The area is also significant for other ecological reasons which will be briefly outlined, along with the major ecological points suggested in the previous chapters, and compared with data and literature generated from saltmarsh research elsewhere in the world. In this way it is hoped that a better appreciation of the ecological significance of the saltmarshes in the Peel-Harvey Estuary will be reached.
Aside from structural complexity, saltmarshes are an environment with a dynamic melange of unique physical and chemical characteristics (Mann, 1982). These characteristics predispose the marsh to providing a number of important physico-chemical functions. Combined with the biota, they create a number of critical processes which heavily influence the estuarine ecosystem, for example nutrient processing and detritus and carbon fluxes (Odum, 1988). Altogether, this strongly suggests that the saltmarshes of the Peel-Harvey are extremely dynamic and complex and are probably affected by varying spatial and seasonal patterns, just as the aquatic flora and fauna of the estuary are already known to be so affected (McComb and Lukatelich, 1986; Loneragan et al., 1987; Rose, 1994). Spatial and seasonal patterns exist in either plant growth or production and nutrient processing or detrital loading, all of which can affect estuarine waters, suggesting that saltmarshes provide important signals that stimulate plant and animal communities found in the whole estuarine ecosystem (Odum, 1988; Kennish, 1990).
Many of the surveyed saltmarshes displayed elevations between 0 and 2.0 metres AHD (Table 6.1). Measuring topographical features or contours and rates of sediment deposition or loss in active areas around the estuary fringe can lead to estimates of plant species diversity and the stage of plant community succession (Kennish, 1990). In general, it has been found that accretion rates of 3 mm or less are very low while rates greater than 10 mm are high (Kennish, 1990). Contour features and rates of sedimentation provide information on the value of saltmarshes to dissipate wave energy and therefore moderate erosion processes. They also provide a record of tidal inundation and propensity of the area to be flooded which in turn indicates how salty or fresh the area will be for plants.
Previous chapters also reported that the sediment composition of the saltmarshes varied but was primarily composed of silty or muddy and detrital based sands (Table 6.1). The presence of sands provides strong evidence of the coastal origins of the sediment in the marshes while silts, muds and detritus (organic material) provide evidence of riverine and in situ or within system origins (Odum, 1988). These sedimentary characteristics also influence important features such as Redox profiles or vertical depth cross sections of the sediment, where oxygenated and de-oxygenated sediments lie. The depth where oxygenated or de-oxygenated sediments can be found influences chemical processes such as the pH of the soil, sulfide gas production, nutrient binding, plant root penetration and animal locations. Generally the more de-oxygenated the sediment, the more the gas production and the less the pH (more acidic) and presence of plants and animals (Kennish, 1990). Sediment composition can also influence sub-surface hydrology, so influencing the duration of surface flooding or standing water, sediment porosity and the nature of flooding water by influencing its salinity. Furthermore, the silt and clay content in the soil affects the ability of the sediment to bind nutrients (Kinhill, 1988).
The literature suggests that the extent and proportions of flats and pans or higher relief Halosarcia, shrubs, rushes and trees provide a measure to estimate the age or stability of the saltmarsh (Frey and Bassan, 1985, cited in Kennish, 1990; Adam, 1993). For example, a saltmarsh such as the Creery Wetlands near the Mandurah Entrance Channel (Figure 1.1), which has roughly equal proportions of Sarcocornia flats and higher relief Halosarcia and trees, could be considered a relatively mature and stable saltmarsh. (Admittedly, the fractionation and road rutting of the low lying Sarcocornia flats indicates that "degradation" is destabilising this relatively mature saltmarsh.) In contrast, the wide expanses of low flats and Sarcocornia around the Harvey River mouth and delta suggest that these areas are young and probably the result of recent sediment deposition caused by upstream erosion.
This leads to our first Ecological Significance Point. Physical features of saltmarshes leave a geological record of events, identify areas of the estuary undergoing erosion or accretion , influence chemical and nutrient dynamics, influence sub surface hydrology, influence the composition of plant and animal communities and provide a valuable service in buffering erosion and siltation events. Saltmarshes provide a physical link between land and estuary water and influence nutrient and sediment exchange between the two.
Overall, more than 40 species of saltmarsh plants (Table 6.1) were recorded in this and other previous studies (for example Bridgewater et al, 1981) which indicated considerable species diversity for the total saltmarsh habitat. Most of these species also displayed seasonality, the ephemeral grasses, perennial rushes (e.g. Bolboshoenus spp.) and occasionally low lying Sarcocornia dying back considerably during winter while higher elevation perennials, such as Halosarcia, did not display noticeable changes in biomass (weight).
