3.1 Introduction

"obtain a better understanding of the significance of the marshes to the general ecology of the area, including their role in shoreline stabilisation, nutrient dynamics, and bird life. "   Hodgkin et al. (1985), p35

 
It was clear that much of the study would of necessity have to rest on a firm understanding of the extent and composition of the marsh, and the required mapping of the marsh was accomplished by interpretation of aerial photographs combined with field visits. The photointerpretation approach used was described in Chapter 2. That chapter described the distribution of samphire flats in the study area using April 1994 colour aerial photography, which was flown at low tide three days before the opening of Dawesville Channel.

In this study both the historic and current distributions of saltmarsh were mapped. A common approach was followed in interpreting all available aerial photographs, which included black and white, normal colour and false colour infrared photography.

As explained in detail in Chapter 2, attempts to map all saltmarshes using the photointerpretation technique proved difficult and prone to misinterpretation, especially with black and white photographs. A decision was therefore taken to map the extent of saltmarsh dominated by samphire, and exclude areas with less than 30% cover and areas dominated by terrestrial shrubs and trees.

3.2 Extent of the Saltmarsh

Austin Bay area                 37.758 ha
Creery Wetlands               140.156 ha
Harvey Delta                   144.783 ha
Goegrup Lake area            45.289 ha
Roberts Bay                      80.749 ha
other                                188.405 ha
total                                 630.140 ha

Although Harvey Delta had the largest area of samphire, the cover was very fragmented because of the geomorphology of the area. Creery Wetlands formed the largest continuous cover of samphire in the Peel Harvey region, although this was showing fragmentation due to human disturbances such as vehicle tracks.

The total area of samphire marsh, 630.140 ha for 1994 was only half of the area estimated made by Hodgkin et al. (1985). However their estimate included Juncus kraussii, and other saltmarsh vegetation. It is also not clear from that reference on what basis their estimates were based. Our estimates from the historical data (Chapter 2) show approximately 9 km2 at the time of the estimate reported by Hodgkin et al. (1985). The difference of 5 km2 between the two estimates is attributed to different definitions of what has been mapped as samphire/saltmarsh.
 

3.3 Composition of the Samphire Saltmarshes of the Peel-Harvey System

3.3.1 Methods

To check the health of the saltmarsh and understand the tolerances and growth rates of the plants, several investigations were undertaken to document the environmental tolerances and growth rates of key species. The first was to examine the percentage cover of saltmarsh species in 30, randomly-distributed quadrats in autumn and winter. Secondly, plant biomass was determined at six of the ten sites. For this purpose each transect was divided into three areas based on perceived similarity of vegetation in each section. Area 1 was closest to the water's edge and area 3 the furthest (Figure 1.2).

The above ground biomass of saltmarsh species in nine randomly distributed quadrats was determined by harvesting all the above ground plant material and obtaining the dry weight. Samples were harvested in autumn and winter. The total nitrogen and total phosphorus content of the biomass samples was also measured. In addition the above and below-ground biomass of the major species in these transects were determined and total nitrogen and total phosphorus concentration measured.
 

3.3.2 Results and Discussion

Species Occurrence

Suaeda australis
This branching understorey shrub was up to 30 cm tall, with slender fleshy leaves which are light green, turning red or purple in autumn (Plate 3.1). It was found with S. quinqueflora, and often had obvious accumulations of organic debris.
 
Bolboshoenus caldwellii
This ephemeral introduced species bears long, grass-like leaves from rhizomes in the winter/spring period, when salinities are low; it senesces over summer and autumn as salinities increase (Marchant et al., 1987; Pen, 1983). In this study, small stands were found interspersed with shoots of S. quinqueflora, forming a closed sedgeland.
 
Halosarcia species
Several species from the genus Halosarcia were found in the marshes. The procumbent stems of these perennial species are hard and woody, but branches contain fleshy segments with succulent branchlets are which appear articulate (Plate 3.1). They form an open heath in the high marsh, which reaches high salinities during summer, with lower salinities, close to that of freshwater, during winter (Pen, 1983; Marchant et al., 1987). Halosarcia indica subspecies bidens is a large, green shrub, up to 60 cm tall (Marchant et al., 1987). An apparently different growth form Halosarcia indica subspecies aff. bidens was found to conform to the species description, except that the branchlets were more succulent and appeared elongated, reaching about 20 cm tall. This was less common than Halosarcia indica subspecies bidens and was found at higher elevations, generally fringing saltmarsh concavities. Halosarcia indica subspecies leiostachya, which possesses cylindrical to ellipsoid spikes was also found (Marchant et al., 1987). However, it was only found on the Creery Wetland transect at Site 5 and did not appear to have been mentioned in other studies of Western Australian saltmarshes. Halosarcia halocnemoides is a smaller, bushy shrub some 30 cm tall. It has reddish-green stem segments with many slender branchlets which lose their succulent tissue on maturity, and become woody (Pen, 1983; Marchant et al., 1987). This species tended to occur in the drier, most saline regions of the saltmarsh, and was present on most transects.
 
