The Saltmarsh Environment

Introduction.

The saltmarsh environment is an extremely harsh one which includes abiotic factors such as high (variable) salinity in soil solution; essential nutrient ions present as a low proportion of the total present in soil solution; anaerobic soil and sulphide toxicity; temperature shock on immersion; changes to photoperiod; scour and drag (that famous music hall double act) and burial.

Consequently saltmarshes are generally species poor but biodiversity increases with height.

Plants that complete their life cycle in saline environments are referred to as halophytes, there are approximately 45 species in Britain in 12 families including 10 Poaceae (grasses), 7 Chenopodiaceae (goosefoots) and 6 Cyperaceae (sedges). There are also ‘normal’ plants (Glycophytes or Mesophytes), which have genetic variants that are adapted to live on saltmarshes.

Summary of factors affecting saltmarsh species.

A.        Salinity.

Salinity varies with the ebb and flow of the tide and with input from freshwater sources. Actual amounts of salt vary daily, seasonally and with the exact nature of the site. The relationship of salt concentration in the soil with height up the saltmarsh is not a simple one. Although the bottom of the marsh receives sea water more often salt will be flushed through the system fairly regularly, in the middle marsh although salt input is less frequent the ground dries out (especially in summer months) between floodings and the salt concentration rises accordingly. At the top of the marsh flooding with saltwater is rare so even though the ground is dry salt concentration still drops off until eventually reaching normal terrestrial levels at around or just above extreme high water of spring tides (EHWS).

Salt in the environment is a problem for organisms for a number of reasons

1. direct toxicity of Na & Cl

2. interference with uptake of essential ions

3. impact on osmotic balance

 

If plants are to avoid water loss to the environment by osmosis they must increase solutes in plant cells to compensate by

1. synthesis of organic solutes e.g. proline and other amino-acids

2. uptake of inorganic salts from the external environment (usually Na and Cl)

3. dehydration tolerance (more common in monocotyledons).

 

The second option seems to be the favoured one as it is least metabolically expensive; plants do it anyway just more so. In general halophytes’ low water potentials are generated by ion accumulation but the ions are probably sequestered in vacuoles and are balanced by organic solutes in the cytoplasm. Many mechanisms exist but they all seem to separate enzyme systems from areas of high salt concentration.

BUT salt levels in shoots still need to be regulated and many cunning strategies are used such as

1. salt secretion by special glands in the leaf e.g. cord grass

2. salt leaching (accumulation of salt on the leaf surface) e.g. orache

3. removal of salt saturated organs e.g. sea purslane

4. salt retransportation (salts accumulated in aerial tissues are retransported back to the roots, via the phloem, and released) e.g. glasswort

5. accumulation of salt in hairs (removal of salts from metabolically sensitive areas into special epidermal hairs) e.g. orache

6. development of succulence (stored water within plant tissue s helps to reduce the concentration of toxic salt) e.g. seablite

 

B.         Flooding.

Under this heading there are a number of related problems as flooding affects the oxygen concentration in the substrate (with associated effects of sulphide toxicity), the quantity and quality of light reaching the plants and also causes scour and drag.

Lack of oxygen.

Roots need to respire so anaerobic substrate conditions can be fatal. Many saltmarsh plants (cord grass in particular) have air filled tissue (aerenchyma) in the roots to offset this problem. Air is conducted down to the roots from the aerial parts of the plant, it also leaks out of the roots and oxygenates the surrounding sediment.

Toxicity.

Many products of anaerobic microbial metabolism are toxic, particularly sulphides. Toxicity can be direct or indirect (as it reduces the availability of sulphate which can lead to sulphur deficiency) and can also reduce the availability of Fe, Mn, Cu, and Zn, which are all essential trace elements for enzyme production. Some plants can incorporate toxic chemicals into harmless compounds and thereby achieve sulphide tolerance.

Effects of the above on photosynthesis.

Salinity fluctuations affect plant water relations and flooding affects gas exchange, for these reasons there may be advantages to plants if they can photosynthesise when their stomata are closed (and/or when they are under water) and do so in a water efficient way. As nitrogen is used in making organic compounds that help offset osmotic problems this means that plants often achieve this at the expense of growth e.g. glasswort. A photosynthetic pathway that is nitrogen efficient would also make sense.

Several halophytes e.g. cord grass, use the C4 photosynthetic pathway. This method allows carbon to be stored for use in photosynthesis when conditions are favourable, it is a water efficient method and uses N efficiently (which frees more up for use with osmotic control). The advantages of this method decline at lower temperatures though which is probably why most British halophytes still use the C3 pathway.