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Jayatissa et al 2002
CHANGES IN VEGETATION COVER AND SOCIO-ECONOMIC
TRANSITIONS IN A COASTAL LAGOON (KALAMETIYA,
SRI LANKA), AS OBSERVED BY TELEDETECTION AND
GROUND TRUTHING, CAN BE ATTRIBUTED TO AN
UPSTREAM IRRIGATION SCHEME
L.P. JAYATISSA1 , M.-C. GUERO2 , S. HETTIARACHCHI1 and N. KOEDAM3∗
1
Department of Botany, University of Ruhuna, Matara, Sri Lanka
2
French Institute of Pondicherry. P.O. Box 33, Saint Louis Street 11, 605001 Pondicherry, India
3
Laboratory of General Botany and Nature Management, Mangrove Management Group, Pleinlaan 2,
B-1050, Vrije Universiteit Brussel, Brussel, Belgium
(∗ author for correspondence, e-mail: nikoedam@vub.ac.be; fax and tel.: +32 2 629 3413)
(Received 6 July 2001; accepted 23 April 2002)
Abstract. According to some non-scholarly reports, Kalametiya lagoon (dry zone of southern Sri Lanka,
formerly 8.9 km2 , now 7.5 km2 ) had been a moderately or high salinity water body and a very important
centre of prawn fishery until the late 1960s. Most of the lagoon area had remained open water until then.
An upstream irrigation project, the Udawalawa irrigation scheme, came into operation in 1967, increasing
the freshwater inflow to the lagoon. The flora, fauna and water quality of the lagoon was reported to have
changed since then.
The lagoon now is a shallow coastal water body with low salinity water. More than 75% of the lagoon is
covered by freshwater species or mangrove species characteristic for water with a low salinity: Eichhornia
crassipes, Typha latifolia resp. Sonneratia caseolaris. There is actually no commercially important fishery
in the lagoon.
The present study was carried out to assess scientifically the said changes in the vegetation within a GIS,
using aerial photographs taken in 1956 and 1994 and IRS IC, PAN + LISS III satellite images of 1997 in
combination with ground surveys and information from a questionnaire-based survey.
It appeared from this work that the aerial cover by Sonneratia caseolaris has increased by more than 30 times
over the period from 1956 to the recent dates. Also, the lagoon area with open water has been drastically
reduced during the same period as a result of spreading of freshwater and low salinity plant species. The
results strongly suggest that the locally reported changes (fisheries decline, water salinity decrease) can be
corroborated by the observed profound changes in plant cover and that upstream water works may have had
strong impacts on this ecosystem, thus causing these changes.
This study couples data obtained from retrospective aerial photograph series, from spaceborne imag-
ing, from actual ground surveys and from questionnaires amongst elderly people to reconstruct decadal
environmental change, thus attempting to fill the gap of lacking historical environmental data.
Key words: fisheries, GIS, irrigation, mangrove, Sonneratia caseolaris, remote sensing, Typha latifolia.
1. Introduction
Mangroves are characteristic plant formations dominated by a set of taxonomi-
cally diverse species of trees and shrubs adapted to grow on intertidal areas of
lagoons, estuaries and sheltered bays in tropical and subtropical areas (Ball, 1988;
Environment, Development and Sustainability 4: 167–183, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
168 L.P. JAYATISSA ET AL.
Saenger et al., 1983). These highly productive ecosystems provide a variety of ser-
vices and goods. Nevertheless, these values have been insufficiently recognized and
vast areas of mangroves are being degraded and destroyed, either intentionally or as
a secondary result of other activities, as assessed globally (Saenger et al., 1983) as
well as locally (Abeywickrama, 1960; De Silva and Balasubramaniam, 1984–1985).
In Sri Lanka, the total area of mangrove forests is reported to amount to around
10,000 ha, fringing about 75 riverine estuaries and 45 basin estuaries along the
coastline of 1,740 km. However, a total of 21 species of true mangroves and more
than 20 species of mangrove associates have been reported from these fragmented
mangrove forests (Dahdouh-Guebas, 2001; Jayatissa et al., 2002) implying that the
species richness of mangroves in Sri Lanka is comparatively high.
Kalamatiya lagoon (dry zone of Southern Sri Lanka) is a shallow coastal water
body, which supports a mangrove forest. At present there is no commercial shellfish
or finfish fishery in the Kalametiya lagoon at a significant level. According to some
non-scholarly reports, Kalametiya lagoon had been an important centre for prawn
fishery prior to the late 1960s. It is also reported that the cover by the mangrove
tree Sonneratia caseolaris (L.) Engler and marsh plants in the lagoon was very
small and was confined to the landward edge of the lagoon, leaving most of the
lagoon area with open water. These species now cover more than 75% of the lagoon
area. Intermittent netting takes place only at the southern corner of the lagoon, by
local people for their own consumption. According to the people in the vicinity,
these changes have taken place after the onset of Udawalawa project, a large-scale
upstream irrigation scheme, in 1967 (40 km north of the lagoon).
The major objective of this study is to assess scientifically the said changes
of the vegetation in the lagoon. As there are no scientific data on the vegetation,
water quality and fishery for the period prior to the inauguration of the Udawalawa
irrigation scheme, remote sensing by aerial photographs of the past and information
collected from the local users and residents by way of a questionnaire were used
to study the previous situation. The present situation was studied with recent aerial
photographs, satellite images and ground surveys. The effective cause of the changes
is inferred from these data, because it cannot be addressed directly.
2. Materials and methods
2.1. Study site: Kalametiya lagoon
Kalametiya lagoon is located on the southern coast of Sri Lanka extending from
latitudes 6◦ 04 26 N to 6◦ 07 19 N and from longitudes 80◦ 54 43 E to 80◦ 57 25 E
covering an area of 8.9 km2 formerly (now 7.5 km2 ). More than 75% of the lagoon
is shallow (<0.5 m) and muddy and covered by marsh vegetation, except the
southern corner at the mouth, which has open water of about 1.5 m depth (Figure 1).
Kalametiya lagoon is connected to another small lagoon, viz. Lunama lagoon that
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 169
Figure 1. Kalametiya lagoon showing silted marshy area (depth <0.5 m) and the open lagoon (depth >0.5 m),
with isobaths (m). Inset: Map of Sri Lanka showing the study area. Adapted from Jayatissa (1987) for the
1994 situation.
does not have a direct opening to the sea and is located 2 km east of Kalametiya,
by a shallow canal (Figure 2). The Udawalawa irrigation scheme lies 40 km to the
north of Kalametiya lagoon.
