Mangroves play an Important role in coral

reef ecosystems. The fringe and basin type mangrove

forests act as depositional areas and are

effective binders of sediments. This reduces the

sediment loads in coastal waters. Trapping and

stabilizing sediments is important for adjacent

coral reefs because it prevents abrasion and burial

of reefs during stormy conditions

Aboobaker, P M

Kunhikoya, V A

Nasser, A K V

CMFRI Digital Repository

ISSN 02'i4-3aCl X ICAR MARINE FISHERIES INFORMATION SERVICE No. 159 JANUARY, FEBRUARY, MARCH 1999 8 9 4 MANGROVE ECOSYSTEMS OF MINICOY ISLAND, LAKSHADWEEP A.K.V. Nasser, V.A. Kunhikoya and P.M. Aboobaker Minicoy RC ofCMFR Institute, Minicoy - 682 559, India Introduction Mangroves play an Important role in coral reef ecosystems. The fringe and basin type man-grove forests act as depositional areas and are effective binders of sediments. This reduces the sediment loads in coastal waters. Trapping and stabilizing sediments is important for adjacent coral reefs because it prevents abrasion and burial of reefs during stormy conditions. Mangroves also regulate freshwater flows into coastal areas and buffer salinity changes. Mangrove vegetation is positively related to high nutrient inputs. They export excess nutrients to nearby waters as dis-solved and particulate organic matter, some of which nourishing the reef organisms. Most of the commercially important prawns and fishes of coral reefs utilise mangrove areas as nurseries due to the presence of shelter and food. Mangroves of coral reefs, like their counter-parts in coastal areas are exploited for wood and stripping of bark for tanning. They are also sub-jected to oil, thermal and chemical pollution. Destruction of mangroves will allow terrigenous sediments to flow into coral reefs resulting in per-manent damge. There can also be excessive out-flow of organic matter which may cause shading and eutrophication. Refuges and nursery grounds for fishes and invertebrates will be destroyed and secondary production of reefs will be reduced. Mangroves of Minicoy Realising the importance of mangroves in coral reefs, a study to monitor these ecosystems was undertaken seasonally, at Minicoy from 1994 to 1998. Mangroves are noticed only at Minicoy Island in Lakshadweep and occur in two patches each of about I hectare in area. Unlike the man-groves of the mainland, the mangroves of Minicoy are in the formative stage and free from serious human pressure. The mangroves situated on the south-eastern side of the island near Well number 3 is constituted by a single species, Bruguiera qjlindrica . This mangrove has developed into a fully developed community in the past three years. The second group fringes the cove on the southwestern side of the island near helipad (Fig. 1). The predominant species is Ceriops tagal with a solitary tree of Avicennia marina. Preliminary studies were conducted at these sites to under-stand the hydrography and phenology of man groves. Fig. 2. Seedlings of Bmguiera cylindrica. Fig. 1. Ceriops tagal site in the background and its connection with the Minicoy Lagoon. The Brufifuiera site is landlocked and ground water is found to be saline or fresh depending on the amount of rainfall received (Table 1). The Ceriops site is, however, inundated by normal high tides which influence the zonation, distri-bution and species composition. Majority of mangrove species grow best in low to moderate salinities (25 ppt) which may explain the phe-nomenal growth at the Bmguiera site. Nutrient values are normal for coral reef areas except for high values of silicate. The depth of water at the first site varies from a few centimetres in dry sea-son to a mziximum of 0.65 m during monsoon, while the depth at Ceriops site depends on the tide and varies from 0.25 to 1.75 m. Total rain-fall received ranged from 47.2 to 380.4 mm in a month. Phenology Permanent plots were established at the Bruguiera site to study the phenology df this spe-cies. Seedling of height less than 0.5 m (Fig. 2) is the prominent group forming 40 % of all plants (Table 2). The number of propagules in second-ary branches of trees varied widely (Table 3). Flowering of Bruguiera trees begin by August and fruiting is noticed in January with mature seeds in April. C. tagal starts to flower in September and ripe seeds are observed in Februa ry Fi'e. 3.Ripe propagules of C. tagal Fig. 4. Flowers of Avicennia marina. (Fig. 3). Flowers of Avicennia are first seen in October and continues upto February and fruit-ing is not observed in this species (Fig. 4). Litterfall estimated from quadrats indicate that Bruguiera produce litter of 7.24 g/m^/day while Ceriops and Avicennia contribute respectively 1.18 and 1.59 g/mVday. The low values of Ceriops site is due to the transport of litter to the lagoon at low tides. Twigs formed the main component of litter at Bruguiera site while at Ceriops site both twigs and fruits were impor-tant (Fig. 5). Leaves were the major constituent Fig. 5. Seedlings of C. tagal near the Avicennia tree forming 47 % of total litterfall. Ceriops seeds were planted at the Bruguiera site to understand their survival in low saline conditions. Survival rate was high (70 to 92 %) for the first six months but seedlings did not survive after the 8 leaved stage. The propagules of Ceriops were longer (maximum length 28.1 cm) than those of Bruguiera (maxi-mum length 7.0 cm). Conservation and management needs A national committee on wetlands, man-groves and coral reefs constituted in 1993 by the Ministry of Environment and Forests, recom-mended certain guidelines for management of mangroves. The recommendations Include : na-tionwide mapping of mangrove areas, to study the grovi^h rate and productivity of mangrove trees, seasonal variation in climate and environ-ment, afforestation, identification of mangrove areas as "National Park" and to create aware-ness among the public on Importance of man-groves. Some of these recommendations are ap-plicable to the mangroves of Minicoy and if im-plemented would help in preserving this unique ecosystem. Introduction of the species available at Minicoy to suitable areas in other islands such as Kalpeni, Kavaratti, Bangaram and Suheli should be attempted. Mangrove areas of Minicoy which are presently under other departments or private ownership has to be transferred to con-cerned agencies for protection and preservation. Virgin mangroves of Minicoy may be declared as protected areas to prevent any modification or alteration. The local people and students have to be educated about the importance of man-groves through the media, lectures, exhibition, posters etc. TABLE 1. Hydrography of mangrove sites at Minicoy (Seasonal average) Parameters Bruguiera Ceriops site site Atmospheric temperature (°C) 31.80 Water temperature (°C) Salinity (ppt) Dissolved oxygen (ml/1) Phosphate (jUg.at/l) Nitrate (/ig.at/1) Silicate (|Ig.at/l) .  31.60 18.80 4.42 1.21 0.36 16.81 30.30 30.40 34.46 4.45 1.20 0.65 13.97 TABLE 2. Seasonalfrequency of plants inB. cylindrica plot Plant Tree Sapling Seedling In % Height . plot (cm) 33 183 27 71 40 29 TABLE 3. Phenology q/"Bruguiera trees at Minicoy Parameters Number of primary branches Number of secondary branches Number of tertiary branches Number of propagules Length of propagules (cm) Plot 1 40 160 Nil 544 4.0 Plot 2 Plot 3 32 16 192 176 . Nil 14 346 211 4.9 3.7 10 

Mangrove ecosystems of Minicoy Island, Lakshadweep

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Mangrove ecosystems of Minicoy Island, Lakshadweep

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