Invertebrates play an important role in the food chains of the saltmarshes, and are important for the health and function of saltmarsh ecosystems. Within the benthic habitat, vascular plants such as samphire, are grazed by herbivorous insects including plant hoppers and grasshoppers. These are in turn fed upon by carnivorous organisms, for example, spiders. The breakdown of organic matter in the marsh is performed by microbial fungi and bacteria, which subsequently comprise a food source for the larval stages of larger invertebrates and microscopic invertebrates such as copepods. The larger invertebrates; polychaetes, molluscs and crustaceans live within or near the sediments and commonly become a food source for wading birds.
The saltmarshes that occur in selected areas of the Peel-Harvey Estuary are considered to be an important attribute of the system, and contribute to the invertebrate biomass and abundance (Plates 5.1, 5.2 and 5.3). The marine component of the fauna associated with saltmarsh sediments are generally common species of adjacent mudflats, where they form the food supply of internationally important gatherings of wading birds and waterfowl. They also play a significant role in decomposition, which in some circumstances results in a net export of carbon and nutrients into adjacent waters (Mason et al., 1991).
The invertebrate species occurring on the samphire flats of the Peel-Harvey Estuary have not been well documented, and little is known about the distribution and composition of these invertebrate communities.
Boulden (1970) surveyed the shallow waters of the Peel Inlet whilst Rose (1992, 1994) investigated and compared macrobenthos and benthic zooplankton communities respectively for the Peel-Harvey and Swan Estuaries, suggesting that species abundance and diversity were lower in the Peel-Harvey. Chalmer and Scott (1984) studied fish and benthic faunal of the Leschenault and Peel-Harvey estuaries and found that both estuaries were dominated by bivalve molluscs, polychaete worms and amphipods. Wells et al. (1980) studied the biology of molluscs in the Peel-Harvey and recorded 34 species of molluscs, the dominant species being Hydrococcus grandiformis and Arthritica semen. A number of other studies have concentrated on the molluscs of the Peel-Harvey system (Wells and Threlfall, 1980, 1981, 1982a, 1982b).
Several overseas studies have dealt with invertebrate communities and distributions on saltmarshes. Mason et al. (1991) investigated the invertebrate assemblages of Essex saltmarshes and suggested that a holistic approach to their management is required. Ranwell (1972) suggests that the dimensions, population size and behaviour of a few species, and the degree of diversity of the remaining species seem to be the more significant biological elements of the saltmarsh ecosystems. A study by Clancy (1994) of the Sydney region found 574 species of insects, 17 species of spiders and 12 species of mites in the mangrove and saltmarsh communities.
Due to the importance of the Peel-Harvey Estuary, and the limited invertebrate information available, a seasonal sampling regime was undertaken on selected areas of the samphire marshes, to find out which species occur within the different zones of the marsh.
The transects were separated into three defined areas (Figure 1.2). Area 1-low marsh; area 2-middle marsh; area 3-high marsh. These three areas were considered to be substantially different due to characteristics of tidal inundation, vegetation type and distance from the shoreline.
The transect areas were sampled once during the summer of 1993/94 and winter 1994 as these represented periods of differing water availability. Two types of invertebrate sampling techniques were used; core samples were taken on both sampling occasions while sampling with a sweep net was performed in winter, when more water was present.
The core samples were collected with a 10 cm diameter corer. The sediment was collected to a depth of 25-30 cm of which the top 10 cm was retained. The samples were placed in plastic bags, labelled and saturated with 70% ethanol to preserve any invertebrates which may occur within the sample. Four random replicate core samples were collected from each area within the transect.
The sweep samples were collected with a 250 µm sweep net. Two replicate sweeps, taken over a distance of 10 m, were collected at sites when sufficient water was available. Generally insufficient water was available to collect separate sweep samples at areas 2 and 3, and due to this, these areas were combined and called area 2. The samples were then placed in plastic bags, labelled and saturated with 70% ethanol.
In the laboratory, samples were washed through a series of sieves, decreasing in size from 2000 µm to 250 µm. The respective fractions were then sorted and all invertebrates present were collected, counted and later identified.
