Distribution and Zonation of Mangroves in the Northern Part of the Negombo Lagoon ( Sri Lanlra )

14 typical nlangrove species were encounterd in this locality. The species dversity in the mangal of Negombo is remarkable. The overall scaled Simpson diversity index was 0.424. Some islets wete completely covered by Acailthus ilicifolim, while the others had a Rhizophova border and a mixed vegetation, dominated either by Avicennia marina or Brlrgllicra gymnorhiza. The vegetation pattern and the elevation suggest that mud Aat is the precursor of the mangal. A. ilicifolius maybe the early colonizer in boggy soils. A. marina occurrec! on firm soils. The presence of a Rlzizophora border on the shore may be due to its morphological adaptations in resisting water currents with the help of prop roots. Daily variation in the tidal level was too small (neap tides 2 cm and spring tides 18.4 cm) to show a spectacular pattern in zonation. Soils from the mud flat, Rlzizoplzora border and the interior of the islets showed the characteristic features of each region with respect to pH, organic matter, particle size, Na+ and Ca++ concentrations. Ca++ content was high in the mud flat but Na+ was the predominant exchangeble cation in the mangrove soil. Measurements of dnrneters and helghis of plants showed that 3 gymnorlr~za and C, taga! are in thew early stages of growth while L. racemosr! has been overexploited


Introduction
Mangroves appear to be an evil smelling, treacheious bog, infested wit11 mosquitoes.Recently the value of mangroves has been realized by many scientists.They are the breeding grounds of crustacean and fish larvae, and they constantIy supply the coastal waters with nutrients.Mangroves in Sri Lanka in general, and those of Negombo in particular, are fast disappearing due to popuIation expansion and industrial pressure.Aruchelvam2 estimated tlte extent of rnp-ngroves in Sri Lanka to be 8,000 -10,000 acres.In the eighties, the mangrove area may be less than 8,000 acres.Other than taxonomic studies and general descriptions, very little work has been done on the mangrove communities of Sri Lanka.This paper is an attempt to study the distribution pattern and zonation of mangroves in selected islets in the northern part of the Negombo lagoon in relation to some environmental factors.

M. L. Pinto
This study was carried out in selected mangrove islets (Figure 1)in the Negombo lagoon latitude 7" 11' N, and longitude 79" 50' E, in the west coast of Sri Lanka.The lagoon opens to the sea from the northern end while the southern end, receives fresh water from Ja-Ela, Dandugam Oya and a Dutch canal.The three islets, Islet I (Kakaduwa) Islet 2 (Mandagas alamba) and Islet 3 (Kadolnallala) had areas of 2.24 ha, 5.88 ha and 2.92 ha, respectively.Between Islet 1 and Islet 3 was the mudflat which was submerged only during the high tides.

Materids And Methods
After making superficial observations on the vegetation around the lagoon, it was thought best to select the uninhabited islands for this study.The vegetation pattern was constructed by chain surveying.In order to study the zonation in islet 1, a transect was chosen a-ross the middle of the islet and different plant species occurring in 2 x 2 m2 quadrats along the transect was noted as well as their heights and diameters at ground level.
Soil samples were collected from the mudflat, Rhizophora border and the interior of islet 1 and the salinity was determined by titration, organic matter by Walkly and Black method4 Ca and Mg by titration with 0.005M EDTA and Na and K .' , using a calibrated flame Photometer.Tidal levels were measured daily at low tide and high tide for one year and the cumulative percentages in the water level for each month was calculated.From these results, the 50% submergence for each month was constructed as shown by the broken line in Figure 2. I'h.is study was mad.e from December 1976 to December 1977.

Results
Figure 1 gives the general distribution pattern of the vegetation in the northern part of the lagoon.Figure 3 gives the distribution of mangroves in detail in 3 selected islets.Table 1 shows the approximate areas of plant cover due to predominant vegetation type.Zonation of different plant species along a transect in islet 1 is shown in Figure 4 A and their heigl-ts and diameters given in Table 2. Some edaphic factors were also measured from the different zones in the environment with the view to correlate with zonation and the results are given in Table 3.

