Saltwater Wetlands - Implications for Stormwater Management
This article is copyright, and should be referenced as shown: Coleman PSJ, 1996, Saltwater Wetlands - Implications for Stormwater Management, Delta Environmental Consulting, Adelaide.
This report was funded by the City of Salisbury, South Australia.
The city council has been foresighted in its use of wetlands to treat stormwater
and urban runoff. Their concern for the marine environment that the stormwaters
ultimately discharges into has led them to investigate saltwater wetlands
as the final ponding areas for runoff in the coastal zone. This report
provided Council with background information during the planning stages
of the Swan Alley Saltwater Wetlands at the mouth of the Little Para River
near Adelaide in South Australia. I would like to acknowledge Barrie Ormsby's enthusiasm, which rekindled
my love of "puddles" as well as Mark Coleman's input in the areas of monitoring
programs and salinity issues.
1.1 Ecology of saltwater wetlands, 1.1.1 flora, 1.1.2 fauna
1.2 Water quality performance of saltwater wetlands, 1.2.1 mechanisms for the improvement of water quality, 184.108.40.206 dilution, 220.127.116.11 degradation, 18.104.22.168 sorption, 22.214.171.124 bioaccumulation, 126.96.36.199.1 homeostatic (primary) accumulation from the aqueous environment, 188.8.131.52.2 trophic accumulation, 184.108.40.206.3 serial accumulation resulting in biomagnification of pollutants, 1.2.2 saltwater wetlands - pathways to improved water quality
1.4 Management issues, 1.4.1 biological issues, 220.127.116.11 distribution of emergent macrophytes, 18.104.22.168 eutrophication, 22.214.171.124 cyanobacterial blooms, 126.96.36.199 production of tubestock for anaerobic conditions, 188.8.131.52 dieback of emergent shoreline vegetation, 184.108.40.206 mosquitoes and other public nuisance issues, 1.4.2 hydrological issues, 220.127.116.11 ground water, 18.104.22.168 evaporation, 22.214.171.124 wetland salinity and flow regime, 126.96.36.199 wetland profile, 188.8.131.52 stratification, 1.4.3 pollutant removal issues
1.5 Management recommendations, 1.5.1 chemical monitoring, 184.108.40.206 chemical monitoring of wetland effectiveness, 220.127.116.11.1 parameters, 18.104.22.168 chemical monitoring of wetland health, 22.214.171.124.1 parameters, 1.5.2 biological monitoring
1. Swan Alley Saltwater Wetlands
The City of Salisbury is constructing a variable salinity stormwater wetlands on the lower reaches of the Little Para River. To assist in designing, constructing and managing these wetlands, Council requested a report providing “a basic understanding of the ecology (plants, fauna, micro-fauna etc) and water quality performance” of saltwater wetlands. Further, the draft specified that information is to be provided on the following topics:
1.1 Ecology of saltwater wetlands
Marine saltwater wetlands are known by various names worldwide. In Europe they are called merselands, washes or saltings, while in the Middle East salt marshes are sabkhas. In Australia they are called by the dominant species present, so in South Australia the marine/freshwater marsh area is referred to as “samphire,” in Queensland “marine couch meadow” and in some parts of Victoria “Spartina flats.”
In the Barker Inlet, saltwater wetlands occur naturally behind the mangrove forests and as the artificially constructed ponds of the saltfields. The naturally occurring wetlands are low lying, and show zonation that is dependent on level above or below tidal influences. At their upper extreme these wetlands grade into dryland coastal zone ecosystems or into freshwater swamps. The latter systems, with their complex patterns of saline/fresh water flows and wide variety of plant species could form a model for the proposed Swan Alley Saltwater Wetlands to be constructed at the mouth of the Little Para River.
The physical characteristics of saltwater wetlands affect the distribution of plants within them. Chapman (1960, p2) in Salt Marshes and Salt Deserts of the World commented on the uniformity of appearance in plants of both saline and alkali lake environments. She postulated that while there was no doubt that the chloride ion was of great significance in the maritime marshes, the sodium ion was the common factor in all the studied environments (salt marsh, inland salt lakes, soda or alkali lakes) and was probably responsible for the similarity in vegetation. Convergent evolution is a common phenomena where plants and animals have similar morphologies in response to a common stress, which in this case is undoubtedly osmotic dehydration.
Saltwater wetlands occur on a variety of substrates. In Australia the majority occur in delta environments on a firm muddy to clayey soil. The terrestrial plant communities most common in South Australia are dominated by samphires of the genera Halosarcia, Sclerostegia and Sarcocornia in the tidal zones, grading to communities of Typha, Bolboschoenus, Phragmites and Juncus where freshwater enters the wetland system. In Europe three major soil types and their associated plant communities have been identified:
Aquatic plants common in the waterways of a saltwater swamp vary from marine algae and seagrasses to freshwater aquatics. In areas of high salinity halophilic angiosperms such as Ruppia and Lepilaena are found, while blue-green algae form mats and colonies in shallow ponds and exposed mud areas.
