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International Journal of Marine Science 2014, Vol.4, No.1, 1-15
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10
al. (1999). The slope between IP vs photon flux in
the coral-seagrass habitat was exactly 45% of the
slope of coral habitat, indicating a direct
relationship between the calcification rate and the
amount of coral in the habitat. Thus, in regards to
calcification, there is no synergistic effect between
coral coexisting with seagrass. On the other hand,
CaCO
3
dissolution was found in the seagrass and
coral-seagrass habitat at night (Table 1). This
suggests that respiration by seagrass decreases the
pH in the surrounding seawater which causes a
decrease in the saturation state of CaCO
3
. This
agrees with Nakamura and Nakamori (2009) who
indicated that the dissolution of CaCO
3
was
observed in seagrass communities. It should be
noted however that some dissolution could occur
due to the biological processes of bacteria or
epilithic and endolithic algae (Islam et al., 2012).
The dissolution of CaCO
3
in the seagrass habitat is
likely not beneficial to the coral with respect to the
calcification-dissolution process and the formation
of a robust reef framework. On the other hand,
the
root systems of seagrass elevate the release of
alkalinity from sediments, which would elevate the
carbonate saturation state of the overlying water
column (
Burdige and Zimmerman, 2002; Burdige et
al., 2008). In the case of the Bise area, the seagrass
community seems to provide a moderate dissolution
environment for
M. digitata
. Here, the most basal
part of
M. digitata
is fragile while the living parts of
the coral are healthy. This suggests that the
respiration of seagrass roots or stems that are
entwined with the basal portion of
M. digitata
could
actually decrease the saturation state of CaCO
3
by
lowering the pH of seawater around that part of the
coral. By utilizing this environment,
M. digitata
may contribute to the development of the
coral-seagrass and coral habitat through the natural
dispersion of fragments as more fragile skeletons
allow for easier fragmentation. CaCO
3
dissolution
was also found in the sand and acorn worm habitat
due to a higher respiration rate than photosynthesis
(Table 1). However, this habitat does not have any
substrate that could be entwined by seagrass, and it
is difficult for coral fragments to survive in the
sandy habitats. This is evident in a study which
reported more than 70% mortality of
Acropora
palmata
fragments transplanted onto a sand
substrate (Lirman, 2000). Therefore, it is possible
that the respiration of the seagrass which can
entwine the basal part of coral may be beneficial to
coral by encouraging the dispersal of fragments via
the partial dissolution of calcium carbonate in the
Bise area.
4.2 Inorganic nitrogen dynamics
The inorganic nitrogen dynamics in the Bise area
are highly impacted by groundwater inflow during
spring low tide. This is evident in the decrease in
salinity and the increase in inorganic nitrogen
concentrations in each chamber area during low vs.
high tide (Table 2). The strong correlations found
between concentration and uptake rate in each
habitat indicate each of the habitats are likely
nitrogen limited and that groundwater inflow
stimulates microbial activity. Thus the habitats with
groundwater inflow will demonstrate higher
inorganic nitrogen uptake rates. This is seen in the
acorn worm habitat, which is located nearest to the
coast, exposed directly to groundwater inflow, and
exhibits the highest NO
x
uptake rate. It should be
noted however, that although the high uptake rate in
the acorn worm habitat during low tide was
prominently due to the correlative increase in the
NO
x
concentration, the uptake rate constant was not
significantly different from the sand area (Figure 8).
The lack of a significant difference between the
NO
x
uptake rate constants in the acorn worm and
sand habitats indicate that the impact of acorn worm
activity on the coupled nitrification-denitrification
processes are negligible. This is surprising as
burrowing
macrofauna
generally
stimulate
nitrification and denitrification processes by
increasing sediment oxygen concentration, redox
potential,
and
bacteria-mediated
processes
(Krantzberg, 1985; Kogure and Wada, 2005).
We attribute our inability to see the impact of acorn
worm activity on the inorganic nitrogen dynamics
to two possibilities. First, there is a lower
population density of acorn worms at this site
(maximum 24 individuals m
-2
), and second, in line
with recent studies, the sediment characteristics
could be masking the impact of the acorn worm.
Mermillod-Blondin
and
Rosenberg
(2006)
demonstrated that in diffusion dominated sediments,
burrowing macrofauna have the potential to
increase oxygen consumption up to 3-fold, whereas