Salt marshes

This article describes the habitat of the salt marshes. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the characteristics, distribution, evolution, zonation, succession, biota, threats, functioning and adaptations of the organisms that live in salt marshes.

Introduction

Salt marshes are defined as natural or semi-natural terrestrial halophytic ecosystems. It is an intertidal zone between land and the salty or brackish water. They replace mangroves in temperate and arctic regions. The dominant flora is composed of halophytic plant such as grasses, shrubs and herbs. The flora is rather species poor. The sediment consists of mud and sand. The marshes are associated with mud flats. These mud flats are dominated by algae. The salt marsh meadows are continuously covered and uncovered by seawater. The drainage of this seawater is controlled by a meandering network of tidal channels. Through these channels sediments, detritus, dissolved nutrients, plankton and small fishes are flushed in and out the salt marshes.

Land of Saeftinghe - Belgium

Distribution

Salt marshes are widely distributed in estuarine systems around the world. They have a range from the Arctic region over Europe, Africa, America, Asia to the coast of Australia. The most extensive development of these salt marshes occurs where rainfall is abundant, sediments are fine-grained and the climate is moderate.

Distribution of salt marshes (green), wetlands (orange and yellow) and mangroves (pink)

Evolution

Salt marshes evolve over time from young marshes to old marshes. The young marshes consist for the largest part of low marsh cordgrass Spartina alterniflora. Nutrients are transported by tidal flooding through the tidal channels. This makes it possible for the grasses to grow thickly and abundantly what weakens the effect of waves and tidal currents so the depositional rate of mud increases. Erosion is reduced by the roots and rhizomes of the plants. At the time that the marsh surface builds up above the high water level, the high marsh plants invade, outcompete and replace the low marsh plants. When the quantity of the low and high marshes is equal, the ecosystem is in a mature stage of development. The continued deposition of mud converts most of the low marshes into high marshes. These are called the old marshes. Little water flows through the tidal channels and the marshes are elevated. At this time, streams and rivers deposit sand and mud on these high marshes and convert it into a dry land that is disconnected from ocean influences.

Requirements for development

The requirements for development of salt marshes are:

• They need fine-grained sediments.
• There may be no strong waves or tidal currents.
• They need salty conditions to grow. They are halotolerant and have adaptations to these conditions.
• They need a temperate or cool temperature. Freezing temperatures can occur, but are not damaging the plants.
• They need a wide tidal range. This is important because it limits the erosion, makes deposition of sediments possible and causes a well-marked zonation.

Zonation

Based on the topography and characteristic plant assemblages, salt marshes are classified as low and high marshes. Below the low marshes are the tidal creeks. These creeks are channels that are flushed with salty or brackish water. The low marshes are the lower limit of the marshes and extend from the mean low water tide to the high water tide. It is dominated by the smooth cordgrass Spartina alterniflora. This grass spreads rapidly. The high marshes extend from the high water tide to the highest spring tide. It is only flooded during extremely high tides and during storms. It resembles more to terrestrial than to a true marine environment. The plant community is more diverse than the low marshes. It consists of Spartina patens, the saltwort grass Salicornia species and the spike grass Distichlis spicata. Distribution, density and activity of invertebrates are mainly controlled by protection, food and frequency of tidal flooding.

Zonation of salt marshes

Succession

Succession is the serial development of different vegetation types at one place in time. The succession starts with unicellular algae such as diatoms sticking the sand together by production of mucus. This causes a brownish biofilm on the substrate.

After this stage, filamentous algae optimize the fixation of sediments. These algae are mostly blue-green algae, Cyanophyta and Chlorophyta. Small gastropods can feed and develop in huge quantities on it. Locally, the filamentous alga Vaucheria can form banquettes or elevations. Brown algae are loosely associated with this stage.

A next stage is the germination of species such as Salicornia. The seeds germinate after some desalination of the soil by rain. Between and around the glassworts takes extra sediment accumulation place. This results in elevating the substrate and makes it more stable. Other species such as Spartina maritime and Spartina townsendii compete for the same place. S. maritima is an indigenous species of continental Europe and S. townsendii is imported from the British Islands. Juncus maritimus is another species that can occur in salt marshes.

In Europe, other plants can occur. These herbs are Common Saltmarsh-grass (Puccinellia maritime), Sea Plantain (Plantago maritime), Sea Arrow-grass (Triglochin maritinum), Sea Aster (Aster tripolium), English Scurvy-grass (Cochlearia anglica), Common Sea-lavender (Limonium vulgare), Sea Purslane (Halimione portulacoides), the red alga Bostrychia scorpioides and Catenella caespitosa.