Saltwater wetlands are part of a changing environment. Sedimentation and erosion provide the framework for a succession environment that starts with seagrass beds in the subtidal zone, passing through the mangrove zone, salt marshlands, freshwater reed swamps, bush colonisation to regional forest. Such environments are termed “seral” and zones within them are not considered to be static. The underlying geography or the activities of humans may both cause interruptions and irregularities in the theoretically smooth transition of biotypes. Figure 1 shows the interrelationships between the abiotic and biotic factors that determine the nature of the wetland.
Figure 1: Determination of Wetland Type - Factors
The combination of factors in Figure 1 determines the character of the wetland. For instance, the Adelaide climate is relatively dry during the high sunshine months, restricting the plant communities to slow-growing types resistant to aridity. Similarly the low rainfall, semi permeable soil and flat profile indicates that the wetland will be hypersaline during summer. The low wave velocity and small tidal range means extensive mud flats.
Areas where the underlying landforms show a sudden increase in relative height may prevent the formation of the next zone. The mangrove/samphire interface zone is commonly mentioned as under “area pressure” in the Barker Inlet area, but a review of the past ecosystems of the Adelaide Plains shows that the transition zone between saltwater wetland and freshwater wetland has been possibly more impacted (Tyler et al, 1976, p123).
While regional geography and human activities result in discontinuous succession, land subsidence and land lifting (along fault lines) or rising sea levels have a more widespread though diffuse effect. Lifted saltwater swamps will rapidly progress to freshwater. Where land subsides or the sea level rises, erosion will occur at the marine edge of the group of coastal ecosystems and plants from each zone will tend to colonise further inland.
Figure 2:Typical zonations in saltwater wetlands
Osmotic stress from high salinity ground and surface water during the extremes of weather, as well as dehydration due to evaporation, results in terrestrial salt marsh plants are typically succulent, mealy, dull or glabrous, with small or grassy leaves. Water storage hairs are present in some genera eg Atriplex and many plants have salt-secreting glands (Limonium). Common representative terrestrial and emergent species found in various zones in the Barker Inlet area are listed below:
Mangrove Zone · Avicennia marina
Samphire Zone · Sarcocornia quinqueflora, · Sarcocornia blackiana, · Halosarcia halocnemoides, · Halosarcia pergranulata, · Sclerostegia arbuscula. In restricted areas only - Halosarcia flabelliformis , Frankenia pauciflora, · Nitraria billardieri, · Suaeda spp
Marine Grasses and Mixed Herbs Zone · Frankenia pauciflora · Sporobolus sp · Disphyma rotundifolia · Enchylaena tomentosa · Halosarcia halocnemoides · Halosarcia pergranulata · Distichlis sp · Wilsonia humilis · Mairiana oppositifolia
Reed Swamp Zone · Phragmites sp · Juncus sp · Bolboschoenus sp · Typha orientalis · Casuarina sp · Eucalyptus camaldulensis · Melaleuca halmaturorum · Melaleuca lanceolata · Cotula crassifolia Atriplex Zone · Atriplex sp · Nitraria billardieri · Disphyma rotundifolia · Enchylaena tomentosa · Halosarcia pergranulata · Halosarcia halocnemoides · Pittosporum phylliraeoides · Acacia spp · Melaleuca lanceolata · Myoporum insulare · Olearia axillaris · native and introduced grasses · mosses and lichens · various monocotyledons - lilies etc
Aquatic plants in the area include seagrasses and hypersaline tolerant seagrasses such as Ruppia and Lepilaena as well as green and red macroalgae in areas where the salinity is high. Benthic mats of algae and bacteria crust the exposed mud surface and the floors of shallow pools. Pelagic plankton including diatoms, chlorophytes, dinoflagellates and filamentous blue-green bacteria are found in samphire pools as well as in the mangrove creeks and freshwater reed swamps. Where the freshwater/saltwater interface occurs, floating plants such as the duckweeds (Lemna and Wolffia) and water buttons (Cotula crassifolia) are common. Azolla is only found in the freshest areas. Submerged saltwater/freshwater species include algae such as Enteromorpha and Chaetomorpha. Pondweeds (eg Potamogeton, Myriophyllum) prefer the freshwater end of the zones.
If the proposed Swan Alley wetlands are subject to tidal influx, mangrove accession may take place. Even without tidal movement, mangroves may establish where water salinities approach that of seawater and there is a regular depth of water. Plants of Avicennia marina are established in the northernmost Penrice Dry Creek Saltfield ponds at Middle Beach in South Australia, where there is no tidal variation but a constant supply of water of seawater. This is not an unusual occurrence and all saltfields in Australia that are in areas where mangroves occur have mangroves growing in their initial ponds.
Should mangrove accession be visualised in the design of the wetlands, consideration should be given to planting out methods. Recent research in Florida (Riley, 1996) using encapsulation to protect young propagules while their anchor roots are establishing shows promise for revegetating areas of high water velocity.
The faunal mix of a saltwater wetland depends on whether its salinity regime is predominantly saline or freshwater. Fauna from the mangrove zone will be found in the samphire zone, particularly at times of very high tides, while freshwater molluscs and other invertebrates will be found in the interface zone between the fresh and saltwater wetlands.
