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The
POM in the lagoons consists of living plant and
animal material and waste products (Figure).
This provides an efficient index to the abundance
of a lagoon's natural resources (Figure).
Pearl oysters and other organisms filtering the
lagoon waters feed on this particulate matter.
The
POM can be recovered by screening the sea water
through fibreglass filters
(Figure).
The concentrations of the various elements present
(such as carbon, phosphorus and nitrogen) can then
be determined and the molecules of substances such
as ATP, phytoplankton and chlorophyll on which
living organisms depend can also be quantified.
The
mean size of the organic particles collected is
determined by filtering the samples using filters
with various mesh sizes. (Figure). |
Temporal
and spatial variability of the particulate matter
present in a lagoon
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The
studies carried out at Tikehau and Takapoto showed
that the POM, as assessed in terms of the particulate
organic carbon (POC) levels present, showed little
spatial variability but varied greatly with time,
mainly due to the churning motion of the waves, which
increases the numbers of suspended particles. The
accompanying figure clearly shows the effects of the
cyclones which hit the Tuamotu atolls in 1983.
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The
variability of the POC levels in the Takapoto lagoon
is also due to the effects of the wind (Charpy L.
& al. 1997). In a study on parameters other than
the POC levels, Pouvreau (1999) also concluded that
the wind is the main factor responsible for the
variations observed.
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The
mean POM concentrations recorded in the two lagoons
mentioned above were 192 and 82 mg C m-3,
respectively (Charpy L. & Charpy-Roubaud C.J. 1991, Charpy L.
& al. 1997).
The
POM in the other lagoons
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Dans tous les
atolls prospectés dans le cadre du programme TYPATOLL,
on observe que les concentrations en COP sont
généralement plus faibles que celles observées dans
l'atoll de Tikehau.
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Some
of the parameters responsible for
the POM levels (moptab.htm) (mg m-3) have
been determined on 16 of the Tuamotu atolls (Charpy L.
& al. 1997).
Taille des
particules
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We
established that 81 % of all the organic
particles present in the Tikehau lagoon were smaller
than 3 µm (Blanchot & al. 1989) and that
80 % of the particles were of this size in
the Takapoto lagoon (Charpy L. & al. 1997). Small
particles seem to be a common feature, since more than
70% of all the particles in all the lagoons prospected
were <3 µm in size (Charpy L. & al.
1997).
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Sedimentation
of the POM
| The
quantities of organic matter deposited at the bottom
of the lagoons can be determined by measuring the rate
of sedimentation with a sediment trap. For details of
the
calculations
, click here. |
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The
rate of sedimentation of the POM in the Tikehau lagoon
was found to average 350 mg C m-3 day-1
(Charpy L. & Charpy-Roubaud C.J. 1991).
Conclusions
about the POM
The
levels of POM present in the lagoons vary considerably,
depending on the time of year. These variations are
correlated with meteorological events, such as the 1983
cyclone. The monthly and daily variations depend mainly
on the state of agitation of the lagoon waters.
The
POM levels vary considerably from one atoll to another.
The most northerly atolls have higher POM levels than
the others.
The
organic particles present in the lagoon waters are very
small, since 80 % of these particles measure <3 µm.
This
page was based on :
Blanchot
J, Charpy L, Le Borgne R (1989) Size composition of
Particulate Organic Matter in the lagoon of Tikehau
atoll (Tuamotu archipelago). Marine Biology 101 :
329-339
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
L, Dufour P, Garcia N (1997) Particulate Organic Matter
in sixteen Tuamotu atoll lagoons (French Polynesia). Mar
Ecol Prog Ser 151 : 55-65
Reference
sources :
Pouvreau S (1999)
Croissance de l’huître perlière dans le lagon de
Takapoto. Thèse Ec Nat Sup Agro, 217p |
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