Ecology of Sulawesi. Tony Whitten
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the mass of the moon, it has less than half of the moon's gravitational pull because it is 389 times as distant. Tides are greatest when the sun, moon and earth are in a straight line (i.e., on days of the full and new moon). These large tides are called 'spring tides'. At the half-moons, when the sun, earth and moon form a right-angled triangle the combined forces are least. These small tides are called 'neap tides' (fig. 2.2). The moon rises about 50 minutes later ever)' day so that successive high tides are 25 minutes later each day.
Figure 2.2. The tidal cycle.
After Pethick 1984
The arc traced by the sun changes throughout the year, being directly above the equator and in a straight line with the moon on the equinoxes: March 21 and September 21. This produces the strongest tide-raising forces. When the sun is directly above the Tropic of Capricorn or Tropic of Cancer (23.5° S and N respectively) on June 21 and December 21—the solstices—the tides are a minimum. Towards the end of the year the sun is closest to the earth and so high tides in this period, particularly around the September equinox, are among the highest of the year. For example, the extreme tides during early October in South Sulawesi each year cause the death of coral exposed to the air, and flooding in the coastal regions.
There are further complications: the moon swings 28° north and south of the equator every month, the distance from the sun to the earth varies through the year, and the gravitational pull of the other planets also has an effect. As a result there are longer cycles of tidal behaviour including the 18-year cycle and the 1,800-year cycle. For example, tides were particularly high in the 1500s and will reach a minimum around the year 2400.
Figure 2.3. General pattern of tidal exposure up a beach.
After Brehaut 1982
The pattern of periodic inundation of the shore leads to a gradient of exposure (fig. 2.3). Thus the beach at the mid-tide level is covered for 50% of the time and exposed for 50%. At the mean highwater of neap tides, the beach is exposed for about 70% of the time. This exposure gradient determines to a large extent the occurrence of different species of animals and plants up a shore.
Surface Currents
Surface currents are relevant to the coastal regions because they carry detritus, animal larvae, etc., from or between coastal areas. During the north-westerly monsoon (approximately November to April) the currents run approximately anti-clockwise around Sulawesi. From May to November no such simple pattern can be discerned. The currents on the Sulawesi side of the Makassar Straits run southwards throughout the year, and there is also a year-long eastward current along the northern coast of North Sulawesi (fig. 2.4).
Figure 2.4. Surface currents.
After Wyrtki 1961
Salinity
As rocks weather chemically and physically, so the salts that are dissolved from them in the rain are carried by rivers, sub-surface and groundwater flows to the sea. The seas have therefore been getting saltier over time, but the rate of increase is extremely slow. The most common salt is sodium chloride.
The average level of salinity in the world's oceans is about 33.5 ppt (parts per thousand) but in coastal regions just after the onset of the wet season, the concentration falls, and the degree of variation differs between areas, being most marked off the shores of seasonal areas. Pools of seawater left on a muddy shore as the tide falls can sometimes increase their salinity to about 50 ppt as a result of evaporation, but this decreases again to 15 ppt after rain. The challenges of living in such an environment are discussed next.
For trees growing in the intertidal zone, the salinity of the tidal water is less important than the salinity of the water within the sediment where the plant roots are found. The salinity in this sediment is often less than sea water because of dilution by freshwater flowing through the soil from the land to the sea. This is an important factor in the management of mangrove forest (p. 192) and in understanding the effects of agricultural and industrial pollutants that may enter into the ground water. Some mangrove trees and other organisms are resistant to certain types of pollution, but the demise of sensitive species may upset the equilibrium of the whole system (Bunt 1980; Saenger et al. 1981).
While high concentrations of sodium and chloride ions are toxic to plants, the osmotic potential of the water is also most important as it influences the ability of a plant's roots to take up the water on which its growth depends. The osmotic pressure depends on the sediment type, being greater in fine 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