Some invertebrates are adapted to changing salinities by virtue of short life cycles. Brine shrimp (Artemia spp) are notable for this. Brine shrimp can survive in a variety of salinities almost to the point of salt crystallisation. They adjust to salinity changes in the first 24 hours after hatching, and then must remain at that salinity. The presence of cysts in the sediments and along pond edges ensures regular recruitment. With salinity changes over time in a temporary wetlands, regular hatchings of new nauplii are available to maintain the population.
While temporary wetlands are still fresh they support a wide array of invertebrates and small fish. Daphnia, Paracyclops, predatory dragon and damsel fly larvae, beetle larvae and water boatmen abound. As the salinity increases the variety is reduced. Water boatmen have been observed in the drying ponds of the Connector Wetlands at Dry Creek SA, when the salinity increases rapidly. Once the water approaches seawater salinity it is colonised by brine fly larvae and possibly marine invertebrates if the ecosystem is continuous.
1.2 Water quality performance of saltwater wetlands
The ability of a wetland to improve the quality of the water flowing through it is correlated to both physical and biological factors. A toxic element entering the wetland may variously become diluted or concentrated, chemically transformed into a more inert or more active form, may accumulate or dissipate. Ideally, a wetland will isolate and transform the toxic or undesirable elements entering the basin, resulting in a clean outflow.
1.2.1 mechanisms for the improvement of water quality
Herricks (1991) developed the table below, which nominates how the varying physical characteristics of a water body affect the concentration of toxic substances.
Table 1 Physical characteristics of reservoirs and resulting effects on toxics within them
The Herricks (1991) table provides a useful check list for water bodies such as reservoirs, however dilution is not really an option in the relatively small wetlands envisaged unless there is a isolated point source pollutant. The main objective of the wetlands is to remove the pollutants from the water column. They may be stored safely in sediments or in the biomass of the wetland. They may also, or subsequently, degrade into less toxic forms or completely disintegrate into their component elements. Eventually the components of many toxics entering the wetland will be recycled as part of the biomass. Additionally, the sediment plays an important part in the amount of and type of toxins stored in the wetland. The primary process is the reduction oxygenation (redox) reaction which controls the rate of toxin mobilisation and fixing.
Dilution is not the solution to pollution, or at least not to all pollution. Some pollutants need to be diluted before they can be effectively dealt with, notably heavy loads of nutrients. The various nitrogen species in particular may be too concentrated upon first entering a wetland and may actually damage plants. Once diluted by the body of water within the basin the nutrients can be safely taken up by aquatic plants.
When a wetland is the only method of treatment that wastewater receives, a sudden influx of concentrated pollutants that results in damage to the wetland will also result in a large export of pollutants from the wetland. This is very visible when extremely high levels of nitrogen “burn off” the plants of the wetland, which then contribute their own biomass to the pollutant load. In order to ensure effective functioning of the wetland it is necessary that pollutants be “diluted” either in actuality (in the ponds themselves) or virtually, by a reduction in the loads put into the system in the first place. The best way to ensure that pollutants are not in such levels as may harm the wetland is to manage the catchment feeding it. Also, the larger the wetland is, compared to the influx of pollution, the greater the chance that its ecosystem will be buffered from damage by the dilution of harmful substances.
Chemicals within the water column in a wetland may degrade in various ways. Chemicals that can absorb light energy may undergo photolysis. The molecules that absorb light energy become fragile and are accelerated towards breakdown (Matsui, 1991). Under certain pH conditions hydrolysis may take place, where a hydroxyl (OH) radical is incorporated into the molecules of the chemical reducing the structural integrity of the molecule, which subsequently breaks down.. Biological degradation includes microbial degradation and metabolic changes within plants and animals that have taken up the substances. All of the above processes are primarily concerned with the degradation of organic compounds, although they sometimes apply to inorganic compounds. Interestingly, not all breakdown products are less toxic that their parent compound.
Organic chemicals can be sorbed onto the soil or sediments in a wetland basin, resulting in a reduction in the levels of these chemicals in the water column. The rate of sorption depends amongst other things on the aqueous solubility of the organic chemical. DDT and PCB’s sorb to the sediment quite readily, while Lindane and 2,4,5-T do not (Matsui, 1991). Sorbed chemicals are not always permanently retained in the sediments however, and may desorb back into solution. The rate of desorption is dependent on water depth, water turnover time and also on how much of the chemical is being degraded microbially and photolytically. Desorption in saline areas may be slowed by the presence of benthic “algal” mats (Coleman and White, 1993) which partition the nutrient rich sediments away from the overlying brines. Climatic conditions that disturb these mats may result in a sudden influx of nutrients and metals back into the water column.
Frequently when discussing metal pollution it is easier to use categories to describe the state of the metal in the environment that refers to its activity in the environment. For instance, the notions of “sinks” and (re)mobilisation are commonly used in the literature for good reason. Most metals that are of concern to humans are sparingly soluble, and therefore the metal is concentrated in sinks. Often the processes of concern are those that move the metal ions in and out of the sinks.