• 

  Biofilm of unicellular algae

• 

  Fixation of the sediment by blue-green and green algae

• 

  Salicornia perennis

• 

  Spartina townsendii

• 

  Triglochin maritinum

• 

  Aster tripolium

• 

  Limonium vulgare

Functioning and adaptations

Salt marshes have several functions and adaptations to a life in an intertidal ecosystem. They need to conquer some problems to be resistant to the environment. Therefore, it is important to have some adaptations to survive. The first problem is that the plants are freshwater plants. This causes waterstress. For this reason, they have to take up water against the osmotic pressure. To overcome the negative osmotic pressure, they generate a negative hydrostatic pressure (by transpiration processes). They have thin, fleshy leaves and are sensitive to extra stress such as pollution. Anatomically, the plants are adapted through strong lignification, a well-developed epidermis and succulent leaves and stems. Evaporation can be limited by thin leaves with scale-like hairs. Physiologically, plants are adapted by accumulating salt in their tissues. In this way, normal osmosis is possible. Other plants have salt gland cells on the lower surface of the leaves and excrete the salt from its tissue. A second problem is the anoxic environment. The underground in which they root is saturated with water. The tissue of the plants requires oxygen for respiration. Gas diffusion between gas particles can only supply this need in soils that are not waterlogged. Even when the water is saturated with oxygen, its concentration is too low and the diffusion in water is very slow. This is solved by aerenchyma, a tissue that can provide air to the submerged parts of the plants. Roots are superficial systems because of the anoxic sediments. These systems are composed of perennial thick roots with a corky layer and without root hairs. To fixate the substrate, shortliving thin and strongly branched roots with numerous root hairs are developed so they can take up nutrients. Salt marshes have a whole range of functions. It plays an important role as a sediment trap. In this way, it regulates the water quality and helps stabilizing the coastlines. Salt marshes function as filtering systems and retain sediments, excess nutrients, toxic chemicals and disease-causing organisms. They remove nitrates and phosphates from rivers and streams which receive waste water effluents. Another function is the recharge and discharge of groundwater attributing to the water supply. Salt marshes function also as important habitat, nursery grounds for larvae and small organisms, shelter, providing food and a nesting ground for wading birds and other organisms.

Fauna

Small mammals, small fishes, birds, insects, spiders, and marine invertebrates are found in salt marshes. Marine invertebrates include amphipods, isopods, anemones, shrimps, crabs, , turtles, mollusks and gastropods. Fishes such as sticklebacks, silversides, eels and flounders are found in the waters of the salt marshes. The salt marshes are important breeding, feeding and overwintering grounds for waterfowl. These waterfowls consist of ducks, herons, Sharptailed Sparrows, Eurasian Oystercatchers, Reed Bunting,… The benthic community consists of mollusks, polychaeta and oligochaeta. The coastal freshwater marshes are species poor. The organisms that you can find here are water boatmen, flies, mosquitos and snails. Further there are mollusks, ducks, geese, muskrats, raccoons, minks and other small mammals. For some species, a seasonal appearance can be seen. In Saudi Arabia, salt marshes are grazing places for wild dromedaries.

• 

  Eurasian oystercatcher

• 

  Water boatman

• 

  European mink

Threats

The total number and area of salt marshes is declining since a long time. The main reason for severe damage is land reclamation. This is completely eliminating the highly specialized ecosystems. Areas where the level is high enough and the area big enough can become embanked. Areas outside the reclaimed zones can produce new marshes. This is because of the continuously sedimentation that allows growth of new plants.

Other threats are:

• Over-grazing
• Embankment
• Excessive agricultural use
• Urbanization
• Recreation
• Coastal erosion
• Pollution and industrial waste water
• Dominance of Spartina anglica and Erytrigia (in eutrophic salt marshes)

Case-study: Land van Saeftinghe

The tidal area ‘Drowned land of Saeftinghe’ (Verdronken land van Saeftinghe) is located at the eastern boundary between the Netherlands and Belgium, a few kilometers downstream Antwerp in the estuary of the Scheldt. It is an official nature reserve since 1976. Because of this legal protection, permits are obligatory for every intervention and strict entrance restrictions are applied. The land is a crosspoint where the river the Scheldt meets the salty water of the North Sea in the estuary known as the Western Scheldt. In the past, the land was a very fertile polder. The area has a surface of 3484 hectares. Almost 70 % of the area is overgrown by specific plants of the salt marsh. The residual 30 % consists of mud flats, sandbanks and a network of channels. Each tide, the brackish water overflows a large part of the area. The plants there are totally adapted to this and are very unique. For huge quantities of birds, this area is an ideal breeding, rest and wintering place. This makes the area of international importance. Since 1996 it is a special protected area for birds (1979 Directive 79/409/EEC on the conservation of wild birds).

Area description of the Western Scheldt, with the Land of Saeftinghe in green

In the past, a few dikes were created to promote the silting up. The northern dike connects a few stands with the dike. These dikes are still recognizable and are used as wandering path. The stands were used as hills where shepherds retreated when the tide became too high. The flora exists of approximately 50 wild plant species. Algae are not abundant in Saeftinghe because there is too little light that penetrates in the water. Organic matter and a lot of silt make the water turbid. Higher plants are more important in this area. One of the most common plants is pickle weed (Salicornia). But other salt marsh plants such as English Scurvy-grass (Cochlearia anglica) and Common Sea-lavender (Limonium vulgare).

References

The main author of this article is TÖPKE, Katrien Please note that others may also have edited the contents of this article. Citation: TÖPKE, Katrien (2008): Salt marshes. Available from http://www.coastalwiki.org/wiki/Salt_marshes [accessed on 21-11-2024] For other articles by this author see Category:Articles by TÖPKE, Katrien For an overview of contributions by this author see Special:Contributions/Ktopke