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Factors affecting the distribution of organisms on Sediment Shores

This pagfe summaries the main features facing organisms living on sediment shores, the key abiotic factors present and a brief outline of the how the abiota causes a zoning of organisms.

General features organisms face when living on sediment shores:

Summary of the Limiting Factors Effecting Organisms

Shore Type

Very coarse sandy shore

Fine sandy shore

Muddy shore

Turbulence

high

average-low

very slight

Particle size

> 2 mm

0.2 - 0.002mm

< 0.002 mm

Slope

steep

gradual

flat

Water content

low

variable

high

Drying out

rapid

slow

negligible

Oxygen level

high

low

deoxygenated

Sulphide layer

absent

slight

present

Organic detritus

slight

variable

high: dark colour

Number of macro-organisms

low-absent

variable

high

edge of the Solent Shingle & Sand

Top/left image is the edge of the Solent. All types of sediment are present, with shingle on the left, falling to gravels, then sand and, in the far distant lower shore, mud.

Bottom/right image is the same but photographed in infra red. Any living tissue shows up red (e.g. the trees in the background). A few seaweeds have been washed up on to the shingle whilst amongst the gravel are a number of small seaweeds. The sand reflects little infra red as there is minimal algal presence. Mud goes a deep red due to the higher amounts of water encouraging growth of producers. See more detail on the factors determining this below:

TURBULENCE: this sorts the particles so that the greater the wave action the coarser the material deposited on the beach. Sandy is more unstable than mud. Waves tend to scour the sand clean of organic matter. Severe gales may remove the sand and in calmer months redeposit.

SLOPE: the gradient is proportional to the particle size; for example, shingle creates steep banks whilst silt yields mudflats

WATER CONTENT: this too is affected by the size of particles. Mud draws water up by capillarity at low tide. Coarse sands have large air spaces and so drainage is rapid

ASPECT: south facing shores are most likely to dry out; any shade will slow this down as well as giving protection from a drying wind.

OXYGEN and SULPHIDE LAYER: oxygen diffuses into the substrate water and airspace; bacteria feed on organic matter and release hydrogen sulphide. Without oxygen this produces ferrous sulphide, giving black sand. With oxygen, an oxide, similar in colour to clean sand develops. This is the pale, oxidising layer at the surface

TEMPERATURE: high air temperatures will affect drying and kill microfauna at the surface of mud which are usually unaffected by desiccation. In winter interstitial water may freeze, causing a high mortality rate. This is especially so in estuaries where the salinity is lower and the freezing point not so depressed. However, some

organisms are quick to recolonise, particularly polychaetes. Bivalves are much slower Temperature becomes more constant with depth. To avoid extremes of temperature, microfauna can migrate away from the surface.

SALINITY: rain on the shore can affect the microfauna in the interstitial spaces. Macrofauna will burrow deeper. Salinity is more variable on these types of shore than others and animals either conform, regulate or move away. Avoidance of freshwater is the common feature: Inachus, a spider crab, moves into deeper water; Nereids burrows and tolerates the absorption of water by its tissues. Carcinus and other kinds of crustacean can osmoregulate.

 

 


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