In classic sorption non-ionic bonding of metal ions to various substrates occurs. Metals removed from the water column in this form can be rapidly remobilised as the bonding is usually not very strong. Ferric oxide (FeO) for instance, is strongly sorbed to clay. Lead, nickel, copper and zinc are also sorbed to clay in decreasing order of strength.
Often metal ions precipitate in association with other metal ions (co-precipitation). A common pair is ferrous and manganese but often cobalt, zinc and copper are also coprecipitated with ferrous. These complexes are released back into the water column under some anaerobic conditions (redox) commonly found in the sediment of saltfield ponds. It is of interest from a water quality point of view that iron is an efficient scavenger of arsenic.
Complexation and flocculation of metals occurs with organic material. For instance, EDTA is often used in chemical procedures to bond metals. Metals bound in such a manner can either be settled to the floor of the ponds and recycled as the organic matter is decomposed, or taken directly into the food chain. The metals can be made relatively inert as long as the organic material is stable. However, metals are particularly rapidly assimilated into the food chain when bound to organic material. In decreasing order of strength Pb, Cu, Ni, Co, Zn, Cd, Fe, Mn, Mg form associations with humic material.
Metals will independently form inorganic precipitates, for instance as oxides and chlorides. Generally these precipitates are relatively stable. Cadmium forms a precipitate with carbonate in alkaline conditions. Chromium and ferrous/ferric form insoluble hydroxides and have short residence times in media with high oxygen levels. Lead, zinc, copper and chromium also rapidly form hydroxides in that order. In an anaerobic environment metal sulphide salts are stable.
Remobilization of the metals most commonly occurs under the following circumstances: redox changes, complexing with organic acids and microbial activity. All three result in the metals moving from the sediment back into the water column. All are the direct or indirect result of bacterial action in nature. Ponds in saltfields and samphire areas have a very active microbial mat in the sediment. In saltfield ponds prior to the gypsum precipitation point these are quite often oxygenizing, but typically the sediment in gypsum ponds has a very strong reduction potential. Increased temperature also plays a major part in the remobilisation of metals from the sediment.
Pollutants in water may accumulate in the tissues of aquatic organisms inhabiting a wetland. Aquatic organisms take up an organic substance in direct proportion to its aqueous concentration, while the picture for inorganic substances such as heavy metals is not so clear, due to competitive uptake of other ions. This accumulation may have a direct impact on the organism, or it may have no noticeable effect on one organism but may be concentrated up through the food chain until it impacts on another species.
126.96.36.199.1 homeostatic (primary) accumulation from the aqueous environment
Soluble pollutants may migrate into the tissue of aquatic organisms (especially autotrophs and microscopic heterotrophs) by the process of osmosis. In some situations this will occur until a point is reached where the concentration in the organism’s tissues and the concentration in the water are in equilibrium. In other cases the pollutant may take part in the metabolic process in the organism, and may become concentrated or excreted as part of that metabolic process. Where the chemical is used as a metabolite or a substitute, the pollutant is often actively transported into the organisms cell, resulting in very high concentration.
188.8.131.52.2 trophic accumulation
A pollutant may be incorporated into an organism through direct ingestion either of the pollutant itself, as for example when a filter feeder ingests fine particulates that may be carrying heavy metals, or through the ingestion of another organism that has the pollutant already incorporated in it.
Metal ions that are ingested are often passed through the organism and deposited as faecal material. The ions which are incorporated into a plant or animal are also deposited, typically in the bottom sediment when it dies.
184.108.40.206.3 serial accumulation resulting in biomagnification of pollutants
The exchange of toxins between trophic levels can sometimes result in greater concentrations in the higher trophic organism. Two classic examples of biomagnification of a toxic substance through serial accumulation are the outbreak of Minamata Disease in Japan as a result of mercury entering the food chain, and Itai-Itai disease where cadmium from a smelting company entered a river supplying water to rice paddies.
Some authors, notably Odun in 1959 (Matsui, 1991) have theorised that eutrophic systems such as shallow wetlands ensure the minimum quantities of accumulated elements are transferred to the upper trophic levels. This is because the biomass of the producers in such a system is much higher than that required to support the other trophic levels and therefore much of the accumulated toxics will be sedimented with the excess phytoplankton and will become available to the sorption pathway. Where there is a high efficiency of converting producer biomass into carnivore biomass, such as in the open ocean or deep waters, a much larger quantity of the toxins may be found in the higher trophic levels.
It is also the case that a eutrophic water body has a very large amount of polarised surface area, in the form of cell walls, available to sorb pollutants. The soluble organic matter in an eutrophic water is also very high, facilitating further sorbtion and potential flocculation of toxins. Blue-green bacteria produce copious amounts of soluble organics and some of these have been found to be very efficient scavengers of nutrients and some metals (Coleman, unpublished).
1.2.2 saltwater wetlands - pathways to improved water quality
The water quality functions of constructed freshwater wetlands are relatively well understood. Data for saltwater wetlands has been, in the main part, collected from naturally occurring wetlands or has been deduced by extrapolation from marine data. Controlled saltwater environments are not common, with the exception of saltfields.
