Abstract:
A comparative study was made of the phytosociology
o-f mangrove vegetation and physicochemistry o-f forty-three
water-bodies selected along the coast from F'alatupana in
the southeastern arid-zone to Puttalam in the northwestern
arid-zone of Sri Lanka with the major emphasis on the three
lagoons at Rekawa, Kalametiya and Lunama. The floristic
data were statistically analysed by Indicator Species
Analysis (ISA) for c1assification and Receprocal Averaging
(RA) for ordination.
The coast covered all the c1imatic-zones and most of
the major soil-types of the lowland of Sri Lanka. The
multivariate analyses of the floristic data from the
extensive survey of the forty-three mangrove communities
revealed that most vegetational diversity is related to
climate with differences in salinity and shallowness of the
water-bodies being responsible partly for the heterogeneity
between the major communities.
The three lagoons at Rekawa, kalametiya and Lunama
differed geomorphological 1y ,
«hydrographical 1y and physicochemical 1y.
hydrolog i cal 1y ,
At Rekawa both
influx and efflux of seawater as well as freshwater occur
through the same canal, while the Kalametiya lagoon is
almost a running water-body because the inland irrigation
waters flow continuously through it into the sea; the
Lunama lagoon remains almost stagnent, has no connection
Ill
with the sea and receives water -from the Kalametiya lagoon
via a canal. The physiochemical parameters investigated
were salinity, alkalinity, pH, dissolved oxygen content,
.reactive soluble, phosphate and suspended matter, all of
which exhibited marked seasonal variations in all the three
lagoons; so was the diurnal variations in salinity,
temperature and pH while their spatial heterogeneity was
highest at Rekawa and lowest at Lunama.
The mangrove vegetation was most extensive and
luxuriant with the highest spec i es-r i chness and -floristic
zonation at Rekawa and least at Lunama while that at
Kalametiya being somewhat i nteremedi ate. As evident -from
the multivariate results, the vegetational differences
between the three mangrove complexes were associated with
differences in salinity and the status of potassium and
sodium; the floristic heterogeneity within each mangrove
complex was also related to the differential influence of
salinity which was further found to create edaphic,
particularly cation, gradients so causing floristic
zonation from the water's edge at the lagoon to the
high-lying ground.
* The floristic data from the monthly samples of
phytoplanktons of the three lagoons were also analysed for
ISA classification and RA ordination; this is probably the
first such treatment of phytoplankto data. According to
the results the maximum phytoplankton differences between
the three lagoons are related to salinity and the monthly
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■fluctuations within individual lagoons are associated with
variations in salinity, rainfall, total suspended matter
and nutrients such as phosphate. Diatoms are predominant at
R'ekawa and blue green algae at Lunama with Kalametiya
sharing both groups; the Importance Value Indices con-firmed
this observed predominance.
A brief study was also made of the phytoplankton
productivity of the three lagoons. Both the overall
productivity and that of the individual species exhibited
considerable monthly fluctuations in association with
variations in salinity, total suspended matter and
rai nfal1.
The results are discussed with major emphasis on the
edaphic, physicochemical and hydrological control of the
variations of mangrove and phytoplankton flora of the three
lagoons at R'ekawa, Kalametiya and Lunama.
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