The intertidal zone (also called the littoral zone) is the area that is exposed to the air at low tide and underwater at high tide, in other words the area between the low and high tide lines. 

The intertidal zone can consist of many different types of habitats, including rocky cliffs, sandy beaches, mudflats, and saltmarshes. The coastline along the Gwent Levels is mostly saltmarsh and mudflats, with some sections of rocky cliffs around Sudbrook and areas of sand further out in the estuary.

Depending on the tidal range, the intertidal zone can be very narrow, only a few metres, or 100s of metres wide. The Severn Estuary has one of the highest tidal ranges in the world, and so has a huge intertidal area stretching 100s of metres from the shore.

During low tide, around 20,300 hectares of mudflats, sandflats and saltmarsh are exposed, the fourth largest area in a UK estuary.

Harsh environment

The intertidal zone is an extreme environment because it constantly experiences drastic changes. Not only does the water keep rising and falling, but salinity and temperature can fluctuate widely. Strong currents, stormy weather, and silt-laden water add to the difficulties of living here.

One of the primary factors influencing the intertidal zone is the tides, which create constant changes in the environment. As a result, the intertidal zone experiences two different states: one at low tide when it is exposed to the air and the other at high tide when it is submerged in seawater.

Temperature in the intertidal zone varies significantly depending on the time of day and the status of the tide. When exposed to the air during low tide, the temperature may rise or drop dramatically. On the other hand, during high tide, the temperature is relatively stable due to the buffering effect of the seawater.

Wave action can provide benefits such as increased oxygenation and the dispersion of nutrients, but they can also pose challenges such as exposure to strong wave forces and the potential for dislodgment.

Salinity levels in the intertidal zone can also vary, depending on factors like rainfall, evaporation, and the presence of freshwater sources.

Organisms living in this zone must possess the necessary adaptations to survive these fluctuating conditions. For example, some species can regulate their internal salt concentrations while others can withstand significant shifts in external salinity.

Each species found on the intertidal zone has a tolerance level for how long they can survive out of the water This determines where on the shore you might find them. This is called zonation and affects plants and animals.

The harsh estuarine conditions limit the range of species that can live in the intertidal zone. However, those that can survive occur in huge numbers, particularly on more stable mudflats. Sections of rockyshore are usually more diverse, with sheltered rockpools  and seaweed providing a more stable habitat.

Biodiversity

The intertidal zone is a hugely important part of our coast’s ecosystem, providing homes and food for species above and below the water.

Mudflats support dense populations of invertebrates, including worms such as lugworm (Arenicola marina) and molluscs, such as Laver spire shell (Peringia ulvae) and Baltic tellin (Macoma baltica).

These invertebrates provide a food supply for large numbers of migratory birds, for several of which the Severn Estuary is of international importance. 

At low tide the mudflats are used by large numbers of these birds. Dunlin are the most numerous, with flocks of 2,000 frequently feeding here. Other wader species present in significant numbers include curlew, oystercatcher, redshank and whimbrel, whilst shelduck also feed on the exposed mud.

Over the course of the year, the range of species changes, with Spring migrants visiting in April and May, Autumn migrants between August and October and Winter visitors between November and March.

Nutrient Cycling

Intertidal zones play a crucial role in nutrient cycling and overall productivity in coastal ecosystems. As tides rise and fall, they bring with them an influx of nutrients, which fuels the growth of various marine plants.

Microscopic organisms like phytoplankton and larger plants, such as seaweeds, form the base of complex food webs. These primary producers are essential for supporting the numerous herbivores, like sea urchins and periwinkles.

In turn, these grazers provide food for larger predators, such as birds. This cascade of energy transfer allows the intertidal zone to sustain a high level of productivity and biodiversity.

Zonation

The intertidal region can be divided into three zones (low, middle, and high), based on the overall average exposure of the zone. 

• The low intertidal zone, which borders on the shallow subtidal zone, is only exposed to air at the lowest of low tides and is primarily marine in character.

• The mid intertidal zone is regularly exposed and submerged by average tides.

• The high intertidal zone is only covered by the highest of the high tides and spends much of its time as terrestrial habitat.

Above the high intertidal zone is the splash zone.

Archaeology

The saltmarshes and mudflats along the Severn Estuary contain some remarkable archaeology.

The same process that makes these habitats excellent at storing carbon (water-logged and low-oxygen sediments) also helps preserve artefacts in the sediment. Organic material, such as wood, is often well preserved in the anoxic saltmarsh sediments.

Below the present saltmarsh are a number of ancient landscapes dating back over 12,000 years to the end of the last Ice Age. Over the last few decades, many spectacular finds have been made, including human and animal footprints, fishtraps, encampments, dwellings, boats and trackways.