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 3.4). Meso‐tidal (2–4 m tidal range) regions include the northern Arabian Sea, parts of East Africa and the eastern Bay of Bengal, and some portions of Southeast Asia and NE Brazil; micro‐tides (< 2 m tidal range) dominate everywhere else in the tropics.
FIGURE 3.3 Global distribution of semi‐diurnal, diurnal, and mixed tides.
Source: Gerkema (2019), figure 1, p.14. © Cambridge University Press.
FIGURE 3.4 Distribution of tidal ranges across the world ocean.
Source: Image provided by and used with permission of James Chittleborough, Australian Bureau of Meteorology. © Australian Bureau of Meteorology.
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