Anon. 1982aChapter Two
Seashores
INTRODUCTION
This chapter is concerned with those habitats that lie between the reach of the lowest low and highest high tides.1 Thus all these areas are inundated by the sea at some time. Habitats such as seagrass meadows and coral reefs that are found below the level of the lowest low tide, are discussed in chapter 3.
Sulawesi has proportionately more coastline relative to its land area than any other Indonesian island, because of its long, narrow peninsulas (table 2.1). No point on the mainland is more than 90 km from the sea and most locations are within 50 km. In addition there are over 110 offshore islands each with an area in excess of 1.5 km2 within the administrative areas of the four provinces. Coastal ecosystems are of great economic and ecological importance for fisheries and other commercial activities, and those concerned with development on Sulawesi should, therefore, have a grounding in understanding the components, interactions, and mechanisms of coastal ecosystems to ensure that these resources are managed for sustainable production and benefit.
PHYSICAL CONDITIONS
Tides
The process that largely determines the characteristic features of the seashore is the ebb and flow of the tides. The tides around most of Sulawesi are termed mixed prevailing semi-diurnal. This means that each day two high and two low tides occur and that the successive tides are different in height and duration. Around the southwest peninsula the tides are termed mixed prevailing diurnal. This means that each day only one high and low tide occur and that successive tides are different in height and duration (fig. 2.1). In narrow straits and bays, such as the Gulf of Bone (Anon. 1980a), however, the patterns may become rather more complicated.
Figure 2.1. The forms of a typical mixed, prevailing semidiurnal tide (above), and a typical mixed, prevailing diurnal tide (below) over the period of a month.
After Pethick 1984
Tides are enormous waves with wavelengths of half the circumference of the earth. These 'waves' are primarily the result of the gravitational pull of the moon which acts not only on the water closest to it, but also on the mass of the orbiting earth itself, thereby pulling the earth away from the water on the opposite side.2 This is similar to pulling someone towards you by one arm, and seeing his other arm move away from his body.
The sun also has an effect on tides but although it has 27 million times 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
