Astill & Lavery: Benthic macroalgae in the Swan-Canning Estuary 
G. comosa was found in all surveyed regions of the 
system on all sampling occasions, indicating that this 
species is euryhaline, tolerating osmotic change resulting 
from the advent of winter freshwater pulses. However, 
given that the species forms large unattached 
accumulations, the physical force of freshwater pulses 
results in the removal of G. comosa accumulations and 
therefore the disappearance of this species from many 
areas of the system during winter (Astill & Lavery 2001). 
Therefore, in addition to any physiological influence that 
freshwater pulses have on invasive marine species 
colonising lower estuarine reaches, such freshwater 
pulses have a physical influence on unattached species in 
the mid- and upper-estuarine reaches. Thus, it appears 
that the advent of freshwater pulses to the system in 
winter has a considerable effect, both physiological and 
physical, on benthic macroalgal assemblages throughout 
the estuary. 
Macroalgal Biomass 
The distribution and quantity of macroalgal biomass 
in the estuary was very patchy (Fig 4). Mean macroalgal 
biomass ranged between 18 and 629 g.nv 2 dwt, varying 
according to estuarine region and season. Regionally, the 
highest mean biomass (629 g.m' 2 dwt) was in the lower 
estuarine reaches, the combined result of 
Cystoseira trinodis stands and Gracilaria comosa 
accumulations. Mean macroalgal biomass in the mid- and 
upper-reaches was highly variable, ranging between 24 - 
130g/m 2 dwt and 18 - 314 g.m 2 dwt, respectively. 
Seasonally, the highest mean biomass during the survey, 
629 g.m' 2 dwt, was in spring, and the lowest (18 g.m' 2 dwt) 
was in winter. 
Macroalgal biomass was generally dominated by 
rhodophytes, although macroalgae of the division 
Chlorophyta dominated at times in mid- and upper- 
Figure 5 Mean macroalgal biomass at sites within lower (| |; 0 to 15 km), mid (15 to 25 km) and upper (H; 25 to 55 km) estuarine 
reaches of the Swan-Canning Estuary (± SE) 
13 
