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International Journal of Marine Science 2014, Vol.4, No.1, 1-15
http://ijms.sophiapublisher.com
4
(9.91×10
–7
at 25°C), K'
2
is the apparent second
dissociation constant of carbonate ions (7.72×10
–10
at
25°C), K'
B
is the apparent dissociation constant of
boric acid (2.02×10
–9
at 25°C), and S is the salinity.
Photosynthetic and respiratory carbon metabolisms
were estimated using the average value of OP (mmol
m
–2
h
–1
) during the light (n=24) and dark (n=9)
conditions in each habitat. The photosynthesis of gross
production (P
gross
) rate and the 24-h respiration (R
24h
)
rate were estimated as:
(Eq. 6)
(Eq. 7)
where T
light
is the time of the light period (13 hours in
this study) and the units of P
gross
and R
24h
are mmol
m
–2
d
–1
. The duration ratio of daylight and night was
13-h (6:00-19:00) and 11-h (19:00-6:00) respectively,
during the chamber experiment.
Note that in the present study, we assume that the
daytime respiration is equal to the night time
respiration as reported in previous studies (e.g.
Tribollet et al. 2006, Bensoussan and Gattuso 2007).
This neglects the possibility that light-respiration may
be higher than dark-respiration, as shown in a large
scale coral reef mesocosm (Langdon et al. 2003).
The calcification rates were calculated for each light
and dark period (G
light
and G
dark
), respectively. G
light
and G
dark
were estimated by the average value of IP
(mmol m
–2
h
–1
) in the light (n=24) and dark (n=9)
conditions in each habitat.
(Eq. 8)
(Eq. 9)
Net calcification (G
net
) can be expressed as the sum of
G
light
and G
dark
(Eq. 10)
where T
dark
is the time of night period (11 hours) and
the units of G
light
, G
dark
and G
net
are mmol m
-2
d
-1
. A
negative calcification value indicates CaCO
3
dissolution.
The rate of inorganic nitrogen flux was also calculated
using the ∆C
biological
during the chamber incubation
periods.
(Eq.11)
where ∆C
biological
is the change in either NO
x
or NH
4
+
by biological activity of the benthic organisms within
a given chamber. A positive flux value indicated
nutrient uptake whereas a negative value indicated
release.
To calculate uptake/release rate constants of inorganic
nitrogen, the total abundance of either NO
X
or NH
4
+
which potentially affects a habitat was estimated as:
C
total
= C
initial
+ ∆C
inflow
(Eq.12)
Using the slope of the linear regressions, we
calculated the NO
x
and NH
4
+
uptake/release rate
constants, to compare the uptake efficiencies within
each habitat.
To compare the differences in habitat nutrient
metabolism in relation to the tidal cycle, we classified
the experimental times into either low (<100 cm) or
high (>100 cm) tide. Statistical analysis was
conducted using one-way analysis of variance
(ANOVA) and Student’s t-tests (JMP 8, SAS).
Analysis of covariance (ANCOVA) with a Bonferroni
adjustment was conducted to compare the uptake rate
constants of the inorganic nitrogen. To compare the
differences in habitat organic and inorganic carbon
production, Tukey–Kramer honestly significant
difference (HSD) tests were conducted.
3 Results
3.1 Groundwater inflow
The negative relationship between total alkalinity,
nitrate + nitrite (NO
x
) and ammonium concentrations
with salinity (R
2
=0.905, 0.921 and 0.723, respectively)
are characteristic of mixing with groundwater (Figure
3). The high values of total alkalinity in the
groundwater are attributed to the CaCO
3
that is
dissolved by groundwater passing through the
limestone dominated underlying geology. Also, as
Okinawa uses nitrate based fertilizers for agriculture
(Kawahata et al., 2000), and there are sugarcane fields
near the coastal area (Figure 1), the dissolution of these
compounds in freshwater and hence, in the
groundwater, causes high NO
x
concentrations.
However, despite the residential area next to the study
area (Figure 1), higher ammonium concentrations
which are generally caused by human activities were
)
OP ( 24
R
avg.
dark
24h
−× =
light
avg.
dark
avg.
light
gross
T)
OP
OP(
P
×
−
=
light
avg.
light
light
T IP
G
× =
dark
avg.
dark
dark
T IP G
× =
dark
light
net
G G G
+ =