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Conservation and Management of
Maldivian Mangrove Habitats – Baseline Study

Haa Alifu FILLADHOO

Table of Contents

2 Introduction
3 Overview of the island and social setting
3.1 Population and housing stats
4 Ecological Setting
4.1 Overview
4.2 Flora
4.2.1 Rhizophora apiculata (Thakafathi)
4.2.1 Lumnitzera racemosa (Burevi)
4.2.2 Bruguiera cylindrica (Kan’doo)
4.2.3 Floral abundance and diversity
4.3 Fauna
4.3.1 Overview
5 Threats
5.1 Climate change
5.2 Anthropogenic
5.2.1 Existing
5.2.2 Potential

2 Introduction

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Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. There may be no other group of plants with such highly developed morphological and physiological adaptations to extreme conditions.

Mangrove forests occupy about 15 million hectares of tropical and subtropical coastline worldwide.

Although they amount to only 1 per cent of the total area of tropical forests, mangroves are highly productive ecosystems rich in biodiversity, consisting of a wide variety of plant species that provide important habitats for a wealth of fauna, including mammals, birds, reptiles, fish and molluscs. They contribute to livelihoods locally and globally by providing forest resources such as timber, firewood and thatching materials as well as non-timber products. Mangroves are also recognized as an important greenbelt and carbon sink that protects coastal areas from natural disasters such as tsunamis, cyclones and erosion resulting from sea-level rise, especially in small island countries.

Mangroves create unique ecological environments that host rich assemblages of species. The muddy or sandy sediments of the mangal are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates. Channels within the mangal support communities of phytoplankton, zooplankton and fish. The mangal may play a special role as nursery habitat for juveniles offish whose adults occupy other habitats (e.g. coral reefs and seagrass beds).
As the mangroves are surrounded by loose sediments, the submerged mangroves’ roots, trunks and branches are islands of habitat that may attract rich epifaunal communities including bacteria, fungi, macroalgae and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character.

Living at the interface between land and sea, mangroves are well adapted to deal with natural stressors (e.g. temperature, salinity, anoxia, UV). However, because they live close to their tolerance limits, they may be particularly sensitive to disturbances like those created by human activities. Because of their proximity to population centers, mangals have historically been favored sites for sewage disposal. Industrial effluents have contributed to heavy metal contamination in the sediments. Oil from spills and from petroleum production has flowed into many mangals. These insults have had significant negative effects on the mangroves.

Maldives is known for its coral reefs and beaches; mangrove ecosystems in Maldives are over-shadowed by these environments and oftentimes neglected and under constant anthropogenic threats despite its crucial ecological and geomorphological function to this small island nation.

This study aims at establishing a baseline of environmental conditions of significant mangrove habitats throughout the country. Data and information collected via this study will provide a foundation for conservation efforts to build upon and help future environmental monitoring of said environments.

3 Overview of the island and social setting

3.1 Population and housing stats

HA. Filladhoo is located on the North-eastern rim of HA. Atoll at 6°52’25.37″N and 73°13’49.11″E. With a land area of 264.6 ha, this island is among the largest of HA. atoll. However, according to the latest available census data, the island of Filladhoo only account for 3.98% of the total atoll population. With a recorded population of 584 in 2014, the population density of the island was estimated to be 2.2 people per ha. Based on 2006 and 2014 census data, the annual population growth rate for Filladhoo was determined as 0.15.

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Figure ‎3‑1: Satellite image of HA. Filladhoo (source: GoogleEarth)

The island shown in Figure ‎3‑1 (left) used to be geographically two separate islands; Filladhoo (Right) and Dhapparu; and as such even at present these two are considered two separate islands administratively. Roughly 17Ha of the island is used for residential purposes; starting on the western side; using naturally sheltered lagoon area as the main access to the island.

4 Ecological Setting

4.1 Overview

There are 5 different wetland areas (Figure ‎4 -2) on Filladhoo; however, three of these are not mangrove habitats. Sites Marked A, B and D did not have any flora or fauna associated with mangrove habitats while Sites C and E had a significant amount of Mangrove flora.

