Laminaria digitata on moderately exposed sublittoral fringe rock
Image Keith Hiscock - Kelp forest exposed at low tide with rocky shore in background. Image width ca 3 m (foreground).
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Ecological and functional relationships
Kelp habitats are dynamic ecosystems where competition for space, light and food result in patchy distribution patterns of flora and fauna. Kelp beds are diverse species rich habitats and over 1,200 species have been recorded in UK moderately exposed kelp biotopes (MIR.KR) (Birkett et al. 1998b). Kelps are major primary producers, up to 90% of kelp production enters the detrital food web and is probably a major contributor of organic carbon to surrounding communities (Birkett et al. 1998b). Major interactions are thought to be the effects of competition for space, shading, herbivory and predation.
In most kelp biotopes there is evidence of strong competition for space, especially for space on a favourable substratum. Competition may occur between individual plants of the same species, between kelps and substratum-colonizing species of animals and other algae and between colonial animals and encrusting algae. Competition for space between individuals and species is dynamic, resulting in a constantly changing patchwork of species covering any suitable substrata within the biotope.
The blades of Laminaria digitata plants form a canopy layer which may cut off much of the incident irradiance. This restricts the development of species with high light demands so that the understorey of plants becomes dominated by shade tolerant red algae. It also allows species normally restricted to the lower infralittoral in kelp-free areas to compete more effectively in the reduced light levels of the kelp bed and so are found at shallower depths.
Within kelp beds there are relatively few species that are directly grazing either the kelp or the understorey algae as the enzymes required to directly utilise algae as food are not common. Those species able to graze directly on the kelp include the gastropods: Gibbula spp., Littorina spp., Haliotis tuberculata (in the Channel Islands only), Helcion pellucidum, Lacuna spp. and the Rissoidae, together with some amphipods and isopods. Helcion pellucidum grazes epiphytes and the kelp tissue directly, forming pits similar to the home scars of intertidal limpets. The larger, laevis form excavates large cavities in the holdfast of Laminaria spp. which creates tissue damage weakening the adult plant and possibly contributes to its loss due to wave action and storms (Kain, 1979). Infestation with Helcion pellucidum varies between sites and decreases with depth.
The molluscs Patella vulgata and Gibbula spp. and chitons (especially in the north) graze the rock below leaving, with the effects of frond-sweeping, extensive bare crustose coralline algae on the rock.
Predation within kelp beds has not been well studied in the UK. Although some species are known to prey on others, such as starfish on mussels, very little is known of the predator-prey relationships for the many species occurring in kelp beds.
The abundance of filter feeding organisms such as sponges, bryozoans and tunicates which colonize both the rock and kelp plants indicates the importance of planktonic input to the benthic community of the biotope. Although very little information is available about planktonic communities in kelp beds it can be assumed that there will be larger inputs of larval stages from species with bentho-pelagic life cycles than in the general plankton (Birkett et al, 1998b).
Kelp plants are also exploited as a habitat; the holdfast, stipe and frond each support a different type of community although only the oldest Laminaria digitata plants will have epiphytic life on the stipe (which is smooth in all but the oldest individuals); however, holdfasts shelter a particularly rich diversity of animals from a wide range of taxa (see Habitat complexity).
Seasonal and longer term change
Most species in the biotope are perennial and seasonal changes are likely to be in condition of individuals rather than presence or absence.
- Growth rate of Laminaria digitata is seasonally controlled with a period of rapid growth from February to July and one of slower growth from August to January. Increased wave exposure and storms in winter months are likely to erode Laminaria digitata blades so that they appear tattered in winter months and overall standing biomass is reduced. Periodic storms are also likely to remove older and weaker plants creating patches cleared of kelp and increasing the local turbidity. Cleared patches may encourage growth of sporelings or gametophyte maturation. Growth of understorey algae is also reduced in the winter months.
- Some species of algae have seasonally heteromorphic life histories spending a part of the year as a cryptic or encrusting growth form and only becoming recognisable in the foliose phase of their life cycles. The occurrence of such algae is often seen as the occurrence of 'ephemeral algae'. Some hydrozoans may be present in the kelp bed in their attached, colonial form only for a part of the year, spending the rest of the year as medusae.