2.2. Remote sensing
Two sets of aerial photographs of the lagoon area; one taken in 1956 with a scale
of 1 : 40,000 and the other taken in 1994 with a scale of 1 : 20,000, were used
for mapping the lagoon and its vegetation. Both sets of aerial photographs were
scanned and digitized using Arc view 3.2 (Esri, USA) to subsequently identify
the lagoon, mangrove area and adjoining land uses. Differences in crown charac-
teristics viz. texture, structure and tonality, are recognizable in aerial photographs
and hence can be used to distinguish different mangrove species as reported by
Dahdouh-Guebas et al. (2000). The characteristics, which were used to identify
five assemblages or stand types of mangroves, are given in Table I. The accuracy of
the identification was checked by field visits. Line coverage of aerial photographs
showing the vegetation and land uses, were geocoded with standard topographical
170 L.P. JAYATISSA ET AL.
Figure 2. Catchment area of Kalametiya lagoon including major canals, streams and manmade reservoirs.
Adapted from Liyanarachchi et al. (1995).
sheets (1 : 50,000 in scale) by using GIS software. The coverage of 1956 and that
of 1994 were superimposed to obtain area statistics of changes in the mangrove
cover.
Moreover, an IRS-1C PAN image of 1997 was merged with the simultaneously
acquired LISS III image. The Kalametiya lagoon area was extracted from the
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 171
TABLE I. The identification key used to distinguish different assemblages/stands of mangrove species.
Tonality Texture Structure Other attributes Species
Light grey Fine grain and Discontinuous canopy None Avicennia spp.
and blurred
aspect
Medium grey Coarse grain Discontinuous canopy None Excoecaria agallocha
(or light grey)
Grey to Coarse grain Discontinuous canopy Often higher than Sonneratia caseolaris
dark grey with irregular shapes surrounding trees
Dark grey Very fine grain Continuous canopy None Lumnitzera racemosa
with crowns hardly
distinguishable
resulting image and georectified to be overlaid with the aerial photographs. The
resolution merge between the images was carried out to bring together the better
spatial resolution of the PAN image (pixel size is 5.8 m) and the multispectral infor-
mation (green, red and near infrared bands) of the LISS III image. The extent of
different types of mangrove cover was obtained by visual interpretation based on
thorough ground knowledge of the study area.
2.3. Vegetation and environmental data
Salinity and the water level of a sampling point at the deepest point of the open
water area were monitored regularly with monthly intervals during the year 1999.
Profile diagrams of the mangrove vegetation were prepared according to Davis
and Richards (1933, 1934) to show the physiognomy of the mangrove forest and
required data were collected along a 1 m wide band transect running from the outer
margin of the mangrove cover to the inside. Five of 4 m wide transects running across
the mangrove areas were also selected from physiognomically different parts of the
mangrove vegetation and all the inhabiting species were recorded.
2.4. Questionnaire-based survey
A questionnaire was used to obtain information on the situation of the lagoon in
the past from the local residents and users around the lagoon. All men and women
whose age was not less than 60 years and living in the proximity of the lagoon
were selected to get information. Questions were asked to get their response on the
following:
1. Identity (name and address) and age,
2. Period of living in association with the lagoon,
3. Occupation,
4. Source of water used for their daily needs (i.e. drinking, bathing and washing),
5. Involvement in lagoon fishery and (if involved) the purpose of fishing,
6. Common species of fish and shellfish caught,
7. Amount of the catch per day,
172 L.P. JAYATISSA ET AL.
8. The total number of fishermen involved in lagoon fishery,
9. Total income from lagoon fishery,
10. Other sources of family income,
11. Other uses of the lagoon,
12. Frequency of opening the lagoon mouth (breaching the sand bar),
13. Species of birds and other animals living in the lagoon ecosystem,
14. Species of mangroves and mangrove associates.
The question nos. 3–14 were asked in order to gather information to assess the situa-
tion before and after the 1965–1970 period and responses were recorded separately.
3. Results
3.1. Remote sensing
Mangrove cover on the seaward shore and silted marshy lands differ floristically.
Seaward fringes of mangroves consisted of three species, Lumnitzera racemosa
Willd., Avicennia marina (Forsk.) Vierh. and Excoecaria agallocha L. (hereafter
referred to as ‘mixed mangrove’) whilst the mangrove cover on marshy area was
dominated by a single species, Sonneratia caseolaris.
In 1956, the area covered by mixed mangroves was 19.5 ha whilst that covered
by S. caseolaris was 4.5 ha (Figure 3, Table II). However, over a 38-year period
(from 1956 to 1994), the mixed mangrove area has decreased down to 17 ha and the
S. caseolaris-cover has increased dramatically resulting in 139 ha, by this species’
invasion of the shallow area of the lagoon. The comparison of two maps prepared
for 1956 and 1994 revealed that the area of mixed mangroves has decreased on the
land between the lagoon and the sea where fishermen’s houses are located, but it has
increased on non-populated areas of the same strip (Figures 3–5). Area statistics of
these chronological changes of the mangrove cover are given in Table II. The total
area of the lagoon (892 ha) has also decreased by 146 ha which is mainly due to the
expansion of agricultural lands.
In aerial photographs, it was difficult to distinguish areas with densely grown
reedmace, Typha latifolia L. from the other areas of the marsh with helophytes
(marsh plants). In contrast, the merged product (PAN + LISS III) of satellite images
of 1997 helped to distinguish areas covered by reedmace T. latifolia as it gives a
different reflection compared to areas covered by S. caseolaris and other marsh
plants. The areas dominated by S. caseolaris were seen in a purple colour on the
merged image with a coarse texture and in irregular shapes which represent joined
or large crowns, whilst areas dominated by reedmace, T. latifolia, were seen in a
dark pink colour with a fine texture (Figure 6). More than 50% of the lagoon area
was covered by dots in a purple colour with coarse texture, representing areas being
invaded by S. caseolaris. Patches of T. latifolia were also observed in these areas.
Only about 20% of the lagoon area remained open water without the two species
in remarkable densities.
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 173
Figure 3. The former distribution of mangrove areas and other landuses at Kalametiya lagoon, based on the
aerial photograph of 1956. Inset: The mouth area of the lagoon.
3.2. Ground surveys
The lowest and highest water levels recorded during the study period at the deepest
point of Kalametiya lagoon were 2.25 and 3.00 m, respectively. Fluctuations of the
174 L.P. JAYATISSA ET AL.
TABLE II. Area statistics, showing the changes of the mangrove cover of Kalametiya
lagoon over a 38-year period from 1956 to 1994 (based on aerial photographs).
Area (ha) Area (ha) in 1994
in 1956 Disappeared Remaining Newly grown
Mixed mangroves 19.5 13.0 6.5 10.5
Mangrove dominated by 4.5 0.0 4.5 134.5
Sonneratia caseolaris
water level within this range resulted from the influx of irrigation water, rainfall
and the tides or their combined effects.
The observed salinity ranges of water near the bottom and surface were 1–5.5‰
and 0–1.0‰, respectively. Monthly variations of the salinity were more appreciable
near the bottom than at the surface. The mean of bottom and surface salinity values
was 2.1‰.
The most common plant species recorded from selected transects are given in
Table III. The profile diagram of newly grown mangroves (along the line demar-
cated as x-x in Figure 4) shows the predominance of tall trees of S. caseolaris with
abundant pneumatophores in between (Figure 7). The elevation range of the land
along the transect is less than 25 cm. The last 25–30 m of the inward end of the
transect shows rather sparsely distributed trees with lower heights compared to the
other areas of the transect.
