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- Field and Laboratory measurements -
The deep site (Ron Courtney Island; RCI) and the shallow site (Tonkin Highway Overpass; TOP) are located in the upper reaches of the Swan River (see thumbnail). Water samples from these sites were collected at intervals of 1 to 4 weeks over a period of 384 days in 1993-1994. Periods of higher sampling frequency were designed to coincide with periods of rapid advance or retreat of the salt wedge.
Water samples for major ion, nutrient and organic carbon analyses were collected in acid-washed, 4 L dark glass bottles from 0.2m below the water surface and 0.2m above the bed of the estuary. Samples for chlorophyll a analysis were collected at 0.5m intervals in acid washed, 0.5 L plastic bottles. All water samples were refrigerated at 4 degrees celcius until analysis within 24 hours of sampling. Sub-samples for major dissolved ions, dissolved organic carbon (DOC) and dissolved nutrients were centrifuged in acid-washed centrifuge tubes at 12000 rpm (~17 000 g) for 30 minutes to remove particles >0.2um (Douglas et al. 1996).
Centrifuged samples were analysed for major cations (Ca, Mg, Fe, Na, K) using an Atomic Absorption flame spectrophotometer (Varian SpectrAA 40). Major anions (Cl, HCO3 and SO4) and nutrients (PO4, NO3, NO2, NH4 and Si) were analysed colourimetrically using a Technicon Autoanalyser II. DOC was analysed with a Beckman Organic Carbon Autoanalyser following persulphate/UV digestion. Total phosphorus was analysed on unfiltered samples as for PO4-P but following alkaline persulphate digestion (APHA 1985). Samples for chlorophyll analysis were filtered through GF/C filters, and pigment was extracted from filters following overnight immersion in 90% acetone, and analysis of the extract spectrophotometrically after correction for chlorophyll b and c, and phaeophytin (by acidification) (APHA 1985).
At the deeper RCI site, potentially bio-available phosphorus (BAP) was measured using the Fe-strip 'infinite sink' method (Oliver et al. 1993). BAP is considered to be representative of filterable PO4 and the pool of rapidly desorbed (<24 h) PO4 bound to the surface of suspended particles.
A Grant/YSI Model 3800 water quality probe, calibrated according to manufacturer instructions, was used to measure dissolved oxygen (DO), electrical conductivity (EC), salinity, turbidity and water temperature at the two sample sites. Rainfall, wind speed and air temperature data were obtained from Perth Airport, 3 km east of the study sites. Daily flow data were obtained for the two major inflows above the study sites, Avon River (Northam Station) and Ellen Brook (Upper Swan Station), from the Water and Rivers Commission.