It should be emphasised that present understanding of the natural fluctuations in the species assemblages, populations, distribution and diversity of species in kelp beds is very limited.
Habitat structure and complexity
The structure of kelp beds is complex with many different habitats i.e. bedrock, crevices, sediment pockets, the holdfast, stipe and blade of the plants themselves.
- Holdfasts provide refuge to a wide variety of animals supporting a diverse fauna that represents a sample of the surrounding mobile fauna and crevice dwelling organisms, e.g. polychaetes, small crabs, gastropods, bivalves, and amphipods.
- Kelp fronds may be colonized by encrusting bryozoans and hydroids and are grazed by molluscs such as the blue-rayed limpet Helcion pellucidum.
- Older rougher stipes of Laminaria digitata provide a substratum for a large number of epiphytic flora and fauna and it has been estimated that rugose stipes provide one and a half times that surface area provided by the bedrock (Jones et al., 2000).
- The bedrock offers surfaces for settlement and shelter of species and is colonized by encrusting and foliose red algae with a variety of tubicolous animals and ascidian species attached.
Kelp plants are the major primary producers in the marine coastal habitat. Within the euphotic zone kelps produce nearly 75% of the net carbon fixed and large kelps often produce annually well in excess of a kilogram of carbon per square metre of shore. However, only about 10% of this productivity is directly grazed. Kelps contribute 2-3 times their standing biomass each year as particulate detritus and dissolved organic matter that provides the energy supply for filter feeders and detritivores in and around the kelp bed. 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.
Most species in this biotope produce planktonic propagules annually and so recruitment is often from distant sources and is frequent.
- Benthic species, plant and animal, that possess a planktonic stage: gamete, spore or larvae, are likely to be influenced by kelp mediated alteration of fluid and particulate, and consequently larval fluxes. Kelp canopies also affect the physical environment, such as the substratum, experienced by actively settling planktonic larvae. The substrata beneath kelp plants for example, are often dark and sediment laden, conditions likely to affect larval settlement and post settlement survival. Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates driven by such factors.
- Laminaria digitata plants are fertile all year round with maximum production of spores in July - August and November - December. Young sporophytes (germlings) appear all year with maxima in spring and autumn. Chapman (1981) demonstrated that substantial recruitment of Laminaria digitata plants to areas barren of kelp plants was possible up to 600m away from reproductive plants.
- Among sessile organisms, patterns fixed at settlement, though potentially altered by post settlement mortality, obviously cannot be influenced by dispersal of juveniles or adults.
- Some of the species living in kelp beds do not have pelagic larvae, but instead have direct development producing their offspring as 'miniature adults'.
Time for community to reach maturity
Kain (1975) examined the recolonization of cleared concrete blocks by kelp plants and other algae and found that Laminaria digitata
plants were re-established within 2 years and that red algae returned with a year. Many other characterizing species have planktonic larvae and/or are mobile and so can migrate into the affected area. Colonization of most species of fauna inhabiting kelp holdfast, for example, were found as early as one year after kelp trawling of Laminaria hyperborea
plants in Norway, although numbers of both individuals and species, especially isopods and amphipods, increase with a corresponding increase in holdfast size (Christie et al
, 1998). However, although these species colonize the biotope quite rapidly maturity of the overall community is likely to be longer. For example, encrusting coralline algae such as Lithophyllum incrustans
are slow growing (2-7 mm per annum - see Irvine & Chamberlain 1994) and recruitment of other species to the kelp bed may take longer. In dredged kelp beds in Norway for example, although the rock between Laminaria hyperborea
plants was uniformly covered with coralline algae after 3 years, the more diverse community of cnidarians, bryozoans and sponges associated with coralline algae seen on undredged plots was absent (Birkett et al
., 1998b). Within five years, however, the biotope is likely to have reached maturity.
This review can be cited as follows:
Laminaria digitata on moderately exposed sublittoral fringe rock.
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=297&code=1997>