Rocky Reefs

2 The Physical Environment

The slope of the rock platform determines the extent of tidal exposure. Steep platforms have narrow bands of algae and non-moving animal communities like mussels whereas gentle slopes have broad bands of these communities.

Generally, the greater the slope of the shore and the lesser the variation in the surface form of the rock, the more clearly delineated the distribution pattern will be. On a flat or gently sloping shoreline, distribution patterns will often be less distinct from one end of the shore to the other.

Tides cycling twice daily play a vital role alternatively covering and exposing organisms in the intertidal zone with water. Tides also vary in height daily. The top of the rock platform may remain uncovered for extended periods.

Waves exert a powerful physical force on organisms in the intertidal zone hence animals and plants need some means of preventing themselves being washed away by the waves. Waves also erode or carve the rocky shores undercutting cliffs and in some case depositing sediment from offshore.

The type of rock (eg. basalt or sandstone, boulder field or flat ledge) influences the way in which the rocky shore is eroded. Some rocks like basalt break up easier than others such as sandstones and this in turn gives rise to a wide range of habitats such as cracks or crevices within the rocks providing shelter and basins for rock pools.

The type of material a reef is made from also has a great influence on the animals and plants which live on it. Subtidal sandstone reefs, for example, are often undercut and have many ledges and crevices worn into them. The surfaces of sandstone reefs often teem with sponges, feather stars, urchins and anemones, while basalt reefs are more likely to support mobile animals such as Pacific Octopus, abalone and chitons. The number of caves and crevices in a reef will also contribute to the number of fish which seek shelter there.

Intertidal reefs also show differences in the marine life they support. Basalt rock platforms are warmer than sandstone and the plants and animals that live on basalt must be able to cope with higher temperatures. Basalt is also more likely to form boulders. These boulders provide microhabitat for many intertidal animals.