Download CD accompanying Saltwater Wetlands Rehabilitation Manual

10 Chemistry
10.1 Introduction
The chemistry of saltwater wetlands is principally governed by local tidal inundation.
The mixing of fresh and salt water in the estuary gives the wetlands unique chemical and
biological properties, which help dictate the development of plant and animal communities.
The main chemical factors that influence plant growth in saltwater wetland areas are salinity,
oxygen availability and nutrients.
Macroalgae within a coastal lake (DECC)
10.2 Salinity
Salinity is the defining and most important feature of saltwater wetland chemistry. The high
salt content of the tidal waters and of the soil affects species selection and the productivity of
the plants and animals. With the exception of hypersaline areas, estuarine salinity ranges
from 0.5 to 35 ppt. This range allows a number of different plants and animals to survive in
particular niches to which they are specially adapted. Salinity is extremely important for the
germination success and survival of many estuarine plant species and accounts for the
zonation patterns seen commonly in these areas. The adaptations of estuarine species allow
them to inhabit this zone with relatively little competition from terrestrial species.
The salinity of intertidal soils is governed by several factors:
•
•
•
•
Local tidal regime: The height of the preceding tide determines whether an area of wetland
has been flooded or not. Lower elevations will be flooded more often, and their salinity
often reflects that of the estuarine water. Higher elevations will dry out between
inundations, and evaporation may cause salinity to increase in these areas.
Drainage: The nature of the soil and the proximity of creeks affect the drainage of the
wetland and the soil salinity. Fine silty mud retains more water and more salt than do
sandy sediments.
Gradient: The wetland slope influences the penetration of tidal waters and the rate at
which the water can drain away.
Vegetation: Plant cover reduces evaporation and lowers the salt accumulation at the
surface of the wetland.
10 Chemistry
133
•
•
•
Water table depth: An elevated water table may reduce soil salinity by dilution or may carry
salt to the surface.
Freshwater inputs: The inflow of fresh water from natural or anthropogenic sources tends
to dilute the salinity within the wetland environment.
Climate: Rates of evapotranspiration depend on temperature, and the relationship
between seasonal rainfall and tides affects dilution of soil salinity.
10.3 Oxygen
Plants and animals need oxygen. In the estuarine environment, frequent tidal inundation
commonly leads to waterlogging of the soil that can deplete the available oxygen within the
sediments, a situation referred to as anoxia. Water fills the soil pore spaces and reduces the
exchange of oxygen between the air and soil. The oxygen remaining is consumed by
microbial respiration within the soil. The lack of oxygen in the root zone poses particular
difficulties for intertidal vegetation. All species that are able to live in this environment have
specific adaptations, such as pneumatophores, to cope with the low soil oxygen in the
waterlogged areas of the intertidal zone. Fauna that lives in these areas can improve the
aeration of the soil by burrowing into the sediments. This in turn aids the survival of many
plant species in this zone. The activity of crabs is particularly important.
10.4 Nutrients
Nutrients are very important in the coastal environment, and nutrient cycling is probably the
main ecological function that drives the processes within the saltwater wetland ecosystem.
Owing to the substantially different chemical environment in saltwater wetlands, nutrient
cycles have different characteristics from those in freshwater wetlands. The two most
important nutrients to living organisms within estuaries are nitrogen and phosphorus, which
are present in concentrations of three to four orders of magnitude lower than those in typical
freshwater wetlands. However, excessive inputs of nitrogen and phosphorus from sewage,
urban, agricultural and industrial effluents can lead to eutrophication, resulting in algal
blooms if the wetland is not well flushed by the tides.
Nutrient uptake and litter decomposition are important to the functioning of ecosystems
because they provide essential components to sustain life in the estuary. A great number of
animals found in saltwater wetland habitats can be classified as detritivores or decomposers
(for example, protozoans and nematodes). These fauna convert the wealth of plant matter in
wetlands to detrital food sources for a rich and diverse invertebrate community which may in
turn support other marine and terrestrial species.
Nitrogen
Nitrogen (N) generally arrives in most intertidal areas via tidal water, precipitation,
groundwater and sediments carried by the tide. Where tidal influence is infrequent (for
example, in saltmarshes), groundwater often carries more N than any other source. The
processes associated with N cycling are shown in Figure 10.1. Plants extract nutrients from the
sediments via their roots and use them for growth and reproduction. The supply of N may
not always be sufficient to maintain the productivity of these ecosystems after denitrification,
the process whereby nitrate or nitrite is converted to gaseous nitrogen (N2).
Denitrification (and thereby the loss of N) is generally enhanced in waterlogged anoxic soils
that are typical of habitats constantly inundated by the tides. Consequently, other sources of
N are important. In N fixation, bacteria convert atmospheric N2 to organic N (Figure 10.1).
134
Part 3 Characteristics and processes
In nitrification, aerobic bacteria (Nitrosomas spp. and Nitrobacter spp.) convert ammonium,
produced by proteolytic bacteria and fungi in the soil, to nitrate. Plant decomposition
recycles N.