Coleman, in his paper Phosphate Mass Balance in a Constructed Saline Wetland (to be published 1996) comments that,
“ Limitations in researching nutrient cycling within waterbodies are a lack of control when working with large bodies, and for smaller water bodies the uncertainty of applying conclusions reached from small scale experiments... there are several reasons why saltfields are a valuable source of data... the closed pond system is monitored for physical as well as biological characteristics... in some fields the data extends back 30 to 40 years... the system is normally maintained at a constant salinity over many years... the saltfield system is designed and exhibits a consistency that is not found in natural systems.”
His research, conducted at two Queensland saltfields over eight years, shows that the initial ponds with high species diversity removed phosphates efficiently. As species diversity fell in the succeeding ponds, phosphates were not removed. Next, the more hypersaline ponds rich in filter feeders removed a “second helping” of phosphates. Any remaining phosphates were available to cyanobacteria growing in the most saline ponds.
Saline wetlands have a decided advantage in their ability to reduce the phosphates in their discharge waters. Seawater is relatively high in calcium which is sparingly soluble with phosphate. Under alkaline conditions the majority of the phosphate is precipitated into the sediment.
In a constructed saltwater wetland designed to treat stormwater runoff, much of the water quality improvement will be as a result of the removal of turbidity (by slowing down water flow) and nutrients (by uptake into plants). Therefore the productivity of the macrophytes selected for planting is of major importance. Mangroves and estuarine swamps are generally highly productive. Naturally occurring wetlands in the area variously support populations of mangroves, samphires, rushes, reeds and sedges, with some areas of sea couch.
Recent research (Clarke and Jacoby, 1994) at Jervis Bay compared the productivity of a species of sea-rush (Juncus kraussii) with the productivity of other salt marsh plants. They established an annual above ground production of 0.81 kg/m2 for the J. kraussii growing in Jervis Bay in 1989. This was higher than the productivity of neighbouring stands of Avicennia marina (0.31 kg/m2/yr).
The authors compared the estuary they were studying to several in Western Australia and discovered that while standing crops of Juncus at Jervis Bay seemed similar to those at the Western Australian sites, the standing crops of other salt marsh plants were about half of those in the West. Sarcocornia was measured as having a standing crop of 317 g/m2 and Sporobolus (marine couch) had a standing crop of 349 g/m2. It is likely that environmental parameters such as water flow and salinity produced the wide variation in results. As a general rule even halophilic plants are more productive with regular freshwater flushing.
Research at Lake Biwa in Japan (Kurata and Kiro, 1991) showed that stands of Phragmites communis growing around the lake utilised up to 250 kg of nitrogen per hectare every year and 25 kg of phosphorus. Typical blooms of submerged and planktonic plants utilised nitrogen at a rate of 0.35g/cubic metre of lake water annually. Saltwater wetlands tend to be shallower than Lake Biwa, which averages 3.5 metres deep, and so exhibit correspondingly higher productivity.
1.3 Positive attributes of saltwater wetlands
The major ecological advantage of constructing a saltwater wetland at the mouth of the Little Para River is that it mimics the natural system found in the area prior to human intervention. Large discharges of freshwater into the marine environment, even when “cleaned” by a freshwater wetland are a shock to the mangrove and seagrass ecosystems. The ideal situation is where a river or drainage system discharges through an estuary or coastal swamp. This allows for gradual changes in salinity while turbidity and nutrient loads are stripped from the water.
A saline wetland has all of the advantages of a freshwater wetland, with some additional features. The greater diversity of environments means greater diversity of habitats for birds and other animals. It provides a wildlife corridor from the coastal fringe to the hinterland. Estuaries are typically more productive, in fisheries terms, than most other marine habitats and from a conservation point of view the construction of saltwater wetlands means the construction of more suitable habitats for breeding. Lastly, the chemical changes at the marine/freshwater interface provide an unique opportunity for the precipitation of chemicals before they are deposited into the sea.
Such natural ecosystems are not static, and the location of the varying salinity regimes does move depending on climatic factors such as droughts or wetter than normal winters. Many of the plants and animals found in such environments can cope with such changes, however there are occasional events that may result in the death of stands of plants. The Orange Mangroves (Ceriops tagal) of the Calliope River anabranch in Queensland have suffered considerable dieback during the last four years of drought in Central Queensland. The stumps of the dead trees will prevent rapid water velocities in the area when the next wet period occurs, ensuring rapid recruitment of new trees. The stumps form perching areas for fishing birds such as cormorants in the meantime.
1.4 Management issues
1.4.1 biological issues
220.127.116.11 distribution of emergent macrophytes
A well balanced ecosystem needs a variety of macrophytes. Some constructed wetlands (and some natural ones) become monocultures. Monocultures cannot support the same variety of faunal resources and are more prone to “catastrophic” events.
Typha orientalis has a reputation for invasive growth and could become a monoculture if not kept in check. The planting plans for the Swan Alley Saltwater Wetlands will contain a variety of emergent macrophytes, but they should be encouraged to establish prior to the introduction of Typha. Typha has established rapidly in parts of the Connector Wetlands and will doubtless colonise the Swan Alley wetlands from the Little Para River.