Ground surveys showed that there is a liana, Cayratia trifolia (L.) Domin. grow-
ing in areas dominated by S. caseolaris in association with Sonneratia trees. In
some areas it has grown extensively, covering crowns of Sonneratia trees, changing
textural and spectral properties of the formation.
3.3. Questionnaire information
Most of the area of the periphery of the lagoon is still not highly populated and
covered by scrub forests. Although 23 houses from the periphery of the lagoon were
visited, only five knowledgeable men older than 60 years who had been involved in
lagoon-associated activities including fishing were found. Interestingly the response
of all five for the questionnaire was very similar. Information revealed by them and
data collected from field studies were used to compare the present and previous
(before 1965–1970 period) situations of various aspects and the comparison is
given in Table IV.
4. Discussion
Nowadays remote sensing and GIS are extensively used in a variety of spatial stud-
ies, including vegetation analysis, and they have proved effective in assessing the
extent of surface features and their variations over time. Spatial studies on man-
grove vegetation rather than many other vegetation types, especially need remote
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 175
Figure 4. The distribution of mangrove areas and other landuses at Kalametiya lagoon, based on the aerial
photograph of 1994. Transect X-X in the newly invaded mangrove area at the eastern bank of Kalametiya
lagoon. Inset: The mouth area of the lagoon.
176 L.P. JAYATISSA ET AL.
Figure 5. The changes of the mangrove cover and other landuses at Kalametiya lagoon from 1956 to 1994
(generated by overlay of maps in Figures 3 and 4).
sensing as they are ‘difficult lands’ for in situ studies due to the boggy nature of
the soil and the extensive system of aerial roots. In this regard, Kalametiya lagoon
is one example, for which even the area had not been exactly established, although
its importance had been identified since 1938 when the lagoon was declared a
sanctuary (Ceylon Government Gazette no. 8370 of 27 May 1938). This declara-
tion was cancelled in 1946 because of the opposition of local residents, but it was
re-declared a sanctuary in 1984 (The Gazette of the Democratic Socialist Republic
of Sri Lanka No. 303/7 of 28 June 1984). At that time the approximate area of
the two interconnected lagoons, Kalametiya and Lunama, was given as 1,760 acres
(∼712 ha) (Scott, 1989). Nevertheless, this study shows that the area of Kalametiya
lagoon within the boundary declared for the sanctuary is 810 ha and Jayatissa (1987)
showed that the Lunama lagoon itself is about 140 ha in extent. Except for the survey
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 177
on phytoplankton and mangrove vegetation of the lagoon carried out by Jayatissa
(1987), no other scientific studies on the flora have been conducted so far.
The aerial photograph of 1956 shows the Kalametiya lagoon as a water body
with a mouth closed by a sand bar. Mangroves or other emergent trees were not
observed in the northern sector, except in a small (4.5 ha) patch of S. caseolaris
close to the eastern bank. Nevertheless, on the seaward bank, where the soil salin-
ity could be expected to be comparatively higher due to seepage of seawater,
a fringe of mangroves comprising three species, A. marina, E. agallocha and
L. racemosa, covering 19.5 ha, was observed. Questionnaire information confirmed
these findings.
Aerial photographs of 1994 showed that the area covered by mixed man-
groves increased remarkably in non-populated areas over the last 3–4 decades and
decreased in populated areas. More interestingly, the area covered by S. caseolaris
alone had increased up to about 17% of the total area of the lagoon over the 38-year
period from 1956 to 1994. This did not include the area with sparsely distributed
young, isolated plants of S. caseolaris, as they were hard to distinguish unequivo-
cally in aerial photographs particularly when located among reedmace, T. latifolia.
However, the satellite images gave a particular reflectance for those areas with reed-
mace and S. caseolaris, and therefore, it was possible to establish that the lagoon
area has been invaded further by sparsely distributed young plants of S. caseolaris
up to about 53% of the total area previously declared a sanctuary. At present, alto-
gether more than 70% of the lagoon has been invaded by a mature forest or sparsely
distributed young plants of S. caseolaris.
According to the questionnaire information, floral change is not the only change
that took place during the last few decades. The lagoon fauna, and hence fishery has
also changed. Prior to the commencement of Udawalawa irrigation scheme in 1967,
the closing of the lagoon mouth by forming a sand bar and opening it again had been
taking place periodically. Moreover, the freshwater inflow was low and seasonal
(Liyanarachchi et al., 1995). Therefore, when the lagoon mouth was open, the flow
could be reverted by tidal action and seawater could enter the lagoon, resulting in
a higher salinity in lagoon water. This was confirmed by the residents’ observation
that crystallization of salt had occurred before the new system of canals made in the
Udawalawa project that grossly increased the inflow of freshwater into Kalametiya
lagoon. With this increased inflow, the flooding of the northern area of the lagoon
was severe and frequent. As a solution, a reinforced artificial outlet, was constructed
to enable a year round outflow of lagoon water (Figure 4). Since then, the lagoon
rarely directly receives seawater, only when the outflow of the lagoon to the sea
gets reversed in a drought. The seepage of seawater through the seaward bank of
the lagoon is not enough to increase the salinity of water in the whole lagoon to a
substantial level. As a result of all these events and processes, the lagoon has been
converted into a running freshwater body.
The foregoing is further supported by the information collected by the ques-
tionnaires. Besides the observed lack of salt crystallization at present, people are
using the waters for bathing and washing etc., which was not done previously.
178 L.P. JAYATISSA ET AL.
The reported change in the types of fish living in the lagoon is also meaningful
and proves that the salinity has changed. Data on bird populations were unreliable
as residents are unable to name most of them correctly. The reported biological
changes appear to be a result of hydrological changes in the lagoon created after
completion of an upstream irrigation scheme, the Udawalawa project where water
was diverted to agricultural lands especially to paddy lands. This is inferred because
it is the chronologically most plausible single causal event.
Figure 6. (a and b) Map of Kalametiya lagoon based on the merged product of IRS-1C, PAN + LISS III
images taken on 10 January 1997, showing the distribution of S. caseolaris and T. latifolia (categorization
was based on image interpretation).
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 179
Figure 6. (Continued)
The irrigational manipulations not only changed the hydrology and salinity
regime of the lagoon but must also have increased the nutrient inflow to the lagoon
and siltation, as the drainage influx comes through paddy fields. Prior to these
changes, S. caseolaris had been restricted to 4.5 ha of the eastern corner of the
lagoon. The new “environment” appears to be more favourable for S. caseolaris
and it has started to spread dramatically throughout the lagoon. Jayatissa (1987)
calculated the area covered by S. caseolaris in Kalametiya lagoon by using the
aerial photograph taken in 1975 as approximately 25 ha. It was shown that it had
been limited to the eastern boundary of the lagoon. According to the present study,
it is more than 139 ha. A large number of incompletely canopied small patches of
180 L.P. JAYATISSA ET AL.
TABLE III. The list of mangroves, common mangrove associates
and most abundant marsh species recorded from the mangroves of
Kalametiya lagoon. (Nomenclature according to Bandaranayake et al.,
1974 and Dassanayake, 1981, 1983, 1991, 1997).
