142
where C chl is the chlorophyll a concentration of phytoplankton on the measurement
day, V is the average water volume of the north salt marsh (45,700 m
3
), r is the conversion ratio of phytoplankton biomass carbon to chlorophyll a (= 56; Lü et al.
2009), and A is the average water surface area of the north salt marsh (40,000 m
2
).
C chl ranged from 1.47 to 23.81 μg L
−1
, and C chl was higher in September because of
a red tide. CS p ranged from 0.094 to 1.524 g C m
−2
. Excluding the data gathered
during the red tide, the average carbon stock of phytoplankton at this bird sanctuary
site was estimated to be 0.167 g C m
−2
.
The carbon flow by phytoplankton was estimated as the net ecosystem production (NEP) of the water column including phytoplankton and bacteria, because the
net primary production of phytoplankton alone could not be estimated. Water quality parameters and phytoplankton biomass were measured at 17 sites corresponding
to a 50 m × 50 m mesh covering the north salt marsh, and the daily net ecosystem
production ( NEP ) of the water column at each site was calculated by using Eqs.
(5.10), (5.11) and (5.12):
GPP
G z dz h t
i
n
h
i
i
g
= å ò ( )
×
=1 0
/
(5.10)
R
R z dz h t
i
n
h
i
i
r
= å ò ( )
×
=1 0
/
(5.11)
NEP GPP R
=
- ,
(5.12)
where GPP is the daily gross production rate in the north salt marsh, n is the total
number of measurement sections (= 17), G(z) is G at the depth z estimated by using
Eqs. (5.5), (5.6) and (5.7), h is water depth in each measurement section, i is the
measurement section, t g is photosynthesis time (8 h), R is the daily respiration/
decomposition rate in seawater, R(z) is the respiration/decomposition rate at depth z
estimated by using Eq. (5.9), and t r is respiration time (24 h). The NEP values were
estimated as −1.59 g C m
−3
day
−1
on 15 July 2015, 16.6 g C m
−3
day
−1
on 16
September 2015, −0.74 g C m
−3
day
−1
on 25 November 2015, −0.09 g C m
−3
day
−1
on 27 January 2016, −0.76 g C m
−3
day
−1
on 9 March 2016, and 4.60 g C m
−3
day
−1
on 25 May 2016. A large amount of carbon was absorbed by phytoplankton on 16
September 2015 due to the red tide. NEP was negative in the season of low water
temperature because R was larger than GPP . However, the averaged value of
NEP without including the red tide data implied a net uptake of inorganic carbon.
These findings suggest that the photosynthesis of phytoplankton affects the carbon
cycle of the tidal flat. The average annual carbon flow of phytoplankton per area was
estimated as 118.3 g C m
−2
year
−1
based on GPP , which is likely smaller than that
of previously reported carbon flow of phytoplankton (see Table 5.1) because this
value includes not only respiration of phytoplankton but also that of bacteria in the
water column.
T. Endo and S. Otani
where C chl is the chlorophyll a concentration of phytoplankton on the measurement
day, V is the average water volume of the north salt marsh (45,700 m
3
), r is the conversion ratio of phytoplankton biomass carbon to chlorophyll a (= 56; Lü et al.
2009), and A is the average water surface area of the north salt marsh (40,000 m
2
).
C chl ranged from 1.47 to 23.81 μg L
−1
, and C chl was higher in September because of
a red tide. CS p ranged from 0.094 to 1.524 g C m
−2
. Excluding the data gathered
during the red tide, the average carbon stock of phytoplankton at this bird sanctuary
site was estimated to be 0.167 g C m
−2
.
The carbon flow by phytoplankton was estimated as the net ecosystem production (NEP) of the water column including phytoplankton and bacteria, because the
net primary production of phytoplankton alone could not be estimated. Water quality parameters and phytoplankton biomass were measured at 17 sites corresponding
to a 50 m × 50 m mesh covering the north salt marsh, and the daily net ecosystem
production ( NEP ) of the water column at each site was calculated by using Eqs.
(5.10), (5.11) and (5.12):
GPP
G z dz h t
i
n
h
i
i
g
= å ò ( )
×
=1 0
/
(5.10)
R
R z dz h t
i
n
h
i
i
r
= å ò ( )
×
=1 0
/
(5.11)
NEP GPP R
=
- ,
(5.12)
where GPP is the daily gross production rate in the north salt marsh, n is the total
number of measurement sections (= 17), G(z) is G at the depth z estimated by using
Eqs. (5.5), (5.6) and (5.7), h is water depth in each measurement section, i is the
measurement section, t g is photosynthesis time (8 h), R is the daily respiration/
decomposition rate in seawater, R(z) is the respiration/decomposition rate at depth z
estimated by using Eq. (5.9), and t r is respiration time (24 h). The NEP values were
estimated as −1.59 g C m
−3
day
−1
on 15 July 2015, 16.6 g C m
−3
day
−1
on 16
September 2015, −0.74 g C m
−3
day
−1
on 25 November 2015, −0.09 g C m
−3
day
−1
on 27 January 2016, −0.76 g C m
−3
day
−1
on 9 March 2016, and 4.60 g C m
−3
day
−1
on 25 May 2016. A large amount of carbon was absorbed by phytoplankton on 16
September 2015 due to the red tide. NEP was negative in the season of low water
temperature because R was larger than GPP . However, the averaged value of
NEP without including the red tide data implied a net uptake of inorganic carbon.
These findings suggest that the photosynthesis of phytoplankton affects the carbon
cycle of the tidal flat. The average annual carbon flow of phytoplankton per area was
estimated as 118.3 g C m
−2
year
−1
based on GPP , which is likely smaller than that
of previously reported carbon flow of phytoplankton (see Table 5.1) because this
value includes not only respiration of phytoplankton but also that of bacteria in the
water column.
T. Endo and S. Otani