Froend and McComb (1994, pp123-140) studied the distribution of Baumea articulata and Typha orientalis in four wetlands in Western Australia. Transects showed that both species could, and did, exist in the same range of conditions occupied by the other. Water regime per se was not the determining factor in which species occupied a specific position in the littoral zone of a particular wetland. The authors felt it likely that invasion and displacement of Baumea stands by Typha were due to physical disturbance of the Baumea stands allowing the Typha to establish a foothold.
Excessive nutrients accelerate the process of eutrophication. In a eutrophied system large blooms of planktonic algae may result in wide day/night excursions in dissolved oxygen, due to respiration and photosynthesis. Hot, still nights exacerbate these excursions. The extremes may have disastrous effects on other aquatic life in the wetland.
Algal blooms become a possibility once inorganic nitrogen levels in the water exceed 0.3 mg/L and phosphorus levels exceed 0.01 mg/L (Metcalf & Eddy, 1991, p 1213). Early warning of eutrophication is signaled by rapidly lowering dissolved oxygen levels in deeper water.
Eutrophied systems are a common problem in aquaculture. Prawn farmers in tropical Queensland use floating paddlers to maintain night-time oxygen levels in their growing out ponds. In wetlands prone to eutrophication, the ponds should be designed to ensure maximum wind mixing occurs in the summer months.
18.104.22.168 cyanobacterial blooms
Coleman and White (1993) in The Role of Biological Disturbances in the Production of Solar Salt, examined the conditions most likely to result in blooms of Synechococcus in the closed constructed wetlands forming the Central Queensland and Port Alma saltfields. Many salt tolerant blue-greens have the ability to fix nitrogen, leaving manipulation of the phosphate levels in the ponds as the most effective control method in situations where a bloom is occurring.
Synechococcus elebans is a solitary microalga that grows in colonies and forms a benthic mat lining the high salinity ponds. Under certain climatological conditions the algae separate from the mat and become planktonic, causing production problems for the saltfield operators. Burnard and Tyler, in their paper Brine Quality Management in Solar Salt Operations (1993), found that shallow, clear water above the mats was a contributing factor in maintaining firm, continuous mats. Reducing the depth of turbid ponds at Dampier saltfield, and reintroducing the filter feeding brine shrimp Artemia clarified the water column and stabilised the benthic algal mats.
22.214.171.124 production of tubestock for anaerobic conditions
Sorrell’s research (1994, p172) on root aeration in Eleocharis sphacelata has implications for the nursery rearing of tubestock for wetland planting. Plants produce long roots with poor aeration potential if they are grown in well aerated soils. Later flooding will cause death of most of those roots and may be fatal for the plant. At a minimum it will cause a setback that can be avoided by growing tubestock in anaerobic conditions initially.
126.96.36.199 dieback of emergent shoreline vegetation
Many emergent macrophytes have roots that oxygenate the sediment around them to prevent the roots from dying. Studies show that such plants have a marked effect on the redox potential and sediment oxygen compared to nearby unvegetated areas (Sorrell, 1994, p170). The survival of the plant may depend upon its ability to produce oxygen for its roots at a rate greater than the sediment around the plant can absorb it. Actions that either impede the plant’s ability to produce oxygen or increase the local biological oxygen demand may lead to dieback in emergent macrophytes.
Fluctuating water levels may suddenly expose a plant with low aeration potential in its roots to an anaerobic environment, leading to rapid drowning. Rhizomatous plants simply resprout after such an episode, but plants without storage devices may not survive.
Eutrophication may raise the local biological oxygen demand to an unsustainable degree. Recent spillage from a sewage works into a reed bed in the Barker Inlet north of Adelaide demonstrated this amply. Bolboschoenus growing in a freshwater/saltwater wetland were flooded with highly sediment-laden primary treatment sludge. Within days the growing parts of the plants were dead, but some weeks later the plants resprouted from tubers.
Overfrequent harvesting of wetland plants in the growing season reduces their ability to supply oxygen to their roots. Harvesting is best done at the end of the growing season.
188.8.131.52 mosquitoes and other public nuisance issues
Saltwater wetlands are prone to infestation by the saltwater mosquito, Aedes vigilax. This mosquito is implicated in the transmission of Ross River Fever. Naturally occurring saltwater wetlands near the proposed Swan Alley Wetlands at the Little Para River are monitored for the presence of mosquito larvae, and ponds hosting them are treated with Abate. The ecological implications of Abate treatment are reduction in predator numbers. Predator numbers take longer to recover than the mosquito numbers, leading to a situation where constant applications of the insecticide are required. Newly developed growth regulating hormones are being trialled in the samphire area as a treatment to prevent emergence of adult mosquitoes.
Tests in Queensland and America have shown the hormones to have little impact on predators of the mosquitoes, making this form of control one that may be very useful in constructed saltwater wetlands. Presentation of the hormone in hard blocks makes it an ideal medium for areas that are intermittently inundated.