No. Species Family
1. Acanthus ilicifolius L. (Acanthaceae)
2. Acrostichum aureum L. (Polypodiaceae)
3. Avicennia marina (Forsk.) Vierh. (Avicenniaceae)
4. Azima tetracantha Lam. (Salvadoraceae)
5. Cayratia trifolia (L.) Domin. (Vitaceae)
6. Clerodendron inerme (L.) Gaertn. (Verbenaceae)
7. Eichhornia crassipes (Mart) Solms. (Pontederiaceae)
8. Excoecaria agallocha L. (Euphorbiaceae)
9. Hibiscus tiliaceus L. (Malvaceae)
10. Lumnitzera racemosa Willd. (Combretaceae)
11. Salvinia molesta Flyer (Salviniaceae)
12. Salvadora persica L. (Salvadoraceae)
13. Sonneratia caseolaris (L.) Engl. (Sonneratiaceae)
14. Typha latifolia L. (Typhaceae)
Figure 7. Profile diagram of the vegetation along transect X-X representing the newly invaded mangrove
area at the eastern bank of Kalametiya lagoon. Interrupted sections have a continuous S. caseolaris cover.
Lr = Lumnitzera racemosa, Sc = Sonneratia caseolaris, Tl = Typha latifolia.
S. caseolaris can be seen at the periphery of completely canopied area implying
that the spread is continuing further into the lagoon. In Sri Lanka, S. caseolaris is
mainly found in estuaries and low saline lagoons particularly in the wet zone and
some reports also give evidence or consider that S. caseolaris is a low saline man-
grove tree species (Abeywickrama, 1964; Banerjee et al., 1989; Aksornkae et al.,
1992; Duke et al., 1998). This also provides circumstantial evidence for the change
in vegetation cover in line with the salinity changes caused by diverting irrigational
drainage water through this lagoon.
It can be marginally remarked that the fruit of this expanding mangrove,
S. caseolaris, is a source of a popular fruit drink in Sri Lanka. Also, pneumatophores
of S. caseolaris are used as an alternative for cork. However, from a commercial
point of view, no attempt has been taken to use these resources.
This study clearly indicates that a decrease in salinity has occurred in the
Kalametiya lagoon at a decadal time scale and the most probable and identifiable
cause is the Udawalawa irrigation scheme. Very important changes in the vegetation
and also the fauna caused a severe disruption in the livelihood of those people who
depended on lagoon fishery.
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 181
TABLE IV. Comparison of important aspects of Kalametiya lagoon before 1970 and actually, based on
questionnaire information received from Liveris Wijesuriya (age 71), Seedin Andaraweera (age 74), S.K.D.
Piyadasa (age 68), W.G. David Singho (age 72) and W.G. Peter (age 60), and field surveys. (Nomenclature
according to Lovett, 1981 and Munro, 1955.)
Previous situation (before 1970) Present situation
A large portion of the lagoon was with open water. As seen in the aerial photographs, mangroves and
Few small patches of S. caseolaris, and T. latifolia marsh plants cover more than 75% of the lagoon
were found only in the northern end of the lagoon. area. E. crassipes S. caseolaris, and T. latifolia are
E. crassipes was rare. the dominant plants. S. molesta and Azolla sp. were
also rather common.
Lagoon water was not used for drinking, bathing or Lagoon water is used for bathing and washing.
washing due to high salinity. People in the vicinity
travelled a few kilometres for bathing and washing.
In dry seasons, salt crystallized in the southern Salts never crystallize in the lagoon even in severe
(i.e. seaward) part of the lagoon. Naturally crystal- droughts.
lized salt had been collected by the people and used
in cooking and salting of fish.
The catch of shellfish in the season was in thousands The catch of shellfish even during the season is few
of individuals per effort. The measure used in selling individuals per day. Usually used for own
them was not a weight unit but volumes of basketfuls. consumptions. If sold, priced by number.
Shellfish species caught were: Only two species of shellfish are present, but in low
densities.
a. Penaeus indicus H. Milne-Edwards
b. Penaeus monodon Fabricius a. Penaeus indicus H. Milne-Edwards
c. Macrobrachium rosenbergii de Man b. Scylla serrata Forsk˚ l
a
d. Scylla serrata Forsk˚ l
a
Commercially important fish species in the lagoon Fish caught in the lagoon fishery (no commercial
were: significance):
Liza oligolepis Bleeker Cyprinus carpio Linnaeus
Elops machnata Forsk˚ la Oreochromis mossambicus Peters
Chanos chanos Forsk˚ l
a Oreochromis niloticus L.
Macrura kelee Cuvier
Sillago sihama Forsk˚ l
a The first is a native fresh water species and the
Gerreomorpha setifer Hamilton-Buchanan other two are introduced freshwater species but now
Ambassis commersonii Cuvier also found in brackish waters (Pethiyagoda, 1991).
Therapon jarbua Forsk˚ la
Except C. chanos all the other species are
marine/brackish/coastal water species (Munro,
1955).
More than 25 families were involved in lagoon Only 6 families are involved in the lagoon fishery. It
fishery and depended primarily on that. is mainly for their personal consumption. The other
fishermen depend on sea fishery.
The objective was to reconstruct a past ecological setting at a decadal scale
by combining sets of remotely sensed data and circumstantial but independent,
questionnaire-based evidence, thus attempting to fill the gaps in historical data.
Comparison with the recent (1994, 1997) and actual situation offers a probable
causal relationship to changes in vegetation and to socio-economic transitions.
Though a spontaneous background succession leading to the expansion of a
helophytic species, particularly (as here) in a shallow lagoon, must be expected,
182 L.P. JAYATISSA ET AL.
the scale and the rate of this change, seen in the light of independent sources of
information on salinity and faunal shifts, point at the anthropogenic nature of this
change. The changes may also affect the importance of this site as a bird sanctuary,
though data for this aspect could not be obtained. Predicting these changes might
have been impossible, but as this system has exemplified, it should now be possible
to speculate on what changes and losses could occur when diverting fresh water in
significant amounts to a dry zone brackish water system.
Acknowledgements
The authors wish to acknowledge the support of the European Union through the
project ‘Assessment of mangrove degradation and resilience in the Indian subconti-
nent: the cases of Godavari Estuary and South-west Sri Lanka’ contract ERB IC18-
CT98-0295. The help and advice of Dr Danny Loseen and Mr G. Muthu Sanker
(French Institute of Pondicherry) at several stages of the research were highly appre-
ciated. The late Prof P.A. Pemadasa, University of Ruhuna, is acknowledged for
initiating work on Kalametiya lagoon.
