Journal of the Royal Society of Western Australia, 87:9-14, 2004 
Distribution and Abundance of Benthic Macroalgae in the Swan-Canning 
Estuary, South-Western Australia 
H L Astill '* & P S Lavery 2 
’Aquatic Sciences Branch, Dept of Environment L2, Hyatt Centre, Plain St, East Perth 
2 School of Natural Sciences, Edith Cowan University, 
100 Joondalup Drive, Joondalup 6027 
121 helen.astill@environment.wa.gov.au 
12 p.lavery@ecu.edu.au 
Abstract 
The macroalgal communities of the Swan-Canning Estuary were investigated between August 
1995 and May 1997. Thirty-six macroalgal species were recorded, predominantly of the division 
Chlorophyta. Macroalgal species number was highest (15 to 22 species) in the lower, more marine 
reaches of the estuary, and in the seasons of hydrological change when freshwater flows were 
commencing or abating (i.e. spring and autumn). Macroalgal biomass was patchy both spatially 
and temporally: highest biomass was in the lower estuarine reaches (629 g.m' 2 dwt), where mixed 
soft and hard substrata were present in spring (November 1996) after winter freshwater pulses had 
subsided. Macroalgal biomass was generally dominated by rhodophytes, especially the species 
Gracilaria comosa, although high biomass was also attributable to the phaeophyte Cystoseira trinodis 
at sites characterised by a hard substratum in the lower reaches of the estuary. 
Keywords: Swan-Canning Estuary, benthic macroalgae, biomass, species number, Gracilaria comosa 
(Manuscript received June 2003; accepted November 2003) 
Introduction 
The Swan-Canning Estuary is a large south-western 
Australian system that stretches more than 50 km inland 
from the coast at Fremantle. The system contains 
extensive areas of shallow sand-flats. Two rivers 
discharge into the system: the Swan-Avon River to the 
east, and the Canning River to the south, with catchment 
areas of approximately 119 000 km 2 and 20 000 km 2 , 
respectively (Thurlow et. al. 1986). The suburban and 
industrial development of Perth city occupy a large 
proportion of the estuary's banks (Hodgkin 1987). 
The Swan-Canning Estuary is a highly seasonal 
system, where the main influence is large freshwater 
pulses driven through the waterbody in winter. With 
the onset and dissipation of freshwater flows, the 
physico-chemical environment of the estuary changes. 
During winter (July to September), relatively high and 
sporadic rainfall events (over 100 mm per month) 
generate highly variable and pulsing river flows 
(>100 m-^.s' 1 ). Associated with the pulses are low 
salinities throughout most of the estuary (approximately 
5 units at the surface although bottom waters near the 
mouth are up to 32 units), low temperatures (13-14 °C) 
and high light attenuation (Secchi disk depth 35 to 70% 
of the water column). The freshwater pulses lead to an 
elevation of nutrient concentrations; dissolved inorganic 
nitrogen concentrations are commonly between 0.6 and 
1.65 mg.L' 1 in fresher surface waters, and 0.15 to 
1.25 mg.L’ 1 in the more marine bottom waters (Astill 
2000). As rains and the freshwater input to the system 
subside, there is a gradual transition in the estuary's 
© Royal Society of Western Australia 2004 
physico-chemical environment, largely brought about 
by marine waters penetrating into the estuary as a salt 
wedge, reaching some 55 km upstream from the estuary 
mouth by autumn (Astill 2000). At this time, salinity, 
temperature, and light penetration increase in all 
regions of the estuary (20 to 35, and 19 to 21 °C, 40 to 
80%, respectively), and dissolved inorganic nitrogen 
concentrations decrease (<0.3 mg.L' 1 throughout the 
system). 
Macroalgae are a conspicuous biological component of 
the Swan-Canning Estuary. Allender (1981) recorded 29 
species when he surveyed the estuary in the summer of 
1968 between the junction of the Swan and Helena Rivers 
and the mouth of the estuary. Allender proposed that 
changes in the assemblages within the estuary could be 
spatially defined by the seasonal salinity regimes. Many 
of the species present were typical of eutrophic estuaries 
and have been recorded in the estuary since at least the 
1970s (Allender 1981). 
This study investigated the populations of attached 
and unattached macroalgae present in the Swan-Canning 
Estuary, almost thirty years after Allender's (1981) 
preliminary and qualitative investigations of this 
biological component of the Swan-Canning Estuary. The 
primary aims of this study were to investigate the 
macroalgal species now present in the system in 
comparison to Allender's findings, and to quantitatively 
describe the temporal and spatial variation within the 
macroalgal populations present in the estuary. 
During the study, macroalgal assemblages were 
surveyed seasonally over two years (August 1995 to May 
1997). Assemblages were investigated for a number of 
characteristics including species numbers, temporal 
presence and absence, and macroalgal biomass. 
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