Nutrient
fluxes at the sediment water interface (SWI)
Flux
calculation
Rates
of efflux of N, P and Si from the sediments were calculated
using the equations of Rutgers van der Loeff et al. (1984) (Fick’s
first law of diffusion) :
|
Equation
:
F
= Ø x
Ds x dC/dz
where
:
-
F
= flux (µmol m-2 day-1)
-
Ø
= sediment porosity (dimensionless)
-
Ds
= effective diffusion coefficient (in m2
day-1)
-
dC/dz
= the concentration gradient across the SWI* (µmol m-4
)
|
* SWI : Sediment Water Interface
Fundamental
values:
-
The
measured porosity (ø)
of coral sand sediment was the same from one station
to another: 0.63 ± 0.01.
-
The
apparent diffusion coefficients in deionized water
(D0), taken from Manheim (1970) and Li
and Gregory (1974) for a water temperature of 25°C,
were corrected for tortuosity using a porosity of
0.63 and porosity-tortuosity relationships reported
by Sweerts et al. (1991). The apparent diffusion
coefficients for the flux across the SWI thus
calculated are respectively:
Ds
for NH4, 9.85 m2
day-1
Ds
for PO4, 3.72 m2
day-1
Ds
for Si , 5.01 m2
day-1.
Since
the evaluations of Ds,
ø
and dC/dz are independent, the cumulative error on the flux
calculation can be estimated at ± 40 %.
Results
|
Table1
: Nutrient fluxes (µmol m-2 day-1) calculated using
pore water concentration gradients and N/P atomic ratio
of efflux. The maxima and minima are in bold letters. |
|
Date
Station Depth
NH4
PO4
N/P
Si
May
1987 1
13
10.0
0.8
13.1
nd
May
1987
8
36
188.6
6.0
31.6
nd
May
1987 4
24
18.3
0.9
20.1
nd
April
1993 3
21
4.2
0.6
25.1
14.8
April
1993
2
21
78.8
3.0
25.9
2.7
April
1993 8
36
600.7
8.9
67.6
53.4
April
1993 6
27
133.4
10.3
13.0
34.1
April
1993 5
25
nd
2.3
nd
57.8
April
1993 7
31
273
7.7
35.4
79.2 |
Comparison
with others lagoonal N and P
fluxes
In
spite of the large range of the flux results, we try to compare them
with other lagoonal N and P fluxes. N and P deposition rates (in term of
particulate organic nitrogen -PON- and total phosphorus -POP-) were
measured at station 6 (close to station 4) between 1986 and 1987 (Charpy
and Charpy-Roubaud 1991). The results are 2,575 µmol N m-2
day-1 and 104 µmol P m-2 day-1.
Therefore,
| Average N (163 µmol N m-2 day-1) and P
(4.5 µmol P m-2 day-1) fluxes from sediment
represented respectively 6% and 4% of the average N and P deposition
rates.
|
The
average atomic N:P ratio of the efflux at SWI was 29:1, a value very
close to the PON:POP (27:1) ratio in trapped material (Charpy and
Charpy-Roubaud 1991). Therefore, there was no sink or loss for
regenerated nitrogen or phosphorus in sediments. However, the deepest
stations (7 and 8) presented high N:P ratio (32-68). This implied that
| There was a sink for regenerated nitrogen.
|
Charpy
& Charpy-Roubaud (1990) have demonstrated that the total primary
production (benthos + plankton) was independent of depth of the station
and depends only on the light energy reaching the surface. Therefore,
| We
can compare the fluxes at the SWI, which represent potential nutrients
fluxes available for lagoonal primary producers, with mineral N and P
requirements for this production. |
Average
P-PO4 requirement for phytoplankton + phytobenthos is 603 µmol
P m-2 day-1 (Charpy and Charpy-Roubaud 1989).
Nitrogen requirement may be estimated to 8,849 µmol m-2 day-1
using an average uptake of carbon of 58,329 µmol C m-2
day-1 (Charpy-Roubaud et al.
1989) and a Redfield C:N ratio (6.6). Therefore, the flux of ammonium
from the sediment represents between 0.1 % and 6.8 % of the N
requirement and the flux of phosphate between 0.1 and 1.7 % of the P
requirement. Part of N and P requirements must then be supplied by the
zooplanktonic excretion; in Tikehau, Le Borgne et al. (1989) have
estimated, for the micro + mesozooplankton, this contribution to be
respectively 32 % and 18 % of phytoplankton nitrogen and phosphorus
requirements, i.e. respectively 18 % and 10 % of phytoplankton +
phytobenthos N and P requirements. The remainder can be supplied by
other excretions and mineralization in the water column and nitrogen
fixation.
Conclusions
-
The
sediments of Tikehau lagoon are sources of nitrogen
(4 - 601 µmol NH4 m-2 day-1), phosphorus (0.6 -
10.3 µmol PO4 m-2 day-1)
and dissolved silica (3 - 79 µmol SiO2 m-2
day-1) to the overlying water column.
-
N
and P effluxes from sediments represented 6% and 4%
of N and P deposition rates. In general trend, there
was no sink of regenerated N and P.
-
N and P fluxes represent between 0.1 % and 6.8 % of
the N requirement and between 0.1 and 6.8 % of the P
requirement of lagoonal primary production.
|
This
page was based on :
Charpy-Roubaud
C., Charpy L., Sarazin G. (1996) Diffusional nutrient fluxes at
the sediment-water interface and organic matter mineralization
in an atoll lagoon (Tikehau, Tuamotu Archipelago, French
Polynesia). Mar Ecol. Progr. ser. 132: 181-190
References :
Charpy
L, Charpy-Roubaud CJ (1989) Phosphorus budget in an atoll lagoon.
In Choat JH et al (eds) Proc 6th int Coral Reef Symposium,
Townsville, Australia, Vol 2: 547-550
Charpy
L, Charpy-Roubaud CJ (1990) A model of light-primary production
relationship in an atoll lagoon (Tikehau,
Tuamotu
Archipelago
,
French Polynesia
). Journ of
Mar Biol Ass 70: 357-369
Charpy
L, Charpy-Roubaud CJ (1991) Particulate Organic Matter fluxes in
a Tuamotu atoll lagoon (
French
Polynesia
). Mar Ecol
Progr Ser 71: 53-63
Charpy-Roubaud
CJ, Charpy L, Lemasson L (1989) Benthic and Planktonic primary
production of an open atoll lagoon (Tikehau, French Polynesia).
In Choat JH et al (eds) Proc 6th int Coral Reef Symposium,
Townsville, Australia, Vol 2: 551-556
Le
Borgne RP, Blanchot J, Charpy L (1989) Zooplankton of the atoll
of Tikehau (Tuamotu Archipelago) and its relationship to
particulate matter. Marine Biology 102: 341-353
Rutgers van der Loeff MM, Anderson LG, Hall POJ, Iverfeldt A,
Josefson AB, Sundby B, Westerlund SFG (1984) The asphyxiation
technique. An approach to distinguish between molecular
diffusion and biologically mediated transport at the
sediment-water interface. Limnol Oceanogr 29: 675-686
|
Atoll_site_