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The effect of a saline wedge on the macrobenthos of the upper Swan River Estuary

Papas, Philip (1994) The effect of a saline wedge on the macrobenthos of the upper Swan River Estuary. Honours thesis, Murdoch University.

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The benthic macroinvertebrate fauna of deep (>3.0m) and shallow ( <0.5m) regions at four sites in the upper Swan River estuary were sampled over a six month period from September 1993 to December 1994. Changes in the benthos were examined in order to determine the effect of a number of physico-chemical parameters pertaining to the progression of a saline wedge from the middle to upper regions of the estuary during spring and summer.

The saline wedge was characterised by a higher salinity than overlying waters, and low dissolved oxygen concentration. With the arrival of the saline wedge, salinities increased rapidly and dissolved oxygen concentrations fell to below 2.0mgL·1. Oxygen recovery was evident two to three weeks after the arrival of the saline wedge, however, the magnitude of oxygen recovery was highly variable between sites. Turbidities at all sampling sites increased abruptly with the arrival of the saline wedge. This was due to the coagulation of colloidal particles caused by an increase in metallic ions, particularly calcium and magnesium in more saline water. Turbidities fell as abruptly as a more homogenous salinity profile was established. The movement of the saline wedge through the upper estuary was affected by a number of factors including surface freshwater discharge, barometric pressure and tidal variation.

Forty seven benthic macroinvertebrate species were collected from the upper Swan River Estuary. Shallow sites supported a diverse, species rich fauna comprising bivalves, polychaetes, crustaceans and other taxa. Deep sites were comparatively depauperate, being dominated by one species of polychaete. Classification and ordination techniques revealed further separation of shallow sites into upstream/downstream groups. Downstream sites had a more diverse and species rich fauna, whereas the fauna upstream was dominated by a small leptonid bivalve.

The differences in the faunal assemblages were explained by habitat variation between deep and shallow and upstream and downstream sites, and physical stress induced by the saline wedge. Shallow sites, in general, were characterised by fine to medium sands which provided suitable attachment sites for a number of bivalves and a tube building media for some amphipods and polychaetes. Dissolved oxygen concentrations in the shallows were generally high throughout the sampling period at all sites. Deep sites however, were characterised by black, organically enriched anoxic mud, which was inhabited only by a spionid polychaete.

Salinity was an important factor in the distribution of relatively few species, predominantly the amphipods and isopods. This may be due to the euryhaline nature of the species inhabiting this region and their adaptations to extremes in salinity and temperature. A negative correlation between species richness and turbidity suggested that the higher turbidities present in the deeper regions may have impacted on the benthic fauna. Absence of a number of deposit feeding species contributed to the decrease in richness, which suggested these species were intolerant to large particulate fluxes.

The main effect of the saline wedge appeared to be deoxygenation, which explained much of the spatial and temporal variation of the benthos. The dominant species (particularly Prionospio cirrifera) in the deep regions were largely opportunistic and flourished in a very stressful environment characterised by hypoxia and on some occasions anoxia. Larger, non opportunistic species, intolerant to low dissolved oxygen concentrations, were either absent from the deeper regions or suffered population collapses during periods of oxygen stress.

Further research is required to observe whether many of the trends recorded in this study persist under different annual climatic conditions. In particular, this study did not encompass autumn and winter periods, where investigation into the response of the benthos to winter flushing and the subsequent development of riverine conditions is still needed.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you
Supervisor: Davis, Jenny and McComb, Arthur
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