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The nutrient status of Wilson Inlet 1984-1985

Lukatelich, R.J., Schofield, N.J. and McComb, A.J. (1986) The nutrient status of Wilson Inlet 1984-1985. Department of Conservation and Environment, Perth, Western Australia.

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In response to the concern of local residents about the condition of Wilson Inlet, a study of the inlet and its catchment was undertaken during 1982-83. While the results for that study were clear, it occurred during an atypical year; the winter had a particularly low rainfall and the sand bar between the estuary and the ocean was breached for the shortest period of any winter. To obtain some understanding of the variability likely to be encountered in the estuary between years, another, smaller study was undertaken in 1984-85.

The main conclusions about nutrient concentrations in plant tissues, the importance of phosphorus, total plant biomass and general ecosystem behaviour were si111ilar to those reached in the earlier study, despite very different winters. Catchment behaviour, in relation to nutrient losses to streamflow was also generally similar, with somewhat higher concentrations of nutrients associated with a winter of higher runoff. The main contrast was the large amount of nitrogen, especially in the form of nitrate, which came from the larger subcatchments during the present study.

Rainfall was much closer to the long-term mean in 1984 and there was a 2.8 times increase in streamflow for the whole Wilson Inlet catchment compared to 1982. The phosphorus load of 19 tonnes was lower than the 30 tonnes predicted for an average year on the basis of the 1982 results. The nitrogen load of 340 tonnes was slightly higher than predicted for an average year from the 1982 study.

Salinities were lower and nutrient concentrations and water column loads higher in winter in the present study due to the higher streamflows. Chlorophyll 'a' concentrations and water column load were also higher, presumably in response to the higher nutrient loads. Estimates of total plant biomass in winter were similar, and the results of both studies show that Ruppia biomass is one of the major nutrient banks in the Inlet. Sediment nutrient loads were similar in both studies.

A nutrient budget was calculated for one cycle of bar opening and closing. There was a net retention of phosphorus of 9 tonnes (49% of total input), and net retention of nitrogen of 217 tonnes (63%). The percentage of total riverine phosphorus load retained by the inlet was similar in both studies. In contrast, it was estimated that there was net export of nitrogen in the previous study.

Item Type: Report
Series Name: Department of Conservation and Environment; Technical Series 9
Publisher: Department of Conservation and Environment
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