most abundant because of high river discharge and precipitation. The global distribution of sandy shorelines agrees with the earlier determination of latitudinal variation of sediments on the inner continental shelf (Figure 4.1).
Tidal flats have a range of complex sedimentary structures, such as cross bedding, lenticular bedding, and mud/silt couplets that reflect depositional history. Mud flats can be sheltered or moderately exposed and are commonly found in tropical estuaries, tidal inlets, and river deltas. Tidal flats occur in macro‐tidal settings where local areas of deposition occur where sedimentologic processes are active and stratigraphic sequences are developing. There are several types of tidal flats in macro‐tidal regions: low tidal sand flats, mud/sand slopes encompassing the low to mid intertidal zones, mangrove‐fringed mud flats, and high intertidal and supratidal salt flats. Hypersaline tidal flats have recently been found to be important storage sites for salt, sediments, carbon, and nutrient elements (Brown et al. 2021). Unlike tidal flats in micro‐ and meso‐tidal settings, physical processes dominate biological processes such as bioturbation in salt flats.
While tidal flats occur throughout the marine geosphere, roughly 60% (about 75 000 km2) of sand, rock, and mud flats occur within the low latitudes. Of a global total area of 127 921 km2 (Murray et al. 2019), 49.2% of the world’s tidal flats are in Indonesia, China, Australia, the United States, Canada, India, Brazil, and Myanmar; about 70% are found on three continents: Asia (44% of total), North America (15.5%), and South America (11%). Tidal flats are declining in global area; 16% of tidal flats were lost between 1984 and 2016 (Murray et al. 2019). In addition to direct losses from coastal development, increased subsidence and compaction of intertidal sediments, reductions in sediment supply, altered sediment deposition and erosion rates, vegetation loss, coastal eutrophication, and sea‐level rise are also likely drivers of intertidal flat loss. Tidal flats have responded by local migration, but not quickly enough to offset ongoing losses. For example, the highly dynamic tidal flats of the Meghna River estuary in Bangladesh have migrated extensively since 1984, but now occur within only 17% of their initial extent despite expanding in area by 21% due to the rate of sediment delivery exceeding the rates of subsidence and sea‐level rise; seaward migration of tidal flats has been slow, influenced by altered sediment deposition patterns due to coastal development and local expansion of mangroves (Murray et al. 2019).
FIGURE 4.3 Global distribution of sandy shorelines. The coloured dots along the world’s shores represent the local percentage of sandy shorelines (light brown is sand, dark brown is non‐sand). The subplot to the right presents the relative occurrence of sandy shores per degree latitude where the dashed line shows the latitudinal distribution. The lower subplot presents the occurrence of sandy shores per degree longitude. The curved grey lines in the main plot represent the boundaries of the ice‐free shorelines. The underlined percentages indicate the percentages of sandy shores averaged per continent.
Source: Luijendijk et al. (2018), figure 1, p. 4. Licensed under CC BY 4.0. © Springer Nature Switzerland AG.
The global distribution of mangroves (Figure 4.4) indicates a tropical dominance with major latitudinal limits relating best to major ocean currents and the 20 °C seawater isotherm in winter (Bunting et al. 2018) with most mangroves occurring in Southeast Asia and the Americas, including the Caribbean. Both mangroves and seagrasses grow best in quiescent environments where hydrology is favourable for their development (Chapter 2). Estimates of global mangrove area range from 83 495 km2 (Hamilton and Casey 2016) to 135 870 km2 (Worthington et al. 2020). A global typology of mangroves has found that as of 2016, 40.5% of mangrove ecosystems were deltaic, 27.5% were estuarine, and 21.0% were located on open coasts, with lagoonal mangroves occupying only 11% of global mangrove area (Worthington et al. 2020); mangroves in carbonate settings represent just 9.6% of global coverage.
In contrast, the known area of tropical seagrass meadows is poorly constrained by large areas remaining unmapped and inconsistent methodology being used (McKenzie et al. 2020). The global area of seagrasses most likely is 160 387 km2 with a moderate to high confidence (Figure 4.5), but possibly 266 562 km2 with lower confidence (McKenzie et al. 2020). Seagrass meadows in the tropical Atlantic (44 222 km2 with moderate–high confidence) and in the tropical Indo‐Pacific (87 791 km2 with moderate–high confidence) make up 82% of the global total and tropical seagrass meadows make up 85% of the global total if the low confidence estimates are included (McKenzie et al. 2020).
Like seagrasses and mangroves, the global distribution of coral reefs (Figure 4.6) reflects the influence of long‐term environmental conditions that are most suitable for establishment and growth. Species richness of corals is greatest in the Coral Triangle in Southeast Asia (Figure 4.6, dark red area). Corals are found in three principal areas: the Caribbean, the Red Sea, including the Indian Ocean islands such as the Seychelles, and in the Indo‐West Pacific. Corals are found in a broad band throughout the tropics, although there are areas out of this band where warm currents permit the existence of corals, such as on the west and east coasts of Australia and as far north as the southernmost islands of Japan. Coral reefs cover about 600 000 km2 of the global ocean, comprising only about 0.17% of total ocean area and roughly 15% of the world’s sea floor <30 m (Spalding et al. 2001). This small area belies their importance to the structure and function of the tropical coastal ocean as they are important areas of fisheries, marine biodiversity, and carbonate production.
FIGURE 4.4 Global distribution of mangrove forests. The green bars outside the box indicate relative distribution of mangrove with latitude (right) and longitude (bottom).
Source: Bunting et al. (2018), figure 4, p. 10. Licensed under CC BY 4.0. © MDPI.
FIGURE 4.5 Global seagrass area relative to the maximum potential seagrass area within each of the global seagrass bioregions which are represented by scaled circles. Seagrass area in each bioregion: 1. Temperate North Atlantic = 3229 km2; 2. Tropical Atlantic = 44 222 km2; 3. Mediterranean = 14 167 km2; 4. Temperate North = 1866 km2; 5. Tropical Indo‐Pacific = 87 791 km2; 6. Temperate Southern = 9112 km2.
Source: McKenzie et al. (2020), figure 4, p. 7. Licensed under CC BY 4.0. © IOP Publishing.
FIGURE 4.6 Global distribution of coral reefs. The coloured areas indicate species richness of hermatypic coral reefs in each region.
Source:
