| THE SEASHORE ENVIRONMENT |
The
seashore environment is a very formidable one. During high tide, the shore
is covered by seawater, and is part of the sea. When the tide recedes however,
the shore becomes exposed, and for several hours, is reclaimed by the land. The
shore (or littoral/intertidal zone) is the region between the highest level
of high water spring tides (HHWST) and the lowest level of the low water
spring tides (LLWST). The area above the HHWST is termed the supralittoral
zone and that below the LLWST is the subtidal or sublittoral zone.Spring tides occur once a fortnight during the new and full phases of the moon, when it exerts its greatest gravitational forces on the sea. In between these maximal and minimal tides, the high and low waters are referred to as high water neap tides (HWNT) and low water neap tides (LWNT) respectively. The mean tidal level is simply the average of the high and low tides over a long period of time. The difference in height between the low water and high water levels is known as the tidal range. The largest range occurs during spring tides. Singapore shores generally experience maximal tidal ranges of about 3.7 m (12 feet). Any organism inhabiting the intertidal zone must be able to tolerate the extremes of environmental conditions that occur between tidal cycles. On the average, Singapore shores experience two flow (high) and two ebb (low) tides every 24 hours and 51 minutes. Intertidal animals and plants are thus subjected to an alternation of exposure to the atmosphere and the sea. This raises many problems other than the basic ones of food and shelter for their survival. Each organism has evolved its own special adaptations to overcome these challenges. Its shortcomings are in turn, reflected in the habitat it eventually chooses. In addition to all this, man-made activities continue to threaten to pollute the environment. The composition of animal and plant communities on the shore is therefore determined by various factors such as the nature of the shore substrate, the length of time a particular shore area is exposed to the air and heat, the degree of dilution of seawater by rain and land drainage etc. Most of the resident intertidal shore creatures are found in the lower half of the intertidal zone. There is little animal life in the upper third of the littoral zone. The number of organisms increasing as one progresses clown to the subtidal zone. Burrowing animals can be found below the surface on a sand/mud shore if there is sufficient moisture or interstitial water (i.e., water trapped between the particles). Often, during low tide, when the substrate dries up under the hot sun, they burrow even deeper to escape the crippling effects of desiccation. Hence, to the casual observer, the shoreline may appear barren or impoverished. Unless the beachcomber is familiar with the nomenclature of the shore profile, it would be futile to describe the zonation in terms of what constitutes the upper and lower littoral zones. For this reason, we have dispensed with a classification of the seashore. |
Introduction Shore environment |
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From A Guide to Seashore Life by Dr Leo W H Tan and Peter K L Ng Published by the Singapore Science Centre and sponsored by BP @Raffles Museum of Biodiversity Research and Singapore Science Centre |