1.4.2 hydrological issues
184.108.40.206 ground water
The ground water in this region is typically saline, with a salinity up to 100 parts per thousand, or two to three times seawater. Sodium ions from the salty groundwater act upon soil mechanically, dispersing colloids (Chapman, 1960, p2). The measurable effects include slower water movement, poor aeration, and “physiological drought” resulting from the high rate of water absorbtion by the soil colloids.
The salinity of the ground water is a result of wicking of marine water through the coastal St Kilda formation. The driving force for the marine brine movement through the formation is solar evaporation and the transpiration of water by plants, leaving the salts behind in the soil to increase the local salinity. The effect of St Kilda Formation ground water on the wetland is that the wetland will gradually become more saline during the seasons with reduced flow through the wetland. The impact of the saline groundwater will be greater in the initial setup of the wetland, but as the fine organic sediment forms on the base and sides of the constructed wetland the movement of ground water into the wetland will decrease. Algae mats are very useful in restricting movement of groundwater into water bodies.
Various bores are needed to monitor changes in the groundwater.
Any body of water is subject to evaporation. There are several factors that change the rate of evaporation from a waterbody. Most are to do with meteorological conditions, but some such as water depth and colour are specific to the water body. With all things considered, the evaporation from the Little Para wetland should be in the region of 10 to 20 mm per hot summer day. If the water that is being evaporated is saline, whether from tidal inundation or ground water contamination, the salinity will increase.
This will have an effect on the formation of the long-term ecosystem. For instance if there are marked fluctuations in salinity the ecosystem is more likely to be dominated by cyanobacteria than the more complex plants and algae. In some case a cyanobacterial mat may be preferential to exposed mud, but it is generally considered that emergent foliage is better at nutrient trapping than a mat that would contribute to the nitrogen load of the water body by fixing nitrogen from the air.
A mass balance model would assist in determining the likely quantity of water needed to supply and maintain the freshwater head in the wetlands.
220.127.116.11 wetland salinity and flow regime
The wetland will have marked seasonal variations in flow and salinity. The winter period will be dominated by fast freshwater flows. The volume from a typical downpour will easily over run a small wetland and obviously the salinity will be low for most of the winter/spring period. The salinity will gradually increase over the summer period with minor tidal overflows and ground water movement into the deeper sections of the wetlands. The final salinity will depend on the combination of the effects of tidal inflow, groundwater movement, evaporation and freshwater runoff from the catchment. It is understood that the latter can be controlled from the wetlands further upstream. The autumn period will be characterised by very high tidal flows into the wetlands and it is expected that the wetlands will become predominately marine until the winter rains come. If the winter flow is minor there may be a problem with the salinity of the wetlands during the following summer.
The salinity needs to be monitored and adjusted as the seasons change. It is important to manage the wetland with natural variability in mind. Average flow regimes will be useful for design but a means of modifying the salinity profile will be needed for the non-average year.
18.104.22.168 wetland profile
The profile of the wetland should be carefully considered as it will affect the final biological structure of the wetland. The most appropriate model for a saline wetland is one that occurs naturally. There are basically two natural types of wetlands with a range between the two.
The first is an estuary where the freshwater flows down a channel and the marine waters form a saline wedge beneath the freshwater, and the saline wedge moves back and forth with the tide. The water body is essentially marine with short periods of freshwater during high rainfall.
The second type is the delta or salt marsh where the freshwater is forced to flow over a shallow area covered with hypersaline type plants. The area is periodically covered with marine water during high tides, and there is much less transportation of marine water through the wetland. This form of wetland is much more efficient at capturing nutrients and sediments, but does have a high salinity during low rainfall periods and may impede the flow of freshwater during floods.
It is normal to have a deeper section behind the shallow bar separating the fresh from the seawater. This basin is designed to act as a reservoir or buffer allowing the freshwater to have a longer retention time. This basin will gradually become saline from the groundwater unless the freshwater is higher than the ground water levels and the fresh is allowed to perch through the soil. A freshwater lens will gradually develop but there must be a constant feed of freshwater from the water course upstream of the wetlands, otherwise the deep basin of the wetland will become another “Kellar’s Pit” (Barker Inlet Wetlands), with a salinity stratification from top to bottom.
Water bodies exhibit varying degrees of stratification of temperature and density. In freshwater lakes this becomes noticeable once the depth exceeds 5metres, however saline lakes may show marked stratification even when they are quite shallow.
In a freshwater lake the stratification reflects the ambient air temperatures. In summer a warm, less dense layer of water forms the well aerated epilimnion, there is a thermocline underlying it and a dense, cold, hypolimnion forms in the deeper areas and is characterised by poor aeration. As the water of the epilimnion cools to 4oC in autumn, it reaches the point of greatest density and starts to fall through the lake waters to the lake floor, bringing oxygen to the hypolimnion.
In saline wetlands the density and temperature of the water reflect salinity levels as well as the ambient temperature, and modeling the movement of stratifications in these systems is not uni-dimensional. Computer models have been developed for estuaries and coastal lagoons, which provide for detailed analysis of the systems, however the main features of saltwater stratification are summarised below.