The N cycle varies slightly in saltmarsh environments on account of the infrequency of tidal
exchange. The infrequent wetting and drying of soils favours exchange with the atmosphere
over exporting nutrients and plant matter to the estuary via the tide. Therefore, a relatively
large proportion of plant material is consumed on the saltmarsh by respiratory or burial
processes. Denitrification in the soil is limited, because the soils of saltmarshes are typically
drier than those of nearby mangroves, thereby reducing the potential for N loss to the
atmosphere. Furthermore, saltmarsh vegetation actively transfers oxygen to its roots and
consequently to the soil. This background forms the basis for nitrification processes, whereby
nitrogenous compounds are converted to biologically available ammonium and nitrate,
which are used in plant growth. In addition, during decomposition, nutrients in the plant
tissues are released and recycled into new plant growth.
Figure 10.1 Depiction of the main processes of nitrogen cycling in saltwater wetland environments
N
fix
in
g
ba
ct
er
ia
atmospheric N2
N-
N groundwater N
decomposition
N
uptake by plants
ammonia
NH3
Nitrosomonas
spp.
aerobic process
nitrite
NO2
Nitrobacter
spp.
anaerobic process
nitrate
NO3
denitrifying
bacteria
uptake by plants
tide
N
gaseous N in soil
N + NO2
decomposition
10.4.2 Phosphorus
Phosphorus (P) is very important in the wetland environment, being one of the
macronutrients needed for building living tissue. The P cycle in intertidal soils is much simpler
than the N cycle: P is taken up by organisms as phosphate and is liberated by excretion or the
decomposition of dead matter by microbial activity. By the processes of excretion, death and
sedimentation, P bound to organic matter concentrates in sediments, where microbial
decomposition converts organic P to inorganic forms, which may again be taken up by
organisms. Phosphate ions are also adsorbed to silt and clay particles. Mobilisation of
sediments can contribute considerably to P levels within the water.
10 Chemistry
135
Carbon
Plant material is the most significant source of organic carbon in estuaries and also of
atmospheric carbon dioxide (by respiration). The herbivore, decomposer and detritivore
communities play a pivotal role in processing and mobilising this material (Figure 10.2).
Saltwater wetlands usually support a substantial biomass of coastal macrophytes, most of
which do not support a large number of grazers. Therefore, the majority of the plant material
ends up as leaf litter and, eventually, detritus. This detrital material can either be consumed in
situ by detritivores and decomposers or be exported with the tides to other parts of the
estuary. Organic detritus in all states of decomposition is a very important source of
particulate organic carbon in estuaries.
Figure 10.2
The process of carbon cycling within saltwater wetlands, showing the importance of the
detrital food chain
CO 2
Photosynthesis
Oxygen producers
decomposers
Respiration
First-order oxygen consumers
grazers
detritivores
Second-order oxygen consumers
136
Part 3
Characteristics and processes
filter feeders
Many of the most important chemical reactions occurring in sediments are associated with
the decomposition of organic matter. Decomposition reactions affect the sediment
environment and organic content.
The process of decomposition involves the gradual breaking down of dead material to
smaller and smaller particles and eventually to small molecules. It involves the action of
physical factors, such as weathering and leaching, and the activities of microorganisms and
detritivores. Litter within intertidal areas is derived from several sources. The largest source in
terms of mass is usually the higher plants, such as mangroves and saltmarsh plants. Smaller
detrital particles derived from microphytobenthos, epiphytic algae, phytoplankton and
faecal material may also be significant. In addition, a substantial portion of the organic litter in
intertidal areas may be of terrestrial origin, washed down by rivers or deposited by the tides.
10.5 Sulfur
The water that inundates saltwater wetlands not only is saltier than terrestrial waters, but also
contains relatively high levels of sulfur (S). The lack of oxygen within the soil allows anaerobic
bacteria, which can operate on S respiration, to thrive. However, the microbial action and the
reduction of sulfate to sulfide under anaerobic conditions creates the potential for ASS
(Section 9.9) to form. On exposure to air, these soils can generate potentially harmful levels of
sulfuric acid.
In some estuarine environments, the availability of S combined with certain environmental
conditions will lead to the generation of hydrogen sulfide (H2S), or rotten egg gas. H2S is
produced naturally by sulfate-reducing bacteria within the sediments of all marineinfluenced environments. The bacteria convert sulfate and organic matter into H2S, CO2 and
water. The conditions under which the release of H2S is triggered generally relate to the
availability of oxygen. If there is insufficient oxygen available in the water or if the rate of
oxygen input to the water (from photosynthesis by aquatic plants and atmospheric diffusion)
is not fast enough, the H2S will not be completely oxidised, and it will be released to the
atmosphere. While H2S can prove a nuisance because of its unpleasant odour, it is unlikely to
cause health problems in open environments, where it is quickly dispersed into the
atmosphere.
10 Chemistry
137