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The Seashore

Rocky Shore Trail
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The middle shore (sheltered shores)

The sheltered shore

Sheltered Shore with the middle shore in the lower half of the image, splash zone at the top.

We are taking the part of the shore between the average high water neap tide (MHWN) and the average low water neap tide (MLWN) as the middle shore. As we move down the shore the quantity and variety of living organisms begins to increase. The middle shore shows this big time, particularly towards the lower end of it.

Abiotic issues important here:

Environmental conditions on the middle shore are much more favourable for the majority of inter-tidal organisms. There is an increase in quantity and variety of primary producers (seaweeds). We see an increase in biodiversity because of this and biomass is greater as seaweeds can absorb nutrients and photosynthesise for longer. The image at the top shows the seaweeds here as being bigger. However, all this improvement means greater interspecific competition. The shore is also immersed for a much greater % of the year (50-80% below mid-tide level) providing more food and feeding time for filter feeders like barnacles. Typical seaweeds of the middle shore are the brown alga bladderwrack (Fucus vesiculosus), egg wrack (Ascophyllum nodosum) and the red seaweed pepper dulse (Osmundea pinnatifida).

More food results in more herbivores to eat it and more herbivores mean carnivores to eat them. Limpets are still much in evidence grazing on the small stages of seaweeds. Top-shells (Gibbula umbilicalis, Osilinus lineatus) and different winkle species flat and edible winkles now also begin to thrive. Carnivores like the dogwhelk (Nucella lapillus) may now be found in abundance feeding on the dense assemblages of barnacles (or sometimes mussels if there are some). In pools, under stones and seaweed there are common shore crabs (Carcinus maenas). These are very important predators, particularly on the molluscs. The sheltered conditions will allow many animals that are not attached (like the crabs) to appear here including fish like the blenny or shanny.

Compare this zone (middle shore) with the exposed shore zone.

Table of Zones - click to go

 


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