BS346 Marine Biology

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Zonation

Zonation is one of the most obvious features of life on rock shores. No organism can successfully inhabit every level of the shore due to the fact that the requirements for life in air and water are so different The reasons for zonation are complex and include physiological adaptation and competition for food and living space in the face of a gradient of physical environmental factors between marine and terrestrial ecosystems.

The seashore can be divided into three main zones that are inhabited by particular species, although there is, of course, overlap of species.

The three zones are;

Supralittoral (= supratidal): this zone is never completely covered by the sea, i.e. this zone is at the predominantly terrestrial region of the shore. The supralittoral may be 'spray wetted' occasionally.

Littoral (= intertidal): alternate emersion and submersion are normal in this zone. The littoral is further subdivided into the Littoral fringe and the Eulittoral zone. The former is delimited by the upper limit of barnacles. The Eulittoral may be further subdivided into Upper, Middle and Lower shore. 'Splash' may wet the littoral fringe frequently.

The intertidal zone is the region of the shore showing the highest species diversity.

Sublittoral: never uncovered by the sea.

The extent of these zones is dependent on exposure (wave action), which modifies the conditions on the shore, but the zones can be broadly related to tidal levels as follows:

MHWS = Mean High Water Spring

MHWN = Mean High Water Neap

MTL = Mid Tide Level

MLWN = Mean Low Water Neap

MLWS = Mean Low Water Spring

CD = Chart datum

The following table illustrates that zonation on the shore affects the distribution of species throughout the shore:

The lectures accompanying the field and laboratory work during the field course will go into further detail on the subjects of zonation and tidal cycles. The above information introduces the idea that the seashore is divided into zones which different species may inhabit according to the environmental conditions prevalent within a zone. The presence of a species in a particular zone, rather than any other, is the consequence of long term processes, including environmental factors, settlement etc. It is not possible to fully appreciate or quantify many of these in the limited time available on the one week field course. A longer term study would be necessary, often extending over many years. Literature about such studies is available from the reference list supplied in this tutorial.