unison to influence coastal retrogradation along the coast; mangrove deforestation has contributed greatly to the increased rate of retreat.
Tropical coastlines characterised by migrating mud banks are the exception rather than the rule. Excluding these areas and other regions such as off tropical river deltas, the Peruvian upwelling system, and the Bight of Biafra where highly reducing, sulphidic blue mud persists, the largest area of tropical shelves is sand dominated. Several shelves are dominated by carbonates and in many instances, bordered landward by extensive tidal flats, mangroves, and seagrasses or fringed seawards by coral reefs (Martini 2014).
Modern carbonate shelves in the subtropics and tropics fall into two categories: (i) protected shelf lagoons (the Bahamas, Florida, Belize, Cuba, and the Great Barrier Reef) and (ii) open shelves (Yucatan, western Florida, the Persian Gulf, and northern Australia). Shelf lagoons are characterised by the presence of fringing barrier reefs, islands, and shoals, and commonly have across‐shelf gradients of mixed terrestrial‐carbonate deposits. On the Belize shelf, the Grand Bahamas Bank, and the Great Barrier Reef, the lagoons consist of gradients from inshore terrigenous quartz sand and mud, grading to mixed terrigenous/carbonate deposits and then to carbonate sand and mud out to the edge of the shelf (Vieira et al. 2019). Carbonate shelves are not limited to the low latitudes, but protected shelves are latitudinally restricted because only in the low latitudes has the production of carbonate at the shelf margin sufficient to keep pace with Holocene sea‐level rise. On many tropical shelves, soft sandy‐mud sediments derived from continental drainage dominate inshore areas with varying mixtures of quartz sand and carbonate sand deposits dominating the middle and outer shelf areas, the extent of which vary with shelf width (Vieira et al. 2019).
Many coastal lagoons which formed behind barrier islands are either hypersaline in arid regions due to excessive evaporation (e.g. the Persian Gulf) or form gigantic interconnecting waterways in the humid tropics, as along the Gulf of Guinea off Africa. Sediment composition varies greatly among lagoons depending on their openness to the sea and the presence or absence of rivers and coastal vegetation. Lagoons in highly arid regions are often trapped with aeolian dunes that have been cemented by the precipitation of calcium carbonate, within which biogenic material is rapidly produced. Microbial stromatolites develop particularly in arid Indo‐Pacific areas (e.g. Shark Bay, Western Australia) where other biota is excluded by accretion of precipitated inorganic carbonates. The formation of cyanobacterial mats constitutes the final stages in the formation of coastal gypsum lakes.
On sandy beaches, ecosystem structure and function are highly dependent on the beach type. Beaches are defined by the interactions among wave energy, tides, and the nature of the available sand (McLachlan and Defeo 2018). The beach slope is the simplest index of beach state and is the product of all three variables; beach face slopes flatten as wave energy or tidal range increases or particle size decreases, assuming other factors are kept constant. The flattest beach thus occurs in macro‐tidal regions of high wave energy and fine sand while the steepest beaches occur in micro‐tidal regions with low wave energy and coarse sand. A range of beach morphological types can be distinguished between these extremes.
In a micro‐tidal regime, beaches are wave dominated, and three beach types can be recognised: reflective, intermediate, and dissipative (Figure 4.2). A reflective beach is characterised by a steep face and absence of a surf zone with gentle wave and coarse sand. Dissipative beaches are characterised by a flat beach face and wide surf zone; waves break far out from the beach face and dissipate their energy while traversing the surf zone before expiring as swash on the beach face. Dissipative beaches are thus the product of large waves moving over fine sand. Between these two extremes are intermediate beaches distinguished by the presence of surf zones that are smaller than for dissipative beaches and are generally 20–100 m wide. The surf zone of an intermediate beach has well‐developed sand banks and channels with rip currents.
FIGURE 4.2 Model 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
