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International Journal of Marine Science 2014, Vol.4, No.1, 1-15
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12
availability in this study site, our results contradict
these findings and suggest that when inorganic
nitrogen supplies are sufficient, zooxanthellae do
not need to take up ammonium, resulting in a net
release from coral-zooxanthellae symbiosis.
The ammonium concentration in the coral-seagrass
habitat however was lower than that in the coral
habitat (Figure 7 and Table 2). Stapel et al. (1996)
demonstrated
T. hemprichii
leaves have a high
capacity for ammonium uptake. As we found a
strong linear relationship between ammonium
uptake rates and concentration in the seagrass
habitat, the lower ammonium concentrations in the
coral-seagrass habitat are attributed to the ability of
the coexisting seagrass to take up and retain the
excess ammonium released by coral. Thus, the
coral-seagrass habitat shows a possible beneficial
relationship in terms of ammonium. Although the
lack of a strong correlation between concentration
and uptake rate in the coral-seagrass and coral
habitats demonstrate that the coral community does
not benefit from increased ammonium concentra-
tions, this indicates a somewhat “commensalistic”
biogeochemical relationship between seagrass and
coral in regards to nitrogen nutrient dynamics in the
Bise area.
5 Conclusion
While this study found no synergistic effects
between coral and seagrass with respect to the
photosynthesis-respiration and calcification processes,
it is possible that CaCO
3
dissolution driven by
seagrass could be facilitating coral fragmentation
and fragment distribution. This could benefit
M.
digitata
by furthering habitat development.
In contrast, the NO
x
results demonstrated that there
is a synergistic effect between coral and seagrass in
the coral-seagrass habitat. This is evident in the
higher uptake rate constant of NO
x
in the
coral-seagrass habitat than that estimated from each
habitat alone. In addition to this, significant
differences between the rate constants of the
coral-seagrass and seagrass habitats demonstrated
that the coral-seagrass habitat benefit seagrass with
respect to NO
x
uptake. In the acorn worm habitat
however, the high NO
x
uptake possibly aids in
maintaining a lower nitrate concentration in this
coral reef ecosystem. Overall, the coral reef
ecosystem in Bise assimilates the high input of NO
x
by demonstrating continual uptake regardless of the
benthic composition. On the other hand ammonium
was released by coral because of the sufficient
nitrogen source as nitrate. As ammonium is the
most easily assimilated by all organisms, the
released ammonium was taken up by seagrass in the
coral-seagrass habitat. Therefore, on a strictly biogeo-
chemical scale, the coral-seagrass relationship is
more beneficial for the seagrass in terms of
inorganic nitrogen dynamics.
However, continued research (such as microbial
activity, abundance of microbes in the sediments,
stable isotope analyses or ex-situ experiments)
needs to be conducted to clarify both the physical
and biogeochemical relationships/ mechanisms
occurring within the coral-seagrass habitats.
Acknowledgements
This study was part of the 2008 COE (Center of Excellence) Summer
Program supported by the 21
st
Century COE Program (University of the
Ryukyus). The authors wish to thank the people of the Bise, Okinawa
municipality for their support and cooperation during the research period.
We also thank 3 anonymous reviewers for valuable comments to
improve the manuscript.
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