As a result of Madagascar's long isolation, since splitting from the Indian land mass about 88 million years ago, approximately 90% of all plant and animal species have evolved in relative isolation. However, the island's diverse ecosystems and unique wildlife are now threatened by the encroachment of the rapidly growing human population and other environmental threats.
Madagascar’s population is 24,894,551 (World Bank 2016), with an average annual growth rate of 2.7% (672,153 people per year). Madagascar is subdivided into 22 administrative regions (faritra). The regions are further subdivided into 112 districts, 1,395 communes, and 17,454 fokontany. Most of population lives on the eastern half of the island; significant clustering is found in the central highlands and eastern coastline. The coral reefs, seagrass meadows, mangroves, beaches and intertidal habitats support a high diversity of marine plants and animals that provide critical resources for coastal communities and also species of conservation concern, including dugongs and marine turtles.
Since the arrival of humans around 2,350 years ago, Madagascar has lost more than 90 percent of its original forest. This forest loss is largely fueled by tavy ("fat"), a traditional slash-and-burn agricultural practice2. As human population density rose on the island, deforestation accelerated beginning around 1400 years ago3. According to a conservative estimate, about 40 percent of the island's original forest cover was lost from the 1950s to 2000, with a thinning of remaining forest areas by 80 percent 4. Madagascar’s Marine Protected Areas cover 926,952 ha, and a pledge was made in 2014 by the government to triple this coverage by 2020. Overall, the country has increased its total protected area coverage from 1.7 million to 6 million hectares over the past decade.
Limited information is available on the seagrass habitats in Madagascar. The lack of knowledge makes it difficult for stakeholders to effectively integrate seagrass protections into local, regional or national initiatives.
The most significant seagrass meadows are thought to exist in the north-west of the Madagascar island. The earliest record of seagrass is 1806; Halodule uninervis (originally recorded as Diplanthera madagascariensis Steud.) and Halophila ovalis (originally recorded as Halophila madagascariensis Steud.)5. Over the following centuries, majority of records have been ad hoc herbaria collections, of specific note is the report of Enhalus acoroides from Nosy Bé, at 5-6m depth, in September 18795. Ruppia maritima L. has also been described as a member of the Madagascar floristic diversity by Humbert and Jumelle6, however little information exists as it is more common in brackish waters of coastal lakes and estuaries7.
A number of nearshore habitat surveys have been conducted in northern and western Madagascar over the last two decades, which have included seagrass assessments of varying detail (from anecdotal to accurate field validation).
In northern Madagascar, eight seagrass species have been reported: Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, Halodule wrightii, Syringodium isoetifolium, Halophila ovalis and Zostera capensis. The most extensive surveys of seagrass habitats in Madagascar have been conducted as part of rapid marine assessments along the northern coast. The earliest survey in 2002 was on the northwest coast and covered approximately 200 km of coastline from Nosy Bé to Nosy Hara8. Reef condition was assessed at 30 sites in the vicinity of Mitsio Island, Cape Saint Sebastian, and Nosy Bé. Seagrass information was restricted to anecdotal reports from the lagoonal area in Befotaka Bay, on the inshore fringing reef at Sakatia (northeast side of channel), and Thalassia on the sand with boulders at Nosy Iranja (south)9.
Seagrasses on the northeast coast were first surveyed in 2009. In June-October 2009, four field validation sites (Ampasindava, Andovokonko, Antaravy, and Ramena) were examined and eight species of seagrass were recorded (Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, Halodule wrightii, Syringodium isoetifolium, Halophila ovalis and Zostera capensis10). The validation sites were used to train remote sensed images to estimate the presence of approximately 1,177 km2 of potentially submerged vegetation in waters shallower than 10m10.
In April-May 2010 an additional survey from the town of Diego Suarez to Vohemar was conducted, covering approximately 130 km of coast, to confirm the presence and condition of seagrass11. Five locations (Ambodivahibe, Loky Bay, Nosy Ankao, Andravina Bay and Vohemar) were assessed, which included a mix of deep and shallow bays, and an island system separated by stretches of exposed linear coastline11. The north-eastern coast of Madagascar presents several diverse assemblages of seagrass habitats, ranging from isolated patches to continuous meadows extending over several kilometres. Although meadow extent was not mapped, ten species of seagrass were reported: Thalassodendron ciliatum, Thalassia hemprichii, Syringodium isoetifolium, Cymodocea rotundata, C. serrulata, Halodule uninervis, H. wrightii, Halophila ovalis, H. stipulacea and Zostera capensis11). Larger species (e.g., T. ciliatum and T. hemprichii) were mainly found on stable substrates, mostly in coastal lagoons or on the shallow, inner edge of coral reef flats. In areas where sediment conditions were particularly dynamic and hydrodynamic forces play a major role, smaller, fast growing species (e.g., H. uninervis) were dominant, with considerable spatial and temporal variation. In deeper water in light limited conditions, only Halophila species were found. In the few urban areas present along the coast, seagrass distribution was limited by pollution and eutrophication resulting from sewage outfalls and farming activities11. The two main environmental drivers were sedimentation - particularly in bays - and high exposure to wind and waves from the southern Indian Ocean.
