Topographical structure of the substratum
Slope and Complexity
Hard shores in the UK are composed of a large range of rock types,
dependent on the geology of each region. Man-made surfaces such as coastal defences,
harbour walls and pier pilings also provide a substratum for rocky shore species. The
community present on a rocky shore will be strongly influenced by the structure of the
substratum at several spatial scales.
Hard, igneous rocks such as basalt and granite along with fine sedimentary rocks such
as siltstone and shale will drain and dry rapidly as the tide recedes. More porous rocks
such as sandstone and chalk retain water for longer, reducing desiccation stress. Hard
rocks provide a more secure anchorage for large plants and animals such as fucoids and
limpets. Animals such as piddocks are able to burrow into chalk and other soft rocks as
can endolithtic microalgae, particularly cyanobacteria. Many littoral organisms are more
likely to settle on some rock types than others. Proposed explanations for this include
the surface roughness of the rock and the ability of rocks of different types to sequester
and partition solutes (Holmes et al., 1997).
Slope and Complexity
Rock type influences the slope and topographical complexity of the
shore, and slope determines the area available for littoral species. Barnacles and limpets
are successful on steep shores, while mussels and seaweeds are more common on gently
sloping or horizontal shores. The colonisation of shores by animals is an active process
in those species with motile larvae, whereas algae rely on the retention of passive
spores. Therefore, algae are better able to colonise more horizontal surfaces. Increasing
topographical complexity also increases space and the number of microhabitats available.
These factors have a strong influence on biodiversity.
Numbers of species and abundance increases with topographical
complexity (Kostylev et al., 1996). Greater abundance results from a larger surface
area while the number of species present is influenced by the range of microhabitats
available. Tiny pits in the rock may provide shelter for juvenile littorinids and other
small species. Larger cracks and crevices provide shelter for mobile predators like
dogwhelks. Crevices also collect sediment and trap air. This provides a favourable
environment for a highly specialised fauna of burrowing worms and air-breathing
arthropods. Air-breathing animals include centipedes, millipedes, beetles and
pseudoscorpions, and pulmonates such as Onchidella celtica, foraging only during
emersion and taking refuge in air pockets during tidal immersion. Crevices also provide a
daytime retreat for nocturnally active invertebrates of marine origin, such as members of
the genera Lineus, Eulalia, Ligia and Orchestia. Hypoxic silt accumulating
in deep crevices allows soft sediment biota such as Cirratulu cirratus
to live on rocky shores.
Rocky shores might consist mainly of bedrock or they may be a mixture
of bedrock and boulders, cobbles or pebbles. Boulders and smaller rock fragments can cover
the whole shore or they can occur in lower densities on sediment shores. Bedrock itself is
rarely smooth. Usually, there will be some form of three dimensional structure.
Conglomerates have rounded humps where pebbles are present in the rock with troughs
between and hollows where hard pebbles have been removed. Basalt forms smooth, regular
columns which can collapse to form boulders. Limestone can form extensive ledges dotted
with eroded pools. Soft rocks like chalk are usually rounded by erosion while slate shores
consist of slabs with angular gullies. Each type will have its own degree of topographical
complexity according to the three dimensional features present: cracks, crevices,
overhangs, folds, rock fragments and pools.
Rock pools are a well known feature of many rocky shores although their
ecology is less well understood than that of bedrock (see Chapter VII). Pools provide an
intertidal habitat for obligate water dwellers including many species of fishes. They also
support low-shore organisms, such as seaweeds and anemones, at higher levels than they
would otherwise be found. However, those pools at higher shore levels are subject to
physical and chemical fluctuations which would not be experienced in the sea (Morris and
Taylor, 1982). Pools at about the level of neap tide high water will be separated from the
sea for continual periods of about 11 hours. Higher up the shore, pools experience several
days without inundation (Naylor and Slinn, 1958). Temperatures in these small bodies of
water change in response to air temperatures more rapidly than those in the sea. Water
evaporates from pools and rainwater and freshwater runoff collects in them, causing
salinity fluctuations. Fluctuations in oxygen content and pH also occur with a diurnal
cycle. During the daytime, plants photosynthesise, saturating the water with oxygen and
supersaturation is not uncommon. During the night, a net uptake of oxygen and production
of carbon dioxide occurs. Oxygen concentration drops and pH falls as a result.
Boulders have an upper surface similar to bedrock and, if stable, the
tops are essentially the same as bedrock. Larger boulders will suffer less impact from
waves on their shoreward sides. These sides may therefore be used by species at risk of
dislodgement on more exposed surfaces. Boulders also have interstitial spaces below. Like
crevices, these provide shelter for grazers and foragers. Mobile animals, including
moulting crabs take refuge under rocks. Sediment will collect in the interstitial spaces
and, often, the boulders rest on a sediment substratum. So, while communities on the tops
of boulders are similar to bedrock shores, communities characteristic of sediment shores
can be found below.