Tropical Marine Ecology. Daniel M. Alongi

act in concert with maximum plume outflow, causing sediments to be dispersed farther from the delta mouth. Sediments can be dispersed over relatively wide areas that extend considerable distances from the delta; some rivers do not have a subaerial delta or do not protrude much beyond the regional coastline. Alongshore dispersal of sediments takes place primarily on the inner shelf but near the delta mouth the depth is too shallow and energetic for fine‐grain sediments to be deposited, except temporarily (Hetland and Hsu 2014). Much of the material transported alongshore becomes sequestered within tidal currents, and off the Amazon, muds that are transported alongshore ultimately accumulate, forming expensive, accumulating mud banks hundreds of km to the NW of the mouth. Despite a high level of instability, these mud banks are colonised by mangroves. However, the Amazon is characterised by accumulations of sediments as subaqueous delta deposits at relatively large distances on the mid‐shelf. A large fraction of Amazon sediments reaches the mid‐shelf due to the energetic currents and waves that sustain sediments in suspension until they reach relatively deep water (Wright and Friedrichs 2006). Much of the observed diversity of sediment dispersal and accumulation is attributable to variations in coastal energy regimes and to the temporal sequencing of river discharge relative to oceanographic transport processes. Sediment transport in the southeast section of the Amazon coastal zone is greatly affected by tidal asymmetry, seasonal variation of the wind and wave regime, and river discharge (Gomes et al. 2020). Climate and geological configuration have resulted in numerous estuaries and large‐mangrove‐lined coastal plains that partially divided the estuarine basins, which are connected by tidal channels within the Amazon delta. Convergence of transported sediment occurs within a channel connecting the estuaries, resulting in mud retention and further delivery to the mangrove‐covered plains, with a net flux of suspended sediments between estuaries. The connectivity between estuaries via channels is a key process to redistribute muddy sediments along this coastal sector, which helps to explain the evolution and maintenance of the relatively homogenous and widespread progradation of mangroves along the coast.

      Mechanisms that dominate the short‐term spreading and mixing of riverine sediment may differ from the mechanisms that determine the longer‐term dispersal of sediment. Sediment records from the South China Sea show that strong monsoons are associated with intensified reworking of pre‐existing floodplain sediment over millennial timescales (Clift 2020). Strong monsoons result in deposition of more altered material that is also delivered at higher rates than during drier periods. Millennial‐scale changes in monsoon strength result in changes in the weathering regime but not fast enough to account for the changes seen in the sediments preserved in Asian deltas; instead, monsoon‐modulated recycling dominates. Over longer time periods (>10⁶ year) strengthening of the monsoon is linked to faster bedrock erosion and increased sediment flux to the ocean.

      An additional complication is the fact that most tropical river systems are heavily affected by humans; few, if any, tropical rivers are pristine. Human disturbances such as the construction of dams and deforestation can greatly impact water and sediment discharge. Increased greenhouse gas emissions are projected to impact twenty‐first century precipitation distribution, altering riverine water and sediment discharge. Modelling indicates that increasing global warming will lead to more extreme changes and greater rates of increasing or decreasing changes in fluvial discharge (Moragoda and Cohen 2020). At the end of the twenty‐first century under all IPCC climate change scenarios, mean global river discharge will increase by 2–11% relative to the 1950–2005 period, while global suspended sediment flux will increase by 11–16% under pristine conditions. Combining the effects of climate change with natural and anthropogenic impacts, tropical rivers have and will continue to be affected greatly in future. For example, natural and human‐induced factors have greatly altered the discharge of the Patía River of Colombia (Restrepo and Kettner 2012). In 1972, the river flow was diverted to an adjacent river resulting in several environmental changes, such as coastal retreat along the abandoned delta, the formation of barrier islands with exposed peat soils in the surf zone, abandonment of former active distributions in the southern delta plain with associated closing of inlets and formation of ebb tidal inlets, breaching events on the barrier islands, and accretion on the northern delta plain.

Tropical deltas are sensitive to human encroachment due to regional water management, global sea‐level rise, and climatic extremes (Shearman et al. 2013; Darby et al. 2020). Vertical change within a delta is a function of the change in delta surface elevation relative to sea‐level (Darby et al. 2020), derived as the sum of the rates of natural compaction and anthropogenic subsidence, eustatic sea‐level change, rate of crustal deformation due to local geodynamics, and the rate of surface aggradation (Figure 4.10). A significant fraction of the world’s deltas is subsiding and at risk of imminent drowning, having significant impacts on changes in mangrove area (Darby et al. 2020). In the Ayeyarwady delta, the loss of mangrove habitats and its conversion to cultivation has led to increased salinity intrusion, coastline retreat, and increased flood risk (Kroon et al. 2015). Trends in other rivers of the Asia‐Pacific region indicate deforestation and subsequent destabilisation of coastline (Shearman et al. 2013). Overall, Shearman et al. (2013) observed a net contraction of 76 km², but trends varied among different river systems. Further, some systems such as the Ganges–Brahmaputra are naturally subsiding, resulting in a high‐risk situation in relation to sea‐level rise. Thus, most tropical river systems are at moderate to high risk of anthropogenic change. With increasing rates of sea‐level rise and more intense cyclones, tropical river systems will increasingly undergo environmental and ecological change into the foreseeable future. A model to estimate such future impacts on the Mekong delta (Bussi et al. 2021) indicates that climate change will play a secondary role compared to dams; planned dams will reduce suspended sediment fluxes to the delta by up to 50% over the next two decades.

FIGURE 4.10 Idealised scheme of the factors and processes contributing to vertical changes within river deltas in the face of relative sea‐level rise (RSLR). Most of the world’s deltas currently have low rates of natural sediment supply and high rates of eustatic sea‐level rise (E = 1–6 mm a⁻¹) and often higher rates of human‐induced subsidence (S_A < 250 mm a⁻¹), meaning that many deltas are subsiding or in danger of drowning as sediment accretion (A_N = 0–5 mm a⁻¹) is the only factor that can offset relative sea‐level rise. Natural compaction (C_N = 0–5 mm a^–1) and crustal deformation (G = 0–3 mm a^–1) are also important factors.

      Source: Darby et al. (2020), figure 5.1, p. 105. Licensed under CC BY 4.0. © Springer Nature Switzerland AG.

The geomorphology of continental margins influences the biosphere by helping to mediate genetic connectivity of populations during sea‐level change (Dolby et al. 2020). Combining genetic data, geographical information systems‐based estuarine habitat modelling, and paleobiologic and recent effects of sea‐level change on evolution, Dolby et al. (2020) tested the relation between overall shelf area and species richness using data of 1721 fish species. They found an 82% global reduction of estuarine habitat abundance at low‐stand relative to high‐stand periods and found that large habitats change in size much more than small habitats. Narrow continental margins have significantly less habitat at high‐stand and low‐stand than wide margins and narrow margins significantly associate with active settings, effectively linking tectonic setting to habitat abundance. Narrow margins host greater species richness. Dolby et al. (2020) offer three possible explanations for this finding. First, physical isolation imposed by narrow margins may facilitate the formation of new species over time. Second, the size stability of small habitats, which disproportionately occur on narrow margins, may increase and retain species extirpated in the more variable habitats on wide margins. Third, the smaller habitats on narrow margins may facilitate greater species richness through greater habitat heterogeneity.