Species abundances recorded on the transects in winter (Figure 5.2) tended to be more consistent than in summer (Figure 5.1) over all areas. Area 3 showed lower abundances than areas 1 and 2, and could be an indication of the tidal regime. Areas 1 and 2 were generally similar in abundance, with area 3 being lower. Site 2 showed very low abundances in all areas.
In Figure 5.3 the mean species
abundance for winter sweep sites indicated a substantially greater abundance
at area 1 in most transects. The only exception
to this was at Site 4 where abundance is half that of area 2.
The mean species richness for winter cores at each transect is illustrated in Figure 5.5. This indicates a higher richness at areas 1 and 2, although generally richness was low for all transects. A comparison between Figures 5.4 and 5.5 showed winter to have a slightly higher species richness than summer.
Figure 5.6 displays the mean species richness for winter sweep sites and indicated a substantially higher richness at area 1. Site 4 in area 2 had a higher richness than area 1, which correlates well with species abundance.
During summer, the Peel-Harvey estuary dries significantly, often becoming hypersaline. Flushing in this period is decreased, water levels are low (Wells et al., 1980) and an import of nutrients onto the samphire marshes often occurs (Latchford, 1994). Cores collected in summer indicate abundance and richness to be low, and this may be attributed to the estuary drying out. Higher abundances and richness occurred at areas 1 and 3, which could be due to the proximity of the marsh to water at area 1 or to a greater number of terrestrial and transitional invertebrates such as spiders, springtails and slaters (isopods) occurring at area 3. The occasional inundation of this area may provide an additional food source for invertebrates occupying these areas. Area 2 may be a more difficult environment in which to survive. It receives less water and thus the life span of aquatic invertebrates may be reduced, however, it may receive sufficient water to deter colonisation by terrestrial invertebrates.
Invertebrate abundances and richness were found to be greater at area 1 in winter. This suggests that the conditions within this area are more favourable for habitat and feeding, due to intertidal inundation. The higher abundances and richness would also be attributable to the available water, as this region is frequently inundated due to tidal influences. The lower abundance and richness of aquatic invertebrates at area 2 is due to the irregular flooding of this region. The high marsh generally has a minimum of ten days of exposure to air (Latchford, 1994). Area 2 (areas 2 and 3 combined) is found within this intertidal area (Figure 1.2).
Invertebrates occurring within the saltmarshes are a mixture of terrestrial and aquatic species, occurring within the aerial, benthic and aquatic habitats. The major species occurring in quite high numbers were Oligochaetes. Oligochaete sp 1 dominated Site 2 and Site 6. The bivalve Arthritica semen was prominent at Site 6 and was also found in substantial numbers at Site 7 and at Site 8. Arthritica semen is a filter feeder and exclusively an estuarine species (Thurlow et al., 1986). The location of A. semen at these sites may be due to the influence of rivers and drains providing nutrients for food.
The isopods Syncassidina aestuaria and Haloniscus sp 1 dominated the winter samples, with Haloniscus sp 1 occurring in the majority of sites. The isopods were found within samples containing a combination of plant debris and algae mats and are generally found under mats of dead, decaying plant debris (Kraeuter and Wolf, 1974). Site 10 and Site 4 also contained Haloniscus sp 1 as a dominant summer species. These two sites were quite moist in summer and had not dried out to the extent of Site 2 and Site 7, which may explain the invertebrates' dominance.
Site 4 and Site 8 were dominated by the amphipod species, Ceinidae sp 1 and Ceinidae sp2, as well as Corphium minor and Erichthonius sp 1 which were present in summer and winter samples. Thurlow et al. (1986) suggested that C. minor and certain other amphipod species can tolerate a wide range of salinities. These two transect sites contained water throughout the year which suggested that the amphipod species occurring there can tolerate the varying conditions and suggests that they are a beneficial part of the food web throughout the year.