Frankenia pauciflora
This is a small, prostrate to ascending shrub with small, linear leaves with downturned margins and small pink or white flowers (Bridgewater, et al. 1981). It was found on the drier banks in the marsh, and formed a heath with S. quinqueflora and Suaeda australis, or with Halosarcia species. These communities were restricted in distribution.

 Juncus kraussii
This tall rush- the ‘shore rush’, has cylindrical, pointed, firm culms with spongy pith. The loose, clusters were often found as dense bands of closed rushes, although it was also found growing more sparsely in localised stands in the S. quinqueflora complex. It grew up to 1.5 m high and occurred on the drier, elevated parts of the marsh or in brackish areas where the salinities were lower (Bridgewater et al., 1981; Pen, 1983). It was usually flooded by the highest tides and at some sites reached to the waters edge at low tide.
 
Atriplex species
These are decumbent herbs or shrubs with minute, bladder-shaped hairs which give the plants their characteristic grey colour (Morley & Toelken, 1987). Atriplex hypoleuca is a sprawling decumbent shrub which was found to grow up to 2 m in diameter (Marchant et al., 1987). It was usually found associated with J. kraussii close to the water’s edge. Atriplex prostrata is an introduced annual herb and was found to spread up to 60 cm long. The stems are slender and angular, with arrow shaped leaves (Marchant et al., 1987; Morley & Toelken, 1987). This was usually found on drier, elevated banks.
 
Cotula coronopifolia
This small, fleshy, annual daisy germinates and grows in winter and flowers in September (Stoner, 1976). The toothed leaves loosely sheath the stem, and the solitary flower heads are yellow (Marchant et al., 1987). It was found in the higher damp areas of the marsh and, according to Bridgewater et al. (1981) is influenced by fresh water.
 

Zonation of Plant Communities across the Marsh

Table 3.1. Vegetation units of saltmarsh fringing the Peel-Harvey estuarine system. The first letter of the code defines the complex, and the second letter, the second most dominant species or, if capitalised, the second dominant species.

Several patterns of zonation were recognised in the ten transects (Figures 3.1 ,3.2 , 3.3, 3.4). There were two major sequences in which the complexes were arranged. These were, in order from the water’s edge: bare ground, Sarcocornia, Juncus (Figure 3.1) and bare ground, Sarcocornia, Halosarcia. (Figure 3.3). The main factor contributing to the first sequence was thought to be decreasing salinity, and the main factor contributing to the second sequence was thought to be increasing elevation with a high salinity in summer. Although these general trends were observed at the sites, the sequence was not precisely followed at all. While a definite pattern of zonation with sharp changes between complexes occurred at all sites, the saltmarsh vegetation tended to be a mosaic of the communities represented in Table 3.1.

The Sarcocornia complex was found at the lower elevations, the Juncus complex at the higher, as was the Halosarcia complex. Monospecific stands of Sarcocornia quinqueflora (S1) were usually found at the lowest points on the transect (Figure 3.1), with S. quinqueflora-Suaeda australis (Su) or S. quinqueflora-Bolboschoenus caldwellii (Sb) communities found at slightly higher elevations (Figure 3.1). Communities of Sarcocornia quinqueflora-Atriplex species (Sa) and S. quinqueflora-Juncus kraussii (Sj) had a more scattered distribution on the high elevations and, for the latter, fringing brackish waters and the Juncus complex (Figure 3.2). Sarcocornia quinqueflora-Halosarcia (Sh) communities and S. quinqueflora-Frankenia pauciflora (Sf) were found in isolated areas, usually on high banks on the high elevation sites.

Communities dominated by J. kraussii-S. quinqueflora (Js) and J. kraussii-B. caldwellii (JB) were found largely on the slightly elevated bank of the Serpentine River and, as well as the Juncus kraussii pure community (J1), towards the landward end of some sites at slightly higher elevations (Figures 1 and 2). The J. kraussii-Atriplex hypoleuca dominated (JA) community was usually found on elevated banks occupied by this complex (Figure 3.2).

The Halosarcia halocnemoides (H1) community was usually found at the lowest elevation of the Halosarcia complex, fringing this community on the side closest to the water (Figure 3.3). The H. halocnemoides- H. indica subsp. bidens (Hb) community is found over the higher areas as is the rarer H. halocnemoides grass dominant (Hg) community. The H. halocnemoides-H. indica subspecies leiostachya (Hl) and H. indica subspecies leiostachya dominant (HL) communities were found at Site 5 (Figure 3.4) on the relatively flat concavity surrounding a salt pan, with the H. indica subspecies leiostachya community found on the fringe closest to the Mandurah Channel.

