B Substrate Conditions in Saltwater Wetlands
A wetland is considered to be brackish or saline if the salt concentration is greater than 0.5 ppt (or about 1.4% the concentration of seawater at an average of 35 ppt NaCl). The average concentrations of the most abundant ions in ocean water are 460 mM sodium (Na+), 50 mM magnesium (Mg2+) and 540 mM chloride (Cl-); other ions are relatively scarce. The salinity of soil pore water in saltwater wetlands may be higher or lower than seawater depending on proximity to the tides and on the ratio of evaporation to precipitation. Rainwater or freshwater inputs from inland may dilute seawater while evaporation can cause pore water salinity to be greater than that of the ocean (Flowers et al. 1986; Fitter and Hay 1987). Salinity may also vary with season. For example, in a southern California salt marsh, salinity is low in winter and spring and high in summer and fall when there is less precipitation (Callaway et al. 1990).
The ions in seawater are toxic to most plants and even salt-tolerant plants succumb to very high levels of salt. Salt limits plant growth at about 100 mM in the soil solution. The salt concentration of seawater is around 500 mM and it commonly rises to concentrations of about 1 M in saltwater wetlands (Flowers et al. 1986). The problems faced by plants in highly saline environments are the following:
• It is difficult to acquire water under high salt conditions. Under non-saline conditions, water moves into plants because the external water potential is greater than within the plant. Solutes such as salt decrease the water potential so the plant's internal water potential must be even lower in order for the water to move into the plant. Plants that can acquire water under high salt concentrations are able to decrease their internal water potential below that of salt water (see Chapter 4, Section III.A.1, Water Acquisition).
• There is a different ionic mix in salt water than in fresh water, making the uptake of beneficial ions difficult. For example, where Na+ is in high concentrations, the uptake of K+ is inhibited because the two ions are chemically similar and plants take up Na+ in place of K+ (Tomlinson 1986; Fitter and Hay 1987).
• High salinity interferes with the uptake of carbon dioxide. As rhizosphere salinity increases, the net rate of carbon dioxide uptake declines. Emergent plants open their stomata to take in carbon dioxide, but lose water at the same time. Since the plants are under water stress and cannot afford to lose water through transpiration, taking in carbon dioxide becomes problematic (Pomeroy and Wiegert 1981; Longstreth et al. 1984; Bradley and Morris 1991b).
When salt concentrations rise to extremes, the primary productivity of saline wetlands decreases (Lugo et al. 1988; Srivastava and Jefferies 1996; Teal and Howes 1996). In mangroves, the shortest trees (<1 m) are reported in areas of high salinity (Pool et al. 1977) compared to over 50 m in areas where growth conditions are favorable.
The plants of saline wetlands are also subjected to high sulfide concentrations, which are stressful and potentially toxic (see Section III.A.1.d, Sulfur). Plants of saline wetlands have a number of adaptations which allow them to persist in a high salt, high sulfide environment (see Chapter 4, Section III, Adaptations in Saltwater Wetlands).
Plants in saline wetlands have been shown to be nitrogen-limited in a number of studies (Valiela and Teal 1974; Sullivan and Daiber 1974; Gallagher 1975; Mendelssohn 1979). Nitrogen limitation in marine plants increases with distance from the shore (Ryther and Dunstan 1971) because the major source of nitrogen is from land runoff. In freshwater systems, cyanobacterial fixation of nitrogen provides another important source; however, its contribution in saltwater systems is minimal (Valiela 1984).
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