The copepod Harpacticoida sp 1 occurring at Site 4 and Site 7 in substantial numbers may be a new species. Harpacticoid copepods are strongly influenced by factors such as desiccation and oxygen availability (Dye and Furstenburg, 1981). The frequent tidal inundation of these marshes during the winter months would account for the suitable food source and oxygen availability required by the Harpacticoid species, and suggests that these marshes are nutrient rich during winter. The ostracod Bennelongia sp 1 and the polychaete Ceratonereis aquisetis were not abundant species but were unique as they were only found towards the bottom of the Harvey Estuary in the sampling period. The influence of the fresh water from the Harvey river and agricultural drains as well as the sea water entering the Peel Inlet at Sticks channel may account for the dominance of these species.
Due to the limited literature available on the invertebrate assemblages of saltmarshes it is difficult to compare this work to previous studies. A comparison with Mason et al. (1991) who used a similar sampling technique, indicated that the sweep net samples collected were more variable in taxon richness than cores, and were a good indicator of invertebrate assemblages. Mason et al. (1991) indicated that cores collected low species richness but high species abundances, especially of marine species. This was true for this study, although a comparison of marine species from the invertebrate species collected has not yet been fully determined.
Mason et al (1991) classified the conservation value of the saltmarshes according to community distinctiveness, species richness, species rarity and community functioning, suggesting a holistic approach to management. This approach should be considered as Sites 4, 6 and 10 contained substantial richness and abundances.
A study by Clancy (1994) on the terrestrial arthropods of mangroves and saltmarshes, found a substantial number of invertebrate species occupy the saltmarsh and mangrove areas. The sampling technique involved sweeping across the saltmarshes, collecting invertebrates which occurred on or above the marshes. Clancy’s method differed from the cores and sweeps taken by this study but an interesting comparison was the number of species of spiders found. Clancy collected 17 species of spiders compared to eight species found in this study which suggests that spiders are an important component of the terrestrial make-up of the saltmarshes.
The impact of the Dawesville channel upon the saltmarshes and invertebrate assemblages of the Peel-Harvey Estuary is difficult to predict and remains to be determined. No real impacts upon the invertebrate assemblages can be gauged from this study due to the limited time span over which sampling was undertaken and the lack of previous studies. However, some potential impacts upon the invertebrate distributions and assemblages from the Dawesville Channel are likely due to the altered tidal levels of the Estuary. The Mean Low Low Water Level to the Mean High High Water Level range has changed from 9.7 cm to 27.6 cm in the Peel Inlet and from 8.6 cm to 37.9 cm in the Harvey (Ryan, 1993). The implications of this are that tidal ranges will be more variable and more frequent (Waterways Commission, 1994) which may alter saltmarsh distributions and hence alter invertebrate distributions. The effects of the tidal patterns is predicted to be greater in the Harvey Estuary (Waterways Commission, 1994) where higher invertebrate richness and abundance was recorded.
Other implications include greater ocean/estuarine mixing which will maintain the salinity levels in the estuary more similar to the marine environment for most of the year (Waterways Commission, 1994). This may alter the range of some invertebrate species, especially those who require a fresher aquatic environment. The greater interaction of marine species with estuarine species may also have a significant effect on the food chain, as marine species may out compete estuarine species for food or disrupt ecological niches.
The results from this study have provided useful information on the composition and abundance of invertebrates in the Peel-Harvey saltmarshes, however to obtain a good understanding of the invertebrate assemblages and distributions a longer sampling period is required. Due to the construction of the Dawesville Channel, and limited literature on invertebrate assemblages, it has been difficult to predict how the invertebrates of the Peel-Harvey Estuary will change over time. It may be that the higher tides will reduce the harshness of the low marsh increasing the productivity of this area, on the other hand it may lead to a reduction in specialised invertebrates which are adapted to the extreme conditions.
The major area of invertebrates neglected by the sampling methods of this study were terrestrial species, such as spiders and grasshoppers. The invertebrates collected in this study were generally aquatic in habitat. Terrestrial invertebrates that were collected were either in the water-column or on vegetation when sampling occurred. Studies undertaken in New South Wales indicate that spiders form an important component of the saltmarsh fauna, but little is known about the conditions they require within the marsh (Clancy, 1994).
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