|
Residence
times of the ocean waters in the lagoons
|
The
lagoons are supplied with water originating from the
surface of the ocean, which reaches them via the
navigable passes and other channels (hoas) crossing the
atoll ring.
The
rate of exchange between the lagoon and ocean waters
depends on the number of channels, their depth and their
orientation with respect to the prevailing sea swell.
The residence time is used to characterise the exchanges
between a lagoon and the open sea, given the capacity of
the lagoon. The residence time can be assessed in
several ways, one of which consists of taking the ratio
between the capacity of the lagoon and the net annual
surface exchanges measured. This method requires having
a sufficiently large number of measurements of the
volumes of water exchanged via the channels connecting
the lagoon to the open sea; it was used on the Takapoto
lagoon by Magner & Wauthy (1976) and by Ricard et
al. (1979). The residence times calculated in this way
in the case of this particular atoll were 6.7 and
6.6 years, respectively. This method was also
used by Lenhardt (1991) on the Tiahura lagoon (on the
island of Moorea) and the Tikehau lagoon, where the mean
monthly residence times were found to range between 5h20
and 7h40 in the former case and between 125 and 230 days
in the latter case. Another possible method consists of
taking the difference between the salinity of the ocean
(So) and that of the lagoon (Sl)
and the difference between the precipitation (r) and the
evaporation (e) on a lagoon with a mean depth Z
according to the equation proposed by Smith & Jockiel
(1975) :
If
we apply this equation to the Takapoto lagoon, we obtain
the following residence time:
T
= 4.24 years.
|
The
nutrient salt content of the oceanic waters
surrounding the atolls
|
The
central Pacific waters are part of the great oceanic
Gyre, where the nutrient salt content of the sunlit
water layers is extremely low.
Table: Nutrient salt concentrations (µM) of the
surface oceanic waters surrounding the Tuamotu
Archipelago (Dufour P. & al., 1999).
|
Nutrient |
Mean |
SES |
n |
|
NH4+ |
0.016 |
0.0041 |
19 |
|
NO2-+ NO3- |
0.010 |
0.0036 |
24 |
|
DIN |
0.023 |
0.0048 |
19 |
|
PO4++ |
0.21 |
0.015 |
24 |
|
SiO2 |
1.03 |
0.109 |
24 |
During
a mission carried out in 1985, the nitracline and the
phosphacline were found to both occur at 200 m and
the mineral nitrogen and phosphorus concentrations
recorded in the surface ocean
waters (0 - 50 m) were found to be
distinctly higher in the neighbourhood of the atoll of
Tikehau than 70 km out to sea (Figure).
The
hypothesis was therefore put forward that this enrichment
of the waters surrounding the atolls might be due to
vertical turbulent processes (Charpy-Roubaud CJ. & al.
1990).
|
Does
the horizontal oceanic flux alone suffice to
supply the atoll reefs with their nutrient
requirements?
|
We
compared the P
requirements of the reef with the amounts of P supplied by the ocean.
|
|
An
atoll reef's phosphorus requirements were
assessed, taking the new production
rate on the reef to be 100 mg C m-2 day-1and the C:P ratio to be
550:1 , along with the value obtained for
the area of the
coral ring.
|
|
This
gives 140 mol P day-1
|
|
The
amounts of phosphorus with which the
atoll reef is provided by the horizontal
oceanic flux can be calculated on the basis of
the water flux reaching the atoll multiplied by
p, the P content of the surface ocean waters.
This gives 2.1x107 mol P / day-1
|
 |
|
This
gives 2.1x107 mol P day-1
|
|
The
results obtained in this way show that the
oceanic flux easily provides the amounts of P
required to sustain the reef metabolism.
|
The
research carried out by Eric Basillais on the fractal
aspects of the atoll rings and the micro-turbulent
processes at work has confirmed that the horizontal flux
easily provides the N and P requirements of the outer reef
surrounding the atolls (Basillais 1998).
|
Nutrient
balances between the oceanic and lagoon waters
|
If we take a lagoon ecosystem
to be delimited by its outer barrier reef, the nutrient
elements with which the lagoon is provided via the
oceanic water flux can be said to constitute part of the
new production of the lagoon.
