Sediment attributes of intertidal mudflats and intertidal and subtidal
Porosity, permeability, water content and sediment
Organic content, oxygen content, microbial activity and
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 <63Ám) 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 (<4Ám diameter) together with iron and other
impurities. Particles <2Ám 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).
Intertidal and subtidal sands
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, permeability, water content and sediment
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).
Intertidal and subtidal sands
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).
Organic content, oxygen content, microbial activity
and carbon/nitrogen ratio
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
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.
Sandflats and subtidal mobile sands
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.