To capture the diversity and abundance of seagrass meadows in the region, Di Carlo and Tombolahy(2009)11 described four habitat categories:
Riverine, shallow, intertidal areas: in the proximity of large freshwater inputs, where sediments were mostly muddy sand to mud. Seagrass species include Cymodocea rotundata, C. serrulata, Halodule uninervis, H. wrightii, Halophila ovalis, H. stipulacea and Zostera capensis;
Coastal habitats:
were identified as intertidal to subtidal lagoons located between the shoreline and the reef flats, with a mixture of consolidated (sand) and unconsolidated (coral rubble) substrates and with a max depth of 2-3m. Seagrass species include Thalassodendron ciliatum, Thalassia hemprichii, Syringodium isoetifolium, Cymodocea rotundata, C. serrulata, Halophila ovalis and H. stipulacea;
Reef flat habitats:
generally covered the entire extent of the reef, with a mixture of sand and coral rubble with seagrass growing in the proximity of coral. Reef flats are exposed at low tide, hence seagrasses are mostly found in intertidal pools. Seagrass species include Thalassodendron ciliatum, Thalassia hemprichii, Syringodium isoetifolium, Cymodocea rotundata, Halophila ovalis, H. stipulacea and Zostera capensis;
Deep/open water habitats:
occur at the outer edge of the reef deeper than 8 m, with Halophila ovalis and H. stipulacea being the only seagrass species found in these habitats.
Along the north-eastern coast of Madagascar, seagrasses typically form mixed species assemblages, often dominated by one species, controlled principally by depth (subtidal vs intertidal) and sediment conditions11. Z. capensis was most commonly present in exposed areas, while H. uninervis and C. rotundata colonised tidal pools on mudflats. In the subtidal, T. hemprichii and S. isoetifolium formed extensive meadows on sand or muddy sand (e.g. in Vohemar, Ambodivahibe Bay), largely influenced by sediment composition. All species were associated with mud, sand, muddy sand, and coarse sand substrates, while only T. hemprichii, S. isoetifolium and Thalassodendron ciliatum colonised rubble substrate where the sediment layer was minimal (e.g., Ambodivahibe Bay and Vohemar). At mid depths on the edges of reefs (8-12 m), or among coral patches on sand, Halophila ovalis formed extensive patches often mixed with H. stipulacea. No seagrasses were found below 12 m on the deeper reefs. Due to the limited coastal development, boating operations and other anthropogenic stressors, seagrass meadows were mostly in good conditions11.
The other region of Madagascar where seagrass meadows have been surveyed is along parts of the south-west coast. A survey of 11 seagrass sites reported 8 seagrass species (Cymodocea rotundata, C. serrulata, Halodule uninervis, Halophila ovalis, H. stipulacea, Syringodium isoetifolium, Thalassia hemprichii and Thalassodendron ciliatum) in the 1,000 km2 locally managed marine area (LMMA) of Velondriake16. Of the 11 sites (12 quadrats along 3 transects per site), eight had seagrass cover of over 30%, with the lower abundance sites in the south of the LMMA. Further south, near the town of Toliara, a research study in 2008 identified Halodule uninervis as the most abundant species in the area17. The upper littoral zone of the seagrass meadow located 20 km south of Toliara, consisted of monospecific stands of H. uninervis and Halophila ovalis, whereas the mid-intertidal zone was a mix of H. uninervis, Thalassia hemprichii and Cymodocea rotundata. In the deeper waters of the subtidal zone, Syringodium isoetifolium dominated, whereas the deepest zones consisted mainly of Cymodocea serrulata and Thalassodendron ciliatum17.
Seagrass distribution throughout Madagascar is most likely influenced by shelter, sediment characteristics, water clarity and tidal exposure. To date, no comprehensive survey has mapped the entire seagrass resources of Madagascar and although the global seagrass distribution database estimates Madagascar’s seagrass area at 5,796 km214, the exact area/extent is unknown.
Twelve seagrass species, with an additional two species under review for synonymy, have been confirmed from the waters of Madagascar: Cymodocea rotundata Ehrenb. et Hempr. ex Aschers.; Cymodocea serrulata (R. Br.) Aschers. et Magnus; Enhalus acoroides (L.f.) Royle; Halodule uninervis (Forsk.) Aschers. in Bossier; Halodule wrightii Ascherson; Halophila ovalis (R.Br.) Hook. f.; Halophila stipulacea (Forsk.) Aschers.; Ruppia maritima L.; Syringodium isoetifolium (Ascherson) Dandy; Thalassodendron ciliatum (Forsk.) den Hartog; Thalassia hemprichii (Ehrenberg) Asherson; and, Zostera capensis Setchell. Two additional species (Halodule beaudettei and Thalassodendron leptocaule) are reported from Madagascar, but are under review for synonymy.