References
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CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 183
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TRANSITIONS IN A COASTAL LAGOON (KALAMETIYA,
SRI LANKA), AS OBSERVED BY TELEDETECTION AND
GROUND TRUTHING, CAN BE ATTRIBUTED TO AN
UPSTREAM IRRIGATION SCHEME
L.P. JAYATISSA1 , M.-C. GUERO2 , S. HETTIARACHCHI1 and N. KOEDAM3∗
1
Department of Botany, University of Ruhuna, Matara, Sri Lanka
2
French Institute of Pondicherry. P.O. Box 33, Saint Louis Street 11, 605001 Pondicherry, India
3
Laboratory of General Botany and Nature Management, Mangrove Management Group, Pleinlaan 2,
B-1050, Vrije Universiteit Brussel, Brussel, Belgium
(∗ author for correspondence, e-mail: nikoedam@vub.ac.be; fax and tel.: +32 2 629 3413)
(Received 6 July 2001; accepted 23 April 2002)
Abstract. According to some non-scholarly reports, Kalametiya lagoon (dry zone of southern Sri Lanka,
formerly 8.9 km2 , now 7.5 km2 ) had been a moderately or high salinity water body and a very important
centre of prawn fishery until the late 1960s. Most of the lagoon area had remained open water until then.
An upstream irrigation project, the Udawalawa irrigation scheme, came into operation in 1967, increasing
the freshwater inflow to the lagoon. The flora, fauna and water quality of the lagoon was reported to have
changed since then.
The lagoon now is a shallow coastal water body with low salinity water. More than 75% of the lagoon is
covered by freshwater species or mangrove species characteristic for water with a low salinity: Eichhornia
crassipes, Typha latifolia resp. Sonneratia caseolaris. There is actually no commercially important fishery
in the lagoon.
The present study was carried out to assess scientifically the said changes in the vegetation within a GIS,
using aerial photographs taken in 1956 and 1994 and IRS IC, PAN + LISS III satellite images of 1997 in
combination with ground surveys and information from a questionnaire-based survey.
It appeared from this work that the aerial cover by Sonneratia caseolaris has increased by more than 30 times
over the period from 1956 to the recent dates. Also, the lagoon area with open water has been drastically
reduced during the same period as a result of spreading of freshwater and low salinity plant species. The
results strongly suggest that the locally reported changes (fisheries decline, water salinity decrease) can be
corroborated by the observed profound changes in plant cover and that upstream water works may have had
strong impacts on this ecosystem, thus causing these changes.
This study couples data obtained from retrospective aerial photograph series, from spaceborne imag-
ing, from actual ground surveys and from questionnaires amongst elderly people to reconstruct decadal
environmental change, thus attempting to fill the gap of lacking historical environmental data.
Key words: fisheries, GIS, irrigation, mangrove, Sonneratia caseolaris, remote sensing, Typha latifolia.
1. Introduction
Mangroves are characteristic plant formations dominated by a set of taxonomi-
cally diverse species of trees and shrubs adapted to grow on intertidal areas of
lagoons, estuaries and sheltered bays in tropical and subtropical areas (Ball, 1988;
Environment, Development and Sustainability 4: 167–183, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
168 L.P. JAYATISSA ET AL.
Saenger et al., 1983). These highly productive ecosystems provide a variety of ser-
vices and goods. Nevertheless, these values have been insufficiently recognized and
vast areas of mangroves are being degraded and destroyed, either intentionally or as
a secondary result of other activities, as assessed globally (Saenger et al., 1983) as
well as locally (Abeywickrama, 1960; De Silva and Balasubramaniam, 1984–1985).
In Sri Lanka, the total area of mangrove forests is reported to amount to around
10,000 ha, fringing about 75 riverine estuaries and 45 basin estuaries along the
coastline of 1,740 km. However, a total of 21 species of true mangroves and more
than 20 species of mangrove associates have been reported from these fragmented
mangrove forests (Dahdouh-Guebas, 2001; Jayatissa et al., 2002) implying that the
species richness of mangroves in Sri Lanka is comparatively high.
Kalamatiya lagoon (dry zone of Southern Sri Lanka) is a shallow coastal water
body, which supports a mangrove forest. At present there is no commercial shellfish
or finfish fishery in the Kalametiya lagoon at a significant level. According to some
non-scholarly reports, Kalametiya lagoon had been an important centre for prawn
fishery prior to the late 1960s. It is also reported that the cover by the mangrove
tree Sonneratia caseolaris (L.) Engler and marsh plants in the lagoon was very
small and was confined to the landward edge of the lagoon, leaving most of the
lagoon area with open water. These species now cover more than 75% of the lagoon
area. Intermittent netting takes place only at the southern corner of the lagoon, by
local people for their own consumption. According to the people in the vicinity,
these changes have taken place after the onset of Udawalawa project, a large-scale
upstream irrigation scheme, in 1967 (40 km north of the lagoon).
The major objective of this study is to assess scientifically the said changes
of the vegetation in the lagoon. As there are no scientific data on the vegetation,
water quality and fishery for the period prior to the inauguration of the Udawalawa
irrigation scheme, remote sensing by aerial photographs of the past and information
collected from the local users and residents by way of a questionnaire were used
to study the previous situation. The present situation was studied with recent aerial
photographs, satellite images and ground surveys. The effective cause of the changes
is inferred from these data, because it cannot be addressed directly.
2. Materials and methods
2.1. Study site: Kalametiya lagoon
Kalametiya lagoon is located on the southern coast of Sri Lanka extending from
latitudes 6◦ 04 26 N to 6◦ 07 19 N and from longitudes 80◦ 54 43 E to 80◦ 57 25 E
covering an area of 8.9 km2 formerly (now 7.5 km2 ). More than 75% of the lagoon
is shallow (<0.5 m) and muddy and covered by marsh vegetation, except the
southern corner at the mouth, which has open water of about 1.5 m depth (Figure 1).
Kalametiya lagoon is connected to another small lagoon, viz. Lunama lagoon that
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 169
Figure 1. Kalametiya lagoon showing silted marshy area (depth <0.5 m) and the open lagoon (depth >0.5 m),
with isobaths (m). Inset: Map of Sri Lanka showing the study area. Adapted from Jayatissa (1987) for the
1994 situation.
does not have a direct opening to the sea and is located 2 km east of Kalametiya,
by a shallow canal (Figure 2). The Udawalawa irrigation scheme lies 40 km to the
north of Kalametiya lagoon.
2.2. Remote sensing
Two sets of aerial photographs of the lagoon area; one taken in 1956 with a scale
of 1 : 40,000 and the other taken in 1994 with a scale of 1 : 20,000, were used
for mapping the lagoon and its vegetation. Both sets of aerial photographs were
scanned and digitized using Arc view 3.2 (Esri, USA) to subsequently identify
the lagoon, mangrove area and adjoining land uses. Differences in crown charac-
teristics viz. texture, structure and tonality, are recognizable in aerial photographs
and hence can be used to distinguish different mangrove species as reported by
Dahdouh-Guebas et al. (2000). The characteristics, which were used to identify
five assemblages or stand types of mangroves, are given in Table I. The accuracy of
the identification was checked by field visits. Line coverage of aerial photographs
showing the vegetation and land uses, were geocoded with standard topographical
170 L.P. JAYATISSA ET AL.
Figure 2. Catchment area of Kalametiya lagoon including major canals, streams and manmade reservoirs.
