Tropical Marine Ecology. Daniel M. Alongi

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 of this asymmetry is that there tends to be export of particulate nutrients, rather than net import.

      The velocity of tidal circulation ultimately depends on the geometry of the waterway, that is, the ratio of the forest area to the waterway area as well as the slope of the forest. The ratio appears to be on the order to 2–10 (although few such measurements have been made) with a very small forest slope (Wolanski 2007). The tidal prism of a mangrove estuary thus increases greatly with an increase in the ratio between forest area to waterway area.

      Numerical modelling has determined the importance of the interaction between tidal creek geometry and mangrove forests in causing asymmetry of tides. The dominance of the ebb tide is due to friction in the mangrove forest which is in turn controlled by the density of the forest (Mazda et al. 1995). Inside the forest, the water level and the current velocity are strongly controlled by drag force due to the vegetation (Norris et al. 2019). The denser the forest, the greater the drag resulting in slower current velocity and greater tidal asymmetry in the creek. This relationship, however, is not straightforward. The peak velocity in the waterway decreases at flood tide and increases at ebb tide for increasing levels of drag force. But when the drag force is excessive, the ebb flow is reduced allowing the waterway to silt. There is a natural feedback relationship among the vegetation, water, and sediments. This phenomenon is unique to mangrove estuaries.

      An additional asymmetry of the currents in mangroves is the direction of the currents in relation to forest position (Li et al. 2012). At rising tide, the currents flow perpendicular into the forest, while at falling tide they are oriented at an angle, typically 30–60° to the bank. This lengthens the pathways of water at falling tide reducing the chance that materials such as mangrove detritus can escape the forest.

      Another characteristic feature of mangrove hydrodynamics is the mixing and lateral trapping of water (Wolanski 2007; Mazda and Wolanski 2009). Lateral trapping of water within the forest is a dominant process controlling longitudinal mixing in mangrove waterways. The trapping phenomenon occurs when some of the water flowing in and out of the estuary is temporarily retained in the mangrove forest to be returned to the main water channel later. Trapping of water is enhanced in the dry season when there is little, if any, freshwater to cause buoyancy effects