Pre-marsh processes • Zonation & succession
Abiotic factors • Dynamics & decline

Abiotic Factors

Abiotic factors which influence saltmarsh development include tidal currents, vertical accretion, tidal submergence and salinity.

The effect of strong tidal currents was demonstrated in a classic study in the Dovey estuary almost 60 years ago, in which it was shown that seedling establishment of annual glasswort (Salicornia europaea) was influenced by the current strength and frequency of tidal coverage. No more than 25% of seedlings survived to adult plants on mudflats that were submerged daily, whereas 65% survived in areas which remained tide-free for periods of three or more days. The adult plants in the lower zones showed clear signs of exposure to tidal currents, their stems bent and twisted from being dragged by the tide, (see figure below).

Drawings of glasswort plants from five positions along a transect from landward (0m) to seaward in the Dovey estuary in July. Plants seaward of mean high water neap tides (150m) show the effects of exposure to tidal currents (after Adam, 1990)

The uprooting of seedlings and the tidal transport of seed has also been shown to affect Spartina plants. Transplant experiments in the Dutch Oosterschelde and in southern England indicate that well-established adult plants can withstand the increased amounts of submergence at lower elevations but that wave action on seedlings and small plants prevents the natural seaward spread of the grass. As the velocity of tidal currents is roughly a function of tidal range, the fact that saltmarshes do not extend so far seaward in estuaries with a large tidal range must, to a large measure, be because of the effects of physical forces on plants.

The rates of vertical accretion of sediments on saltmarshes depend on a range of factors but can, at least locally and in the short-term, be very high. Whilst published average rates vary between zero and around 10mm per year, accretion of 30mm, 100mm and, in one study in Bridgwater Bay , up to 150mm of silt has been recorded in a single year. These measurements were all made in seaward marshes, among glasswort or Spartina grass, and in all cases the plants appeared to survive, growing upward through the sediments. Burial by a sudden influx of sediment (often sand), particularly during the growing season, can kill plants in the landward parts of marshes where the species are more sensitive to burial. On the whole, however, high accretion rates per se are unlikely to be a limiting factor in the seaward growth of saltmarshes.

Although tidal submergence cuts down the amount of time available for photosynthesis and both reduces and alters the quality of the light reaching submerged plants, it is difficult to disentangle the effects of these factors from other aspects of flooding. We know from one experiment that Spartina can survive continuously and fully submerged in clear seawater for 4.5 months and that at its lowest limits in Poole Harbour, and in some New Zealand estuaries, it is exposed to daylight in some months for only two or three hours a day (Hubbard & Partridge, 1981).

Saltmarsh soils are more or less permanently waterlogged, at least at the lower elevations. Two major inter-related effects of this are the production of anoxic conditions and the presence of substances normally toxic to plant growth. Saltmarsh plants display adaptations to such conditions ranging from the development of air-filled spaces and pathways (aerenchyma) in the roots and rhizomes, which are aerated via the shoots, to the (at least partial) tolerance of phytotoxins such as ferric ions and sulphide compounds. However, most species are likely to require some periods of soil aeration for successful growth. Measurements of redox potential on a Humber saltmarsh have shown that, even in the lowest zones, the mainly reducing conditions are alleviated temporarily during neap tides, albeit only near the surface. Increasing periods of aeration were found with increasing elevation, and better drainage, of the marsh. In the mid to low zones redox potentials were lowered each month by the spring tides and in the higher areas the mainly oxidizing conditions were affected only by the highest spring tides, producing a general gradient in the degree of soil aeration across the marsh.

In contrast, gradients of soil salinity directly related to the amount of tidal submergence may not always be present and may even be exceptional. The interaction of tidal submergence with rainfall and evapotranspiration from the marsh surface can produce extensive and local fluctuations in soil salinity. Salinities more than three times that of seawater have been recorded during mid-summer on high saltmarshes in north Norfolk . On such marshes the salinity gradient may, therefore, be reversed during much of the growing season, the lower zones being frequently bathed in (relatively less saline) seawater.

Perhaps not surprisingly, therefore, all saltmarsh plants, whatever their distribution, exhibit salt tolerance; indeed it is their ability to tolerate saline conditions that separates them from most other vascular plants. The mechanisms they have evolved to cope with salinity are manifold and are not described in detail here.