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Metadata
Title
etude de la capacite biotique dans les trois lagunes de petriky (ambinanibe, andranasy et andrantoloharano) de la region anosy a fort dauphin
Authors
LOPE Jean Charles
Research Topic
Abstract
The biogeochemical and phytoplankton study of the three lagoons of Petriky in Fort Dauphin
(Southeast Madagascar) was undertaken before their disturbance through ilmenite extraction. The
lakes of Petriky consist of three contiguous lagoons with an average size of 5 to 10 km2.
From East to West, there is Ambinanibe lagoon which is the largest (9,5 km2) and shallowest (2,5m
± 0,2 m) of the three. It is an open lagoon, which receives the Ifaho River from the North and has a
temporary pass linking to the sea in the South. The central lagoon is Andranasy which has a surface
area of 8,2 km² and an average depth of 3,2 ± 0,3m. It has a permanent communication with the
lagoon of Ambinanibe. It also has a temporary link with the third lagoon, Andrantoloharano.
Andrantoloharano is the smallest (5km²) and the deepest (4,5 m ± 0,3 m) lagoon and is almost a
closed system. The average water temperature of the three lagoons Ambinanibe, Andranasy and
Andrantoloharano is 25,3± 2.6°C, 25,4 ± 3.0°C and 25,35 ± 3,0°C respectively.
For the three lagoons, the average pH values were measured at 7,7 ± 0,5 at Ambinanibe, 7,7 ± 0,2 at
Andranasy and 7,9 ± 0,2 at Andrantoloharano.
The three lagoons of Petriky are brackish. Ambinanibe has two different layers of salinity whilst the
pass is open during the three first months of the year. The surface layer has a salinity of 8.7 ± 5.2‰
and the bottom layer 14.9 ± 6.5‰. The other lagoons are homogeneous and have a salinity of 10.4 ±
3,2 ‰ (Andranasy) and 8,8 ± 4,6 ‰ (Andrantoloharano).
The oxygen values do not vary significantly between lagoons (7,14 ± 0,11 mg/litre) and there is
never anoxia. All major enrichments are from the Ifaho River. Thus, the particle charge and
turbidity decrease depending on the distance from the mouth of the Ifaho River. The average depth
of disappearance of the Secchi disk was 1,51 ± 0,34m at Ambinanibe, 2,03 ± 0,45m at Andranasy
and 2,16 ± 0,62m at Andrantoloharano. In all cases, the depth of disc disappearance was less than
that of the water column.
Throughout the year, the lagoons have quantifiable concentrations of nutrients. This enrichment
results from flooding of the Ifaho River. The average ammonium concentrations differ between the
lagoons, with 1,2 ± 0,6 mol / l at Ambinanibe, 2,9 ± 2,0 mol / l at Andranasy and 4,8 ± 3,1 mol / l at
Andrantoloharano. The nitrate enrichment begins in January in the Ambinanibe lagoon and spreads
gradually to the other lagoons after an interval of one month. After the flood, there is a high peak in
nitrate concentrations, with variations between the lagoons. The maximum concentration can be
200-400 times higher than the minimum values. The nitrate concentrations of the water surface
ranged from 0,4 to 158,5 mol/l at Ambinanibe, 1,7 to 299,7 mol/l at Andranasy and 0,6 to 123 mol/l
at Andrantoloharano.
The nitrite concentrations for the three lagoons Ambinanibe, Andranasy and Andrantoloharano
were 0,15 ± 0.10 mol/l, 0,12 ± 0,07 mol/l and 0,21 ± 0,15 mol/l respectively.Silicate also follows this pattern of enrichment; beginning in November in Ambinanibe and ranging
from East to West after a time interval. The average silicate concentrations were 60.64 ± 24.24
mol/l at Ambinanibe, 59,06 ± 25,83 mol/l at Andranasy and 31,23 ± 15,10 mol/l at
Andrantoloharano.
The mean concentration of phosphates was 0,22 ± 0,07 mol/l at Ambinanibe, 0,18 ± 0,04 mol/l at
Andranasy and 0,14 ± 0,05 mol/l at Andrantoloharano. The enrichment is not very marked and
more spread with the time.
The lagoons are Mesohaline and oligo-mesotrophic according to the annual season with a marked
enrichment in nutrients during the rainy season.Temporal monitoring of chlorophyll-a
concentrations and phytoplankton density was undertaken. Primary production was studied during
punctual missions. An in situ method monitoring Carbon-14 incubation was used. The results show
that the lagoons differ in their primary production and that the largest production is linked to
external inputs. The open lagoon of Ambinanibe produces 5 times more Carbon than the half open
lagoon of Andranasy and 8 times more than the near closed lagoon of Andrantoloharano. The
values were recorded at 64 ± 21 g C l-1 day-1, 13.7 ± 5μg C l-1 day-1 and 8.5 ± 2 g C l-1 day-1
respectively.
The chlorophyll-a concentration in the lagoon of Ambinanibe was three times more than the lagoon
of Andranasy and Andrantoloharano. The average values are 0,9 ± 0.4 µg Chl-a /l, 0,3 ± 0,2 Chl-a
µg /l and 0,3 ± 0,1 Chl-a µg /l respectively. The abundance of phytoplankton cells was highest at
Ambinanibe (1139 ± 354 cells / ml) and found at similar levels in Andranasy (787 ± 438 cells / ml)
and Andrantoloharano (821 ± 321 cells / ml). Cosmopolitan genera such as Amphora, Entomoneis,
Pleurosigma, Navicula, Nitzschia (Diatoms) and species as Gonyaulax polyedra, Peridinium
bispinium, Peridinium willei (euryhaline dinoflagellates) were found in abundance.This study
shows that primary production, concentration of chlorophyll-a and phytoplankton density varies
seasonally and with increased inputs of freshwater.
Volume
1
Date of publication
May 25, 2009
Institution (University)
Institut Halieutique et des Sciences Marines
Page numbers (pp)
337