plume of 20–30 m depth (Hu et al. 2004). ‘Estuarisation’ is also important in the Bay of Bengal, the Gulf of Panama, the South China Sea, and the Gulf of Guinea, in addition to discernible plumes proximal to other major tropical rivers. ‘Estuarisation’ on many tropical shelves is seasonal, depending on the onset of the monsoon, and the transition from unstratified to stratified conditions is sharp, delimited by a tidal front.
Even small‐ and medium‐sized rivers can produce impressive shelf ‘estuarisation’. For example, in the Great Barrier Reef lagoon during and immediately after the wet season, diluted seawater migrates out to the Great Barrier Reef proper (Schroeder et al. 2012). These plumes can persist for weeks especially after a severe rainy season or a cyclone.
Subsurface water masses below the thermocline in the eastern tropical coastal oceans frequently contain an oxygen minimum layer. Several explanations have been offered to account for this poorly understood phenomenon, including minimal circulation or mixing of water to replenish oxygen consumed or that detritus accumulates in stagnant areas because of increases in water density with depth, leading to the depletion of oxygen (Stramma et al. 2008). Irrespective of the cause, low oxygen concentrations have important consequences for demersal fish and the benthic fauna (Chapter 7). Small rivers, creeks, and estuaries are also characterised by waters of low oxygen content with presumably similar biological consequences. These oxygen‐minimum zones are expanding as a direct result of global warming (Stramma et al. 2008).
Coastal upwelling is another major feature of the tropical oceans. Such events occur at all latitudes, but within the tropics and subtropics, physicochemical differences between upwelled and surface water masses are greatest. Upwelling is dominant along the subtropical‐tropical boundary coasts of Peru‐Chile (Peru Current), Morocco‐Mauritania (Canary Current), Angola‐Namibia (Benguela Current) and California‐Mexico. Upwelling events also occur on the Malabar coast of India, off the Andaman Islands, Western Australia, the Gulf of Panama (the Costa Rica Dome), the Gulf of Nicoya and Tehuantepec, off NE Venezuela and Brazil south of Cabo Frio, from Ghana to Togo (Gulf of Guinea), on the Somali coast, and off southern Arabia (Longhurst and Pauly 1987). Not all upwelling occurs off eastern boundaries of continents, as some upwelling events are driven by events an entire ocean away. For example, in the Gulf of Guinea and along the Somali coast of Africa, a diversity of mechanisms drives coastal upwelling (Valsala 2009). Seasonality of upwelling in the tropics is well described, but the actual circulatory patterns are poorly understood. Seasonal changes in current patterns occur, driven mainly by the movement of the ITCZ across the equator every six months. Upwelling events and monsoons are thus ultimately linked to seasonal changes in the equatorial climate.
3.4 Estuarine Circulation
The shores of many tropical estuaries are inhabited by mangrove forests. Their presence results in unique circulation patterns, which lead to distinct chemical and biophysical characteristics that are quite different from those in temperate estuaries (Mazda et al. 2007). Water flows in mangrove forests are strongly influenced by the presence of the trees and their aboveground roots as well as by the geomorphology of the tidal creeks. These unique characteristics have important ecological consequences. Tidal and wave energy in any estuary constitutes an auxiliary energy subsidy as tides allow mangrove forests to store and pass on new fixed carbon and benefits animals adapted to make use of subsidised energy. Tides thus do the work of bringing nutrients, food, and sediments to mangroves and their food webs as well as exporting their waste products. This subsidy is an advantage in that organisms do not have to expend energy on these processes and can shunt more energy into growth and reproduction.