Adapted from Liyanarachchi et al. (1995).
sheets (1 : 50,000 in scale) by using GIS software. The coverage of 1956 and that
of 1994 were superimposed to obtain area statistics of changes in the mangrove
cover.
Moreover, an IRS-1C PAN image of 1997 was merged with the simultaneously
acquired LISS III image. The Kalametiya lagoon area was extracted from the
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 171
TABLE I. The identification key used to distinguish different assemblages/stands of mangrove species.
Tonality Texture Structure Other attributes Species
Light grey Fine grain and Discontinuous canopy None Avicennia spp.
and blurred
aspect
Medium grey Coarse grain Discontinuous canopy None Excoecaria agallocha
(or light grey)
Grey to Coarse grain Discontinuous canopy Often higher than Sonneratia caseolaris
dark grey with irregular shapes surrounding trees
Dark grey Very fine grain Continuous canopy None Lumnitzera racemosa
with crowns hardly
distinguishable
resulting image and georectified to be overlaid with the aerial photographs. The
resolution merge between the images was carried out to bring together the better
spatial resolution of the PAN image (pixel size is 5.8 m) and the multispectral infor-
mation (green, red and near infrared bands) of the LISS III image. The extent of
different types of mangrove cover was obtained by visual interpretation based on
thorough ground knowledge of the study area.
2.3. Vegetation and environmental data
Salinity and the water level of a sampling point at the deepest point of the open
water area were monitored regularly with monthly intervals during the year 1999.
Profile diagrams of the mangrove vegetation were prepared according to Davis
and Richards (1933, 1934) to show the physiognomy of the mangrove forest and
required data were collected along a 1 m wide band transect running from the outer
margin of the mangrove cover to the inside. Five of 4 m wide transects running across
the mangrove areas were also selected from physiognomically different parts of the
mangrove vegetation and all the inhabiting species were recorded.
2.4. Questionnaire-based survey
A questionnaire was used to obtain information on the situation of the lagoon in
the past from the local residents and users around the lagoon. All men and women
whose age was not less than 60 years and living in the proximity of the lagoon
were selected to get information. Questions were asked to get their response on the
following:
1. Identity (name and address) and age,
2. Period of living in association with the lagoon,
3. Occupation,
4. Source of water used for their daily needs (i.e. drinking, bathing and washing),
5. Involvement in lagoon fishery and (if involved) the purpose of fishing,
6. Common species of fish and shellfish caught,
7. Amount of the catch per day,
172 L.P. JAYATISSA ET AL.
8. The total number of fishermen involved in lagoon fishery,
9. Total income from lagoon fishery,
10. Other sources of family income,
11. Other uses of the lagoon,
12. Frequency of opening the lagoon mouth (breaching the sand bar),
13. Species of birds and other animals living in the lagoon ecosystem,
14. Species of mangroves and mangrove associates.
The question nos. 3–14 were asked in order to gather information to assess the situa-
tion before and after the 1965–1970 period and responses were recorded separately.
3. Results
3.1. Remote sensing
Mangrove cover on the seaward shore and silted marshy lands differ floristically.
Seaward fringes of mangroves consisted of three species, Lumnitzera racemosa
Willd., Avicennia marina (Forsk.) Vierh. and Excoecaria agallocha L. (hereafter
referred to as ‘mixed mangrove’) whilst the mangrove cover on marshy area was
dominated by a single species, Sonneratia caseolaris.
In 1956, the area covered by mixed mangroves was 19.5 ha whilst that covered
by S. caseolaris was 4.5 ha (Figure 3, Table II). However, over a 38-year period
(from 1956 to 1994), the mixed mangrove area has decreased down to 17 ha and the
S. caseolaris-cover has increased dramatically resulting in 139 ha, by this species’
invasion of the shallow area of the lagoon. The comparison of two maps prepared
for 1956 and 1994 revealed that the area of mixed mangroves has decreased on the
land between the lagoon and the sea where fishermen’s houses are located, but it has
increased on non-populated areas of the same strip (Figures 3–5). Area statistics of
these chronological changes of the mangrove cover are given in Table II. The total
area of the lagoon (892 ha) has also decreased by 146 ha which is mainly due to the
expansion of agricultural lands.
In aerial photographs, it was difficult to distinguish areas with densely grown
reedmace, Typha latifolia L. from the other areas of the marsh with helophytes
(marsh plants). In contrast, the merged product (PAN + LISS III) of satellite images
of 1997 helped to distinguish areas covered by reedmace T. latifolia as it gives a
different reflection compared to areas covered by S. caseolaris and other marsh
plants. The areas dominated by S. caseolaris were seen in a purple colour on the
merged image with a coarse texture and in irregular shapes which represent joined
or large crowns, whilst areas dominated by reedmace, T. latifolia, were seen in a
dark pink colour with a fine texture (Figure 6). More than 50% of the lagoon area
was covered by dots in a purple colour with coarse texture, representing areas being
invaded by S. caseolaris. Patches of T. latifolia were also observed in these areas.
Only about 20% of the lagoon area remained open water without the two species
in remarkable densities.
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 173
Figure 3. The former distribution of mangrove areas and other landuses at Kalametiya lagoon, based on the
aerial photograph of 1956. Inset: The mouth area of the lagoon.
3.2. Ground surveys
The lowest and highest water levels recorded during the study period at the deepest
point of Kalametiya lagoon were 2.25 and 3.00 m, respectively. Fluctuations of the
174 L.P. JAYATISSA ET AL.
TABLE II. Area statistics, showing the changes of the mangrove cover of Kalametiya
lagoon over a 38-year period from 1956 to 1994 (based on aerial photographs).
Area (ha) Area (ha) in 1994
in 1956 Disappeared Remaining Newly grown
Mixed mangroves 19.5 13.0 6.5 10.5
Mangrove dominated by 4.5 0.0 4.5 134.5
Sonneratia caseolaris
water level within this range resulted from the influx of irrigation water, rainfall
and the tides or their combined effects.
The observed salinity ranges of water near the bottom and surface were 1–5.5‰
and 0–1.0‰, respectively. Monthly variations of the salinity were more appreciable
near the bottom than at the surface. The mean of bottom and surface salinity values
was 2.1‰.
The most common plant species recorded from selected transects are given in
Table III. The profile diagram of newly grown mangroves (along the line demar-
cated as x-x in Figure 4) shows the predominance of tall trees of S. caseolaris with
abundant pneumatophores in between (Figure 7). The elevation range of the land
along the transect is less than 25 cm. The last 25–30 m of the inward end of the
transect shows rather sparsely distributed trees with lower heights compared to the
other areas of the transect.
