Geology Reference
In-Depth Information
are good recruiters but poor spatial competitors
(Butler, 1991). Sim (1991) showed that the num-
ber of epiphyte species increases with increasing
seagrass height above the seafl oor, with corallines
most abundant in the upper 30% of the stem, while
bryozoans, e.g.
Celleporaria
, are most numer-
ous near the base. It is not obvious why stems
are better, but several reasons are possible. First,
Amphibolis
stems provide a local and constant
epiphyte source to the blades (cf. Keough, 1983),
a situation not present in
Posidonia
. Second, the
clustering nature of
Amphibolis
blades provides a
more sheltered environment for recruitment than
do
Posidonia
blades. This nodal area is above
the sediment surface whereas it is at the sedi-
ment surface for
Posidonia
. Studies by Harvey &
Bourget (1997) indicate that calcareous epiphyte
propagules settle most frequently in nodal areas.
Third, the structure of
Amphibolis
plants is more
conducive to calcareous epiphyte recruitment and
survival due to their effect on current velocity.
Strap-like
Posidonia
blades reduce water move-
ment more effi ciently than cylindrical shoots of
Amphibolis
, which would be thought to increase
settlement rates. The
Posidonia
blades, however,
tend to fl atten parallel to the current forming a
relatively impermeable dense mat, hence redu-
cing the amount of epiphyte propagules exposed
to the surface of the blades.
Amphibolis
blades
on the other hand remain relatively exposed to
recruiting propagules. Fourth, surface texture
of
Amphibolis
blades provides a more favour-
able site for recruitment than the surface tex-
ture of
Posidonia
blades.
Posidonia
is smooth
and
Amphibolis
is rough. It is easier to stick to a
rough surface.
Calcareous epiphyte production
Studies from other areas are used to determine
turnover rates. For these calculations there is no dif-
ference in turnover rates within genera and because
Amphibolis
stems are biennial, a turnover rate of
0.5 yr
1
was used (Walker, 1985; Coupland, 1997).
Amphibolis griffi thii
for example, has a growth
rate of 0.038 blade clusters/day, and so it takes
26.3 days for a full new blade to grow, or 14 blades/
year. Each blade cluster consists of ~4 blades,
hence the cluster turnover rate is 3.5 yr
1
.
Contrary to standing stock values, blades of
A. antarctica
are the most important substrate in
terms of epiphyte productivity and
Amphibolis
stems are the least important. The abundance of
epiphytes on
Posidonia
is slightly greater than on
other grasses.
Average epiphyte productivity over all sites
is 210
26 g m
2
yr
1
or 750 g kg
1
of seagrass
per year (
n
= 35). The range is 49-661 g m
2
yr
1
.
Detailed study at West Island reveals a range of
99-402 g m
2
yr
1
(
n
= 10), average 254.4 g m
2
yr
1
,
for the shallow site and 98-295 g m
2
yr
1
(
n
= 4),
average 187.4 g m
2
yr
1
, for the deep site (Table 6).
Calcareous epiphytes on seagrasses along the
south Australia coastline are calculated to pro-
duce 2,018,520 t of carbonate per year.
The values from this temperate water environ-
ment are comparable to those from tropical settings.
Nelsen & Ginsburg (1986) calculated an average of
118 g m
2
yr
1
(range 30-846 g m
2
yr
1
) for grasses
from the inner part of the Florida Reef tract near
Tavernier Key. In Florida Bay Frankovitch &
Zeiman (1994) calculated 119 g m
2
yr
1
(range
1.9-282.0), while Boscence (1989) calculated
281.5 g m
2
yr
1
(range 55-1042 g m
2
yr
1
). Land
(1970) calculated 180 g m
2
yr
1
from inshore
Jamaica localities. They are, however, much
higher than the subtropical, siliciclastic domi-
nated grass banks of Mozambique where produc-
tion is calculated to be from 33.4 to 44.9 g m
2
yr
1
(Perry & Bevington-Penny, 2005).
Effect of nutrients
The one location (Semaphore - Fig. 4) with high
nutrient levels shows a correlation between high
nutrient values, high benthic foraminifera num-
bers and low coralline algae abundance. This site
also correlates with the lowest seagrass abun-
dance but highest epiphyte abundance. Because
of its favourable attributes for epiphyte settle-
ment
Amphibolis
spp. in anthropogenic nutrient-
rich environments is one of the fi rst species to
decline. They are covered with epiphytes to such
an extent that they cannot photosynthesize. Thus
increased nutrients, as shown by the Semaphore
site, rapidly reduce the seagrass biomass and thus
the epiphyte production and hence carbonate
sedimentation.
Accumulation rates
The average accumulation rate in the South
Australia seagrass bed area (Table 7) is 7.4 cm kyr
1
assuming (1) CaCO
3
specifi c growth of 2.18 g cm
2
,
(2) zero porosity and (3) zero recycling. Estimates
are based on
Posidonia
having a rate of 3 crops/yr
and
Amphibolis
having turnover rate of 5 blade
crops/yr and 0.25 stem crops/yr. Turnover rates
are not yet known for South Australia and so rates
