Tropical Marine Ecology. Daniel M. Alongi

of sandy beaches from reflective to dissipative types with differences with tidal and/or wave heights. Distances are in metres.

      Source: McLachlan and Defeo (2018), figure 2.16, p. 26. © Elsevier.

      A beach type can be altered by storms, moving towards dissipative conditions over such circumstances and towards reflective conditions during calm weather (McLachlan and Defeo 2018). Tides also play a role in these transformations as spring tides during storms can foster dissipative conditions and neap tides can permit the development of a reflective beach. Simply, sand erodes or accretes on the beach face as wave height and tide range rises or drops.

      A useful index to describe the state of a micro‐tidal beach is called Dean’s parameter (McLachlan and Defeo 2018):

      wave energy is given by breaker height (cm) divided by wave period (seconds) and sand fall velocity is the sinking rate (cm per second) of the mean sand particle size on the beach. Values for Ω < 2 indicate reflective beaches and values > 5 indicate dissipative beaches.

      In macro‐tidal regions, the beach type is more complex as tides play a role that is like waves in that increasing tide range tends to make beaches even more dissipative because increasing tide range allows the surf zone to work back and forth over a wider area. In fact, fully reflective beaches will not occur when the tide range exceeds 1–1.5 m. Reflective beaches only occur on beaches with larger tides at the top of the shore between the neap and spring high‐water swash lines. Under large tidal regimes, beaches are generally tide dominated whereas in intermediate beaches they are mixed and either waves or tides can dominate.

      A useful index of the relative importance of waves and tides is the relative tide range (RTR) which is derived by the mean spring tide range divided by the breaker height. Thus, a two‐dimensional model (Figure 4.2) is produced of beach states of Ω versus RTR which span the entire range of tidal and wave conditions.

4.2 Distribution of Major Habitat Types

      Wide variations in tropical rainfall, hydrography, geomorphology, and tectonics lead to the formation of many sedimentary habitats peculiar to the tropics. Expansive sandy beaches, mud banks, green and blue anoxic mud regions, mixed terrigenous‐carbonate bedforms, hypersaline lagoons, stromatolites and, more generally, extensive intertidal sand‐ and mud flats, mangroves, coral reefs, and seagrass meadows are characteristic of shallow, tropical seas. These habitats are created and altered by processes peculiar to themselves and linked to climate and oceanographic factors and the rate of terrigenous sedimentation.

Extensive sandy beaches and flats, mud flats, mangrove forests, coral reefs, and seagrass meadows are among the most iconic of estuarine and marine habitats and are distributed widely throughout subtropical and tropical latitudes. Intertidal sand and mud flats develop in conditions more quiescent than sandy beaches, fostering deposition of fine‐grain sediment (Eisma 1997). The global distribution of sandy shorelines (Figure 4.3) shows that 31% of the ice‐free world shoreline is sandy, with Africa having the highest presence (66%) of sandy beaches (Luijendijk et al. 2018). The global distribution shows a distinct relation with latitude and hence to climate, while there is no relation with longitude. The relative occurrence of sandy shorelines increases in the subtropics and from 20 to 40° latitude with maxima near 30 °S and 25 °N. They are relatively less common (<20%) in the humid tropics where muddy substrates are 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