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.
Contents
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.
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.
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. [3]
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. [4]
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. [6]
Biofilm of unicellular algae [7]
Fixation of the sediment by blue-green and green algae [8]
Salicornia perennis [9]
Spartina townsendii [10]
Triglochin maritinum [11]
Aster tripolium [12]
Limonium vulgare [13]
References
- ↑ http://www.marbef.org
- ↑ http://cache.eb.com/eb/image?id=6576&rendTypeId=4
- ↑ Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508
- ↑ Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508
- ↑ http://geology.usgs.gov/connections/fws/landscapes/blackwater_model.htm
- ↑ Eric Coppejans – Course Biodiversity of aquatic food webs: from algae to marine mammals UGent
- ↑ Eric Coppejans
- ↑ Eric Coppejans
- ↑ Eric Coppejans
- ↑ Eric Coppejans
- ↑ Eric Coppejans
- ↑ Eric Coppejans
- ↑ Eric Coppejans
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