Environmental Engineering Reference
In-Depth Information
affinity of various chemical substances to the sediment and the biotic composition of different
regions of a salt marsh (Vernberg, 1993). The nutritional state of salt marsh soils depends on
series of external and internal processes: input of nutrients, spatial distribution of these
nutrients, mineralisation and humification in organic materials, fixation of nutrients onto
particles and living materials and output of organic debris offshore all belong to these
processes (Beeftink, 1977). Besides mineral particles, marsh soil consists of carbonates and
organic matter. Organic matter levels of the soil are very variable but frequently low,
although often increasing with elevation (Gray and Bunce, 1972). The organic matter content
may decrease with the estuarine gradient. In an estuary a decrease of one third of the total
amount from the mouth upwards to the oligohaline zone was detected (Beeftink, 1965). In the
sediments of the salt marshes typically contain less organic carbon than sediments of tidal
freshwater marshes due to the greater annual inputs of riverine and terrestrial carbon (Odum,
1988). In the salt marshes, the average salinity in the overlying water is in the range of 5-38
‰ (Boaden and Seed, 1985). The salinity of surface and soil water varies depending on a
number of factors such as rainfall, fresh water input, tidal flooding, evapotranspiration, soil
type and vegetation (Vernberg, 1993). In the lower salt marsh soil salinity is relatively
constant and exceeds that of the flooding water. At higher elevations, there is an interaction
between the influence of flooding and climate, leading a greater variability in soil salinity
(Adam, 1990). In many places, however, high evaporation during summer causes the salinity
to rise to extremely high values in the surface soil (Beeftink, 1977).
Hydropedology is an advantageous integration of hydrology and pedology for studying
the intimate relationships between soil, landscape, and hydrology. There is a close relation
between hydrological and pedological factors. However, combination of soil physical theories
with theories of soil genesis and the related observations results in formation of the subject of
hydropedology (Lin et al.
,
2006). Hydrologic factors play an important role in the formation,
composition and structure of salt marshes. Tidal exchange between the marsh and estuary
promotes the necessary exchange of sediments, nutrients and organic matter, drainage, and
vegetation zonation (Mitsch and Gosselink, 1986). The hydrologic cycle in salt marshes is
dominated mostly by tides, but is also influenced by oceanic currents, evaporitic processes,
winds, freshwater flow, and catastrophic events. Freshwater flows from mainland to coastal
areas through drainage basins, groundwater, and rainfall. Strong discharges occur during the
wet season and during storms and hurricanes. The amount of fresh water is controlled by
several factors, such as levels of rainfall, proximity to salt water, permeability of subsurface
sediments, and elevation above sea level.
The environmental feature which distinguishes coastal saltmarshes from terrestrial
habitats is tidal submergence (Adam, 1990). The range of the tide, the difference between low
water and high water, is dependent on the geography of the coastline. There are regular
patterns to the magnitude of the tides experienced at a particular place. The observed tidal
range is subject to a number of cyclical changes. The first of these is the fortnightly cycle
between ‗spring tides', when the range is at a maximum, and ‗neap tides', when it is at a
minimum. This cycle is associated with the phases of the moon, the spring tides occurring a
day or so after the full or new moon and neaps just after half moon. Typically a neap tide will
be two-thirds to a half of the range of the corresponding spring tide. The distribution of the
salt marsh vegetation is determined by these water levels. Vegetation will only become
established and survive in places where the ground surface is more often exposed than
flooded, i.e., it is significantly above the mean sea level for a particular site.
