Tides are the regular rise and fall of the oceans, caused by the gravitational pull of the moon and sun on the ocean. The strength of this gravitational pull is dependent upon the location of the earth, moon and sun relative to one another and leads to a variety of tidal ranges, from 'neap' tides, where tidal range is at it's smallest, to 'king' tides where the range is at its maximum.

The difference between high tide and low tide is called the tidal range. The biggest tidal range is found in the Bay of Fundy, Canada where sea level rises and falls as much as 16m in just over 6h, while the smallest tidal ranges may be less than 1m. Minnamurra River typically experiences a tidal range of approximately 2m.

Abiotic factors

The width of the shoreline strip that is affected by the tides is known as the 'intertidal zone'. This is the location which is fully inundated by water at high tide, and is fully exposed at low tide. These conditions place a number of abiotic pressures upon the plants and animals which survive in the intertidal zone including:

• Decreased soil oxygen

Typically, terrestrial plants rely upon small air pockets within the soil to allow their roots to gain oxygen for respiration. However, in the intertidal zone, these small air pockets are filled with water as the soil becomes waterlogged at high tide. Such a lack of soil oxygen is a significant hurdle which plants must be adapted to overcome if they are to survive in the intertidal zone.

• Inundation

Across the tidal range, the intertidal zone may be inundated with water for many hours at a time. In order for animals to survive, they must be able to respire underwater. Typically, animals of the intertidal zone breathe with gills, which must remain wet to allow for gas exchange.

• Desiccation

Inversely, at low tide, the sand and mud flats of the intertidal zone are exposed. During summer, they can reach extremely high temperatures and quickly dry out (desiccate). In order to survive, animals must have adaptations which maintain moisture, such as a shell or burrow.

• Increased salinity

Regular tidal inundation with brackish water leads to the intertidal zone being a highly saline (salty) environment. Plants must have adaptations and mechanisms in order to cope with such conditions in order to survive.