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General
background
Nutrient
salts are dissolved mineral salts, the contents of which
include phosphorus (P), nitrogen (N) and silica (Si).
The nitrous salts are nitrates (NO3),
nitrites (NO2) and ammoniac (NH4);
these substances provide plants with the nitrogen they
require to synthesize amino acids, which are the main
components of proteins. Phosphate (PO4) is
the form in which phosphorus can be most readily used by
plants to synthesize energy-rich molecules (ATP), for
example. Ninety-five per cent of the silica dissolved in
the marine waters is in the form of silicic acid (Si(OH)4)
or silicate; this substance is required by the siliceous
algae such as the diatoms, as well as by the
silicoflagellates and radiolarians for their development.
These
nutrient salts are all present in very small quantities
in the sunlit ocean layers
(the euphotic zone). The production of
phytoplankton in the ocean waters is strongly dependent
on their concentrations. Other elements such as iron
seem to play an important part in the primary production
processes at work in these waters.
The
annual and spatial patterns of distribution of nutrient
salts in the lagoons.
| The
nutrient salt levels were studied for several
years in the Tikehau lagoon. Low mean
annual nitrate concentrations of less than 0.1
µM were recorded, except in 1986, when they
reached 0.15 µM. Phosphate levels of less than
0.2 µM were recorded. The silicate
concentrations were found to range between 0.4
and 0.9 µM. |
|
Nitrates
|
Phosphates
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Silicates
|
| On
Takapoto, the nitrate, phosphate and silicate
concentrations recorded were of the same order
of magnitude as on Tikehau, except for the
nitrate levels measured at station 1, which was
located near the village: these amounted to 0.4
µM. These abnormally high water nitrate levels
were probably due to the lagoon water being
mixed with the groundwater, which was polluted
by wastewater from the local septic tanks. The
ammoniac levels measured were around 0.2 µM. As
on Tikehau, the phosphate and silicate levels
were found to be distinctly higher in the
surface ocean waters than in the lagoon. |
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The
mean nutrient salt concentrations recorded on these 2
atolls are given in the following table:
|
Means
± standard
errors: nutrient salt concentrations
(µM)
on
Takapoto (September and February) and Tikehau (January
and March) |
|
Atoll |
PO4 |
NO2 |
NO3 |
Si(OH)4 |
|
Takapoto |
0.07
± 0.012
(n=44) |
0.05
± 0.005
(n=35) |
0.19
± 0.023
(n=44) |
0.53
± 0.044
(n=22) |
|
Tikehau |
0.14
± 0.006
(n=351) |
0.02
± 0.001
(n=309) |
0.07
± 0.006
(n=313) |
0.83
± 0.025
(n=249) |
The
inter-atoll and inter-seasonal variablity
The
other lagoon waters studied were all found to have very
low nitrous and phosphoric contents, except for
Reka-Reka, where the dissolved mineral nitrogen content
(DIN=nitrate+nitrite+ammoniac) was as high as 0.9 µM.
The shallowness of this lagoon (<1 m) has probably
led to the
lagoon waters being mixed with the interstitial waters
present in the sediments, which have a high mineral
nitrogen content (see the section on interstitial
waters). Some abnormally high silicate
values were recorded on completely enclosed atolls (Taiaro
and Reka-Reka) and on one very small atoll where little
renewal of the lagoon waters occurs (Tepoto).
No
significant differences were observed between the wet
and dry seasons. (See table).
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. (1996) Phytoplankton biomass and production in two
Tuamotu atoll lagoons (French Polynesia). Mar Ecol Progr
Ser 145 : 133-142
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Atoll_site_