle Littorina littorea is a herbivore and commonly feeds on fine brown, green and red algae. The chiton Lepidochitona cinerea also grazes on algae and micro organisms.

The common shore crab Carcinus maenas is the largest mobile predator frequently associated with this biotope and is likely to move between the boulders and pebbles feeding primarily on small molluscs, especially Littorina sp. and Mytilus edulis, annelids and other crustacea. It is a true omnivore and will also consume algal material.

Autotrophs in the biotope are varied and include representatives from the brown, green and red algal groups such as Fucus serratus, Bryopsis plumosa and Chondrus crispus respectively. The algae themselves may provide substratum for epiphytes including hydroids, sponges and ascidians. In addition, clumps of algae are likely to provide refuge for smaller crabs and periwinkles which may otherwise be washed away by the strong water flow.

Due to the eulittoral position of this biotope, the associated fauna are likely to experience some predation from birds, when exposed at low tide and shallow water fish at high tides.

The plants in this biotope are likely to experience some seasonal change in abundance, the general pattern being a lower percentage cover over the winter months. However, this biotope is limited to habitats that are sheltered to extremely sheltered from wave exposure and, therefore, increases in wave exposure during winter and the occurrence of winter storms are unlikely to affect it to the same extent that more exposed habitats would be affected. In some habitats, the surface cover of Fucus serratus may reach 95% in the summer months. Ephemeral green algae especially, increase in abundance over the summer months.

Any increase in wave exposure and storm frequency over the winter months is likely to result in the resuspension and subsequent redeposition of the mixed sediment substratum. If the forces were strong enough, the pebbles and boulders may also be moved around. Redistribution of the larger pebbles and boulders may be to the detriment of the epilithic fauna. For example, if colonies of sponges and ascidians on the rock landed face down into the sediment, their feeding would be interrupted and that part of the colony would die. In the Bay of Fundy, Canada, the abundance of the hydroid Dynamena pumila declined dramatically over the winter months as a result of ice scour (Henry, 2002) which is likely to have a similar effect to the scour of the mixed sediment in this biotope. Similarly, if plants were trapped under the boulders they may also die unless subsequent movement of the boulders released them. Periodic storms may remove older and weaker plants and reduce the overall biomass of the plants.

The substratum within this biotope is mixed and structurally complex, offering a wide variety of potential habitats including boulders, cobbles, pebbles and muddy sediments. This means that there will be a mix of both infauna, epifauna and epilithic species. In addition, Fucus serratus and red seaweeds offer a substratum for colonization by epiflora including bryozoans and sponges. 91 taxa of associated fauna were found on 65 specimens of Fucus serratus in Strangford Lough, Northern Ireland (Boaden et al., 1975). Clumps of seaweed also offer refuge for Carcinus maenas and the grazer Littorina littorea. Small patches of the mussel Mytilus edulis may provide refuge for a diverse range of small invertebrates including polychaetes and Littorina sp.. The empty shells of the molluscs also provide some heterogeneity to the substratum.

Very little is known about the productivity of this biotope, or indeed of intertidal mixed sediment communities in general. However, the small patches of macroalgae associated with this biotope can exude dissolved organic carbon, which is 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. Many of the species associated with this biotope make a contribution to the food of many marine species through the production of planktonic larvae and propagules, which contribute to pelagic food chains. There is likely to be less productivity than on fucoid dominated rocky shore such as MLR.MytFves and MLR.BF.

For the majority of important characterizing species and other important species within this biotope, reproduction and recruitment is an annual process. For some of the species, such as the common mussel Mytilus edulis, common shore crab Carcinus maenas and common periwinkle Littorina littorea, various stages in the reproductive process, including gametogenesis, the timing of spawning and/or recruitment, are variable depending on, for example, environmental factors and geographic location. Recruitment in the major groups present is summarized below.

• Reproduction in Fucus serratus commences in late spring and continues until November, with a peak in August and October. Eggs and sperm are produced separately and fertilized externally to form a planktonic zygote. Recruitment is therefore possible from sources outside the biotope.
• Chondrus crispus has an extended reproductive period (e.g. Pybus, 1977; Fernandez & Menendez, 1991; Scrosati et al., 1994) and produces large numbers of spores (Fernandez & Menendez, 1991). Although growth of sporelings is not rapid in comparison to other macroalgae, maturity is probably reached approximately 2 years after initiation of the basal disc (Pybus, 1977). The spores of red algae are non-motile (Norton, 1992) and therefore entirely reliant on the hydrographic regime for dispersal. Hence, it is expected that Chondrus crispus would normally only recruit from local populations and that recovery of remote populations would be much more protracted.
• There is some debate as to the nature of reproduction in the breadcrumb sponge Halichondria panicea but it is likely that it has a short, annual season of reproduction (see MarLIN review).
• The larvae of the sea squirt Ascidiella aspersa have a short free-swimming planktonic stage. However, time taken from fertilization until settlement and metamorphosis is only about 24 hours at 20 °C (Niermann-Kerkenberg & Hofmann, 1989) and, therefore, widespread dispersal seems unlikely.
• Mytilus edulis has a protracted spawning in many places and fecundity is affected by many factors (see MarLIN review). The planktonic life of larvae may exceed two months and, hence, there is a good chance of wide dispersal and recruitment from external sources. Due to the highly gregarious nature of Mytilus edulis settlement, persistent mussel beds can be maintained by relatively low levels of recruitment.
• The larvae of the sand mason Lanice conchilega can spend up to sixty days in the plankton therefore providing good potential for dispersal and recruitment from external sources.
• The duration of the breeding season in the common shore crab Carcinus maenas depends on the geographic location of the population. Females on the south coast of Britain can bear eggs all year round and fecundity is high (see MarLIN review).
• Fecundity in the common periwinkle Littorina littorea can reach 100,000 eggs in large females. The reproductive season is annual and episodic with a pelagic phase of up to six weeks. Littorina littorea can also breed all year, although the length and timing of the season depend on climate.
• Dispersal of the hydroid Dynamena pumila is restricted to the planula stage which usually settles and starts to metamorphose within 60 hours of release (Orlov, 1996). Orlov (1996) that long-distance dispersal was further restricted by the dense bushes of neighbouring algae which serve to trap the larvae in the area.

No information was found concerning the development of this biotope. However, the important characterizing species all reach sexual maturity within three years and have annual reproductive episodes suggesting that the time taken for the community to develop is likely to be less than five years. However, if adverse environmental conditions prevail, time taken to reach maturity could take significantly longer.