Vulnerabililty of mangroves, seagrasses and intertidal flats in the tropical Pacific to climate change (Chapter 6)

In addition to the extensive coral reef habitats described in Chapter 5, the shallow subtidal and intertidal zones around the coasts of Pacific Island countries and territories (PICTs) often support large areas of mangroves and seagrasses. Intertidal sand and mud flatsii with their associated microalgae and infauna are also common features of most PICTs. Due to their varying responses to light, temperature and hydrology, and the capacity of mangroves and seagrasses for dispersal1 -4, these three ecosystems usually form a mosaic. Typically, mangroves are located along the shore, whereas seagrasses and intertidal flats can extend long distances away from the shoreline in lagoons and sheltered bays, and often adjoin coral reefs. In many places, coral reefs buffer waves coming ashore to create suitably sheltered environments for the establishment of mangroves and seagrasses5. Mangroves and seagrasses are of special interest to coastal fisheries worldwide because of the role they play in providing nursery areas for commonly harvested fish and invertebrates6 -9. Although the ecology of fish and invertebrates associated with mangroves and seagrasses in the tropical Pacific is not well understood compared with other parts of the world, the connectivity among mangroves, seagrasses, intertidal flats and coral reefs indicates that mangroves and seagrasses throughout the region provide a similar function to such habitats elsewhere. In addition to their roles as nursery areas, mangroves, seagrasses and intertidal flats provide feeding habitats for many species of adult demersal fish, some of which reside on reefs during the day and forage over seagrasses and intertidal flats at night10. Seagrasses and intertidal flats are also permanent habitats for several species of sea cucumbers, the main group of invertebrates targeted as an export commodity in the region11, and for a wide range of molluscs gleaned for subsistence. Overall, the range of coastal fisheries resources that depend on mangroves, seagrasses and intertidal flats is extensive, with many of these species important to the food security and livelihoods of coastal communities in PICTs (Chapters 1 and 9). The separate values of each ecosystem are surpassed by the productivity that results when they are inter-connected7,8,12. In particular, movement of nutrients, detritus, prey and consumers between habitats can have major effects on the structure and productivity of food webs, with nutrient and detrital subsidies increasing primary and secondary productivity both directly and indirectly13. In addition to supporting fisheries, mangroves provide protection against wind and wave energy, and stabilise shorelines; and both mangroves and seagrasses improve water quality by trapping sediments, nutrients and other pollutants. Ecosystems dominated by mangroves and seagrasses are being eroded in some PICTs due to their proximity to developments in the coastal zone19,20. For example, increases in the turbidity of coastal waters and higher rates of sedimentation, resulting from poor land management in the catchments of high islands, are reducing the area and health of seagrass habitats21,22. The problem is not unique to the region - the range and intensity of anthropogenic effects on coastal habitats have been increasing worldwide, reducing the extent and quality of mangroves23,24 and seagrasses20,25. Climate change is expected to exacerbate anthropogenic impacts on mangroves, seagrasses and intertidal flats3,26,27. Further losses are expected to occur as a result of greater heat stress, increased sedimentation and turbidity due to higher rates of runoff, changes in suitable sites for growth of mangroves and seagrasses due to rising sea levels, and possibly more physical damage from the combination of sea-level rise and more severe cyclones and storms. In this chapter, we assess the vulnerability of the mangrove, seagrass and intertidal flat habitats in the tropical Pacific that support coastal fisheries. We do this by examining the effects that changes to surface climate and the tropical Pacific Ocean (Chapters 2 and 3) are expected to have on the plants that define these habitats. This exposure to change is used in the framework described in Chapter 1 to assess the vulnerability of the habitats under representative low (B1) and high (A2) emissions scenarios from the Intergovernmental Panel on Climate Change (IPCC) for 2035 and 210028. We commence by describing the diversity and distribution of mangrove, seagrass and intertidal flat habitats in the tropical Pacific (25°N -25°S and 130°E -130°W), outlining the role they play in supporting coastal fisheries in the region, and summarising the critical requirements for establishing and maintaining these habitats. Next, we summarise the limited information on the observed effects of climate change on mangroves, seagrasses and intertidal flats, and assess the expected vulnerability of these habitats to the projected changes in solar radiation, air and sea temperatures, rainfall, nutrients, cyclones and storms, ocean acidity and sea-level rise. For mangroves and seagrasses, we integrate these assessments to estimate changes in area under the various scenarios. We conclude by identifying the uncertainty associated with these assessments, the important gaps in knowledge, the research required to fill these gaps, and the key management measures needed to maintain the important roles that the mangroves, seagrasses and intertidal flats of the region play in supporting coastal fisheries.