van Tussenbroek et al. (2010)15 and Hammel et al. (2003)16 suggested that Halodule beaudettei and Halodule wrightii were conspecific, recognising that the plasticity of leaf apex morphology can be attributed to local conditions and that reproductive structures are unknown. Similarly, the taxonomic authenticity of Thalassodendron leptocaule remains under review as it and Thalassodendron ciliatum may be conspecific. The description of T. leptocaule is based predominately on shoot stature and its presence on rocky shores prone to strong swells and winds in southern Mozambique and north-eastern South Africa18. However, T. ciliatum is commonly found on rocky or hard substrates throughout the Indo-West Pacific, in locations of considerable wave action, e.g., reef edge or reef crest19. The current consensus is that there is insufficient information to support the establishment of T. leptocaule as a distinct species, as there is no conclusive genetic information or distinct reproductive structure that be used to assign new species status (FT Short, UNH, Pers. Comm., 18Feb18). For example, seagrasses evaluated for the IUCN Red List require a clear species description and either genetic or reproduction evidence to be included on the Seagrass Species List (www.iucnredlist.org).
With the exception of the species under review for synonymy, the rarest species in Madagascar is Enhalus acoroides. The only published records are from Nosy Bé, at 5-6m depth in September 1879 and Baie du Cratère in July 19625. The other seagrass species reported from Madagascar are not unique from those found along the coast of other countries in the Western Indian Ocean, including: the mainland countries of Somalia, Kenya, Tanzania, Mozambique, and South Africa; and the remaining Island states of Seychelles, Comoros, Reunion, and Mauritius20. The only other additional species in the WIO not reported from Madagascar is Halophila minor (Zoll.) den Hartog. However, Halophila minor is considered synonymous with Halophila ovalis19, as it is difficult to distinguish the species visually in the field and phylogenetic studies indicate either none or some potential divergence21,22,23.
Seagrass meadows are important economic assets in Madagascar on both regional and local scales. Seagrass meadows represent important nursery and feeding grounds for many commercially important fish in Madagascar12. Some species of fish and sea cucumber are export products that bring foreign income fundamental for the economic development of the region24. Seagrasses are valuable at local levels as they contribute to the provision of protein and cash income to the different human populations. For example, in south western Madagascar traditional fishers belonging to the Vezo tribe, who fish from pirogues or by walking on reef flats and seagrass meadows at low tide, target finfish, octopus, squid and lobsters25. In other regions, local communities glean during low tides targeting invertebrates such as cockles, cowries, and other molluscs. Crabs and lobsters are highly demanded as well as sea urchins and sea stars. The most extensive commercial activity in seagrass meadows of south western Madagascar is the collection of sea cucumbers20. Apart from fisheries production, seagrasses provide a range of goods and services from attenuating wave energy and reducing coastal erosion / sedimentation to cultural importance.
The seagrass meadows of Madagascar also provide food and critical habitat for green sea turtle (Chelonia mydas) and dugong (Dugong dugon) which are listed as threatened or vulnerable to extinction in the IUCN Red List (www.iucnredlist.org). Madagascar is the south western region of the dugong range. The dugong population in Madagascar is patchy, with the animals mostly occurring along the western coast, particularly in the north between Mahajanga and Antsiranana where seagrass meadows are thought to be more abundant. Limited information exists on dugong populations in Madagascar and the broader East African region, with most estimates of population size based on limited aerial surveys (e.g., conducted in 2010) and anecdotal evidence. These estimates suggest a population in the hundreds. Anecdotal information on population trends suggest recent declines in Madagascar, coinciding with the introduction of monofilament nylon gill net use.
As a partner in the GEF Dugong and Seagrass Project, a number of country projects focus on the west and north-west of Madagascar in the areas of the Barren Isles, the Sahamalaza Natural Park, and the Anakarea and Ankivonjy Marine Protected Areas, which are dugong and seagrass hotspots. In north-west Madagascar between Mahajanga and Sahamalaza, local community members are trained to undertake monitoring of dugongs, while instruction in participatory mapping of seagrasses is planned with local community members at selected locally-managed marine areas (LMMAs) in the “Mihari” network. In the Nosy Hara Marine Park, training of rangers and local community members in scientific and community-based seagrass surveys is planned, and the appointment of junior “ecoguards” to produce environmental awareness materials and conduct village events to disseminate conservation messages. The majority of the data gathering will contribute to eradicating critical knowledge gaps and provide data for evidence based policy decisions to mitigate threats and improve management practices.
Existing threats to Madagascar’s marine biodiversity include overfishing (increase in seagrass-consuming sea urchins as a consequence of predator removal), destructive fishing techniques (e.g. beach seining or shellfish harvesting, digging up meadows and trampling), turbid flood waters from rivers of poorly managed catchments, coastal development, pollution and localised logging of mangrove forests (resulting in excessive siltation). Research also indicates that climate change will likely affect seagrass ecosystems in the Western Indian Ocean, through increase in sea surface temperature (expected to rise up to 0.6 °C in this area) and sea level rise (predicted up to 50cm by 2100) and changes in storms/cyclone patterns, frequency and intensity. Thus, promoting and enabling an adaptive approach to seagrass management will not only maintain seagrass diversity, ecological functions and ecosystem services, but also enhance the resilience and adaptive capacity of seagrass ecosystems to cope with climate change impacts.
