Figure 3
Graph of site class by time interaction for Acteocina, Arthritica, Bedeva, Hydrococcus, Nassarius, Spisula and Tellina for the sampling period February 1982 to February 1987. X-axis is time. Y-axis approximates log of abundance.
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Five years of abundance data for the molluscs of the Leschenault Inlet estuary at 22 sites were analysed to provide preliminary results on patterns of their temporal and spatial variably and their potential associations. Only a limited number of the most commonly occurring species were selected for analysis, and only those collected in February and...
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Thirty-one species of mollusc were collected in Leschenault Inlet during 1982-1987. Seven species were common, with the remaining 24 species occurring sporadically, rarely or only once during the study. These seven most common species in order of general abundance were: Arthritica semen, Tellina deltoidalis, Nassarius burchardi, Spisula trigonella,...
Distribution of mollusc and polychaete communities in the Dese estuary were analysed in spring, summer and autumn. The benthic communities follow a distribution pattern which is affected by salinity gradient and established through different assemblages of species. This pattern, evident in spring, is modified from summer to autumn because of the ex...
Baseline data of bioaccumulation of heavy metals by molluscs in Bay of Bengal estuary
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... Populations are known to rapidly decline and later re-establish in large numbers equally suddenly (Ludbrook 1984). In a study of molluscs in a Western Australian estuary, Cresswell et al. (2000) noted that the abundance of this species decreased markedly with increased salinity and that it was intolerant of hypersaline conditions. Thus the sporadic and concentrated occurrence of the species as shell bands in the core (Fig. 2.5.5) is probably an expression of such population dynamics and signifies hyposalinity. ...
... Populations are known to rapidly decline and later re-establish in large numbers equally suddenly (Ludbrook 1984). In a study of the spatial and temporal distribution of molluscs in a Western Australian estuary, Cresswell et al. (2000) recorded a substantial reduction in numbers of S. (N.) trigonella with time and also noted that the abundance of this species decreased markedly with increased salinity , showing intolerance of hypersaline conditions. Thus, the sporadic and concentrated occurrence of the species as shell bands in the core is probably an expression of such population dynamics and signifies hyposalinity. ...
Nearly 6 m of uncompacted muddy sediment was recovered from the floor of the northern Coorong
Lagoon in the core Coorong #5. Radiocarbon analyses of molluscan shells indicate that sedimentation
at the core site commenced before 6830 � 90 yr cal BP, and the presence of Pinus pollen confirms a
modern age for the uppermost 0.5 m. Microfossils extracted from the core sediment samples, 2 cm slices
at 10 cm intervals, included the foraminifera Ammonia sp., Elphidium excavatum and Elphidium
gunteri; the ostracods Osticythere baragwanathi and Leptocythere lacustris; and charophyte oogonia.
Shell fragments of the estuarine bivalve Spisula (Notospisula) trigonella in the lowermost 0.7 m of the
core are evidence that these sediments were subject to some marine influence, but the absence of
foraminifera and ostracods from this same interval indicates that at the core site salinity was not sufficient
to support populations of these organisms. Thus, prior to 6830 � 90 yr cal BP the Younghusband
Peninsula was in place, in part isolating the northern lagoon from the Southern Ocean. The initial
recorded salinity event is signified by abundant Ammonia sp. at a core depth of 5.2 m. The duration of
this event was relatively brief; foraminifera were mostly absent in the immediately overlying 2 m, representing
ca 700 yr of sedimentation. This observation is attributed to substantial inflow of freshwater from
the River Murray. In the upper 3.0 m, Ammonia sp. was present in most core samples indicating that
for most of the past 6000 years the Coorong Lagoon has been sufficiently saline to support a continuing
population of this species. At a core depth of 1.3 m, the sediment sample yielded >2000 tests of
Ammonia sp., and they were accompanied by maximum pre-modern numbers of E. excavatum,
O. baragwanathi and oogonia. Taken together, these data signify the maximum pre-modern salinity
event recorded in the core sediments, probably correlating in time with regional drought conditions at
ca 3500 yr BP. Elphidium gunteri is confined to the modern sediments where it is abundant and accompanied
by equally large numbers of Ammonia sp., E. excavatum, O. baragwanathi and L. lacustris.
These data collectively indicate water conditions that are significantly changed from those that prevailed
in the Coorong Lagoon for most of the Holocene.
... Further, the general trends in abundance are different for each species. Obviously, data from different times would result in different assemblages being identified (cf Cresswell et al. 2000). However, using long-term data available in this study, seven molluscan assemblages/populations, determined by the most abundant and/or temporally persistent components, are provisionally noted below as to their occurrence in the various habitats (minor molluscan components of an assemblage or population are not incorporated into the nomenclature): ...
