Ecology of Sulawesi. Tony Whitten
Читать онлайн книгу.
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.
Figure 2.6. Physical changes across the black layer.
After Brafield 1978
Figure 2.7. Comparison between steep, coarse-grained beaches (a) and shallow, finegrained beaches (b).
After Brafield 1978
Figure 2.8. Typical limpets, a - Cellana testudinaria, b - Patella exusta, c - Patella pica, d - Cellana radiata.
The primitive gastropods known as top shells or limpets (fig. 2.8) cope with exposure in at least two ways. Some species return to a 'scar' in a rock when the tide falls and the shells grow to match this scar exactly. Others are not restricted to occupying a single home site and instead secrete a mucus sheet between the shell margin and the rock surface to reduce water loss. The effectiveness of their adhesion is soon realized when attempts are made to pry them off rocks. It is water loss, rather than temperature which is the main danger to limpets even though they are more tolerant of desiccation than most animals: they can lose about 80% of their water and still recover when water becomes available. These animals, as well as mussels, and Littomrid3 snails, can also lower their metabolic rate thereby allowing them to survive periods of exposure when the only oxygen available is in the water held within their shells (Brehaut 1982).
Animals and plants can survive short periods at high temperatures which would be lethal over a longer period. This may have a significant effect on the degree of exposure, or distance up a rocky shore, that an animal can endure (Brehaut 1982). In addition, most marine organisms are only able to tolerate very minor variations in salinity because they do not have mechanisms of regulating the salt and water balance of their body fluids except within narrow limits. The crabs and snails cope with this problem in different ways: crabs can regulate the concentration of salt in their body tissue, whereas snails are remarkably tolerant of a wide variation in the concentration of salts in their body fluids.
Figure 2.9. Percentages of total aquatic animal taxa recorded at three types of shore (omitting microscopic forms). Note that crustaceans and gastropod molluscs account for 74% of the total on the mangrove shore. Note that these data are not based on complete lists or on equally exhaustive surveys but they serve to illustrate the major differences.
Data from Berry 1972
The success of crabs and snails in the intertidal zone is illustrated by their predominance in mangrove, rocky and sand/mud environments (fig. 2.9).
MANGROVE FOREST VEGETATION
Mangrove forests would once have fringed much of the coast of Sulawesi (fig. 1.20) but major expanses of mangroves are now found in relatively few locations (fig. 2.10), the remainder having been largely felled and used for timber or fibre or converted into brackish fish and prawn ponds (p. 187). South Sulawesi has more mangrove forest than the other three provinces combined (Darsidi 1982) and small patches can be found along most shallow beaches and river mouths away from centres of human habitation.
Figure 2.10. Present distribution of mangrove forests around Sulawesi (indicated in black).
After Salm and Halim 1984
Composition