Geology Reference
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between low carbonate quantities/high dominance
of
Posidonia
and high carbonate quantities/high
dominance of
Amphibolis
, a relationship also
noted by Jernakoff & Nielsen (1998). The greater
abundance of epiphytes on
Amphiboli
s is attrib-
uted to the longevity of the stems; they are bien-
nial (Walker, 1985; Coupland, 1997), whereas
Amphibolis
and
Posidonia
blades have life-spans
of 60-100 and 65-130 days respectively. Hence,
epiphytes have a longer time to accumulate on
stems. Longer immersion times result in higher
species abundance and higher species diversity
(Turner & Todd, 1993).
Epiphyte carbonate increases signifi cantly,
independent of biomass, with increasing water
depth to a 8-10 m w.d. maximum associated with
Posidonia
and to a 46 m w.d. maximum associ-
ated with
Amphibolis
, and then decreases. This
is interpreted to be a function of reduced compe-
tition from non-calcareous epiphytes below 10 m
w.d. Furthermore, below 10 m w.d. light for coral-
line algae and food for other epiphytes becomes
limited. Reduction in current speed with depth is
also a possible control on epiphyte distribution
(cf. Eckman, 1983). Calcareous epiphytes appear
to be more affected by depth when associated with
Posidonia
than
Amphibolis
, probably because of
the longer times available for accumulation on
Amphibolis
stems. This is also because with time
calcareous epiphytes out-compete non-calcareous
epiphytes (Borowitzka
et al
., 1990).
There is no apparent correlation between salin-
ity or nutrient concentration at levels prevailing in
the local environment, but temperature data indic-
ate optimum temperature as 16-18°C, because
maximum variation occurs at 16-18°C, and there
is a decrease in abundance over 20°C.
patchiness, unfavourable sediment conditions,
blowouts and anthropogenic infl uence, e.g. sand
harvesting for beach replenishment.
Seventy-four per cent of all seagrass biomass
samples lie between 50 and 500 g m
2
(Fig. 6) sug-
gesting that conditions enabling higher biomass
are rare. Nevertheless, Chinaman Creek, at the
head of Spencer Gulf has the highest biomass of
P.
australis
recorded in the literature. This is prob-
ably because it is located at the head of an inverse
estuary, and the seagrass is subject to high temper-
atures, high salinities and low current velocities
suggesting all of these conditions are favourable
for the growth of
P. australis
.
The higher biomass of
Amphibolis
-dominated
communities is similar to that in Western Australia
(Kendrick
et al
., 1998a,b). This higher biomass
is because
Amphibolis
has a greater number of
shoots per square metre and the biomass of each
individual shoot is greater than
Posidonia
shoots.
On the other hand,
Posidonia
has a higher shoot
spatial density than
Amphibolis
. The implication
is, therefore, that there should be a greater of num-
ber of epiphytes per square metre on
Amphibolis
.
Biomass peaks at 2-4 m (Fig. 6) and decreases
thereafter. This relationship, tied to light, is
well documented (Burkholder & Doheny, 1968;
Bulthuis & Woekerling, 1981; Dennison
et al
.,
1993; Walker
et al
., 1999). The peak at 2-4 m is
attributed to photosynthetic inhibition and desic-
cation in shallower water (Seddon, 2000). In gen-
eral seagrass does not penetrate below 30 m w.d.
In this study the biomass of
Amphibolis
decreases
more rapidly than
Posidonia
with depth, suggest-
ing that
Amphibolis
is more sensitive to lower
light levels.
Photosynthesis and respiration by seagrasses is
affected by temperature (Walker, 1991). Optimum
temperatures for
P. sinuosa
have been measured at
13-23°C in comparison with 23°C and above for
P. australis
,
A. griffi thii
and
A. antarctica
(Masini &
Manning, 1997). Peak biomass for
A. antarctica
is
42.5‰ salinity (Walker, 1985).
DISCUSSION
Seagrass biomass
Results largely agree with studies elsewhere in
western, southern and eastern Australia (West &
Larkum, 1979; Silberstein, 1985; Walker, 1985;
Walker & McComb, 1988; Hillman
et al
., 1990;
Cambridge & Hocking, 1997; Kendrick
et al
.,
1998b; Sim, 1999; Udy & Dennison, 1999; Seddon,
2000). Variance at the 50-100 m scale at each site
is thought to refl ect patchiness (cf. Kendrick
et al
.,
1998a,b). Causes of seagrass patchiness include
competition between species, lack of equal sea-
grass growth expansion, limited dispersion of
seedlings, non-availability of resources, grazing
Epiphytes
The main parameters controlling epiphyte abun-
dance include seagrass biomass, seagrass genera,
water depth and seasonality.
Amphibolis
has
3.5 times more carbonate g kg
1
of seagrass than
does
Posidonia
. Also, the components are mark-
edly different, with 3.4 times more epiphytes in
terms of g kg
1
associated with the stems rather
than the blades (Table 5).