Thirty-one species of mollusc were collected in Leschenault Inlet during 1982-1987. Seven species were common, with the remaining 24 species occurring sporadically, rarely or only once during the study. These seven most common species in order of general abundance were: Arthritica semen, Tellina deltoidalis, Nassarius burchardi, Spisula trigonella, Hydrococcus brazieri, Acteocina sp and Bedeva paivae. The molluscs of Leschenault Inlet can be classified as follows: (1) a stenohaline marine component: Bittium granarium, Mytilus edulis, Polinices conicus, Pholas australasiae, Nassarius nigellus, Solemya australis, Irus crenata, and Venerupis anomala; (2) a euryhaline marine component: Tellina deltoidalis, Tellina sp, Theora lubrica, Sanguinolaria biradiata, Philine angasi, Nassarius burchardi, Bedeva paivae, Spisula trigonella, Epicodakia sp, and Laternula creccina; and (3) a true estuarine component: Acteocina sp, Arthritica semen, Xenostrobus securis, Hydrococcus brazieri, Fluviolanatus subtorta, Assiminea sp, and Salinator sp. Across the inlet in general, molluscs inhabited tidal sand or tidal mud, shallow water platform sand or muddy sand, or deep water basin mud, within lower, middle, or upper estuarine salinity fields. In this context, seven broadly recurring assemblages or populations of mollusc could be discerned: (1) a mixed molluscan assemblage inhabiting the tidally flushed environments of the Preston River Delta and the tidal delta leeward of “The Cut”; (2) a Tellina (+ Spisula) assemblage inhabiting the deep water central muddy basin; (3) a Tellina-Nassarius (+ Bedeva) assemblage inhabiting seagrass-vegetated platforms; (4) Tellina populations inhabiting shallow subtidal mud flats, (5) Hydrococcus populations inhabiting tidal sandy beaches; (6) Acteocina populations inhabiting tidal mud flats; and (7) Arthritica populations inhabiting low tidal to shallow subtidal sand flats. Population structures were found to be different for the three common genera. Tellina populations appeared to be maintained by a relatively continuous low level of juvenile recruitment. Nassarius populations were dominated by a mature age cohort, with a low intermittent level of juvenile recruitment. Spisula populations were numerically dominated by one age cohort, and were not maintained by further recruitment. This study provides a five-year perspective of changes in mollusc populations in Leschenault Inlet, providing insight into the variability and longevity of the fauna. Some species are consistently present in the estuary although abundances varied seasonally, while others fluctuated markedly in their presence or absence. While the overall character in terms of diversity/abundance, and population structure of the mollusc assemblages may have remained similar, there were also changes in composition from year to year and from season to season. For many species there was a decrease in abundance, such that the relative abundance of species within an assemblage changed with time. Abundances of each species fluctuated largely independently of other species or a given habitat, and did not apparently occur in response to seasonal patterns in oxygen concentration, temperature, or salinity. Available at http://www.rswa.org.au/publications/Journal/83%284%29/Semeniuk_Wurm.PDF
... There were also episodic increases in population abundance for some species superimposed on any seasonal fluctuations (viz Capitella, Corophium, Spisula). For the mollusc data, Cresswell et al. (2000) corroborate these patterns mathematically, and conclude that inter-annual variation in molluscan composition is so marked that it would be difficult to categorise biota in terms of consistent assemblages. ...
The Leschenault Inlet estuary is an elongate shore-parallel, shallow water estuarine lagoon with distinctive patterns of bathymetry and geomorphology, framed to the east by the Mandurah-Eaton Ridge, to the west by a dune barrier, and to the south by two deltas. The estuary is diurnally micro-tidal, wave dominated and wind current driven. Estuarine waters are close to marine salinity but annually poikilosaline. Four salinity fields are recognised, corresponding to the delta zone, the lower estuary, the mid estuary, and the upper estuary. Large scale stratigraphic relationships within the system are relatively simple, with estuarine sediments to the east onlapping a quartz sand ridge, and a dune barrier encroaching over estuarine sediments to the west. Sedimentary patterns are underpinned by geomorphology, linked to the nature of shorelines, reworking sources and hydrodynamics, with muddy sediments accumulating in deeper water basins and semi-sheltered environments, and sand accumulating on exposed platforms, dune margins, or in deltas. The dune barrier, underlain by fresh water, separates oceanic and estuarine waters, with a saline/freshwater interface on both sides. Freshwater discharges from the barrier form shore seepages, which are important for shore vegetation and faunal use. Leschenault Inlet estuary is unique in South-Western Australia for several reasons. Formed behind a shore-parallel dune barrier, and wholly Holocene in age, its estuarine geomorphology and hydrologic structure are different to other local estuaries such as the Swan River Estuary and the Peel-Harvey Estuary. The estuary does not represent a simple river-to-sea transition, but has rivers entering at one end of a long coastal lagoon formed by marine processes rather than fluvial erosion. Leschenault Inlet estuary also has had a complicated Holocene sea level history, resulting in complexity of its shores. The western shore is further complicated as parabolic dunes encroach into the estuary, producing a varied assemblage of shore types and stratigraphic/hydrologic situations. The complex of shores and wetland types peripheral to the estuary support five types of fringing vegetation. Consequently, Leschenault Inlet estuary is a classic area for studies of ecology of estuarine peripheral vegetation, and the system ranks as one of the most significant in southern and South-Western Australia. Through its proximity to the Leeuwin Current, the estuary also supports the most southern occurrence in Western Australia of the mangrove Avicennia marina, and the array of landforms and vegetation in and around the estuary combine to create an important classroom for palynology. With numerous low-tidal to subtidal benthic habitats in the estuary, there is a rich diversity of flora and fauna: 3 species of seagrass, 7 species of algae, 31 species of mollusc (6 common species), 21 species of small benthic crustaceans, several species of larger crustaceans including the Blue swimmer crab (Portunus pelagicus), 15 polychaete species, and a normally oceanic ophiuroid echinoderm. The foraminifera assemblages in the estuary also are unusually diverse. Forty two species of fish have been recorded from nearshore and shallow waters. The estuary also is important for waterbirds, and as a dry season refuge in mid spring and summer it ranks amongst the top wetlands in South-Western Australia. Land reclamation, dredging, harbour reconstruction and urbanisation have impacted the estuary in the past 60 years. The estuary currently also is mildly enriched in nutrients introduced during winter freshwater influx. Studies on the phosphorous content of mud, phytoplankton and aquatic macrophytes of the estuary, however, suggest that the impact of nutrients on the estuary to date has been small. The long-term effects of development impacts on the unique characteristics of the estuary, however, are currently unknown.
