than in coarse-grained sediments. Fine-grained sediments are capable of withholding more pure water against gravity due to the small size of the pore spaces, and therefore their osmotic pressure is higher than in coarse sediments. If it is not practicable to measure osmotic potential, then salinity and conductivity are a good second best.
Temperature
Tropical coastal waters usually have a temperature of between 27° and 29°C but can be much warmer in shallow areas. The temperature on the surface of mudflats or rocks can be so high that it is uncomfortable to walk on them in bare feet. Inside a shady mangrove forest, however, the air and soil surface temperature is much more equitable (table 2.2).
Dissolved Oxygen and Nutrients
In general, concentrations of neither dissolved oxygen nor nutrients impose any limits on productivity in coastal environments although the concentrations vary between locations. The greatest concentration of dissolved oxygen in the coastal environment is at the water's edge where wave action constantly agitates the water. The abundance of life in most coastal environments and the general abundance of nutrients in coastal environments (except sandy shores), results in a very high biological oxygen demand and this tends to lower the concentrations of available oxygen. Thus there is a gradient of increasing nutrient concentrations and decreasing oxygen concentrations moving from the water's edge through a mangrove forest. This is a result of dilution in the greater volume of water at sea, and the greater incorporation of nutrients into the sediments in the upper tidal areas where the litter is retained for longer (p. 131) (Davie 1984).
Data from an EoS team
Sediment
It is a matter of debate whether new sediments should be termed soils but they can nevertheless be defined by standard soil classifications. Thus, in the Malangke mangrove forest area, the sediment is primarily a grey hydro-morphic alluvium but towards the terrestrial margin merges into a gley humus reflecting alternating periods of aeration and flooding. These sediments have very low fertility but a high organic content. This was highest (4% -5.8% carbon) in the drier parts of the forest where the vegetation was older and the trees faller, in the foreshore under Sonneratia alba there was much less organic matter (0.5%-2.7%). The sediments are generally acidic and this increases with depth although different regimes occur under different species. Conductivity (which is directly proportional to salinity) at Malangke was highest (5.9-6.4 mhos/cm) under Rhizophore forest somewhat inland; a situation probably related to the fact that the percentage of sand is higher near the foreshore (due to greater wave action) and this does not bind the salt (Anon. 1981a, b).
The grain size of sediments is measured by passing the substrate through a series of sieves and calculating the percentage of the total retained by each sieve. If the grains were identical and perfect spheres then 26% of the volume of the sediment would be pore spaces (i.e., a porosity of 26%), regardless of whether the grains were large or small. In nature, of course, grains are neither spherical nor packed as closely as possible, and in many cases small grains fill the spaces between large grains.
The water content of a beach sediment depends on its grain size and porosity, but not all pore spaces will always be filled with water. When the tide is out, the water table falls faster in coarse-grained sediments than in fine-grained because the average pore size is greater. These coarse sediments therefore have a higher permeability (i.e., are better drained than fine sands or muds [fig. 2.5]). In fine sand or mud beaches the water table may stay at or near the surface, even when the tide is out, due to the massive surface tension resulting from the very large surface area in a finegrained sediment, and to the low permeability.
When the pore spaces remain filled with water the sediment may become thixotropic, or liquid when agitated or subjected to pressure. As the water drains away, external pressures are met by increased resistance and the sediment may become dilatant or solid when agitated or subjected to pressure. This is why sand saturated with water feels soft and sloppy, whereas drier sand, whitens and becomes firm when walked upon. These properties are important to burrowing animals since thixotropic sediments are easily entered but burrows are hard to maintain whereas dilatant sediments are hard to burrow into but the burrows are easily maintained. Certain shorebirds follow the waters edge up and down the beach in order to remain in an optimal feeding zone (p. 149). In general, heavy wave action is associated with steeply sloping beaches and coarse-grained sediments. Whereas shores subjected to little wave action slope gently and are comprised of fine grains.
Figure 2.5. Time taken (minutes) to drain a 50 cm column of water through 10 cm of different sand mixtures. The sand mixture is assessed by the percentage of each sample passing through a 0.28 mm sieve.
After Brafield 1972
Oxygen within the Sediment
Oxygen concentrations are affected by three major factors. First, concentrations drop rapidly with depth. In a fine-grained sediment, oxygen concentration at 2 cm may be only 15% of the saturation concentration and virtually zero at 5 cm, and supersaturated in small surface puddles as a result of photosynthesis by diatom phytoplankton. Second, oxygen concentrations fall with an increase in temperature; for example, an increase in temperature from 25°-30°C reduces saturated dissolved oxygen concentrations by nearly 10%. Third, in coarse-grained sediments, which are relatively well oxygenated when the tide leaves, the concentration can fall rapidly in the first few hours after exposure due to the respiration of the animals within it. Should an oil slick drift onto the shore and settle on the sediment at low tide, oxygen cannot diffuse into the sediment pores with obvious extremely adverse effects on the animals below (Ganning et al. 1984).
Bacteria
As mentioned above, fine-grained sediments have a much larger surface area per unit volume of particles than coarse-grained ones thereby providing more attachment sites for micro-organisms such as diatoms and bacteria. Fine-grained sediments also tend to contain more organic debris and so it is not surprising that chemical processes involving bacteria are more complex and proceed faster in these than in coarse-grained sediments (Brafield 1972).
Only near the surface can the organic matter be broken down by oxidation processes. Below this zone, in an anaerobic environment at a level called the redox (reduction-oxidation) potential discontinuity layer, anaerobic bacteria break down organic matter by fermentation or reduction processes producing alcohols and fatty acids. Sulphate ions are reduced to hydrogen sulphide and much of this is fixed as iron (ferrous) sulphides which gives the sediment below this layer a black or dark grey colour (fig. 2.6).
The boundary of the black layer is found closer to the surface where the sediment is fine-grained and where organic matter content is high (i.e., mudflats in front of mangrove forest). Differences between the features of steep and shallow beaches are shown in figure 2.7.
Adaptations of the Fauna
Organisms in the intertidal area experience cycles of wetting and drying quite unlike those in any other ecosystem. Most animals of marine origin are unable to live in such extreme conditions because they quickly dry out, cannot breathe gaseous oxygen, can feed only on water-borne food, and are bound to the sea for reproduction. Two groups, however, the crabs and gastropod snails, have members which have met the challenges by having exoskeletons of impervious shell to restrict water loss, they are able to breathe gaseous oxygen, feeding on damp organic material or microorganisms, and climb into trees to find food. In addition, by fertilizing eggs internally, they can care for the young in brood pouches, or capsules, rather than having to let them take their chances among the plankton.