Saline, dense water contains considerable heat, and the majority of pools in constructed wetlands are shallow enough for this water to gain heat from solar radiation. As a result, fresher water overlying the denser water will exhibit a lower temperature than the deeper water. This warm, dense, deeper water will not have the capacity to carry as much oxygen as the deep water in a freshwater lake.
Water entering a wetland flows down along the bottom of the waterbody until it reaches the zone of denser water. At this point the inflowing water spreads rapidly across the horizontal plane of the waterbody. In a waterbody characterised by shallow saline groundwater intrusion, light freshwater inflows will spread out across the surface of the lake rather than flowing down to the benthic zone. Depending on the height of any weir controlling the outflow, the denser water may remain permanently in the deeper area and the freshwater may be “skimmed” or decanted off before the desired retention time has been achieved.
To prevent this occurrence, inflows into such systems may be through bottom opening weirs, where the floor of the receiving pool is of a similar depth to the bottom of the inflow channel, or through piped ports into the deepest parts of receiving pools that are much deeper than their inflow channels. Outflows should similarly be constructed to ensure that water on the surface of the waterbody is retained for the desired time.
The less dense inflow water leaving a piped port will tend to form a buoyant plume. Ambient water will become entrained in the plume as it rises and travels away from the port. The plume will, as a result, become denser until equilibrium with the surrounding water is reached (Metcalf & Eddy, 1991 p 1227) or, in strongly stratified ponds, until it reaches a layer of water that is less dense (ie the surface freshwater layer) in which case it will remain just under that layer.
1.4.3 pollutant removal issues
Pollution such as nutrients and metals that are removed from the water column must be themselves be removed from the wetland. The pollution may be bound up with organic material such as plants or inorganic salt in the sediment. If the monitoring program determines that the pollutants are being deposited in the sediment, the requirement of removing them is not so urgent as it would be in the case if the pollutants are “temporarily” bound to plants. Even so, sediment will gradually accumulate and the wetland will cease to function as one, necessitating the removal of the sediment. Dredging is the obvious solution but a strategy must be put in place to minimise the effort to remove the bulk of the sediment and its destination once it has been removed.
The organic material is easily harvested but again the strategy is not to remove organic material but the pollutants that they have accumulated. Some plants accumulate metals more efficiently than others and these should be harvested more regularly than others. On the other hand those species should be encouraged to grow and not be unduly setback by overharvesting.
The design of the wetland should include long term planning for the removal of sediments and plant material that is cost effective and consistent with the better operation of the wetland as a pollutant trap.
1.5 Management recommendations
1.5.1 chemical monitoring
The chemical monitoring of the wetlands should have two basic objectives. It should monitor the effectiveness of the wetland in removing nutrients and pollutants from the water stream, and it should monitor the “health” of the biological system. The monitoring system must be effective and cost efficient.
22.214.171.124 chemical monitoring of wetland effectiveness
The water should be monitored as it enters and leaves the wetland, and the sampling regime should be biased to capture an accurate picture of the nett export into and out of the wetland.
For instance there is not much point in sampling the outflow when there is little water movement. Most sampling should take place during peak flow but access is often difficult during periods of high rainfall. Monitoring on a time basis suffers the same basic problem, with the high flow rates being under sampled per unit volume flow through the wetland. Time based sampling has some value. However, without the data being correlated with water flow it provides little valuable information, and becomes merely an exercise in data collection.
At times the chemical characteristics of the wetlands will change radically. These changes will often be of a cyclic nature, either seasonal or diurnal. Whereas it is impractical to monitor the wetlands extensively for cycles, some understanding of the cyclical nature of the water body is needed. For instance, at least one 24 hour sampling regime is needed per season, more if automatic equipment is available.
Most, if not all, of the nutrients and heavy metals will be mobilised from the wetland sediment during the night, and in particular the early hours of the morning. The wetland can mobilise large quantities of pollutants during very short periods of time (less than 4 hours). However, the mobilisation of pollutants is not the same as the export of pollutants as the chemicals are often incorporated back into the sediment without leaving the waterbody. This happens during periods of stagnation.
The following parameters should be measured on a weekly basis from the inflow and outflow as part of the routine monitoring program:
126.96.36.199 chemical monitoring of wetland health
The wetland will gradually change its character over the years. Some of the changes will be beneficial, such as a reduction in seepage. In general, the wetland will become more mature with a more stable ecosystem. How this will affect the function of the wetland as a filter is unknown. The knowledge of wetlands is still very basic and any information that will help the design of future wetlands both locally and internationally should be documented. For these reasons the sampling locations need to be more extensive than just at the inlet and outlet. The actual locations can only be chosen after the design has been reviewed but a sample should be taken at the major chemical and biological interfaces within the system.
The parameters measured for the wetland effectiveness should duplicate the samples taken for monitoring the wetland health plus the following:
The nature of the flow through the wetland is as important in monitoring the biology as it was for the chemistry. Some of the biological changes can be inferred from the chemical changes in parameters such as the nutrient concentrations. However, it is still important to monitor species’ population dynamics and distribution through the serial flow of the wetlands. Typical monitoring should include:
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