Percentage Cover

The Sarcocornia quinqueflora occurred sparsely along transect 5 across the Creery wetlands (Figure 3.5), and only formed a large percentage of cover where there was little bare ground, such as in the first area of the marsh, by the water’s edge (Figure 3.5). Halosarcia species dominated most of the marsh, which had a large proportion of bare ground. In particular, Halosarcia halocnemoides dominated most of the marsh from the second area close to the water and Halosarcia leiostachya was found at the driest, most saline area of the marsh. This species was not found at any other site.

The areas of marsh closest to the water’s edge at Site 7, east of Peel Inlet contained a higher percentage cover of S. quinqueflora as well as Suaeda australis and Atriplex prostrata than did other sites (Figure 3.6). Halosarcia species dominate the higher marsh, away from the water, in particular, Halosarcia halocnemoides. The grasses and daisy Cotula coronopifolia were also more abundant in these areas, with the rush Juncus kraussii being found on the landward edge of the marsh. This site on the east of Peel Inlet, displayed different communities on both the higher and lower elevation marshes.

An example of the lower marsh in the region was found east of Harvey Estuary (Figure 3.7). This had mostly bare ground in the area edge, consisting mainly of a pioneer zone of S. quinqueflora. Most of the marsh was dominated by S. quinqueflora, with some Atriplex species and Suaeda australis, and the landward edge was dominated by the rush Juncus kraussii.

Biomass of Saltmarsh Vegetation

The first was found at Site 2, Lake Goegrup. There was a striking decrease in biomass in area 1 and a moderate decrease in area 2, over the winter period, but no substantial change at area 3 (Figure 3.8). This decrease in areas occurred because the two dominant species in area 1, Bolboschoenus caldwellii and Atriplex hypoleuca, and a codominant species of area 2; B. caldwellii are ephemeral, and die back substantially during winter, while the dominant species in area 3 are perennial.

In contrast, there was little seasonal change in the higher elevation marshes such as Site 7, east of the Peel Inlet (Figure 3.9). This was because the dominant species, Halosarcia halocnemoides and Sarcocornia quinqueflora, were perennial and because the high elevation ensured little wave damage to plants. The high statistical variance in areas 1 and 2 during autumn suggest a greater difference in the biomass in these areas in this season.

The third pattern was that of a general decrease in biomass of area 2, but with little seasonal change at areas 1 and 3. This occurred because of the larger component of the annual, Atriplex prostrata, in area 2; this species dies back in the winter, while the other zones were dominated by the perennial species S. quinqueflora and J. kraussii (Figure 3.10).

Below ground biomass during autumn was substantially higher than that of above-ground material (Figure 3.11).

The weights and the ratios of above : below ground material differed considerably with time. This variation was even greater in winter, (Figure 3.12) when the above ground components were larger, so that Atriplex hypoleuca and both growth forms of Halosarcia indica subspecies bidens had slightly larger above ground material compared with below-ground material.

The occurrence of higher weight in the below ground component agreed with studies performed in Australia and Europe (Knox, 1986a).
 

Seasonal changes in the nutrient content of saltmarsh vegetation.

There was a higher nitrogen concentration in the above-ground material in winter (Figure 3.13). This is supported by the literature which states that most nitrogen leaching from saltmarsh plants takes place in autumn, and most absorption of nitrogen occurs in early summer before maximum biomass is achieved (Knox, 1986a).

In area 3 at a number of sites, the total nitrogen concentration in vegetation was approximately the same in both seasons (Figure 3.14). This could be related to the very dry winter prior to the winter sample, and the high elevation of area 3. Both factors would ensure less inundation by nutrient rich waters, which could limit the amounts of nutrients available for plant uptake for absorption by the plants. It could also account for the relatively lower nitrogen content of plants in area 3 of most sites, compared to other areas, of most sites during both seasons, and the greater decrease in winter.

The similarity in nitrogen content of plants at different sites, could suggest that the marsh plants concerned have similar phenology, or that physical factors acting upon them are similar at the various sites around the system. There were slightly smaller concentrations at two sites, (Figure 3.14), and these may result from their close proximity to the mouth of a river. At such sites the salinities of open water and sediment salinities would be relatively low, which may affect nitrogen transformations.

There was also a higher concentration of total phosphorus in the above ground component at most sites in winter, (Figure 3.15 & 3.16) similar results have been reported for J. kraussii in the Blackwood Estuary (Congdon & McComb, 1980).