Ground surveys showed that there is a liana, Cayratia trifolia (L.) Domin. grow-
ing in areas dominated by S. caseolaris in association with Sonneratia trees. In
some areas it has grown extensively, covering crowns of Sonneratia trees, changing
textural and spectral properties of the formation.
3.3. Questionnaire information
Most of the area of the periphery of the lagoon is still not highly populated and
covered by scrub forests. Although 23 houses from the periphery of the lagoon were
visited, only five knowledgeable men older than 60 years who had been involved in
lagoon-associated activities including fishing were found. Interestingly the response
of all five for the questionnaire was very similar. Information revealed by them and
data collected from field studies were used to compare the present and previous
(before 1965–1970 period) situations of various aspects and the comparison is
given in Table IV.
4. Discussion
Nowadays remote sensing and GIS are extensively used in a variety of spatial stud-
ies, including vegetation analysis, and they have proved effective in assessing the
extent of surface features and their variations over time. Spatial studies on man-
grove vegetation rather than many other vegetation types, especially need remote
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 175
Figure 4. The distribution of mangrove areas and other landuses at Kalametiya lagoon, based on the aerial
photograph of 1994. Transect X-X in the newly invaded mangrove area at the eastern bank of Kalametiya
lagoon. Inset: The mouth area of the lagoon.
176 L.P. JAYATISSA ET AL.
Figure 5. The changes of the mangrove cover and other landuses at Kalametiya lagoon from 1956 to 1994
(generated by overlay of maps in Figures 3 and 4).
sensing as they are ‘difficult lands’ for in situ studies due to the boggy nature of
the soil and the extensive system of aerial roots. In this regard, Kalametiya lagoon
is one example, for which even the area had not been exactly established, although
its importance had been identified since 1938 when the lagoon was declared a
sanctuary (Ceylon Government Gazette no. 8370 of 27 May 1938). This declara-
tion was cancelled in 1946 because of the opposition of local residents, but it was
re-declared a sanctuary in 1984 (The Gazette of the Democratic Socialist Republic
of Sri Lanka No. 303/7 of 28 June 1984). At that time the approximate area of
the two interconnected lagoons, Kalametiya and Lunama, was given as 1,760 acres
(∼712 ha) (Scott, 1989). Nevertheless, this study shows that the area of Kalametiya
lagoon within the boundary declared for the sanctuary is 810 ha and Jayatissa (1987)
showed that the Lunama lagoon itself is about 140 ha in extent. Except for the survey
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 177
on phytoplankton and mangrove vegetation of the lagoon carried out by Jayatissa
(1987), no other scientific studies on the flora have been conducted so far.
The aerial photograph of 1956 shows the Kalametiya lagoon as a water body
with a mouth closed by a sand bar. Mangroves or other emergent trees were not
observed in the northern sector, except in a small (4.5 ha) patch of S. caseolaris
close to the eastern bank. Nevertheless, on the seaward bank, where the soil salin-
ity could be expected to be comparatively higher due to seepage of seawater,
a fringe of mangroves comprising three species, A. marina, E. agallocha and
L. racemosa, covering 19.5 ha, was observed. Questionnaire information confirmed
these findings.
Aerial photographs of 1994 showed that the area covered by mixed man-
groves increased remarkably in non-populated areas over the last 3–4 decades and
decreased in populated areas. More interestingly, the area covered by S. caseolaris
alone had increased up to about 17% of the total area of the lagoon over the 38-year
period from 1956 to 1994. This did not include the area with sparsely distributed
young, isolated plants of S. caseolaris, as they were hard to distinguish unequivo-
cally in aerial photographs particularly when located among reedmace, T. latifolia.
However, the satellite images gave a particular reflectance for those areas with reed-
mace and S. caseolaris, and therefore, it was possible to establish that the lagoon
area has been invaded further by sparsely distributed young plants of S. caseolaris
up to about 53% of the total area previously declared a sanctuary. At present, alto-
gether more than 70% of the lagoon has been invaded by a mature forest or sparsely
distributed young plants of S. caseolaris.
According to the questionnaire information, floral change is not the only change
that took place during the last few decades. The lagoon fauna, and hence fishery has
also changed. Prior to the commencement of Udawalawa irrigation scheme in 1967,
the closing of the lagoon mouth by forming a sand bar and opening it again had been
taking place periodically. Moreover, the freshwater inflow was low and seasonal
(Liyanarachchi et al., 1995). Therefore, when the lagoon mouth was open, the flow
could be reverted by tidal action and seawater could enter the lagoon, resulting in
a higher salinity in lagoon water. This was confirmed by the residents’ observation
that crystallization of salt had occurred before the new system of canals made in the
Udawalawa project that grossly increased the inflow of freshwater into Kalametiya
lagoon. With this increased inflow, the flooding of the northern area of the lagoon
was severe and frequent. As a solution, a reinforced artificial outlet, was constructed
to enable a year round outflow of lagoon water (Figure 4). Since then, the lagoon
rarely directly receives seawater, only when the outflow of the lagoon to the sea
gets reversed in a drought. The seepage of seawater through the seaward bank of
the lagoon is not enough to increase the salinity of water in the whole lagoon to a
substantial level. As a result of all these events and processes, the lagoon has been
converted into a running freshwater body.
The foregoing is further supported by the information collected by the ques-
tionnaires. Besides the observed lack of salt crystallization at present, people are
using the waters for bathing and washing etc., which was not done previously.
178 L.P. JAYATISSA ET AL.
The reported change in the types of fish living in the lagoon is also meaningful
and proves that the salinity has changed. Data on bird populations were unreliable
as residents are unable to name most of them correctly. The reported biological
changes appear to be a result of hydrological changes in the lagoon created after
completion of an upstream irrigation scheme, the Udawalawa project where water
was diverted to agricultural lands especially to paddy lands. This is inferred because
it is the chronologically most plausible single causal event.
Figure 6. (a and b) Map of Kalametiya lagoon based on the merged product of IRS-1C, PAN + LISS III
images taken on 10 January 1997, showing the distribution of S. caseolaris and T. latifolia (categorization
was based on image interpretation).
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 179
Figure 6. (Continued)
The irrigational manipulations not only changed the hydrology and salinity
regime of the lagoon but must also have increased the nutrient inflow to the lagoon
and siltation, as the drainage influx comes through paddy fields. Prior to these
changes, S. caseolaris had been restricted to 4.5 ha of the eastern corner of the
lagoon. The new “environment” appears to be more favourable for S. caseolaris
and it has started to spread dramatically throughout the lagoon. Jayatissa (1987)
calculated the area covered by S. caseolaris in Kalametiya lagoon by using the
aerial photograph taken in 1975 as approximately 25 ha. It was shown that it had
been limited to the eastern boundary of the lagoon. According to the present study,
it is more than 139 ha. A large number of incompletely canopied small patches of
180 L.P. JAYATISSA ET AL.
TABLE III. The list of mangroves, common mangrove associates
and most abundant marsh species recorded from the mangroves of
Kalametiya lagoon. (Nomenclature according to Bandaranayake et al.,
1974 and Dassanayake, 1981, 1983, 1991, 1997).
