Rockpools occur where the topography of the shore allows seawater to be retained within depressions in the bedrock producing 'pools' on the retreat of the tide. As these rockpool communities are permanently submerged they are not directly affected by height on the shore and normal rocky shore zonation patterns do not apply. For this reason rockpools have been dealt with as a separate habitat type, apart from the scheme of wave exposure and shore height. Four main rockpool biotopes have been described, and although it is accepted that an enormous variety of rockpool communities exist, it is hoped that these biotope descriptions are broad enough to adequately encompass most types. It would be meaningless to include the characterising species in a description at the habitat type level. Rockpools on the upper shore which are subject to rainwater influence and wide fluctuations in temperature are included in A1.42. Shallow rockpools in the mid to upper shore characterised by encrusting coralline algae and Corallina officinalis (A1.411); several variants of these coralline pools occur in south-west Britain and Ireland (A1.4112, A1.4113 and A1.4114). Deeper rockpools on the mid to lower shore can support fucoids and some sublittoral species such as kelp (A1.412). Those rockpools influenced by the presence of sand are characterised by sand-tolerant seaweed such as Furcellaria lumbricalis and Polyides rotundus (A1.413). Where more stable sand occurs in the base of the rockpool sea-grass beds can occur. Shallow rockpools on mixed cobbles, pebbles, gravel and sand may be characterised by hydroids (A1.414). A very rough guideline to the terms "shallow" and "deep" rockpools: "shallow" rockpools do not support kelp, whereas "deep" rockpools do. A1.41 does not include shallow standing water on compacted sediment or mixed substrata. Situation: Rockpools occur in the littoral zone where the topography of the shore allows seawater to be retained within depressions in the bedrock producing 'pools' on the retreat of the tide.
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