Journal of the Royal Society of Western Australia, 87(1), March 2004 
estuarine reaches (Fig 5). A large proportion of the 
macroalgal biomass was attributable to Gracilaria comosa. 
In winter, this species constituted over 90% of the total 
macroalgal biomass at the sampling sites, especially at 
those lacking hard substrate (i.e. predominantly mid- and 
upper-estuarine reaches). At freshwater-dominated sites 
in the upper region, a loose-rubble substratum was 
present and macroalgal biomass at these sites was 
dominated by the freshwater chlorophyte, CJiara sp. 
Biomass measurements in Leschenault Inlet showed a 
similar seasonal variation in macroalgal biomass, varying 
between 46 g/m 2 dwt in autumn/winter to 209 g/m 2 dwt 
in spring (Hillman et al. 2000). Macroalgal biomass in the 
Leschenault Inlet was dominated by chlorophyte and 
rhodophyte species (11 to 43% of total biomass, and 20 to 
30% during spring, respectively). Similarly, in Peel Inlet, 
minimum and maximum macroalgal biomasses of 25 g/ 
m 2 dwt (November 1988) and 237 g/m 2 dwt (April 1993) 
were recorded, although this biomass was predominantly 
(>85%) composed of chlorophyte species (Hillman et al. 
2000 ). 
Previous studies have attributed seasonal variability 
in macroalgal biomass to factors such as changes in light 
and nutrient availability (Kinney & Roman 1998; Collado- 
Vides 1994; King & Schramm 1976), while studies on drift 
macroalgae attribute a proportion of seasonal variation 
in biomass to water movement (Talbot et al. 1990; 
Virnstein & Carbonara 1985). Bell & Hall (1997) found 
drift macroalgal biomass in Tampa Bay (USA) to vary 
between almost nothing to more than 150 g/m 2 dwt. 
They suggested that variations in macroalgal biomass 
and distribution were related to local hydrodynamic 
regimes, such as wave action, currents, wind and tides. 
In this study, G. comosa was commonly found in large, 
unattached accumulations overlying soft sediment, 
dominating macroalgal biomass in the system. However, 
this species was noticeably reduced or absent with the 
onset of winter freshwater flows, resulting in subsequent 
reductions in total macroalgal biomass throughout the 
system. From these observations it is proposed that 
seasonal freshwater flows in the system strongly affect 
the presence/absence of unattached macroalgal species 
through both physical and physiological means, and the 
quantity of macroalgal biomass present. Winter 
freshwater flows appear to be a dominant environmental 
factor controlling the presence, diversity and abundance 
of macroalgal assemblages in the Swan-Canning Estuary. 
Acknowledgements: The assistance of M Pennifold in the field and of 
M Vanderklift in the laboratory, is gratefully acknowledged. The authors 
also thankfully acknowledge A. McComb's advice on the manuscript. 
Financial assistance was provided by the Western Australian Estuarine 
Research Foundation. 
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