Fluid dynamics is the study of the movement of fluids. Among other things, it addresses velocity, acceleration, and the forces exerted by or upon fluids in motion (Daugherty et al., 1985; White, 1999; Kundu and Cohen, 2002). Fluid dynamics affects every aspect of the existence of seagrasses from the smallest to the largest scale: from the nutrients they obtain to the sediment they colonize; from the pollination of their flowers to the import/export of organic matter to adjacent systems; from the light that reaches their leaves to the organisms that live in the sea-grass habitats. Therefore, fluid dynamics is of major importance in seagrass biology, ecology, and eco-physiology. Unfortunately, fluid dynamics is often overlooked in seagrass systems (Koch, 2001). This

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chapter provides a general background in fluid dynamics and then addresses increasingly larger scales of fluid dynamic processes relevant to seagrass ecology and physiology: molecules (^m), leaves and shoots (mm to cm), seagrass canopies (m), seagrass landscapes (100-1,000 m), and seagrasses as part of the biosphere (>1,000 m). Although gases are also fluids, this chapter is restricted to water (i.e. compressed fluids), how it flows through seagrasses, the forces it exerts on the plants, and the implications that this has for seagrass systems. Sea-grasses are not only affected by water in motion, they also affect the currents, waves and turbulence of the water masses surrounding them. This capacity to alter their own environment is referred to as "ecosystem engineering" (Jones et al., 1994, 1997; Thomas et al., 2000). Readers are also encouraged to consult a recent review by Okubo et al. (2002) for a discussion on flow in terrestrial and aquatic

A. WD. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp. 193-225. © 2006 Springer. Printed in the Netherlands.

vegetation including freshwater plants, seagrasses, and kelp.

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