Basic HTML Version
International Journal of Marine Science 2014, Vol.4, No.1, 1-15
http://ijms.sophiapublisher.com
11
in advection dominated sediments, their impact is
much lower (-20% to +30%). The coral rubble that
is the primary foundation of the benthos in the Bise
ecosystem, and the high influx of groundwater
implies that it is an advection dominated system.
Thus the coastal sands of Bise are neither nitrogen
nor oxygen limited and the impact of the acorn
worm on the inorganic nitrogen cycle in Bise is less
apparent. Instead, the NO
x
uptake in the sand and
acorn worm habitats may be dominated by
autochthonous microorganisms and epilithic algae.
Although less effective, it is possible that the NO
x
uptake by sand-related microbes in the acorn worm
habitat may suppress further increases in the NO
x
concentration found in the nearby seagrass, coral,
and coral-seagrass habitats. By mitigating NO
x
increases, the sand in the acorn worm and sand
habitats may aid in maintaining a healthy coral
community.
The coral-seagrass and coral habitats demonstrated
a significantly higher uptake rate constant than the
other habitats (Figure 8). These rate constants are
comparable to estimates (4.4±0.4 m d
–1
for NO
3
–
)
derived from a coral reef in Biosphere 2 (Atkinson
et al. 2001). When the rate constants of the sand,
seagrass, and coral habitats (0.6, 1.0 and 1.9 m d
–1
,
respectively) were used to calculate the expected
rate constant for a 5% sand, 50% seagrass and 45%
coral mixed habitat, we estimated 1.4 m d
–1
which
is lower than the observed constant of the
coral-seagrass habitat (Figure 8). This estimate
indicates that the co-existence of coral and seagrass
can remove more NO
x
from the water column than
would be expected.
Moreover, because of the significant difference in
rate constants between the seagrass and coral-
seagrass habitats, it is possible that the coexistence
of coral and seagrass may benefit seagrass with
respect to NO
x
. It is known that the most important
factors of nutrient exchange between water and the
benthic community are the nutrient concentration,
water velocity, and friction with the benthos
(Atkinson, 2011). Falter and Atkinson (2004)
formulated the rate constant which is a molar
mass-transfer coefficient as a function of the
roughness of a community, the diffusivity of the
nutrients and the ambient flow conditions. Since the
diffusivity and flow conditions are similar across all
the habitats enclosed by the chambers, the
coexistence of coral and seagrass may increase this
friction by creating the complex structure of
entwined coral and seagrass. Additionally, since this
system is not nutrient deficient, it is not necessary
for coral and seagrass to compete for nutrient
acquisition. If the system were to exist under
nutrient deficient conditions, different mechanisms
would likely be involved in the competitive
co-existence between seagrass and coral.
Overall, the ammonium uptake rates were relatively
low due to the low concentration of ammonium in
Bise. Previous studies in the seagrass area indicated
the ambient concentration of NH
4
+
ranged between
0.5 – 5.4 μM (Romero et al., 2006). Although the
present study found a lower average of 0.43 ± 0.20
μM than that of a general seagrass area, the value
was within the range of a typical coral reef area
which was reported as 0.7 ± 0.7 µM at the One Tree
reef lagoon, Australia (Steven and Atkinson, 2003)
and 0.76 ± 0.49 at Rukan-sho atoll off Okinawa
Island, Japan (Ohde and van Woesik, 1999). The
uptake rate constant of the seagrass habitat seemed
to be the highest among all the habitats, however
there were no significant differences between
habitats (Figure 8). This may be due to a lower
correlation between the uptake rate and
concentration in the coral-seagrass and coral
habitats (R
2
=0.03 and 0.05, respectively, Figure 7)
which is mainly caused by the release of ammonium
from coral.
More than half of the ammonium flux values in the
coral habitat were negative, indicating a net release
of ammonium. Most of these rates corresponded to
samples collected during the night and morning
incubations. Muscatine and D’Elia (1978) and
Muller et al. (2009) reported ammonium release by
the coral host and uptake/ retention by
zooxanthellae, resulting in a net uptake of
ammonium via the coral-zooxanthellae symbiosis.
They also demonstrated that under normal seawater
conditions, which are nitrogen limited, a net release
was produced by
Pocillopora damicornis
after more
than 12 hours incubation in the dark, resulting in a
depletion of the photosynthetically produced energy
in zooxanthellae. However, due to the high NO
x