Tropical Marine Ecology. Daniel M. Alongi

are weathering, the presence or absence of large rivers and coral reefs, glaciation, tectonics, authigenic mineral and detrital formation, and ice‐rafting (Flemming 2011). Recent changes in sedimentary habitats are due to extensive degradation from coastal development, reduced sediment delivery from rivers due to damming, increased coastal erosion, and sea‐level rise. Mud and coral/carbonate deposits of biogenic origin are most abundant in the tropics, whereas sand is globally dominant, decreasing with higher latitudes to be proportionally displaced by glacial rock and gravel (Figure 4.1).

      The latitudinal pattern of inner shelf sediment types is somewhat deceptive because a large proportion of mud in the tropics occurs in proximity to the major rivers, particularly the deltaic systems of the Amazon, Orinoco, Mekong, Ganges, and Brahmaputra Rivers. Muds along coastlines near major river plumes are stirred up sufficiently due to cyclones that disrupt wave trains and form mud banks. This phenomenon occurs predominantly in the tropics and is well documented along the coast of SW India (Muraleedharan et al. 2018), northern South America (Proisy et al. 2021), and northern Australia (Caitcheon et al. 2012). On the Kerala coast of SW India, certain inshore areas produce zones of calm water by dampening wave action. Large quantities of riverine‐derived matter in colloidal suspension lead to the dissipation of wave energy. These mud banks, locally known as ‘chekara’, extend over areas of at least 25 km² and are characterised by silty‐clay sediments, oxygen‐deficient bottom waters, and possibly by generation of gases. These ‘chekara’ are generally 1–2 m thick.

Formation and migration of the Kerala coast mud banks are associated with high‐period waves and their refraction pattern along the sea bottom (Muraleedharan et al. 2018). The size of each bank is determined by the location of converging intertidal currents and offshore flow. The mud bank is supplied continuously with mud from both directions and compensates for losses due to settlement and from export by currents moving offshore. Particulate nutrient concentrations in mud bank deposits are generally high and positively correlated with decreasing grain size.

FIGURE 4.1 Latitudinal distribution of sediment types and composition on the world’s inner continental shelves.

      Source: Flemming (2011), figure 9, p. 15. © Elsevier.

      The coast of the Guianas (French Guiana, Surinam, British Guiana) of South America is also characterised by migrating mud banks. The coast is bordered by the sources of mud in the region, in the east by the mouth of the Amazon, and in the west by the Orinoco River (Jolivet et al. 2019). Mud banks attached to the shore are gigantic (about 200 km²) and are composed of fluid mud, forming a temporary storage for silt and clay, and colonised and recolonised by mangroves after alternating episodes of accretion and erosion in relation to river discharge (Proisy et al. 2021). As off the SW coast of India, the subtidal nearshore seabed is shallow, gently sloped, and fronted by mangrove forests at the water’s edge, leading to similar wave and tidal current patterns which foster accretion and migration of fluid mud. These linear mud shoals change rapidly in space and time and are transported westward by wave‐induced currents on the inner shelf and by the Guiana Current on the outer shelf (Jolivet et al. 2019).

      Land‐use change is an important factor in changing mud coastlines. For example, along the Mahin mud section of the Nigerian coast, Gulf of Guinea, 58% of the coastline experienced serious erosion, with a rapid rate of coastline retreat triggering a land loss of 10.6 km² to the Atlantic over the last three decades (Dada et al. 2019). Although the changing wave climate had a strong influence on the observed patterns of erosion and accretion along the Mahin mud coast, both marine and anthropogenic processes act in 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