A. PEEL-HARVEY ESTUARY
The Peel-Harvey Estuary, which was formed by the coalescing of three estuarine systems as a result of the sea level rises that started occurring about 6000 years ago, assumed its current morphology about 2000 years ago.30,31 This Estuary, with an area of 133 km2, is one of the largest estuarine systems in temperate Australia. 25 It comprises two large basins, the Peel Inlet and the Harvey Estuary, which are generally less than 2 m deep, and the saline reaches of three main tributary rivers (Figure 1 ). The Murray and Serpentine Rivers discharge into the Peel Inlet, while the Harvey River flows into the Harvey Estuary (Figure 1). These river systems collect water from a greater catchment with an area of 11,378 km2. Water supply dams have been constructed on the Harvey and Serpentine Rivers, but the potential reduction in total riverine discharge into the Peel-Harvey 8 has been offset by a considerable increase in runoff due to clearing in the catchment.
The climate in the region of the Peel-Harvey Estuary, and therefore also of the Swan Estuary and Leschenault Inlet, is Mediterranean, with hot dry summers and mild wet winters. 28,29,31 Average annual rainfall in the area of the estuary is 825 mm, most of which falls between May and October. 31 Direct rainfall accounts for 23-30% of the freshwater input into the estuary, ground water discharge less than I%, with riverine inputs accounting for the rest. Evaporation is high (approximately 1980 mm y-1), 31 producing hypersaline conditions in some shallow regions during the dry summer and autumn. 32
Water circulation of the Peel-Harvey Estuary is driven by tidal currents, wind, density-induced circulation and river flow. 33 Wind exerts an important influence on the hydrodynamics of the estuary and causes resuspension of sediments and thus nutrient exchange between the sediments and the water column. 34 The estimated residence time is 75 days, which means that between 2.5 and 4.9 times of the volume of the estuary is flushed each year, depending on the extent to which mixing occurs. 33
Prior to the 1970s, the dominant macrophytes in the Peel-Harvey Estuary were aquatic angiosperms, such as Ruppia megacarpa and Halophila ovalis. 30 The extreme manifestations of eutrophication exhibited by the Peel-Harvey Estuary since the late 1960s, as a result of nutrient enrichment through runoff from surrounding agricultural land, include the production of massive growths of green macroalgae and very large blooms of phytoplankton such as diatoms and blue-green algae. 8,35,36 However, the massive growths of macroalgae occur mainly in the Peel Inlet, while the blooms of phytoplankton, such as diatoms, and the extensive blooms of blue-green algae are found predominantly in the Harvey Estuary. 5,8 Macroalgal production in the Peel-Harvey system rose sharply in the 1970s, mainly as a result of growth of the green alga Cladophora montagneana 4This alga was subsequently replaced by other green algae, such as Chaetomorpha linum, Ulva rigida and Enteromorpha intestinalis which, during the 1980s, accounted for over 85% of the total macrophyte biomass in the Peel-Harvey. 37 In contrast to the situation with macroalgae, conspicuous blooms of the blue-green alga Nodularia spumigena did not start appearing until 1978 and these are highly seasonal, being largely restricted to the late spring and early summer. 4,8
The Peel-Harvey Estuary supports the most important commercial and recreational
estuarine fishery in Western Australia. 9,38For
example, the commercial catch in the Peel-Harvey Estuary in 1990/91 was 317
t, which represented 37% of the total catch taken by the commercial estuarine
fisheries of the state in that year. 39
The main commercial teleost species are yelloweye mullet (Aldrichetta forsteri),
sea mullet (Mugil cephalus) and cobbler (Cnidoglanis macrocephalus).
9 Seasonal catches
of two crustaceans, i.e., blue manna crab (Portunus pelagicus) and western
king prawn (Penaeus latisulcatus), are also important.40,41
The main recreational species are cobbler, blue manna crab, western king prawn,
mulloway (Argyrosomus hololepidotus), tailor (Pomatomus saltatrix)
and King George whiting (Sillaginodes punctata). 40-42