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The Seashore

Rocky Shore Trail
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The lower shore (exposed shores)

kelp

Lower shore on an exposed shore (B.E.S. 3) looking towards Dale Fort. The view shows kelp (particularly L. hyperborea) dominating the foreground.

For our purposes the lower shore is that part from the average low water neap tide (MLWN) to the lowest the tide ever get ie. extreme low water spring tides (ELWS).

As we move down into the lower shore abiotic (physical) factors become less and less of a problem for most inter-tidal species. Most of them do better when immersed (underwater) and worse when emersed (out of the water). The sea is so enormous that it is very slow to change in temperature, pH, salinity (and anything else you can think of). By the time you get to the bottom part of the lower shore it's almost like being permanently under the water. Most inter-tidal organisms originated in the sea and do better wetter and in this relatively cushioned constant environment. Abiotic environmental factors now start to become of less importance in controlling what can live where. The better environmental conditions means that there are more species (increased diversity) and much greater abundance of those species. There are still not as many species as you'd expect to get on a sheltered shore though. This means that biotic factors (interactions with other living organisms) become more important than abiotic factors (interactions with the non-living part of the environment) in determining the distributions of organisms.

Most algae are shade plants and are adapted to absorb low amounts of light energy. This applies especially to red algae and they may begin to appear on the lower shore. Most of the time the light they receive there is attenuated by its passage through the covering sea water. Lots of wave action does not usually equate with success for seaweeds though and so there will not be as many species or as greater quantity as on a sheltered shore. You may find coral weed (Corallina officinalis) and lots of the encrusting red algae of many species but usually called Lithothamnion spp. In the lower zone of sheltered shores the saw wrack (Fucus serratus) was common. With increased wave action it may be replaced by the thongweed (Himanthalia elongata), a species that is narrow with less surface area.

Large brown algae (Kelps (Laminaria digitata, Alaria esculenta, L. hyperborea) begin to dominate the shore towards the lower end of this zone. There will be least wave action at the very bottom part of the shore because breaking waves will hardly ever break there (the middle shore should get most). Even so because the kelps are large organisms with lots of drag they need massive holdfasts to keep them in position against the surge of the water. Greater productivity of algae means more herbivores can survive (eg. Blue rayed limpets (Helcion pellucida), grey topshell (Gibbula cineraria), and the kelp holdfasts themselves form a sheltered microhabitat with food that may support dozens of species. All this means that factors like competition and predation and interactions between organisms become more important as we move down the zone.

Compare this zone (lower shore) with one from a sheltered shore.

Table of Zones - click to go

 

 


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