This production was calculated by taking the
amount of organic carbon exported via the Tikehau atoll
pass, and was found to amount to approximately 26 mg C m-2 /
day-1 , i.e., 6 % of the total
phytoplankton production (Charpy L.& Charpy-Roubaud
CJ. 1991).
|
To
determine the contribution of the horizontal
oceanic flux to the new production, we drew
up nitrogen, phosphorus and silica budgets
between the oceanic and lagoon waters.
|
The phosphorus budget turned
out to be perfectly balanced: equal amounts of
phosphorus enter and leave the lagoon. A change of state
occurs, however, since the phosphorus is initially
mineral and becomes organic. The nitrogen budget was
found to be much less well balanced: more nitrogen was
leaving the lagoon than the amounts being imported. One
possible explanation for this enrichment of the ocean
waters during their stay in the lagoons was based on the
molecular nitrogen fixation processes involved. The fact
that the silica budget was
negative showed that Si was being consumed by the
benthic organisms inhabiting the lagoon (Charpy-Roubaud
CJ. & al. 1990).
The
functional mechanisms at work in the atoll ecosystems
have also been investigated by F. Rougerie, who
developed a functional model in collaboration with B.
Wauthy, based on the upwelling of waters carrying
abundant supplies of nutrients in the vicinity of the
atolls (Rougerie & Wauthy 1986).
Measurements
carried out at drilling sites located on the outer
perimeter of the atoll rings have shown that the nutrient
salt concentrations
are distinctly higher here than in the oceanic waters,
possibly due to a process of remineralisation. It was only
quite recently that a model was developed for the fluxes
occurring in interstitial coral environments, which showed
that the hydro-geological features of the atolls play only
a minor role in the nutrient budget of the reef ecosystems
(A.M.Leclerc 1998).
|
Upon
applying these findings to the nitrate, phosphate
and magnesium transport processes, it was observed
that the hydro-geological features of the atolls
play only a minor role in the nutrient budget of
the reef ecosystems, but that they may be involved
in the long-term geological transformations which
occur.
Extract
from the Abstract of A.M. Leclerc's Doctoral thesis
|
|
Conclusions
about the exchanges occurring between the
lagoons and the open sea
|
-
The
residence times in the lagoons of the waters
originating from the open sea range between
a few hours and several years
-
The
oceanic waters surrounding the atolls are
enriched with N and P as the result of vertical turbulent processes.
-
Although
the nutrient salt concentrations are low,
the horizontal oceanic fluxes easily suffice
to provide both the outer coral reef and the
lagoon with their phosphorus requirements.
|
This page
was based on :
Charpy-Roubaud
CJ, Charpy L, Cremoux J-L (1990) Nutrient budget of the
lagoonal waters in an open South Pacific atoll (Tikehau
Tuamotu French Polynesia). Mar Biol 107: 67-73
Charpy
L (2001) Phosphorus supply for atoll biological
productivity. Coral reefs 20:357-360
Reference
sources:
Basillais E (1998) Etude
géométrique fractale de trois espèces coralliennes (Pocillopora
eydouxi, Porites lobata, Acropora cerealis). Transfert
d'échelles, modèle fractal des flux naturels d'espèces
chimiques dissoutes vers les récifs coralliens. Thèse de
doctorat de l'Université de la Méditerranée (Aix-Marseille
II). Pp 188
Charpy
L. & Charpy-Roubaud C.J. (1991). Particulate Organic
Matter fluxes in a Tuamotu Atoll Lagoon (French Polynesia).
Mar. Ecol. Progr. Ser, 71 : 53-63.
Dufour
P, Charpy L, Bonnet S and N. Garcia (1999). Phytoplankton
nutrient control in the oligotrophic South Pacific sub
tropical (Tuamotu archipelago). Marine Ecology Progress Series,
179 : 285-290
Leclerc A-M (1998)
Modélisation des écoulements dans le milieu interstitiel
corallien et quantification des échanges de matière avec
l'environnement océanique. Thèse Doctorat, Ecole Nationale
Supérieure des Mines de Paris, pp 199
Lenhardt X (1991)
Hydrodynamique des lagons d'atoll et d'île haute en
Polynésie Française. ORSTOM, Etudes et Thèse, Paris pp 132
Magner Y, Wauthy B
(1976) Esquisse hydrologique du lagon de Takapoto (Tuamotu).
Cah Off Rech Sci Tech Outre Mer, Série Océanographie 14:
279-284
Ricard M., Gueredrat
J.A., Magnier Y., Renon J.P., Rochette J.P., Rougerie F.,
Sournia A., Wauthy B., (1979). Le plancton de Takapoto. J.
Soc. Océanistes, 35(62) : 47-67
Rougerie F, Wauthy B
(1986) Le concept d'endo-upwelling dans le fonctionnement des
atolls-oasis. Oceanol. Acta 9: 133-148
Smith S.V., Jokiel P.L.
(1975). Water composition and biogeochimical gradients in the
Canton atoll lagoon: 1. Lagoon description; design of system
analysis; salt and water budget. Mar. Sci. Comm. 1(1):75-100
|
Atoll_site_
