Effects of Water Velocity and Canopy Morphology on Ammonium Uptake by Seagrass Communities

Abstract

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Ammonium uptake by seagrass communities: effects of oscillatory versus unidirectional flow

“…A total of 27 ammonium uptake experiments were conducted in oscillatory flow, 18 in beds of T. testudinum, and 9 in bare sand as a control for planktonic uptake and flume leakage; 7 experiments were also conducted in unidirectional flow in T. testudinum communities. In addition, data collected from 7 unidirectional experiments conducted previously at the same field site (Thomas et al 2000) were used in the comparison of unidirectional and oscillatory flow on ammonium uptake. Shoot densities ranged from 1600 to 3100 shoots m -2 and canopy heights from 22 to 27 cm, which are values similar to those reported by Thomas et al (2000).…”

“…Canopies formed by seagrass plants collectively attenuate water flow within the canopy (Fonseca et al 1982, Gambi et al 1990, Ackerman & Okubo 1993) and can enhance flow over and around the bed (Gambi et al 1990, Koch & Gust 1999. These effects on water flow influence important ecological processes such as sediment retention (Fonseca & Fisher 1986, Gacia et al 1999, transport and recruitment of larvae (Eckman 1987, Grizzle et al 1996, photosynthesis (Fonseca & Kenworthy 1987, Koch 1994 and the transport and uptake of nutrients (Thomas et al 2000). Our current state of knowledge on the effects of water flow on these processes is primarily based on studies involving flume experiments with unidirectional flow.…”

“…, Larned & Atkinson 1997, Thomas & Atkinson 1997). In addition, field flume experiments conducted in entire seagrass communities and in unidirectional flow have demonstrated that uptake rates of an important nutrient, ammonium, is positively dependent on water velocity (Thomas et al 2000).…”

“…Seagrass canopies are flexible; however, allowing seagrass blades to bend with increasing flow velocity. Thus, the friction imposed by the benthos on the water decreases with increasing water velocity at a faster rate than would be expected for a solid non-flexible surface such as a coral reef (Thomas et al 2000). Flexibility of the grasses also creates regions of variableflow environments within the canopy (e.g.…”

“…In this study, we measured rates of ammonium uptake for a seagrass community in the field using a portable field flume (Thomas et al 2000) and compared uptake rates for experiments conducted under oscillatory and unidirectional flow. Uptake rate constants were analyzed in terms of hydrodynamic characteristics over the seagrass canopy including velocity, speed, and turbulent energy.…”