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The Seashore

Sediment Shores - an Introduction

View of a sandy shore at low tide

Compared to the rocky shore depositing shores look barren and devoid of life. At high tide when water movement is low, finely suspended particles rain down on to the shore. These depositing shores are molded by different degrees of shelter to produce a range from clean, coarse sand (with few organisms) to muddy, bacteria rich ones (with many organisms). In all cases, shelter causes the material to settle and any movement, including water currents, will disturb the surface. Typically, the substrate is unstable. Any disturbance of the surface by water currents or occasional storms will affect the substrate and therefore the communities. Due to the very nature of their formation depositing shores have little slope, and hence the term, mudflat.

Estuaries are typically sheltered shores where the velocity of the water has slowed to allow considerable deposition to occur. Substrates will be a mix of gravels, sands and fine silty mud. They occur where freshwater runs into the sea. The former is less dense, floating on the salty water. With changing tides, mixing gradually occurs.

THE SEDIMENT SHORE SUBSTRATE

• particles may be of quartz, felspar or shell fragments

• particles are of an irregular shape, with pits and crevices; this gives a large surface area for the attachment of bacteria and microscopic algae (e.g. diatoms)

• between the particles (interstitial space) is a micro-community called the MEIOFAUNA, base on bacteria and diatoms

• mud particles are much smaller than those of sand; this gives a larger surface area with small interstitial spaces. The combination of both yields a richer community and a greater productivity

• muds smaller interstitial space means that water does not drain away and at low tide the surface of mudflats remains wet: communities living in the mud will not suffer from desiccation

• sandy shores do drain quickly so that upper shores suffer from desiccation. Fewer species can survive

• water turbulence grades and sorts the particles so that many shores will show a transition from high water mark to low water: e.g. shingle through sand to mud

PROBLEMS/FEATURES OF LIVING HERE

• no fixed substrate for attachment, except for the occasional stone on the upper shore.

• species are burrowers, requiring distinct forms of adaptation to live under the surface.

• few producers are visible and the water is cloudy with silt and organic matter.

• in very sheltered areas saltmarsh may develop.

• depositing shores are typical of estuaries and here salinity changes daily; osmotic problems.

Substrate from a sandy shore

Substrate from a sandy shore; note the clean, large grains (Magnified 100x)

EDUCATION POINTS OF SPECIAL INTEREST

* a contrasting group of organisms to the rocky shore; good source of micro-organisms

 

PRECAUTIONS IN FIELDWORK

Soft mud can be very dangerous (sinking!) and the danger increases towards the lower shore.

Make sure you have checked the tide times as it is easy to get caught by the rising tide. For example, the slope may be so flat that a rise in the tide causes it to rush across the shore faster than people can run. On large areas of sediment shores obtain information loacally for any dangers. On some shores, like Morecombe Bay, visits should attempted only with a local guide.

 


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