A few tropical estuaries are driven by macro‐tides, although these are the exception rather than the rule as most tropical estuaries are micro‐ or meso‐tidal. Along the Brazilian coast south of the Amazon, there are four different types of macro‐tidal estuary: (i) ‘typical’ macro‐tidal, (ii) estuaries with large fluvial discharge, (iii) shallow, frictionally dominated macro‐tidal estuaries, and (iv) estuaries with structural control (Asp et al. 2013). In the ‘typical’ macro‐tidal estuary, ebb tides are longer than flood tides, and this condition is like that found in macro‐tidal estuaries in northern Australia (Wolanski et al. 2006). In the Daly estuary in northern Australia, freshwater becomes dominant up to the mouth and tides can be suppressed during the wet season (Wolanski et al. 2006). The Daly estuary is a ‘leaky’ sediment trap with its trapping efficiency varying with season and between years. In contrast, Darwin Harbour, a wider and much larger macro‐tidal estuary NE of the Daly, is poorly flushed, especially in the dry season; much sediment remains trapped on intertidal flats and in mangroves with little loss to the sea (Wolanski et al. 2006). Within the harbour, complex bathymetry results in the generation of jets, eddies, and stagnation zones that can trap sediments inshore. There may be a feedback between tidal circulation and bathymetry as tidally averaged circulation appears to control the formation and movement of sand banks.
In micro‐tidal estuaries, circulation is similarly complex and greatly influenced by the presence or absence of discharging rivers and the width of the connection between the estuary and the adjacent coastal ocean. Along the western Gulf of Mexico, tropical micro‐tidal estuaries share many characteristics, including a narrow connection between the estuary and the adjacent continental shelf (Salas‐Monreal et al. 2020). In the Jamapa River estuary, for example, surface horizontal displacements of the salinity and temperature fronts during the dry season occur, while during the wet season, the salinity and temperature gradients are observed in the vertical at about 1 m depth. A cyclonic recirculation at the mouth of the estuary occurs when the ratio between the mouth and the estuary width is below 0.4. This should hold true for all tropical micro‐tidal estuaries in the western Gulf of Mexico (Salas‐Monreal et al. 2020).
Not all tropical estuaries are driven solely by tides year round. In some northern Australian estuaries, a salinity maximum zone develops during in the dry season that is driven by high rates of evaporation (Wolanski 1986). This zone occurs near the mouth of each estuary where downwelling occurs, and a classical and an inverse estuarine circulation prevails upstream and downstream of the salinity maximum. This zone acts as a ‘high salinity plug’ inhibiting the mixing of estuarine and open ocean water to the extent that, in some cases, freshwater does not leave the estuary. Similar conditions have been found in estuaries along the SW coast of Ghana (Dzakpasu and Yankson 2015), in the Konkouré estuary of Guinea (Capo et al. 2009) and in the Gulf of Fonseca estuary on the Pacific coast of Central America (Valle‐Levinson and Bosley 2003).
Some tropical estuaries are so complex as to defy simple classification. Good examples of such complexity can be found along the north coast of Brazil (Medeiros et al. 2001; Schettini et al. 2013). These estuaries have multiple riverine systems feeding into a larger lagoon which is ordinarily fronted by coral reefs or coral‐fringed barrier islands. In the Itamaracá estuarine system, there are several estuarine waterways than feed into an ‘inner sea’ via a series of inlets, each considerably different from the other (Medeiros et al. 2001). Most of the freshwater enters the northern branch of the Santa Cruz Channel through the Catuama, Carrapicho, do Congo, Arataca, Botafogo, and Igarassu Rivers, the last three being the main source of freshwater. During the dry season, hypersaline conditions exist at both entrances in the “inner sea” due to evaporation, evapotranspiration by mangroves, and reduced exchange between the channels and reef shelf waters; a series of coral reefs fringe the outer edge of the estuary.
Tidal circulation within most mangrove waterways is characterised by a pronounced asymmetry between ebb and flood tides, with the ebb tide being shorter, but with stronger current velocity than the flood tide (Cavalcante et al. 2013). Current velocities in tidal creeks can often exceed 1 m s−1 but only rarely approach 0.1 m s−1 within the forest proper (Cavalcante et al. 2013). This asymmetry results in self‐scouring of the tidal waterways to the extent that the bottom of most channels is composed of bedrock, gravel, and sand, with little or no accumulation of fine sediment. The ecological implication
