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Tidal flats: Amphibious habitat on soft ground

Wherever tidal variations in sea level are greater than a metre, a broad margin develops on flat coasts, which is alternately submerged under seawater for hours at a time and then left to dry for a similar period. Because the water is only shallow, wave action is less of a factor in shaping the sea floor than the incoming and outgoing tides. The sea washes in sediments, sand, plankton and nutrients, which are deposited when the water flow loses momentum. These amphibious habitats, known as tidal flats, harbour an astonishing diversity of plant and animal organisms. They enrich the soil with organic material and are part of a complicated food web.

 

Some 2,300 or so species live in the shoreline and salt marsh zones influenced by salt water.
Organism groups and their species counts in the constantly and regularly flooded areas of the North Sea-Wadden Sea. (after Wolff, Heydemann 1998)

In the earth's different climatic regions, different types of tidal flat are formed. For example, on the east coast of Tierra del Fuego, persistent strong west winds can keep the sea off the tidal flats for long periods, drying them out completely. This causes groups of organisms to decline, even though they are found in areas with similar climates elsewhere. In tropical regions in particular, the tidal flats are colonised by mangrove trees. In temperate latitudes, only salt-resistant grasses and forbs are capable of encroaching onto the higher levels of the tidal flats.

 

Europe's biodiversity hotspot – the Wadden Sea

The European Wadden Sea, in the temperate climate zone along the coasts of the Netherlands, Germany and Denmark, has been studied intensively. No comparable habitat in Europe covers anywhere near the same area (ca. 5.000 km²), and no ecosystem in Europe has a similar species inventory.

 

The grey or grey-brown soil is a mixture of sand, silt, clay particles, mussel shell and fine organic matter in varying proportions. Some 2,500 marine species live in the tidal zone and the constantly flowing waters of the natural channels and tidal inlets, with a further 2,300 or so species occurring in the shoreline and salt marsh zones influenced by salt water. In addition, over the course of the year but mainly between May and September, 10 to 12 million birds head for the European Wadden Sea in search of food, on migration routes which in some cases span the globe.

 

In the aquatic zone, the Wadden Sea is the habitat area for around 60 percent of the organism species in the entire North Sea – although it accounts for no more than 2 percent of its area. Almost the same number of species are found in the salt marshes above the mean high water line. These flood only occasionally, usually during storms.

 

In comparison to the North Sea, the Wadden Sea as a whole has 25 to 30 times the species density. Nevertheless, not one of these species occurs throughout the Wadden Sea. They only inhabit specific areas – small ecological sectors or niches – and even these are only appropriate for individual phases in their life cycles. The more specialised the habitat, the more the organisms have had to adapt their survival strategies in special ways. One type of strategy is based on migration, arriving for the summer and departing to spend the winter elsewhere. This applies to the majority of bird species and fish, and to seals and larger crustaceans. As a result, there is a close interdependency between the North Sea and the Wadden Sea. The latter is not only vital to the species that spend their immature phases in its shallow tidal zone, but also performs a special ecological function extending far beyond its boundaries.

 

Flora and fauna

The species diversity of many tidal flats results above all from their mosaic-like mix of many discrete habitats (biotope types such as clean sand flats, mudflats and muddy sands). These biotope types are relatively species-rich when they consist of different microhabitats or habitat types, such as sea grass beds, blue mussel banks or zones of taller growing algae.

 

Most parts of the open tidal flats appear free of any vegetation to the naked eye. Only under the microscope is it possible to make out the dense layers of silica algae (diatoms) on the usually brown-coloured soil surface. These microscopic algae are the Wadden Sea's main producers. They can reach densities of up to 3 million diatoms per square centimetre, composed of up to 40 different species.

 

In contrast, sea grass and larger algae species such as sea lettuce (Ulva spec.), and brown algae of the genus Fucus, make a lesser contribution to production by plants in the Wadden Sea. Inputs of organic matter come predominantly from the open North Sea, and lay the foundation for the diversity and high density of the fauna.

 

The zooplankton is not as species-rich as in the open sea, but nevertheless, given the special characteristics of the Wadden Sea, it plays an essential biological role. The adult stages of numerous species live on the bottom, so they classify as bottom fauna.

 

In the soft, nutrient-rich ground, biotic communities of astonishing diversity have developed. Depending on the level and particle size of the sediment, these benthic communities are characterised by the varied types of colony formed by the numerous sessile or relatively immobile animals. Some live as epifauna, firmly attached to a surface or freely mobile. Others are infauna, burrowing into the ground or boring tubes and hollows. In the process, certain species can reach astonishing densities: Take the small mud snail: up to 100,000 individuals per square metre of mudflat have been counted. A square metre of muddy sand can harbour as many as 2,000 common cockles. Some 300 grams of animal biomass are contained in one square metre of tidal flat soil, which equates to three tonnes per hectare – ten times the level found elsewhere in the North Sea.

 

To make sufficient space for such an abundance of life, the animals adopt multi-tiered living arrangements. On the surface of tidal flats, for example, mussels congregate in large banks. The 10-cm substrate below is preferred by the cockle, the Baltic tellin, the Caspian mud shrimp, the estuary ragworm and the mud snail. Sand gapers, sand masons and lugworms are found even deeper in the soft ground.

 

Numerous bird species take advantage of this natural abundance. For around half of the Wadden Sea's nearly 100 bird species, this coastal zone is critically important. This place alone provides the necessary conditions for the survival of substantial sections of their total populations, for a certain period of the year. For the majority of the bird species, particularly migratory types, the Wadden Sea performs a key function, providing them with breeding, feeding, resting and moulting grounds. The catchment area of these species ranges from Alaska to the Taymir peninsula in northern Siberia – an area 1,000 times the size of the Wadden Sea itself.

 

Somewhat more than 100 fish species have been recorded in the Wadden Sea, but only eight are classified as sedentary fish that remain here throughout their life cycles. A further group leaves the Wadden Sea to spawn elsewhere, or to escape extremes of temperature in summer or winter. Others, like herring and plaice, come to the tidal flats specifically to spawn. More than half of the species are only occasional visitors to the coastal zone, however.

 

Threats

The Wadden Sea is subject to considerable human influence. Influxes of nutrients and pollutants, oil and gas extraction, dredging for raw materials and to enlarge shipping channels, coastal impounding works and tourism cause localised or large-scale impacts.

 

Due to more intensive farming, influxes of nutrients are also on the increase, with all the consequences of eutrophication (excessive fertilizer levels), such as more widespread algae blooms. Even despite greater efforts to purify wastewater and reduce the discharge of industrial wastewater into rivers, contamination by pollutants from the water catchment area is problematic. Numerous chemical compounds and heavy metals are adsorbed to suspended matter and, in the long term, deposited in sediments. Under certain conditions these can be mobilised again.

 

In the National Parks, which have existed since the 1980s, traditional forms of use such as crab and mussel collecting are also permitted. After all, Europe's Wadden Sea region on the North Sea has also been a cultural region for centuries, with the attendant conflicts of such use.