Tropical Marine Ecology. Daniel M. Alongi

Чтение книги онлайн.

СКАЧАТЬ 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.

Mangroves grow best where wave energy is low and tidal flow and subsequent attenuation within the forest result in the deposition of fine particles from the incoming tidal water. The transport and deposition of suspended sediment is controlled by several interrelated processes: tidal pumping; baroclinic circulation; trapping of small particles in the turbidity maximum zone; flocculation; the mangrove tidal prism; physicochemical reactions that destroy flocs of cohesive sediment; and microbial production of mucus (McLachlin et al. 2020). The relative importance of these processes is site‐specific. Along the river's edge, mud banks form not just as the result of baroclinic circulation but also as the result of tidal pumping and mixing, especially in the turbidity maximum zone. A turbidity maximum zone is formed within an estuary where the residual inward bottom flow meets the outflow river flow. This zone is usually at the most landward point reached by the saline water flow. The water is generally shallowest at this point because this is a convergence point where sediment accumulates, the bulk of which creeps along the bottom. Some suspended matter settles here as the net current velocities in the zone are low; the zone is not stationary but moves with tidal ebb and flood.

      In relation to the turbidity maximum, flocculation of particles begins at salinities often <1; the largest flocs remain near the river bottom. The small flocs and unflocculated particles move further downstream with the currents where they aggregate with local particles (Gratiot and Anthony 2016). As floc size increases, they move towards the riverbed where they are entrained upstream by the baroclinic circulation. Due to tidal pumping, these flocs are carried further upstream at flood tide rather than downstream at ebb tide. The flocs are a loose matrix of clay and silt particles, typically a few micrometres in diameter, with their small size controlled somewhat by the strength of the tidal currents. Disaggregation starts when tidal velocities exceed 1 m s⁻¹. During spring tides, flocs are typically between 15 and 40 μm in diameter and are larger during neap tides. These flocs are colonised by bacteria, protists, and fungi and their extracellular mucus and threads, which help to cement the flocs and to maintain size when subjected to turbulence. Within the forest, flocs can remain in suspension owing to turbulence generated by flow around the trees. The settling of flocs occurs for a short period when the tides turn from rising to falling and the waters are quiescent. Settling is facilitated by the sticking of microbial mucus and by formation of invertebrate faecal pellets.

      It is thus correct to state that mangroves actively help to settle fine particles and are not just passive importers. The size, shape, and distribution patterns of mangrove trees have a profound impact on sedimentation. Tree species with large above‐ground root systems, such as Rhizophora, facilitate the deposition of particles to a much greater degree than tree species without extensive roots. The flocculation of particles results in faster settling velocities; most flocs settle within 30 minutes just before high slack tide. Until slack water, turbulent wakes created by tree trunks, roots, and pneumatophores maintain particles in suspension. Once in the forest, however, conditions are unfavourable for them to be resuspended as the high vegetation density inhibits water motion.

3.5 Coral Reef Hydrodynamics

The circulation of water in and around an individual coral reef is similarly complex and is central to understanding the high biodiversity and grandeur of these beautiful structures and their food webs. From individual coral colonies to entire coral reefs, hydrodynamics regulate their structure and function. Unlike mangroves, coral reefs thrive in highly physical conditions where water rapidly circulates and where residence times are short (Monismith 2007).

      At the scale of an individual branching coral, denser branching tends to divert more water flow to the exterior, whereas coral geometries with less dense branching allow more flow through the interior (Reidenbach et al. 2006). At a finer scale, branches create wakes that lead to blocking of interior flows. In the presence of waves, the flow through a coral colony is different, with the velocities inside the colony like those outside the colony. As colonies are living structures, flow variations inside the colony may induce localized calcification and a specific growth form or may lead to branch orientation that optimises functions such as the capture of prey or the uptake of nutrients. With higher flow velocities, the coral skeleton invariably becomes uniformly thicker, reducing the spaces between branches. This phenomenon suggests the possible importance of internal translocation of nutrients, etc., in the tissue layer that connects the polyps of a colony. Coral skeletal growth is often positively related to mean current velocity and to velocity fluctuations (Lenihan et al. 2015). Corals thus benefit directly from increased currents, probably through enhanced autotrophy (i.e. photosynthesis of coral symbionts) and/or heterotrophy (feeding on particles and living organisms) and indirectly by reduced feeding on corals by corallivorous fish.

      At the scale of 1–10 m, bottom drag is critical in water circulation. The bottom drag coefficient, C_D, for coral reefs is roughly 10 times greater than for a sandy unconsolidated seabed (Rosman and Hench 2011). Because of this roughness and because of the complex nature of corals, reefs have much higher transfer rates of water flow than predicted by simple engineering models. Waves are the dominant water feature of corals at this scale. Indeed, nutrient uptake is linked directly to the wave regime of the reef. However, as roughness increases, mass transfer is reduced, that is, water movement decreases. Lower parts of the rough coral surface are sheltered from wave action, that is, velocities of water movement are higher near the tops of coral colonies than in the troughs between colonies. As greater roughness leads to greater water motion, this has the practical result of making grazing by corals more efficient. Turbulence produced by the coral reef also affects the uptake and kinetics of the rate that particulates suspended in the water column can be grazed by the reef community. As a reef grows and becomes rougher, it can support denser benthic assemblages, such as sponges, which in turn leads to a richer reef community.

      At the scale of 100–1000 m, waves dominate coral reefs. The basic pattern is that incident waves break over the offshore face of the reef front and pushes water over the reef flat and then into the lagoon (if there is one). There are two regimes of flow behaviour when water flows over the front and the reef flat: (i) reef‐top control, when the reef flat flow is in a friction–pressure gradient balance when the mean water level is sufficiently above the reef flat and (ii) reef‐rim control, when the lagoon end of the reef flat determines СКАЧАТЬ