No. Species Family
1. Acanthus ilicifolius L. (Acanthaceae)
2. Acrostichum aureum L. (Polypodiaceae)
3. Avicennia marina (Forsk.) Vierh. (Avicenniaceae)
4. Azima tetracantha Lam. (Salvadoraceae)
5. Cayratia trifolia (L.) Domin. (Vitaceae)
6. Clerodendron inerme (L.) Gaertn. (Verbenaceae)
7. Eichhornia crassipes (Mart) Solms. (Pontederiaceae)
8. Excoecaria agallocha L. (Euphorbiaceae)
9. Hibiscus tiliaceus L. (Malvaceae)
10. Lumnitzera racemosa Willd. (Combretaceae)
11. Salvinia molesta Flyer (Salviniaceae)
12. Salvadora persica L. (Salvadoraceae)
13. Sonneratia caseolaris (L.) Engl. (Sonneratiaceae)
14. Typha latifolia L. (Typhaceae)
Figure 7. Profile diagram of the vegetation along transect X-X representing the newly invaded mangrove
area at the eastern bank of Kalametiya lagoon. Interrupted sections have a continuous S. caseolaris cover.
Lr = Lumnitzera racemosa, Sc = Sonneratia caseolaris, Tl = Typha latifolia.
S. caseolaris can be seen at the periphery of completely canopied area implying
that the spread is continuing further into the lagoon. In Sri Lanka, S. caseolaris is
mainly found in estuaries and low saline lagoons particularly in the wet zone and
some reports also give evidence or consider that S. caseolaris is a low saline man-
grove tree species (Abeywickrama, 1964; Banerjee et al., 1989; Aksornkae et al.,
1992; Duke et al., 1998). This also provides circumstantial evidence for the change
in vegetation cover in line with the salinity changes caused by diverting irrigational
drainage water through this lagoon.
It can be marginally remarked that the fruit of this expanding mangrove,
S. caseolaris, is a source of a popular fruit drink in Sri Lanka. Also, pneumatophores
of S. caseolaris are used as an alternative for cork. However, from a commercial
point of view, no attempt has been taken to use these resources.
This study clearly indicates that a decrease in salinity has occurred in the
Kalametiya lagoon at a decadal time scale and the most probable and identifiable
cause is the Udawalawa irrigation scheme. Very important changes in the vegetation
and also the fauna caused a severe disruption in the livelihood of those people who
depended on lagoon fishery.
CHANGES IN A LAGOON AND UPSTREAM IRRIGATION SCHEME 181
TABLE IV. Comparison of important aspects of Kalametiya lagoon before 1970 and actually, based on
questionnaire information received from Liveris Wijesuriya (age 71), Seedin Andaraweera (age 74), S.K.D.
Piyadasa (age 68), W.G. David Singho (age 72) and W.G. Peter (age 60), and field surveys. (Nomenclature
according to Lovett, 1981 and Munro, 1955.)
Previous situation (before 1970) Present situation
A large portion of the lagoon was with open water. As seen in the aerial photographs, mangroves and
Few small patches of S. caseolaris, and T. latifolia marsh plants cover more than 75% of the lagoon
were found only in the northern end of the lagoon. area. E. crassipes S. caseolaris, and T. latifolia are
E. crassipes was rare. the dominant plants. S. molesta and Azolla sp. were
also rather common.
Lagoon water was not used for drinking, bathing or Lagoon water is used for bathing and washing.
washing due to high salinity. People in the vicinity
travelled a few kilometres for bathing and washing.
In dry seasons, salt crystallized in the southern Salts never crystallize in the lagoon even in severe
(i.e. seaward) part of the lagoon. Naturally crystal- droughts.
lized salt had been collected by the people and used
in cooking and salting of fish.
The catch of shellfish in the season was in thousands The catch of shellfish even during the season is few
of individuals per effort. The measure used in selling individuals per day. Usually used for own
them was not a weight unit but volumes of basketfuls. consumptions. If sold, priced by number.
Shellfish species caught were: Only two species of shellfish are present, but in low
densities.
a. Penaeus indicus H. Milne-Edwards
b. Penaeus monodon Fabricius a. Penaeus indicus H. Milne-Edwards
c. Macrobrachium rosenbergii de Man b. Scylla serrata Forsk˚ l
a
d. Scylla serrata Forsk˚ l
a
Commercially important fish species in the lagoon Fish caught in the lagoon fishery (no commercial
were: significance):
Liza oligolepis Bleeker Cyprinus carpio Linnaeus
Elops machnata Forsk˚ la Oreochromis mossambicus Peters
Chanos chanos Forsk˚ l
a Oreochromis niloticus L.
Macrura kelee Cuvier
Sillago sihama Forsk˚ l
a The first is a native fresh water species and the
Gerreomorpha setifer Hamilton-Buchanan other two are introduced freshwater species but now
Ambassis commersonii Cuvier also found in brackish waters (Pethiyagoda, 1991).
Therapon jarbua Forsk˚ la
Except C. chanos all the other species are
marine/brackish/coastal water species (Munro,
1955).
More than 25 families were involved in lagoon Only 6 families are involved in the lagoon fishery. It
fishery and depended primarily on that. is mainly for their personal consumption. The other
fishermen depend on sea fishery.
The objective was to reconstruct a past ecological setting at a decadal scale
by combining sets of remotely sensed data and circumstantial but independent,
questionnaire-based evidence, thus attempting to fill the gaps in historical data.
Comparison with the recent (1994, 1997) and actual situation offers a probable
causal relationship to changes in vegetation and to socio-economic transitions.
Though a spontaneous background succession leading to the expansion of a
helophytic species, particularly (as here) in a shallow lagoon, must be expected,
182 L.P. JAYATISSA ET AL.
the scale and the rate of this change, seen in the light of independent sources of
information on salinity and faunal shifts, point at the anthropogenic nature of this
change. The changes may also affect the importance of this site as a bird sanctuary,
though data for this aspect could not be obtained. Predicting these changes might
have been impossible, but as this system has exemplified, it should now be possible
to speculate on what changes and losses could occur when diverting fresh water in
significant amounts to a dry zone brackish water system.
Acknowledgements
The authors wish to acknowledge the support of the European Union through the
project ‘Assessment of mangrove degradation and resilience in the Indian subconti-
nent: the cases of Godavari Estuary and South-west Sri Lanka’ contract ERB IC18-
CT98-0295. The help and advice of Dr Danny Loseen and Mr G. Muthu Sanker
(French Institute of Pondicherry) at several stages of the research were highly appre-
ciated. The late Prof P.A. Pemadasa, University of Ruhuna, is acknowledged for
initiating work on Kalametiya lagoon.
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