Discussion
Out of the 28 mangrove species listed for Sri Lanka1 14 species were fou~ld in these islets.As these islets were not connected with the mainland, the entire comm~ti~ity formed the mangal, with no hinterland vegetation.Compared to the other mangrove patches along the west coast of Sri Lanlka, the mangroves of Negornbo show high diversity in species composition.In Waikkal north of Negombo Nypafrutica~~s is abundant.Xylocarpus granatz~~n and Heritiera littoralis occur in the mangroves of Chilaw.Mundel-Puttalam area is dominated by Tumarix gallica, Acrostic/lunz aureum, Excoecaria agalocha, A~iceiznia marina together with the salt marsh vegetation.Aruwakkalu mangal has tall Rhizophoras and A. marina dominates the d1y zone mangroves.In the south, NJPU fruticans though present is fast disappearing due to cocount fiber industry.Compared to the sparse vegetation elsewhere, the rich vegetation in the mangrove islets is clearly shown in Figure 3 and Table 1.
Scaled Sinlpson index values showed a high degree of diversity along the shores of islet I. (Figure 4B) Zonation within the mangrove community has been fully established by early workers.Watsonlg recognized 5 zones in the Malayan mangroves on the basis of tidal inundation.De Haan5 postulated 6 zones and suggested the soil salinity to be a major factor and tide to be a subsidiaryfactor.Walter and SteinerlS Macnae and Kalk13 and Macnae12 worlcing on the African mangroves realized 6 zones not on the basis of physical factors but on the basis of dom~nating species.Abeywickrama's generalization of 3 mangrove zones, namely Rhizophora or Avicel~nia zone, mixed vegetation zone and the littoral woodland vegetation, holds good for these islets except for the woodland vegetation.Presence of a Rhizophora border instead of an Avicennia border in islet I and islet 2 (Figure 3) may be due to the depth and slope as suggestedl,ll as well as due to the lack of sandy soil and poor aeration.18 Figure 4 shows clearly the high population density of R. mucronata along the border and the high density of B. gymnorhiza towards the interior.Lumnitzera racemosa prefers elevated areas as indicated in the same figure.From table 2 it is clear that most of the B. gyrnnorhiza and C, tagal plants have small diameters and are short.The fact that L. racemosa are short and bushy with large diameters indicate that they have been overexploited.The heights of mangroves observed in these islets are far short compared to the mangroves of Puerto Rico8, Malaysiahnd India.l0It should be dbtedt:hat variation in vegetation pattern occurs within the islet and a single transect is insufficient for any generalization.But the occurrence of a Rhizophora border is true for any transect taken from islet 1 or isIet 2.
On evaluating the reasons for zonation, the approacla has to be one of reductio ad absurdurn.Since zonation 1s not so spectacalar the necessary environmental factors could be accounted for, by their absence.In the lagoon the mean spring tide fluctuation was 0.184 m and the mean neap tide fluctuation was O.02Om.From Figure 2 it is clear that seasonal variation in water level due to rains become an important factor than the daily tidal variation.Tide appears to be an important factor for zonation, since zonation is clear in areas where tldal fluctuation is high.For instance 8m tidal variation was recorded for well zoned mangrove areas of Australia, 5 rn for Malaysia and 3.5 for Mozambique."The soil salinity did not show much v~.riation at different stations in the same islet as ~t showed with different seasons.The presence of Rhizophoras along the shore may be due to their morphological adaptation to withstand water currents by their proproots.However the Bruguiera nixed vegetation in the interior of the islets showed high precentage of organicmatter, high Na content and low pH compared to the Rlzizophora border (Table 3).
The vegetation pattern m the islets and the mud flat shows some trends in the succession of the mangrove communities.The niud flat as a precursor of saltn~arsh communities in temperate countries has been fully established.7>9, 15 From the low elevation of the mudflat (Figure 4) compared to the elevation of islet I, it is possible to conclude that mud flat is the precursor of mangrove comm~~nities in this locality.Islet 3 is often submerged with poorly consolidated soil and dominated by A. ilicifolius.This vegetation type disappears gradually with the elevation as seen from Table

Figure 2 .
Figure 2. The range of tidal fluctuations for each month and the mean water level of the Negombo lagoon.Percentage cumulative frequencies were obtained from the daily measurements of high tide and low tide levels.(1977).

FigureTABLE 3 .
Figure 4A The elevation and plant zonatlon along trdnsecr 1 5 lil ~s l e t I He~ghrs o f the plants do not correspond to the scale In the ord~nare BG B g~ rnnorhrza CT C tagal

1 .+-ISLET 2 A
Islet 2 as well as islet 1 are completely submerged during the rainy season in May and October.Islet 2 has a Rlzizophora border of tall trees (Height 7 +_ 2 n~ and diameter 12 + 5 cm) compared to the RlzizopIzora border of islet 1 (Height 5 + 2m and diameter 5 + 3 cm).A. marina formed the predominant vegetation in islet 2 wit11 different heights as trees (4-7m), shrubs (2-4m) and saplings (less than 2 m) similar to the observations of Bruce et a1 (1975) on A. marina in Australia.From the vegetation type, elevation and the size of the plants the chronology of colonization may have been in the following series.MUD FLAT --+ ISLET 3 ---+ ISLET 1 -community established itself on the mud flat, the prop roots, knee roots and pneumatophores may have accelerated s e d i m e n t a t i ~n .~J ~> ~~ In the same locality were seen pure strands of Rhizoplzora seedlings emerging from mud flats.These were not a result of natural succession but planted by the fishermen with the view of reclaiming land and harves.tingtimber.With the changes in the soil structure and elevation of the mud flat and the msngal proper, not only the vegetation but faunation too has shown marked differences.While Macrophtalzlms ~Ze~pressus and a few other Ocypodid crabs occurred on the mud flat, the mangal was dominated by the Sesarmid crabs as exemplified by Chiromarztes clarwinensis, C.indiart~m, C. bidens, Neoepisesarma versicolor, Neoserrnatizrrn srnttlzi and Neosermatiurn m a l b a r i ~u m ~~.Due to the absence of a vegetation on the mud flat the fauna generally associated wit11 the mangrove leaves, stems and roots were not found in the mud flat.