Particle size

Porosity, permeability, water content and sediment stability

Organic content, oxygen content, microbial activity and carbon/nitrogen ratio

The sediment provides two fundamental resources, space and food, required by the predominantly infaunal organisms which characterise these biotope complexes. It provides the 3-dimensional space for colonisation and the food source for the predominantly deposit and detritus feeders characteristic of the sedimentary communities.

All mud contains some sand hence it has a high sorting coefficient, but the most important features are the high silt and clay content (%S&C, particles <63µm) and thus low median particle diameter (MPD) and deposits >80%S&C are described as mud (Dyer, 1979). Silts are very fine inorganic particles, which are usually held in suspension by slight water movement at the sediment surface. In contrast, clay is mostly colloids of hydrated aluminium silicate (<4µm diameter) together with iron and other impurities. Particles <2µm in diameter are mainly the clay minerals illite, kaolinite and montmorillonite whose flocculation is dependent on salinity (Whitehouse et al, 1960) and turbulence (Kirby & Parker, 1974).

Particle sizes of sands range from coarse sands (0.5-1mm), medium (0.25-0.5mm) and fine sands (0.063-0.25mm), with the substratum structure being rarely homogenous and having a low sorting coefficient, i.e., it is often well sorted. The sorting will be the result of the prevailing hydrodynamic regime including long-shore drift and coastal gyres, in the case of intertidal sediments, and headland gyres in the case of subtidal sandbanks. The sand grains of beaches and subtidal sandbanks are usually quartz (silica) particles derived from erosion (Gray, 1981). Sediments containing >10 %S&C are commonly termed ‘muddy sands’ but ‘sandy mud’ if >30%S&C (Dyer, 1979).

Porosity denotes the amount of pore space in a sediment whereas permeability is the water flow through it. Particle size, its mixture and compaction influence the permeability or percolation rate (Pethick, 1984) especially those with a mixture of particles, i.e. low porosity and permeability in fine grained sediment and vice versa for sands. Porosities in different-sized material may be similar (Taylor et al, 1966) due to interaction between grain shape, the degree of sorting, the length of time since deposition and therefore the degree of settling and compaction.

Clays can have porosities ranging from 65-82% and silts 45-88% (Taylor et al, 1966). However, in extreme cases a mud flat which is composed largely of clay can become sufficiently compacted to supporting sessile fauna and even rock borers such as the burrowing bivalve Pholas (Eltringham, 1971). In contrast, fine and very fine sands have porosities ranging from 40-50% and medium sands 37-42% (Taylor et al, 1966) again depending on compaction coupled with the mixture of particles.

Sediment particles consolidate in low energy environment, e.g. in the middle estuarine mudflat areas of low energy and hence less vigorous mixing. In such cases the weight of overlying sediment forces out pore water and the floc structure collapses (Parthenaides, 1965). The consequence of the rearrangement of sediment particles gives the mudflat increased shear strength and thus resistance to re-erosion.

The water content of mud and sandflats is influenced by the porosity and compaction of the sediment, the shore slope and the potential for draining. Mud and sandflats may be extensive yet retain water at low tide as the result of their a shallow gradient and the capillary attraction of closely packed particles (Gray, 1981). The sediments may be thixotropic due the high water content (Chapman, 1949), thus allowing easier burrowing by infauna applying pressure to the sediment which becomes softer and easier to penetrate.

Silt and clay is more cohesive and when mixed with sand creates a more stable sediment. In the case of intertidal sediments, strong shoreward wave velocities move coarse sediments as bedload (under saltation) and fine particles as a suspension whereas weaker offshore velocities move only the finer bedload and suspended material (McCave, 1979, Buller & McManus, 1979).

The permanent water content in an intertidal sand flat may be low as the interstices between the particles are filled with water which drains during exposure although draining is inversely related to organic and silt content. In water-logged sands, for example subtidal sand banks, particles are prevented from abrasion by a film of water surrounding them. The ease with which infauna can burrow depends upon the amount of water present, for example, dilatant sands (which have a low water content) are difficult to penetrate as the application of pressure causes them to harden.

