Direct effects of the tide

One of the most obvious effects of the tide is the regular replenishment of the soil water across the whole area. The soil of the salt marsh is usually quite wet. Where the soil is sandy with little organic matter, it tends to be free draining and the water table follows the tide downwards so that the surface layers are left damp but not waterlogged however where the soil is high in organic matter (mainly in the areas occupied by the mangroves) the soil is permanently waterlogged. Where waterlogging occurs, the soil rapidly becomes anoxic and takes on the typical sulphide smell of the mangrove swamp. This is a natural phenomenon due to the restriction of oxygen supply by waterlogging and the presence of bacteria which carry out anaerobic forms of respiration where sulphur is the final electron acceptor and is not due to pollution or environmental degradation.

When mangrove and other swampy soils are drained, for example, for farming or other development, the oxidation of these sulphides leads to the formation of 'acid sulphate soils' and the leaching of acid into waterways. The pH of the water can drop to between 2 and 3 causing fish kills and releasing toxic elements such as aluminium into the waterways.