The Tide

All areas adjacent to the open sea experience some degree of tidal movement, that is , the regular daily or half daily alternation of high and low water levels. This change in level can vary from as much as 15 meters (in the Bay of Fundy, Nova Scotia) to a few centimetres in the Mediterranean.

The main causes of the tide are the interaction of the gravitational attraction of the moon and the sun acting on the earths waters, together with the effects of the rotation of the earth round the sun, the moon round the earth and the daily rotation of the earth. The greatest effect is that of the moon where the combined gravitational and centrifugal effects cause a high tide directed towards the moon and another high tide on the opposite side of the earth directed away from the moon. (Remember - the moon does not revolve round the earth but rather the earth and the moon revolve round a common point.) In the same way, the sun produces opposed high and low tides.

When the sun, the earth and the moon are in an approximately straight line as they are at full moon and at new moon, these tides reinforce each other and the extremely high and low tides known as spring tides. Half way between these times, at the first and third quarters, the sun and moon form a right angle and the tides tend to cancel each other. The smaller tides which result are called the neap tides. The cancelling effect is not complete because the tidal effect of the moon is double that of the sun. The exceptionally high 'king tides' are brought about by the elliptical nature of the orbit of the moon, which means that the distance between the moon and the earth varies and thus the gravitational force causing the tide changes.

In most parts of the earth there are two high tides and two low tides every lunar day of about 25 hours. The size of these tides can be the same or they can vary greatly, even to the extent that there is effectively one high and one low tide a day (eg Fremantle WA). In most areas -including Sydney - there is a marked variation between successive highs and between successive lows.

Local effects including the natural frequency of water movements in enclosed bays and seas cause the exceptionally large tides observer in some parts of the world. Although the causes of the tides are fairly well understood, it is not possible to predict tides on an a priori basis. Tide tables are produced by extrapolation from local observations.

Other effects which can cause large though short lived variations in sea level are changes in barometric pressure such as cyclones (up to 50 cm), wind surges caused by friction between strong winds and the sea surface (up to 4 metres) and tsunamis or 'tidal waves' caused by earthquakes which can be very large.

Although mangroves are effected by all of these phenomena, is the regular tidal effects and especially the mean sea level and the mean high water spring tide level which determine the extent of the shore which mangroves can colonise.

The mangrove and salt marsh areas are covered by salt water for some time every (lunar) month. The number, duration and depth of coverings bty the tide depend on the elevation (above mean sea level) of the particular area.