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 on water circulation. In the wet season, the buoyancy effect is important as freshwater is trapped in the forest at high tide, and as a floating lens or boundary layer hugging the riverbank at low tide. This effect means that the forests control the runoff of freshwater, especially at the end of a flood tide. The evaporation of water and the build‐up of salt generated by the physiological activities of the trees helps to generate gradients of salt and other materials, both laterally and longitudinally, especially during a long dry season.
A significant lateral gradient within mangrove creeks during the dry season can be attributed to high evapotranspiration (Le Minor et al. 2021). Weak stratification usually prevails at the headwaters of mangrove waterways in the wet season as the result of freshwater input from rivers. Such gradients have been observed in Gazi Bay in Kenya (Kitheka 1997) and in the Konkoure River delta in Guinea (Wolanski and Cassagne 2000). In arid‐zone estuaries, the salinity structure is inverse due to the lack of freshwater input and the high evaporation rate, especially in relation to salt flats and mangrove bordering the estuary; salinities can be >50 (Kitheka 1997).
The behaviour of tidal water is also longitudinally complex. Longitudinal diffusion is proportional to the square of the water velocity, which means that mixing rates are very small at the headwaters of mangrove creeks where currents are also very small. Water speed decreases from the mouth to the headwaters along the length of a waterway. The longitudinal and cross‐sectional gradients in current speed are partly the result of shear dispersion processes that are magnified by the presence of the forest. This diffusion process drives the intensity of mixing and trapping. These complex processes translate into long residence times for water near the head of the waterway, especially in the dry season.
All estuaries, including those inhabited by mangroves, exhibit secondary circulation patterns superimposed on the primary tidal circulation. This phenomenon is responsible for the often observed trapping of floating detritus in density‐driven convergence fronts during a rising tide (Stieglitz and Ridd 2001). These fronts occur in well‐mixed estuaries due to the interaction between the velocity of water across the estuary and the density gradient up the estuary. Due to friction, the velocity is slower near the riverbanks than in the centre of the estuary, thus causing on flood tides a greater density mid‐channel than at the banks. A two‐cell circulation pattern results from the sinking of water in the centre of the estuary. The existence of these cells has ecological consequences. A net upstream movement of floating debris occurs, on the order of several km per day; mangrove propagules are unlikely to enter the mangrove forest when these cells are present and will accumulate in large numbers in ‘traps’ upstream from the convergence and upstream from the mangrove fringe (Stieglitz and Ridd 2001). Trapping of propagules is not conducive to the natural strategy of maximizing dispersal of seeds.
Within the mangrove forest, trees, roots, animal burrows and mounds, timber, and other decaying vegetation exert a drag force on the movement of tidal waters (Le Minor et al. 2021). The drag force of the trees can be simplified to a balance between the slope of the surface water and the flow resistance due to the vegetation (Mullarney et al. 2017). Water flow in the forest depends on the volume of the trees relative to the total forest area. The momentum of tidal forces is greater than the shear stress induced by the presence of the trees, including friction with the soil surface. Even the presence of dense pneumatophore roots induces turbulent friction near the forest floor (Norris et al. 2019). The presence of mangrove seedlings results in alteration of tidal flow by modifying the vertical velocity and the magnitude of turbulent energy (Chang et al. 2020). The dynamics of tidal forces in mangrove forests changes in relation to different tree species, density of the vegetation, and state of the tides.
Currents in the forest itself are not negligible, and a secondary circulation pattern is usually present due to the vegetation density and the overflow of water into the forest at high tide (Mazda et al. 2007; Mullarney et al. 2017). This secondary circulation enhances the trapping effect of tides. The drag force has two main influences: (i) inundation of the forest is inhibited and this decrease in water volume results in smaller dispersion and (ii) the trapping of water in the forest is enhanced, favouring dispersion. Thus, the magnitude of tidal trapping depends on the drag force due to the vegetation, so the magnitude of dispersion depends ultimately on the vegetation density.
Animal structures also influence water circulation in mangrove forests (Le Minor et al. 2021). Benthic organisms, especially crabs, produce numerous burrows and tubes in the forest floor through which tidal water flows. Tidal water flows through a labyrinth of interconnected crab burrows in the same direction as the surface current (Ridd 1996; Stieglitz et al. 2013). The flow through the burrows is caused by a pressure difference between multiple burrow openings. The total quantity of water that flows through crab burrows can range from 1000 to 1 010 000 m3 representing from 0.3 to 3% of the total volume of water moving through the forest. This percentage is not large but is enough for the burrows to play an important role in flushing salt away from mangrove roots. The transport of salt derived from the tree roots results in variations in the density of water flowing through the burrows, having an impact on flushing time. Burrows, tubes, and other biogenic structures impart some significant delay in water flow, assisting in the trapping of water in the forest (Stieglitz et al. 2013).
Not all water entering an estuary leaves via the surface. Some water leaves via subterranean pathways such that mangrove estuaries often have significant groundwater flow. This flow can have significant biogeochemical and biological effects, such as removing excess salt from mangrove roots and removing high concentrations of respired carbon from the forest to the adjacent coastal zone (Gleeson et al. 2013). Crab burrows and other biogenic structures can facilitate groundwater flow. The flow of groundwater in mangroves usually has three components (Mazda and Ikeda 2006): (i) a near‐steady flow towards the open ocean due to the pressure gradient induced by the difference in height between water levels in the forest and the open sea,