The physical features of saltmarsh plants were highlighted in Chapter Three, where in particular, above ground biomass was found to be lower than below ground biomass during certain seasons. This feature of the Peel-Harvey saltmarsh flora indicates that root systems play a crucial role in influencing the sediment. Root systems would affect sediment porosity and drainage of water, they would affect animals living in the sediment, oxygenation of the soil (Redox) and sediment nutrient dynamics. Perhaps most importantly they would help stabilise sediments and the topography of the saltmarsh. Combined with above ground plant growth, they help to act as baffles and collect suspended sediment in the water. Overseas research has indicated that root systems of saltmarsh plants anchor the sediment, stabilise substrates and mitigate against erosion (Kennish, 1990).
Previous chapters discussed the tolerance of various species of plant seeds to water saturation and duration of exposure to salty or fresh water. In turn, this suggests that the seed bank in the soils of the various zones of the marshes would be extremely important in re-colonisation of new, denuded and degraded saltmarsh habitat. The variety of saltmarsh species found in the seed bank would be important in determining the potential for the establishment of certain plant complexes if altered tidal regimes and salt exposure caused by the Dawesville Channel eliminate current plant communities. The speed by which new saltmarsh plant communities become established because of changes caused by the Dawesville Channel is critical in terms of minimising erosion and influencing nutrient and carbon balances of nearby waters.
The results of Chapter Five indicated that saltmarshes are a haven for some animals, particularly benthic species. However, over nine taxa of spiders were collected and this indicates that the aerial portion (above ground) of saltmarsh plants is very important to this predatory taxon. Although alluded to in previous chapters but not specifically measured in this study, other animals such as snakes, lizards and terrestrial insects such as ants would be directly affected by saltmarsh plant cover. This cover provides shelter, food and substrate for prey animals. Leaves and stems serve as attachment substrate for many animals and together, aboveground and belowground habitat complexity provided by plants would help account for the relatively high densities of animals collected at some sites.
The second Ecological Significance Point about saltmarshes is that their presence increases plant species diversity in the region and maintains biodiversity, provides a pool of plant species to re-colonise salt affected land, stabilises sediment minimising erosion or mobility, provides habitat diversity for animals and organic sustenance or food for bacteria and animals as either detritus or grazing material. Water exchange through tides and flooding ensures that potential food and nutrients are exchanged between the two environments, land and water.
While there are many ways of classifying the animals of the saltmarsh, residence time may be the most appropriate for these habitats in the Peel-Harvey. This is because of the very clear seasonal differences in inundation patterns which used to exist prior to the Dawesville Channel. Winter and spring were seasons of usual inundation and where benthic and water column animals and fish could be dominant components of the marsh. In contrast, summer and autumn fauna were more typified by terrestrial fauna such as insects and birds, as most of the aquatic areas dried out during this time.
It appears that few animals are permanent residents of the marsh and this is undoubtedly due to inundation patterns. Adam (1993) has classified many animals of the marsh as permanent (such as small invertebrates found in the sediment with or without water), visitors, many of which are seasonal (such as migratory birds), daily who use the marsh only at certain tides, essential as completion of their life cycles are dependent on the marsh environment, and opportunistic. This last category may include animals which seek plant food to graze, animals to prey upon or shelter in during the day (such as macropods). The extent to which the animal communities change in relation to the Dawesville Channel is open to conjecture and will be heavily influenced by changes in plant communities and tidal immersion. One trend which has been observed is the use of the saltmarsh as a shelter or habitat by animals not normally seen regularly in that environment and this may be due to the loss of preferred habitat because of the rapid development in the region.
In the work reported in Chapter Five, more than 60 species of invertebrates (Table 6.1) were collected and this was mainly based on benthic and scoop sampling using macro sized sorting screens. It would be safe to suggest that at least another 60 species of macroinvertebrates and vertebrates could be collected throughout the year, apart from bird species. The potential for many estuarine and freshwater species to utilise this environment, including the micro- and meio- sized organisms, makes saltmarshes extremely important to conserve from a habitat and biodiversity point of view.
The third Ecological Significance Point about saltmarshes is that the animals that inhabit or use them contribute to biodiversity, and enhance the food chains and carbon budget of the estuarine ecosystem. Animals acting as primary consumers contribute to the nutrient dynamics of the saltmarsh and estuary and help recycle material throughout the estuary. They do so by acting primarily as prey for higher order consumers and if they leave the saltmarsh after dying, thereby releasing nutrients for bacteria, fungi and plants not found in the saltmarsh.
The fourth Ecological Significance Point about saltmarshes is that the structural complexity provided by the mixture of geological and topographical features and the biological diversity of plant and animal life makes this habitat one of the most important areas in the estuarine and coastal ecosystem. In essence the presence of saltmarshes contributes to the diversity of ecotones in the estuarine ecosystem.
Benthic phytoplankton, macroalgae and vascular plants are affected by seasonal patterns in rainfall, inundation and the salinity of flooding waters. Reference to previous chapters also indicates that seasonal changes occur in plant biomass and fauna production. Since the saltmarsh is so productive and can export much of its productivity to nearby waters or to non-aquatic animal communities, it is very likely that the saltmarsh sends profound signals to other parts of the estuarine ecosystem. These signals may stimulate both the aquatic plant and animal communities found in the shallows and deeper basins of the estuary. Measurements of the fraction of the net primary productivity of saltmarshes to reach adjacent estuarine water range from 20 to 45% (Table 6.1) (Mitsch and Gosselink, 1986, cited in Kennish, 1990).
