Marine Atlas: Maximizing Benefits for Tonga

HABITAT VALUES

HOME, SWEET HOME: COASTAL HABITATS Tonga’s famous hospitality extends to the thousands of species that call its coral reefs, mangroves and seagrasses home. These habitats are home to countless plants and animals that store carbon and help protect Tonga’s coastal inhabitants.

The previous set of maps in the “Supporting values” section of this report took us on a journey from the ocean floor all the way to the surface, demonstrat- ing the colourful biophysical features of Tonga’s waters. While they are fascinating in their own right, the combination of features such as bathymetry, geomorphology, currents, nutrients or plankton are also important factors in the distribution and the health of Tonga’s coastal habitats. These habitats can be highly valuable, as residents of many Tongan islands came to realize in February 2018. Cyclone Gita struck Tonga as a category four, causing devastation to buildings, widespread power outages and flooding. However, without the protec- tion that coral reefs and mangroves provide to most of Tonga’s islands, the outcome could have been a lot worse. Every year, reefs and mangroves mitigate damage to houses and hotels across Tonga’s islands; coastal protection is valued at TOP 20 million every year (Salcone, 2015), demonstrating just how valuable marine and coastal ecosystem services are to Tonga. Coast protection, a key marine and coastal ecosys- tem service, has two components: the prevention of erosion and the mitigation of storm surges. Healthy coastal ecosystems prevent coastal erosion by reducing the effects of waves and currents and they also help regulate the removal and deposition of sediment (erosion and accretion). They also provide increased short-term protection against episodic events, including coastal floods and storm surg- es. The benefits of this protection against extreme weather events include minimizing damage to homes, buildings and other coastal infrastructure and to important resources such as crops. Coastal habitats such as mangrove forests, sea- grass beds and coral reefs play an important role in stabilizing shorelines. As human density increas- es, however, so too does the impact on these important habitats. The role of mangroves in coastal stabilization is well known. They protect coastal areas from erosion, storm surges (especially during cyclones) and tsu- namis. Their massive root systems are efficient at dissipating wave energy and slow down tidal water so that suspended sediment is deposited as the tide comes in, with only the fine particles resus- pended as the tide recedes. In this way, mangroves help build their own environment. Given the unique- ness of mangrove ecosystems and the protection they provide against erosion, they are often the sub- ject of conservation programmes and are common- ly included in national biodiversity action plans. Seagrasses are another important coastal habitat that form extensive meadows in the coastal areas they colonize. Their leaves can also slow currents, and their roots and rhizomes trap the sediments in which they grow, thereby enhancing the stability of the substrate. Seagrasses can also dissipate the energy of waves by up to 40 per cent, which can in turn increase the rate of sedimentation. As such, seagrass beds effectively help protect against waves and limit coastal erosion.

Aerial view of Tongatapu island’s coastline in Tonga.

In addition to protecting the coast, Tonga’s coastal habitats also act as nursery areas for fish and sup- port food security, livelihoods, tourism and other human activities. Tonga is a known whale-watch- ing destination because migration paths take humpback whales through the islands, where they seek shelter in coastal habitats to breed and give birth. Seagrass meadows and mangroves are also recognized as important carbon stores, with the preservation of healthy mangrove systems con- tributing to climate change action. Mangroves are sparsely distributed across Tonga, with the main areas found in the Tongatapu and Vava’u groups of islands. Eight species of mangroves occur in Tonga, the main species being Rhizophora man- gle, Rhizophora stylosa, Bruguiera gymnorrhiza, Excoecaria agallocha and Lumnitzera littorea . While coastal habitats are some of the most pro- ductive and valuable marine habitats, they are, by the same token, some of the most vulnerable to human activities (see also chapters “Reefs at risk” and “Turning sour”). Tonga’s mangrove area has declined significantly in recent decades, from an estimated 1,000 hectares in 1983 to 336 hectares in 2010 (MESCAL, n.d.), under threat chiefly from urban development, waste disposal and agricultur- al expansion.

The map of coastal habitats presents the distribu- tion of coral reefs and mangroves. Shallow coral reefs form some of the most diverse ecosystems on Earth. Despite occupying less than 0.1 per cent of the world’s ocean surface, they provide a home for at least 25 per cent of all marine species, including fish, molluscs, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Coral reefs provide many benefits to people living in coastal areas, including food provision, supporting artisanal and commercial fisheries, tourism opportunities and coastal protection. Tonga is home to a diversity of coral reefs, with most of the islands surrounded by fringing coral reefs. There are also several barrier reefs, with the largest of these on the eastern side of the Ha’apai group of islands. Seagrass beds are highly diverse and productive ecosystems that can harbour hundreds of associ- ated species from all phyla, for example, juvenile and adult fish, epiphytic and free-living macroal- gae and microalgae, molluscs, bristle worms and nematodes. These beds occur in the sheltered waters of many of Tonga’s islands. However, sea- grass maps have not been presented in the map of coastal habitats as there are currently no publicly available data that adequately capture the distribu- tion of seagrass in Tonga.

MAXIMIZING BENEFITS FOR TONGA

HABITAT VALUES

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