In contrast to mudflats, both intertidal and subtidal sands are extremely unstable as the predominant material is unable to form cohesive clumps. This instability prevents the colonisation of vegetation but allows the development of interstitial populations of organisms. Most coastal sediments lack cohesion and have solid particles, i.e. not flocculates, usually greater than 0.06mm in diameter, which are held together by gravitational forces (Pethick, 1984).

These contain a high proportion of organic matter which is deposited and accumulates in low energy areas due to its small and low specific gravity. Allochthonous organic material is derived from both anthropogenic sources (effluent, run-off) and natural sources (settlement of plankton, detritus). Autochthonous organic material on these sedimentary areas is restricted to benthic microalgae (microphytobenthos) such as diatoms and euglenoids and heterotrophic micro-organism production although mats of opportunistic green macroalgae such as Enteromorpha and Ulva will also develop (see Chapter III). The organic matter (measured as organic carbon or nitrogen) is degraded by the micro-organisms and the nutrients recycled (Newell, 1965; Trimmer et al, 1998). In addition, the high surface area to volume ratio of fine particles acts as a surface for the development of microfloral populations. These features, coupled with poor oxygenation of muds and hence low degradation rates, lead to an accumulation of organic matter.

Oxygen content is a function of the degree of oxygenation (aeration) and the inherent oxygen demand of organic matter. Fine sands and muds tend to have lower oxygen levels because their lower permeability leads to the trapping of detritus which, together with the large surface area for microbial colonisation, leads to higher oxygen uptake (Eagle, 1983). Much of the organic detritus therefore undergoes anaerobic degradation, with hydrogen sulphide, methane or ammonia produced, as well as dissolved organic carbon compounds which can be utilised by aerobic micro-organisms living on the surface (McLusky, 1989; Libes, 1992). These features produce a reducing layer (indicted by the redox potential discontinuity layer, RPD) very close (often <1cm) to the surface. In mudflats the carbon to nitrogen ratio is high due to high productivity and microbial activity in these areas (McLusky, 1989; Russell-Hunter, 1970). The C:N ratio reflects a high build up of labile organic matter in relation to a lower degradation rate, as shown by large micro-organism populations.

Microbial activity is high in muds which contain a large amount of detritus and microbes although at depth that bacterial activity will be chemosynthetic (Libes, 1992). Microbial activity has a valuable role in stabilising estuarine organic fluxes by reducing the seasonal variation in primary production, ensuring a relatively more-constant food supply, and allowing the reabsorption of dissolved nutrients (Robertson, 1988). The bacteria living on particulate or dissolved organic matter makes the primary production more readily available for animal consumption (McLusky, 1989). It has been calculated that the biomass of bacteria within mudflats may be of the same order of magnitude as the biomass of animals living in the sediment. Breakdown of organic matter to sulphides and sulphates by bacteria forms the sulphur cycle which determines the redox potential and pH of the sediment.

These have low levels of organic matter and are well oxygenated in the surface layers (Eagle, 1973) with the detritus derived from decaying seaweed, the faeces and remains of animals, and terrigenous sources (as wind blown material). Sands are usually sufficiently oxygenated by seawater which, at high tide, percolates from a few mm in fine, sheltered sandflats to several metres in coarse sand (Eagle, 1983). Interstitial oxygenation, may be poor below the surface layer particularly where the sand is fine or mixed and thus poorly drained or in cases of high concentrations of organic material such as decaying seaweed on the strand line (Hayward, 1994). In cases where the sublittoral sand banks are created at the centre of gyres, they will receive and concentrate organic materials and debris. However, the mobile and unconsolidated nature of the sediments will produce a high oxygenation and thus high biodegradation rate.

Intertidal and subtidal sands are well-oxygenated though the tidal pumping of overlying water. Their mobile nature produces a deeper anaerobic layer (>15cm) and that any organic matter incorporated into the sediment is degraded rapidly. High energy areas have a low carbon to nitrogen ratio due to the low organic content and reduced productivity and the rapid degradation of labile organic material. Microbial activity is low in areas of higher energy as there is limited organic detritus available for bacterial degradation coupled with the particles’ comparatively low surface area to volume ratio providing a surface for microbial populations.

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