The areas of these Sites are as below;

Site Area

(m2)

A 2900
B 640
C 9800
D 3900
E 1400

Site C had a narrow mangrove vegetation belt around the water body while Site E had mangroves in and around the water body. At Site C, the narrowest strip (eastern side) of mangrove fauna belt was found to extend up to 10m while the widest strip (western side) was found to be 20m.

As far as the water quality of the sites are concerned; sites A, and D were found to be saline (over 22ppt) while sites B,C and E found to be brackish (Table ‎4 -1). Decaying plant matter and fine sediments were found suspended (in water column) in great concentrations at all the sites; leading to high TSS values. Typical values from onsite measurements range from 50NTU to 100NTU for turbidity while TSS readings were found to range between 42mg/l to 81mg/l.

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Table 4‑1: Water Quality readings

Sites D and E had an estimated water depth of 0.1 to 0.2m at MSL while Site A is estimated to have a depth of 0.5 to 1m at MSL. Site B is estimated to have a depth of 0.3m while Site C is estimated to have a depth over 2m at the deepest areas. However, it should be noted; due to high amount of lose sediment and organic matter settled at the bottom, it is hard to quantify water depths accurately.

According to locals, Site A is supposedly formed due to tidal waves during Halha, though there were no evidence of such. Apart from a small saline water body disconnected from the lagoon by a narrow stretch of land, there were no features of notable significance at this site.

In contrast to Site A, Site B had brackish water and is used by locals to process coconut husk for coir making. There was a large amount of introduced freshwater fish (Tilapia (Oreochromis sp.) and Bitterlings (Rhodeus spp.)) at this site.

Site C had a deep brackish water body with a fair amount of mangrove flora and fauna. Site D was found to have a thick mud layer as thick as 0.7m around the water body; which is mostly dry apart from the center. These mud layers are expected to contain an enormous amount of bacteria which breakdown organic matter and facilitate nutrient recycling.

Site E had a shallow waterbody with mangrove plants in and around the waterbody. A thin layer of mud was also found at this site.

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Figure ‎4‑2: Wetland Areas and general landuse of HA. Filladhoo

4.2 Flora

Three species of mangrove plants were found in Filladhoo during the field survey; Rhizophora apiculata (Tall stilted mangrove- Thakafathi), Lumnitzera racemosa (Black mangrove- Burevi) and Bruguiera cylindrica (small- leafed orange mangrove- Kandoo).

4.2.1 Rhizophora apiculata (Thakafathi)

Scientific Name: Rhizophora apiculata
Common Name: Tall-stilt Mangrove
Conservation status: Currently categorized as of Least Concern according to the IUCN Red List of Threatened Species.
Habitat: This species is found in the intermediate estuarine zone in the mid-intertidal region. This species tolerates a maximum salinity of 65 ppt and a salinity of optimal growth of 8-15 ppt. Sediment accretion increases the mortality rate of seedlings. This species will not be an efficient colonizer of coastal areas exposed to sudden discharges of sediments such as those of highly eroding watersheds
Key Uses: The timber is used in salt water piling and fencing. It is also used for house posts, boat building and as firewood.
Sites: C
Taxonomy
Class: Eudicots
Order: Malpighiales
Family: Rhizophoraceae
Genus: Rhizophora

Morphology:

Trees are 20-30m tall. Bark dark grey and chequered. Conspicuous arching stilt roots that can extend 5m up the stem. Often also with lots of aerial roots emerging from the branches so that the tree appears to have a skirt of roots under the leaves.

Leaves eye-shaped (8-15cm long), glossy green and stiff, with tiny evenly distributed black spots on the underside. Stipule is usually (but not always) red.

Flowers (1-2cm) in pairs on very short stalks so they appear to be stuck directly onto the branch. Calyx globular hard thick, brown on the outside yellow inside. Petals yellow to white, flat membranous and hairless, falling off soon after blossoming.