While the saltmarsh environment can be very harsh environment for animals, those animals which can survive inundation by brackish or salty water and desiccation during dry periods, can reach very high population densities (Day, 1981). The high population densities of some marsh fauna will influence prey communities such as birds and spiders. This may have a very significant influence on the population sizes of these predators, particularly those that depend on saltmarshes and migrate large distances to reproduce.
Such a profound export of detritus or nutrients to nearby waters may be tempered or modified by the eutrophic status of the estuary. Essentially, these signals may be overpowered by aquatic production caused by excessive phosphorus enrichment of estuarine water. It is expected that over time, the combined effects of flushing to the sea by the Dawesville Channel and catchment initiatives to reduce phosphorous export will reduce aquatic production of algae and opportunistic animals in the estuary. When this occurs the productivity signals caused by saltmarsh export of detritus and nutrients will be very important to the rest of the plant and animal communities of the whole estuary.
The fifth Ecological Significance Point is that the saltmarshes of the Peel-Harvey are an extremely productive environment which must stimulate and influence production in other communities found in the shallows and deeper basins of the estuary. These productivity signals can also affect bird populations that nest or breed overseas and in the region. They can also affect commercially important fisheries. The ramifications of losing such productive environments which export a variety of organic material with a wide variety of quality may be large and should be minimised. Their loss would drastically alter the carbon budget and food chain of the estuary.
The quality of detritus, whether it can be easily assimilated by bacteria and small and large animals, or conversely will take longer to decompose and become available, depends on the proportions of lignin and fibre and various acids (Odum, 1988). In general, algae are easily assimilated while seagrasses and emergent woody vegetation take much longer to decompose and are often poor in nitrogen, or at least in nitrogen which is accessible to bacteria, fungi and animals (Kennish, 1990). Because saltmarshes have a variety of sediment types, bacteria, fungi, plants and animals, they can also act to transform nutrients, changing dissolved oxidised inorganic forms to dissolved organic reduced forms more available to consumption by microbial and animal communities found in adjacent waters. Thus saltmarshes are critical in decomposition processes (Odum, 1988).
The sixth Ecological Significance Point is that saltmarshes are critical in their influence on the release or uptake of nutrients and carbon from adjacent estuarine waters. They are analogous to the human liver which acts a storage and metabolism organ for the human being, thus acting in a critical way upon the estuarine ecosystem. Saltmarshes function as either sinks or sources of nutrients depending upon the age of the marsh, salinity and sedimentary factors, upland and human nutrient inputs, tidal energy, quality and quantity of plant litter and the nature of the nutrient flux in the estuary to which the marsh is coupled.
The future role that saltmarshes will play in the Peel-Harvey estuarine system cannot be readily quantified. It will undoubtedly be just as, if not more influential if the eutrophic status of the estuary is reduced, longer term tidal exposure of the estuary's periphery occurs and further losses of significant portions of this habitat occur because of the impacts caused by urban development and human activity.
Table 6.1. Some major features of saltmarsh
in the Peel-Harvey Estuary
|
|
|
|
|
Found around periphery of estuary and in tidal portions of tributary rivers (mid and lower estuary) |
|
|
Varies from 0 to 2.0 AHD and displays pronounced zonation with three zones reflecting elevation differences. |
|
|
Salinity of inundating waters varies between 0 and 53 ppt (ocean is 35 ppt.) |
|
|
Influenced by lunar cycle, barometric pressure, wind velocity and fetch direction. |
|
|
Silty-clay sands with moderate to high organic content. Redox shallow and strongly reducing with lots of dissolved sulphur, plentiful reduced iron and sulphur compounds. |
|
|
Colonise all three elevation zones, lower zone dominated by halophytic chenopods and upper zone by shrubby chenopods, rushes and trees. Zonation patterns are shown. Seed bank relatively important and asexual (rhizome) propagation important. |
|
|
Lower zone dominated by green macroalgae originating from both in situ growth and growth washed onto the lower areas. |
|
|
Plant species relatively low but high compared to other saltmarshes, animal diversity high, both dominated by several species. Important that both are salt tolerant. |
|
|
Inundation phase dominated by fish and aquatic invertebrates, few snails found. Dry phase dominated by ants, lizards, snakes, birds and grazing macropods. |
|
|
Reported to contribute between 20 and 45% of primary productivity to estuarine ecosystem. Contributes immense material to animal productivity in terms of mosquitoes and birds |
|
|
Acts a significant source and sink of nutrients and organic material. |
|
|
Provides a habitat where a multiplicity of biodiversity exists and where there is a variety of habitat complexity. |
|
|
