Tropical Marine Ecology - Daniel M. Alongi

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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.

      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.

      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, (ii) a reversing tidal flow with a damped amplitude and delayed phase towards the forest, and (iii) a residual flow towards the forest caused by the damped tidal flow. This residual flow reduces the outflow of water from the forest towards the sea.

      Mangroves often receive a significant amount of wave action, even in an estuary. Mangroves attenuate wave energy via two primary mechanisms: (i) multiple interactions of waves with mangrove trunks and roots and (ii) bottom friction. The latter is not well understood, but a significant amount of attention has focused on the effect of the presence of tree trunks and roots. Forces induced by waves on tree stems and roots are inertial and drag‐type forces, with drag force dominating for most mangroves (Hashim et al. 2013). The degree of wave attenuation increases with increasing tree diameter, although interactions between tree stems can influence the extent of drag. Waves within a mangrove forest are strongly dissipated by these interactions. Dissipation of wave energy is a function of total tree area which is in turn a function of both tree diameter and forest density. Water depth can also play a role in wave dissipation. For a very dense forest, wave energy is almost totally dissipated within 40–50 m from the mangrove‐sea boundary, but in less dense forests, about 35% of the incident wave energy is still extant behind the forest (Hashim et al. 2013). In mangrove forests that are small in area due to urban disturbance, such as in Singapore, the percentage of wave height reduction is higher under storm events compared to normal conditions, with vegetation drag being the main mechanism of wave dissipation; mangrove density and width were positively correlated to the percentage of wave height reduction during a storm (Lee et al. 2021). Mangrove roots contributed to a larger percentage of wave height reduction than trunks and canopies, although there were no significant differences in the extent of wave height reduction between forest types, incident wave heights, and water levels. Thus, even comparatively small, disturbed mangrove forests can offer some protection from wave energy.

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