Ecosystems

An ecosystem refers to the collection of biotic (plants, animals and other organisms) and abiotic components and processes that can be considered an entity (system). The North Sea and the Wadden Sea are considered two different ecosystems. The coastal landscape (beaches and dunes) is considered another ecosystem.

Ecosystems can be distinguished at different geographical levels. A table with an explanation of many used terms can be found by the summaries of biotope and habitats (ecosystems on regional and local scale).

The ecosystemsof the Wadden Sea

The ecosystem of the Wadden Sea consists of a coastal sea with channels and mud flats. The channels are formed by the tidal action; the banks are flooded during high tide. The coastal sea is surrounded by marshes on its inland side. The dominant process is the tide. The ecological dynamics of the area is high. This high dynamic is caused by a great turbulence, strong temperature changes and salt gradients. Even the bottom is exposed to high dynamics. Sand and clay is deposited in some places (sedimentation) and eroded away at other places (erosion).

The diatom communities on the mud flats, the existence of eelgrass fields and rich benthic fauna are characteristic of this ecosystem. The system is important for many species of fish which grow up in this area. For a large part, waders and seals are also dependent upon this ecosystem.

In some parts of the wadden region, the influence of the flowing river is also noticeable, creating a brackish water environment. The Dollard is an example of this. You could say these areas are a separate ecosystem. The Zuiderzee used to be such an area. The fresh water flowing in produces a gradual transition from fresh to saline (fresh - saline gradient). This transition is very important for a number of organisms. The greylag goose and the sedge warbler need brackish water marshes to breed. Fish such as eel, sturgeon and salmon use the gradient for orientation during migration. Flounder is a typical brackish water fish.

The ecosystem of the North Sea

The North Sea is considered one ecosystem. This ecosystem is split into five zones. The coast forms the border; behind this is the sea bottom with numerous banks and ridges. The differences in water depths and types of bottom are the criteria for subdividing the zones.

Silty bottoms

The first zone is the deep sedimentation zone. This zone is deeper than 40 meters. The bottom consists of silt and fine-grained sand. The low ecological dynamics give a rich assortment in species. It is an important spawning ground for herring. The deep sedimentation zone is found north of 54°N (with the exception of Doggers Bank).

The second zone, the front zone, has a depth of thirty to 40 meters. This zone forms the border between two different water masses. Due to a difference in temperature or salinity, the two masses of water mix poorly. The place where this separation occurs is called a front. The bottom consists of silty sand, silt and clay. The zone forms the transition area between the erosion area in the south and the sedimentation area in the north. The Frisian Front lies in this zone. A lot of nutrients are deposited here whereby a rich benthic fauna is able to develop. Therefore, the zone is also very important for marine mammals and seabirds.

Sandy bottoms

The areas no deeper than 20 meters and with a sandy bottom are considered the shallow erosion zones. Dogger's Bank and the southern North Sea fall under these zones. In the winter, many auk species and sea ducks are found here. The area is also a spawning ground for smelt, plaice and herring.

The shallow sandy coast is also a zone. This zone is no deeper than 30 meters and the bottom consists of sand waves and sandbanks. Every once in awhile, a fresh surface layer forms in the coastal water due to the flowing rivers. The zone plays an important role as nursery for many fish species.

Gravel beds

The shallow gravel zone is the fifth zone and consists of the shallow parts of the sea with a gravelly bottom, coarse sand or boulder clay. It is important for shellfish with a long lifespan, such as quahog. The Cleaver Bank lies in a shallow gravel zone.

Gravel beds are leftovers from river beds during the time when the North Sea lay dry. The flora and fauna which grow either on or in the gravel are different than that growing on the surrounding sandy or silty bottoms. They are often typical types for a hard bottom, such as sea anemones and polyps. The worm fauna is different from that of soft-bottoms in the area and shellfish such as the quahog are numerous and heavy in weight. Snails, starfish, sea cucumbers, sea urchins, as well as fish such as lesser sandeel are present in large numbers, in and above the gravelly bottom. Herring use some gravel beds as spawning grounds.

The Cleaver Bank in the Netherlands part of the North Sea is a shallow gravel zone, located 200 kilometers northwest of Den Helder. This bank offers a hard underground and therefore has unique ecosystem with a high biodiversity, for the Dutch North Sea. Coldwater corals, unusual sea anemones and cuttlefish can be found here. In 2001, the Ministry for Traffic and Water wanted to allow a limited amount of ecologically responsible exploitation of sand and gravel on the Cleaver Bank. Since it would cause a large amount of damage to the ecosystem and the amount that could be exploited would only be sufficient for a year's need, environmental organizations such as the North Sea Foundation protested. And had success. In August 2003, an environmental effect report concluded that a remnant layer had to remain, which would decrease the exploitable amount even more. The government eventually decided against exploitation.

The ecosystem of the coasts

This ecosystem is very diverse. It contains many elements: beach, dune grasslands, open dune vegetation, lakes, creeks, wet valleys, brushwood, forests, salt marshes and tidal flats. The characteristic processes within all the elements are dune formation, vegetation development and an undisturbed hydrology. Since many elements are involved in the ecosystem, the variety in species is also great. These elements are further discussed in the volume on 'landscape'

Weblinks

Collection of links about marine ecosystems
http://ioc.unesco.org/iyo/classroom/marine_ecosytem.htm

Site about large ecosystems
http://www.edc.uri.edu/lme/intro.htm

Source: de Vleet, Ecomare