The concentration of phosphorus in area 1 at the Lake Goegrup transect (Figure 3.17), differs from an expectation based on the findings of Congdon and McComb (1980) who found the nitrogen to phosphorus ratios to be higher in J. kraussii plants fringing the water than in those at the landward edge. This can be explained by the dominant species of area 1, which consisted almost entirely of large bushes of Atriplex hypoleuca, with some Bolboschoenus caldwellii. These two species, especially A. hypoleuca, had very high concentrations of phosphorus and this plant showed a marked increase in phosphorus concentration in winter.

Apart from these exceptions, there was a general similarity in the concentration of phosphorus at the different sites. The phosphorus content at area 3 at most sites appeared to be less than that of the other two areas in both seasons, as is illustrated in Figure 3.15. This is consistent with Congdon and McComb (1980) who found the nitrogen to phosphorus ratios to be higher in those plants fringing the water than those at the landward edge of the marsh.

The high nitrogen to phosphorus ratio found in all the species concurred with other saltmarsh species data (Congdon & McComb, 1980; Rose & McComb, 1980).

The higher concentration of nitrogen and phosphorus contained in whole plants, and in the above-ground components, of most species during winter (Figures 3.19 and 3.21), was the same as that found in the communities sampled in the three areas of the transects and is similar to that cited in the literature for saltmarsh plants (Congdon & McComb, 1980). This could be because most nutrients are obtained from the estuarine waters and during the winter the marshes are frequently inundated with nutrient rich waters. This similarity may result from the ready availability of nutrients in estuarine water in winter.

There was less variation in the nitrogen and phosphorus concentrations of the below ground component of whole plants, presumably because this component was not subject to the same environmental extremes as above-ground material, and tended to senesce less between major growth periods; in contrast, there was apparently an increase in the concentrations of below-ground nutrients taken from all areas (Figures 3.18 ; 3:19; 3:20; and 3.21).

In most of the species sampled there was a high concentration of total nitrogen and total phosphorus in the above-ground component. This was accounted for by the fact that much of the store of nitrogen in plants is in the protein photosynthetic apparatus which would be more evident in the above ground component. There was a smaller difference in total phosphorus, which usually more equally distributed within plants.

Nitrogen and phosphorus in the below ground material was only slightly higher in autumn than in winter in H. halocnemoides and A. prostrata. The small exceptions in below-ground nitrogen in winter included B. caldwellii, J. kraussii and, marginally, H. halocnemoides. The species with slightly more phosphorus in the below ground component were B. caldwellii, J. kraussii in both seasons and, marginally, H. halocnemoides in autumn. B. caldwellii and J. kraussii use salt evasion strategies such as ephemeralism and shedding of salty culms, and Halosarcia species use succulence, all of which are not as energy expensive as the method used by Atriplex species (Waisel, 1972).

The variation between seasons may have occurred because the plants with a higher content of dead material or old growth such as J. kraussii and H. halocnemoides contained fewer nutrients in above-ground material in autumn. This would be more pronounced in autumn when much of the nutrients would be transported to the roots. It would also be more pronounced in phosphorus for species such as J. kraussii which tended to retain its phosphorus more tenaciously (Congdon & McComb, 1980). The greater below ground nitrogen content in winter and phosphorus content in both seasons in B. caldwellii could be explained by its ephemeral nature. In the winter sample, the shoots were just beginning to sprout and most nutrients would still be retained in the roots, and during the autumn the nutrients were just being leached or transported to the roots as the above ground component senesced. The higher phosphorus retention is likely to be a result of a tendency to retain phosphorus similar to J. kraussii. The annual A. prostrata would likewise senesce first in the above ground component in the autumn.

The mean total nitrogen and phosphorus concentrations in the autumn above ground biomass of S. quinqueflora was similar to that found in S. blackiana by (Congdon & McComb, 1980; Rose & McComb 1980).

The small differences in nutrient concentrations between species, in some areas, may suggest similarities in phenology and physiology, however some trends were clear. The highest concentration of total phosphorus was found in plants such as A. hypoleuca, with the most energy-expensive salt exclusion strategy, and the lowest percentages were for J. kraussii, H. halocnemoides and H. indica subspecies aff. bidens, all of which have less energy-expensive methods of salt reduction. However the high concentrations of phosphorus may also occur because most phosphorus is obtained from the estuarine waters and that those closer to the water’s edge would be most frequently inundated with phosphorus-rich waters.

Similar observations with nitrogen were documented during the autumn, especially in winter. S. quinqueflora, Suaeda australis and A. hypoleuca containing highest, and J. kraussii the lowest, concentration of total nitrogen.

3.4 Conclusions