Rocky shore at high tide.
Toxic to most terrestrial plants and animals, sea water is the stuff of life for intertidal and marine creatures. Its chemistry is important, of course, but so too are the physical aspects of tide and current, which shape our coastline and affect the lives of marine creatures on a daily basis.
TIDES
Manifesting itself on coasts as tides, the rise and fall of sea-level on a regular basis results from the gravitational effect of the Moon. The tidal influence on British shores is profound: every six hours or so, all intertidal seashore plants and animals are exposed to air, then covered by sea water. The extent of the tides (tidal range) is dictated in the main by the phases of the Moon and changes on a daily basis throughout a 28-day period; atmospheric pressure also affects tidal range.
Tidal range is most extreme during full and new phases of the Moon; these periods are referred to as ‘spring’ tides. The tidal range is least extreme when the Moon is half waxing and half waning; these periods are referred to as ‘neap’ tides. The proximity of the Moon to the Earth influences its gravitational pull and hence the range of the tides. The most extreme tides occur when the Moon is closest to the Earth, during the spring and autumn equinoxes, in March and September, respectively.
High tide is never at the same point on any given beach on consecutive days. Tidal range also varies around Britain, affected by the geography of the coast. For example, the funnelling effect of the Bristol Channel means that the tidal range here is 10m or more in places; elsewhere in the region, tidal range can be as little as 1m, although 3–5m is typical of most stretches of shore.
Rocky shore at high tide.
Rocky shore at low tide.
ZONATION
The intertidal zone’s plant and animal inhabitants have evolved to cope with different degrees of exposure to air and sea-water inundation. Consequently, they are usually found at precise zones on the shore; these are most obvious on rocky coasts. The growth of different lichen and seaweed species provides the most immediate evidence of zonation, appearing as bands of different colours. The effect is most noticeable on the west coast of Britain, where the tidal range is large and where rocky shores are often steep rather than shelving.
WAVES AND EXPOSURE
Clearly, waves have a battering effect upon seashore plants and animals, and the degree to which any given stretch of shoreline is exposed to waves has a strong bearing on what lives there. Some plants and animals have adapted to these seemingly hostile conditions while others thrive only where they are sheltered from the full force of the elements, such as by a headland.
Waves are caused by wind – the faster the wind and the further it has travelled over the sea, the larger and more powerful the waves. The prevailing wind in Britain comes from the west, and our most severe gales come from that direction too. Consequently, our most exposed shores are found in western Britain, and it is here that the differences in shoreline inhabitants are most profound when comparisons are made between exposed and sheltered locations on the same stretch of coast.
Rough seas battering the Isle of Portland.
SALINITY
Although traces of every element can be found in sea water, it is the presence of salt – sodium chloride – that is by far the most significant for marine life. Fresh water has a salinity of 0.05%, while true sea-water salinity is 3–3.5% (salinity measures the weight of salt, in grams, per kilogram of water).
In most circumstances, sea-water salinity is a constant to which true marine creatures are adapted and on which they depend. Estuaries are an exception to the rule and present a challenge for the plants and animals found there. With each tide the salinity changes from virtually fresh when the tide is out, to fully marine when the tide is in. These changes in salinity would be enough to kill most species adapted to either fresh water or truly marine conditions. So estuary inhabitants are special indeed, adapted to cope with the daily osmotic challenges their environment throws at them.