The fruit looks like a brown, upside down pear (about 2cm) and is crowned by short persistent sepals. The cylindrical hypocotyl can be up to 38cm long, somewhat smooth, green ripening purple.

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1.1.1 Lumnitzera racemosa (Burevi)

Scientific Name: Lumnitzera racemosa
Common Name: Black Mangrove
Conservation status: Currently categorized as of Least Concern according to the IUCN Red List of Threatened Species.
Habitat: Usually restricted to the landward edge of open mangrove forests, Lumnitzera racemosa is found along coastal shores, lagoons, saltwater and freshwater swamps, swampy meadows and in sandy soils
Key Uses: The timber is used in salt water piling and fencing. It is also used for house posts, boat building and as firewood.
Sites: C
Taxonomy
Class: Eudicots
Order: Myrtales
Family: Combretaceae
Genus: Lumnitzera

Morphology:

Its tree bark is brown colored and ‘knee roots’ are often formed through the extension of the curved aerial roots. The leaves, which are alternately arranged and emerald-colored, grow in clusters at the tip of the branch. The leaf margin is wavy with small serrations. The obovate leaf is either round or caved at the front, similar to a mullet roe. The flower has five white-colored petals and there are two blooming periods each year. The first flowering period occurs between May and July, while the second time occurs between October and November. The fruits of Lumnitzera racemosa are drupes, whose endocarp is solid and expcarp is similar to that of a sponge. This allows the fruit to float on the water surface and be distributed through water dispersal. Lumnitzera racemosa is the most salt-tolerant tree species amongst the mangrove plants. Its roots develop into ‘knee roots’, which provides stability.

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4.2.2 Bruguiera cylindrica (Kan’doo)

Scientific Name: Bruguiera cylindrica
Common Name: Small-leafed Orange Mangrove, Reflexed Orange Mangrove
Conservation status: Currently categorized as of Least Concern according to the IUCN Red List of Threatened Species.
Habitat: This species is found in downstream and intermediate estuarine zones in the mid-intertidal region. This species is also shade tolerant.
Key Uses: Fruit is used for human consumption while timber is sometimes used as firewood.
Sites: E
Taxonomy
Class: Eudicots
Order: Malpighiales
Family: Rhizophoraceae
Genus: Bruguiera

Morphology:

Bruguiera cylindrica is a columnar tree growing to 23 m high. It has finely fissured, greyish bark. There are short buttresses at the base of the trunk and small, knee-like air-breathing roots (pneumatophores).

The leaves are simple, opposite, thin and glossy green, elliptic in shape, 5 – 17 cm long, 2 – 8 cm wide, with a bluntly pointed apex. The leaves occur in clusters at the end of branches. The petiole is often reddish and 1 – 4.5 cm long.

The inflorescence is often three-flowered and axillary. Flowers have a pale-greenish calyx with 8 calyx lobes, 16 stamens and 8 white, bi-lobed petals with 2 – 3 bristles on each apex and 1 conspicuous bristle in the sinus (indentation between the petal lobes) that is longer than the petal lobes. The viviparous propagule grows from within the calyx and is pencil-like and green, with a smooth surface. It is 8 – 15 cm long and 0.4 – 0.8 cm wide.

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4.2.3 Floral abundance and diversity

Site A

No mangrove flora was observed at this site. The flora around water body consist of typical coastal vegetation found in Maldivian islands; mostly iron wood trees on the seaward side while sea hibiscus, sea lettuce and wild screw-pines were dominant on the landward side.

Site B

No mangrove flora was observed at this site. The flora around water body consist of typical coastal vegetation found in Maldivian islands; iron wood trees, sea lettuce and wild screw-pines were dominant.

