Tropical Marine Ecology. Daniel M. Alongi

island indicate large internal waves that result in periodically cool deeper areas of the fore reef.

3.6 Fluid Mechanics in Seagrass Meadows

      Seagrasses, like their mangrove and coral reef counterparts, are ecosystem engineers capable by their very existence of reducing the velocity of currents and attenuating waves to the extent that sediment particles can deposit on surfaces and on the seabed in quiescent zones (Peterson et al. 2004). Other factors play important roles in helping to accumulate carbon in seagrass meadows, such as canopy complexity, turbidity, wave height, and water depth (Samper‐Villarreal et al. 2016). But the essence of what drives the accumulation of sediment particles and associated carbon is fluid dynamics. The movement of water among, between and around seagrass blades is the key feature of carbon capture (Koch et al. 2006).

The main source of energy required to move water is the sun that causes winds that lead to waves and thermal gradients that in turn lead to expansion, mixing, and instabilities in water gradients and thus flow. Seawater, being an incompressible fluid, moves at a flow rate that is defined by the velocity of the fluid that passes through a cross‐sectional area, A. Water flow leads to both hydrostatic and dynamic pressures which are a constant. What this means in practical terms is that the sum of the pressures helps to explain lift that occurs within, around and under seagrass canopies. Drag is another force that operates in the case of water motion and has two components: (i) viscous drag that exists due to the interaction of the seagrass surface with the water and (ii) dynamic or pressure drag that exists under high flows when flows separate from boundaries (Koch et al. 2006).

      Water flow can be either smooth and regular (laminar flow) or rough and irregular (turbulent flow), depending on the velocity and temporal and spatial scale under investigation as defined by the Reynolds number:

      (3.1)

      where l is the length scale under observation and v is the kinematic viscosity. R_e defines four flow regimes that may occur: (i) creeping flow where R_e < < 1 which occurs at very slow flows and spatial scales such as those experienced by microbes, (ii) laminar flow (1 < R_e < 10³) which is smooth and regular, (iii) transitional flow (R_e ≈ 10³) which involves the production of eddies and disturbances in the flow, and (iv) fully turbulent flow (R_e > > 10³). These flows are scale‐dependent; flow is almost always turbulent across entire seagrass meadows, but laminar at the scale of individual seagrass leaves (Koch et al. 2006).

      Flow conditions become more complex when water approaches a boundary, such as the seagrass canopy or seafloor. Water cannot penetrate such boundaries, but slips by it, a condition which leads to the development of a velocity gradient perpendicular to the boundary as the velocity at the boundary will be zero relative to the stream velocity. As water flows downstream, the velocity gradient will get larger and a slower moving layer of water will develop next to the boundary. Vertically, there is a sublayer in which the forces are largely viscous. Consequently, mass transfer in this layer is slow, dominated by diffusion, in what is called a diffusive boundary layer. Such boundary layers can become embedded within one another such that it is possible to define boundary conditions around blade epiphytes, flowers, leaves, and the canopy.

      At the molecular level, a boundary layer develops on the sediment surface as well as on each leaf, shoot, or flower as water flows through a meadow. The faster the water movement, the thinner the diffusive boundary layer and thus the transfer of molecules (e.g. CO₂) is faster from the boundary layer to the water column. When currents are weak, the flux of molecules may be diffusion‐limited, but after a critical velocity is reached, the transfer is no longer limited by diffusion but by the rate of assimilation capacity (i.e., biological or biochemical activity). The mass transfer of molecules also depends on other factors, such as the thickness of the periphyton layer on the seagrass leaves, reactions within the periphyton layer, and the concentration of molecules in the water adjacent to the leaf‐periphyton assemblage.

      At the scale of shoots (mm to cm), a feedback mechanism operates as individual shoots are affected by other shoots and its position within the canopy (that is, edge versus centre of the entire meadow). As water velocity increases, shoots bend minimising drag, but the forces exerted on individual shoots are more complex when waves are involved, as a shoot is exposed to unsteady flows in different directions. This is confirmed by the fact that in wave‐swept environments, seagrass leaves become longer as wave exposure increases (de Boer 2007). Flow around shoots results in bending but also pressure gradients on the leeward side of the leaf such that a vertical ascending flow is generated downstream of the shoot. This water then disperses horizontally at the point where the leaves bend over with the flow; interstitial water is also flushed out at the base of the shoot due to the pressure gradients generated on the sediment surface.

At the whole‐canopy level, reduced flows occur within the canopy due to the deflection of the current over the canopy and loss of momentum within the canopy. Water speed as a result can be two to >10 times slower than outside the meadow. It is this process that allows water and sediment particles to be trapped during low tide; even short seagrass canopies can reduce water velocity. Vertically, however, water flow intensifies at the height of the sheath or stem as these parts are much less effective at reducing water velocity compared with the leaf component. Canopy flow is nevertheless complex because it is a function of the drag or resistance of the leaves on the water.

      Seagrass meadows are where sediments and carbon deposit and accumulate largely due to the reduction in velocity and intensity of turbulence, that is, a reduction in flow strength that leads to a reduction in resuspension within the canopy (de Boer 2007; Gullström et al. 2018). Accumulation of sediment may be seasonal, especially during summer when seagrasses are at their maximum density and in winter then resuspension may be greater than accumulation when seagrasses are minimal, although roots and rhizomes may alone be enough to stabilize the accumulated deposits (Gullström et al. 2018). Epiphytes on seagrass leaves may foster the accumulation of sediment particles by increasing the roughness of the canopy and increasing the thickness of the boundary layer on the leaf surface. However, in highly wave‐exposed locations, seagrasses may not accumulate fine sediments due to resuspension. Indeed, in some cases, sediment may be coarser beneath seagrass patches due to turbulence generated by the leaves themselves.

3.7 Tides

      A variety of tidal types and ranges exist across the tropical ocean as a result not just of the pull of the moon and the sun, but because of the presence of continents and shorelines, and the relative size scales of these coastal features (Townsend 2012). Various tidal types and ranges exist because some areas are more tied to the daily cycle of the sun's gravitational attraction than others. In these areas, diurnal tides exist; in areas more tied to the moon's gravitation, semi‐diurnal tides exist. And still other areas exhibit a combination of both influences, producing mixed tides. Macro‐tides such as in northern Australia result when the resonant frequency of a body of water closely matches the lunar tidal frequency and is modified by geomorphology; lesser tides are produced with a less close match between lunar frequency and basin shape.

Semi‐diurnal tides predominate along the coast of northern South America, the Pacific coast of Central America, most of tropical Africa and along stretches of northern Australia (Figure 3.3). Mixed tides occur everywhere else (Figure 3.3), except some areas of Southeast Asia (western Borneo, eastern Sumatra, Gulf of Thailand), and the Gulf of Mexico (Figure 3.3) where diurnal tides occur.

With respect to tidal ranges, macro‐tides (> 4 m tidal range) predominate along a portion of the northern Australia coast, the southern Great Barrier Reef shelf, the Pacific coast of southern Central America, and a portion of East Africa (Figure