Physical and chemical characteristics
of interstitial waters
peeper
Materiel
and methods
Sampling
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8
stations were surveyed in the lagoon of Tikehau atoll: 3
in 1987 (1,4 et 8) et 6 in 1993 (2,3,5,6,7,8). Station 1
is located close to the internal reef flat. Stations 2
and 3 have been chosen because of their leeward and
windward locations from a pinnacle with respect to the
prevailing wind direction. Stations 4, 5 and 6 exhibit
depths similar to the average depth of the lagoon (25 m)
while stations 7 and 8 are located in the deepest part
of the lagoon. |
We used the in situ peeper
sampling method described by Hesselein (1976). The peepers were
inserted in the sediment and left on the site for at least 15
days in order to equilibrate the initial pore water with the
interstitial medium. According to the size of the peeper
chambers (3 or 10 ml), the spatial resolution is either 1 cm or
2.5 cm. These chambers were covered with a Biodyne A membrane
(pore size = 0.2 µm).
Analysis
The pH was determined at room temperature (T =
22 °C) on
submicrosamples (500 µl) using an Ingold micro-electrode and
the Hansson's calibration method described by Almgren et al.
(1975) with a precision of ± 0.01 pH unit.
Gran's potentiometric titration was used for alkalinity
measurements (Stumm and Morgan 1981).
The accuracy was ± 0.5 %.
Standard colorimetric procedures were used for nutrient
analysis (SiO2, NH4 and PO4)
and total dissolved sulfide (SO4), adapted for submicro samples (Merck Spectroquant
methods; sample volume: 0.5 or 1 ml, precision ±4%).
A known volume of the ZnS suspension is used to develop the colorimetric
analysis of H2S using the conventional
methylene-blue method with a precision of
±
3 %.
Ca
were analyzed by conventional AAS procedures with a precision of
5 %.
Results
Pore
water profiles of H2S, pH, alcalinité et calcium
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Fig. 2
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H2S
(Fig. 2) appears either a
few millimeters or 2 centimeters below the sediment
water interface (SWI) at stations 5 and 8, while this
redox limit is located between 3 and 5 cm below
the SWI at stations 1, 2 and 4. The highest
concentrations (365 µM)
are observed at station 5, 13 cm below the SWI. SO4
concentration profile is available only at station 8 (Fig.
2). It regularly decreases with the depth in the
sediments. |
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pH
in pH unity |
All
the stations exhibit the same general characteristics:
-
The
pH immediately decreases below the SWI (1cm inside
the sediment).
-
The
calcium measured at station 8, shows a relatively
constant concentration between 9 and 12.5 mM.
-
Maximum
of alcalinity are observed between 5 and 15 cm
inside the sediment at stations 3, 5, 6 et 8 (data
of 1987) and between 0 and 5 cm inside the sediment
at stations 2 and 8 (data of 1993).
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calcium and alcalinity
in mM |
Pore
water profiles of nutrients concentrations
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We
observe :
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an
increasing concentration from the SWI to a
maximum ranging from 2 to 6 µM located between 4
and 6 cm downward at stations 2, 3, 6 and 8.
-
an
increasing concentration from the SWI to a
maximum ranging from 2 to 5 µM located between 10
and 25 cm downward at stations 1, 4, 5 and 7.
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We
observe :
-
a
regular but minor increase of the concentration with
depth along the sediment core at stations 2 and 8.
-
an
increasing concentration going through a maximum,
followed by a regular decrease at stations 2, 5, 6
and 7. The highest concentrations being observed at stations 5 (59
µM) and 7 (32 µM).
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We
observe :
-
either
a regular increase as at stations
1, 2 and 6
-
or
a mid-depth maximum concentrations as seen at stations 5 (102 et 180 µM)
and 7 (101µM).
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Relation
between the depth and the nutrients
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The
integrated concentrations in the upper layer of 20 cm
of the sediments, expressed in cm2
of PO4 and NH4, are correlated
to the depths of the stations (R=0.79, n = 13 for PO4
and R = 0.73,
n = 11 for NH4).
The sedimentation rates of the organic matter
from the water column is certainly one of factors of the
influence of the depth on the nutrients concentrations
in the interstitial waters. Consequently the fluxes
calculated of PO4
and NH4 are also correlated to the
depth (respectively R= 0.73
and R= 0.73). |
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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
:
Hesslein
RH (1976) An in situ sampler for close interval pore water
studies. Limnol Oceanogr 21: 912-924
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Atoll_site_