Site C

Only two species of mangrove plants were found at this site; Rhizophora apiculata and Lumnitzera racemosa. Of these two, R. apiculata was found in abundance around the water body; densest plantation was found on the western side of the waterbody. Only a small amount of L. racemsa was found at this site; on the northwest side of waterbody. Mature trees of R. apiculata reaching 6m was observed on the seaward side.

Site D

No mangrove flora was observed at this site. The flora around water body consist of typical bush vegetation found in Maldivian islands; sea trumpet, sea lettuce and wild screw-pines were dominant.

Site E

Only one species of mangrove flora was found at this site; Bruguiera cylindrica. Mature trees of this species, reaching up to 5m were sighted around the waterbody while smaller trees were found inside.

4.3 Fauna

4.3.1 Overview

A fair amount of mangrove fauna, mostly crustaceans were sighted in Filladhoo wetland areas.

Crustacea

In many mangroves a large proportion of the leaf litter is directly consumed by crabs, particularly those in the family Sesarmidae. This dramatically accelerates the incorporation of mangrove biomass into the food chain. The acceleration happens in three main ways:

1. Shredding – as crabs feed on leaf litter they shred it into fine particles, increasing the surface area for leaching and microbial colonisation. An Australian study found that 20% of the material processed by crabs is dropped without being ingested (Camilleri 1989), but even this is shredded into fine particles.

2. Accelerated leaching – 85% of the leaf litter ingested by crabs ends up in faeces. Processing of this material in the crab gut reduces content of unpalatable tannins in faeces to less than 3%, compared to 13% in freshly fallen leaves (Lee 1998). This process means that decomposing microbes can colonise the processed leaves in hours or days, rather than the weeks required without crabs.

3. Assimilation – around 12% of the leaf litter processed by crabs is assimilated as crab biomass. A range of predators then feed on these crabs, including a number of fish species that are of high importance to fisheries (Sheaves and Molony 2000).

Additionally, crabs will invest a proportion of the energy assimilated in reproduction, producing large numbers of crab larvae which are an important food source to smaller predators.

Estimates for the amount of litter consumed by crabs vary. In many mangroves, crabs play a major role: one Australian study found that 70% of Bruguiera leaves and 88% of Ceriops leaves were taken down crab burrows in an Australian mangrove, and those left on the surface were eaten by crabs where they fell (Robertson and Daniel 1989b). Similar results have been found in Brazil, where the crab Ucides cordatus was found to consume 84.2% of the total daily litterfall (Nordhaus et al. 2006).

Mangrove crabs were found in almost all the wetland areas of Filladhoo.

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5 Threats

5.1 Climate change

Among the predicted outcomes of climate change, the most pronounced effect is sea-level rise due to global warming. Mangrove systems do not keep pace with changing sea-level when the rate of change in elevation of the mangrove sediment surface is exceeded by the rate of change in relative sea-level. The understanding of mangroves as opportunistic colonizers with distribution controlled through ecological responses to environmental factors highlights the importance of the geomorphic setting in determining where mangrove ecosystems establish, their structure and functional processes. An understanding of a mangrove’s geomorphic setting, including sedimentation processes (sediment supply and type), hydrology, and energy regime, is likewise important in understanding resistance and responses to changes in sea-level, as these affect both surface and subsurface controls on elevation of the mangrove sediment surface.

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5.2 Anthropogenic

5.2.1 Existing

Almost all the accessible wetland areas of Filladhoo has been altered by the local community;

Site A had a pontoon constructed from one side
Site B is used by community for coconut husk processing; numerous small cages were constructed in the area, filled with coconut husk and a large amount of invasive species introduced
Site C had what appears to be part of a jetty constructed across the water body
Site D is not easily accessible, hence limited anthropogenic alterations
Site E had no noticeable alterations

Overall, Sites A, B and C is greatly impacted by anthropogenic activities while Sites D and E, relatively inaccessible is not significantly altered by anthropogenic activities.

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5.2.2 Potential

If existing destructive modifications to the wetland areas are not properly managed and regulated, there is potential to cause irreversible significant damage to wetland areas of Filladhoo.

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