Tropical Marine Ecology. Daniel M. Alongi

Subarnarekha India 10 3 Indus Pakistan 5 10 Limpopo Mozambique 5 33

      Regardless of size, a great variety of physical processes, ultimately driven by climate, make these tropical coastal margins unique compared to coastal settings of higher latitude. The climate of the equatorial region is characterised by high rates of rainfall, solar insolation, and temperature. By virtue of these characteristics and global position, Coriolis forces are small, and winds are dominated by easterly trade winds (Chapter 2). These physical forces, coupled with the enormous loads of freshwater and sediment draining from the land, produce extensive buoyant plumes, in some instances, extending beyond the shelf edge. Rapid rates of sediment accumulation and high rates of nutrient flux and primary productivity are but a few of the unique characteristics of these river‐dominated coastal systems.

      Tropical rivers worldwide drain a variety of geologic/geomorphologic settings: (i) orogenic mountain belts, (ii) sedimentary and basaltic plateau/platforms, (iii) cratonic areas, (iv) lowland plains in sedimentary basins, and (v) mixed terrains (Latrubesse et al. 2005). These types of rivers show high but variable peak discharges during the rainy season and a period of low flow during the dry season; some rivers show two flood peaks during the year, a main one and a secondary flood peak.

      Tropical rivers exhibit a variety of channel forms and, consequently, a variety of different delta and mouth morphologies (Latrubesse et al. 2005). In most cases, rivers morph from one form to another over time so they are difficult to classify. Two main settings are rivers that discharge onto a tectonically active margin and those that discharge onto a passive margin (Leithold et al. 2016). Active margins are narrow and passive margins are wide.

Exactly how and where the discharged sediment deposits onto the adjacent shelf are not well understood, although it appears that sediments being discharged onto a wide, passive margin deposit in shallow waters often proximal to the river mouths, while a significant, if highly variable, proportion of sediments discharging onto narrow, active margins are transported to the adjacent shelf slope and deep sea (Wright and Nittrouer 1995; Leithold et al. 2016). The fate of sediment seaward of river mouths involves five stages: (i) supply via plumes, (ii) initial deposition, (iii) resuspension and transport by marine hydrographic processes, (iv) sediment that comes back on shore from far away and/or via tidal pumping, and (v) long‐term net accumulation (Figure 4.9). These processes vary greatly with river regime and coastal physics. The Amazon plume extends along the NW portion of the coast and far seaward. And although tide range is large and mixing processes are relatively intense, the enormous volume of outflow results in the effluent filling the entire water column beyond the mouth, before ascending above the seawater. It continues to expand as a thick (5–10 m) buoyant plume which reaches more than 300 km offshore and about 1000 km to the NW entrained by the North Brazil Current (NBC). However, the plume is highly variable over time and space due to wind forcing and tidal variations in bottom drag and vertical mixing. There is also seasonality in discharge, with maximum discharge during May–June; peak sediment discharge precedes peak water discharge by about a month or more, so the volume‐specific rates of discharge vary considerably. Strong tidal currents and waves generated by the easterly trade winds dominate the Amazon shelf resulting in variable spatial and temporal distribution of sediments on the shelf. Intense reworking of sediment on the inner shelf allows only temporary deposition. Once resuspended, the NBC carries sediment far to the NW. Despite high alongshore current flux, erosion occurs along the shore although erosion–deposition episodes depend on the strength of the NBC. At least 50% of sediment accumulation takes place on the mid‐shelf (depth 30–50 m) seaward and NW of the mouth. Remaining sediment is probably stored in the tidal reaches of the lower river.

FIGURE 4.9 Major stages in sediment dispersal of river sediments in the coastal ocean.

      Source: Wright and Nittrouer (1995), figure 3, p. 503. © Springer Nature Switzerland AG.

      The Purari River system on New Guinea is different, being much smaller (Table 4.1) and having a mountainous watershed (Wright and Nittrouer 1995). The Purari delta is heavily vegetated by mangroves and crossed by an intricate network of interconnected channels which trap most of the river sediment load. Saltwater intrusion is prevented by large shallow and mobile sand banks within and outside the river mouth. Fine sediments are carried onto the inner Gulf of Papua shelf as muddy, low‐salinity plumes that are broken up by the coastal oceanographic regime which is dominated by onshore‐directed southeast trade winds for most of the year. Thus, much of the sediment remains trapped relatively close inshore as a turbid band and is advected alongshore. Plumes enter tidal channels on flood tides supporting the extensive mangroves within the delta. Some sediment is transported directly offshore especially during summer when the trade winds are weak, and rainfall is at its peak.

In the Ayeyarwady River system, little modern sediment accumulates on the shelf immediately off the delta. Instead, a major mud wedge with a distal depocentre of up to 60 m thickness has been deposited seaward of the adjacent Gulf of Martaban, extending to about 130 m into the Martaban Depression (Liu et al. 2020). However, no river‐derived sediment has been found in the adjacent deep Martaban Canyon. There is a mud drape wrapping around the narrow western Myanmar shelf in the eastern Bay of Bengal. Unlike other large river systems in Asia, such as the Yangtze and Mekong, there is a bidirectional transport and depositional pattern controlled by local currents and tides, and seasonally varying monsoonal winds and waves.

      In the Ganges–Brahmaputra River system, approximately one‐third of total sediment discharge is sequestered within the flood plain and delta plain (Rahman et al. 2018). The remaining load appears to be apportioned between the accumulating subaqueous delta and the deep‐sea Bengal fan via a nearshore canyon. The roughly equal partitioning of sediment among the flood plain, shelf, and deep sea reflects the respective influence of an inland subsiding tectonic basin, a wide shelf, and a deeply incised canyon system (Rahman et al. 2018).

      Plumes of other large tropical river systems may be dispersed laterally due to local coastal currents and hydrography. For instance, the typical seasonal orientation of the Zaire River plume is northward for most of the year, except during February–March when the plume has a large westward extension onto the narrow shelf (Denamiel et al. 2013). The northward extension of the plume is explained by a buoyancy‐driven upstream coastal flow and the combined influences of the ambient ocean currents and the wind. During February–March, the surface ocean circulation drives the westward expansion of the plume and the presence of the deep Congo canyon increases the intrusion of seawater into the river mouth.

      Off the Mekong delta, a similar lateral plume occurs throughout most of the year, with a net deposition SW of the river mouth down the Ca Mau peninsula (Szczuciński et al. 2013). In summer, a large amount of fluvial sediment is deposited near the Mekong River mouth, but in the following winter, strong mixing and coastal currents lead to resuspension and south‐westward dispersal of previously deposited sediment. Strong wave mixing and downwelling‐favourable coastal current associated with the more energetic NE monsoon in the winter are the main factors controlling post‐depositional dispersal to the SW.

For tropical river plumes, coastal hydrography plays an important role in governing how the discharged sediment is dispersed onto the adjacent shelf (Wright