Substratum

An understanding of the features of sediments of intertidal sand and mudflats and the subtidal sand banks and the inter-relationships between those features is necessary to interpret their influence on the biota. Marine sediments are often heterogeneous in containing particles of many grades and types but their characteristics will vary spatially depending on the nature of the adjacent coastline, the hydrodynamics of the water (which produce areas of high or low energy) and the contours of the sea bed. The underlying geology, topography and physiography will produce the basic shape of the coastline and its ability to be infilled with sediment (Pethick, 1984). The substratum is defined by the size range of its constituent particles and may be classified by the Wentworth scale (Buchanan, 1984; Buller & McManus, 1979; Tait & Dipper, 1998).

Intertidal mudflats are predominantly clay (particles <4Ám), silt (4 - 63Ám) and to a lesser extent very fine sand (63 - 125Ám); intertidal sandflats contain all the grades of sand and to a lesser extent silt and clay, whereas subtidal mobile sand banks contain all the grades of sand (63Ám - 1mm) with a very low silt and clay content.

The settling velocity of particles is dependent on particle size and water characteristics such that sands and coarser materials settle rapidly and particles >15Ám diameter will settle out within one tidal cycle (King, 1975). In contrast, clay and silt (particles <4Ám diameter) are unlikely to settle within one tidal cycle and, in addition, have settling velocities which are influenced by flocculation processes often mediated by surface electrostatic charge. Such phenomena are important in estuarine waters and intertidal muds subject to widely fluctuating salinity and pH.

Sediment deposition within an area is controlled by the type, direction and speed of the currents and the size of the particles. Fine grained material will move in suspension and will follow the residual waterflow, although there may be deposition at periods of slack water. The coarser grained material will travel along the bed in the direction of the maximum current and will be affected most by high velocities (Postma, 1967). Erosion of fine sand of 0.1mm particle diameter occurs at >30 cm s-1, and deposition will occur at <15cm s-1. Particles of 1-10 Ám diameter have a similar relationship, although erosion requires faster current speeds because of consolidation and flocculation (Hedgpeth, 1967).

The distribution of grain sizes within a substratum is indicated by sorting and skewness characteristics (Buller & McManus, 1979). Sorting reflects the range of forces which have formed the sediment and it influences the gradient of slope of intertidal areas. In sediments that have a low degree of sorting, as a reflection of a greater mixture of particle sizes, small particles occupy the spaces between larger grains and thus reduce pore space or porosity. In intertidal areas this lowers percolation rate and creates steeper shore profiles (Pethick, 1984). Pore space also depends on the rate of deposition with rapid deposition leading to cubic packing which maximises the spaces between grains and leads to a more porous sediment (Pethick, 1984). Skewness indicates the shape of the tail of the frequency distribution of the sediment particles.

These characteristics of sediments are interrelated (see figures linked below) to create conditions conducive to supporting infauna.

Factors pertaining to sedimentary low energy areas

Factors pertaining to sedimentary high energy areas

Mudflats and sheltered beaches consist of fine or silty sands and thus reflect low energy conditions. The characteristic features that define the substratum in low energy environments are noted below and illustrates in the first figure linked above:

  • particles of a small median diameter (as the result of settlement by all sizes of particles);
  • shallow slope and high water content (by an inability to drain through sediment packing and low porosity);
  • high sorting coefficient, low permeability and generally low porosity (depending on compaction, but as the result of particles blocking pore spaces);
  • high organic content (as the result of organic detritus settling and being formed, by growth of heterotrophic and autotrophic micro-organisms) and thus high microbial population and high sediment stability (as the result of cohesion); high carbon to nitrogen ratio (as an addition of carbon over its degradation (Russell-Hunter, 1970); and
  • low oxygen content and therefore high reducing conditions (as the result of poor percolation of oxygenated waters together with high heterotrophic activity degrading organic matter).

The characteristic features that define the substratum in exposed sandflats and subtidal mobile sandbanks (areas of high energy) are summarised in the second figure linked above. The main substratum features which are common to these biotope complexes are:

  • particles of a high median diameter with a low sorting coefficient, high permeability and generally high porosity (depending on compaction) and low sediment stability; and
  • low organic content; high oxygen content and therefore low reducing conditions; low carbon to nitrogen ratio and hence small microbial population.

The main features which distinguish these biotopes are aerial exposure, interstitial water movement and the presence and movement of the water table. Although, as indicated above, the biotope complexes share many of the main environmental features in being physically controlled, they differ in the central aspect that subtidal sand banks are highly dynamic and unstable and by definition always have a predominantly sandy substratum, a high median particle diameter and low proportion of silt and clay material. In contrast, the intertidal sand and mud flats have varying amounts of silt, clay and organic material and are generally more stable.

Sediment attributes of intertidal mudflats and intertidal and subtidal sands

 

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