As water flows through seagrass beds, a boundary layer develops on the sediment surface as well as on each seagrass component exposed to the moving water (leaf, short-shoot, flower) (Ackerman, 1986; Fon-seca and Kenworthy, 1987; Koch, 1994; Cornelisen and Thomas, 2002). The faster the water moves, the thinner the diffusive boundary layer (DBL, or 5d) becomes (Massel, 1999; Fig. 1) and, consequently, the faster the transfer of molecules from the water column to the sediment and/or seagrass. It follows that when currents are weak, the flux of molecules to the seagrass surface may be limited by diffusion through the áD (i.e. physical limitation). Under those conditions, many biological sites or enzymes in the seagrass tissue are available to assimilate molecules when/if they reach the plant's surface (Koch, 1994; Cornelisen and Thomas, 2002). After a critical velocity (Uk) is reached (Fig. 2), the transfer
Fig. 2. An example of uptake kinetics by seagrass leaves exposed to increasing current velocities (U). Uk is the critical current velocity at which uptake rate saturates (equivalent to Ik in photosynthesis x irradiance curves). At currents below Uk (1), uptake is mass transfer-limited and at currents above Uk (3), uptake is kinetically limited. A combination of both limitations may occur at flows around Uk (2). If nutrient concentration in the water column increases, the curve is likely to shift upwards. Additionally, other types of responses to water flow are also possible (see text).
of molecules through the SD is no longer the limiting factor. Instead, the capacity of biological uptake sites or enzymes to assimilate molecules that reached the plant surface becomes limiting (Koch, 1994). In this case, the conditions are said to be kinet-ically (and not physically) limiting. When velocities are at intermediate levels, around Uk, a combination of physical (SD) and kinetic (enzymes) limitations may influence the uptake of nutrients (Sanford and Crawford, 2000). At velocities below 3-5 cm s-1 (Uk), photosynthesis (i.e. carbon uptake) in Thalas-sia testudinum and Cymodocea nodosa is SD limited, whereas at velocities above Uk, photosynthesis seems to be limited by the kinetics of Rubisco (Koch, 1994). Interestingly, a similar Uk value was found for the kelp Macrocystis integrifolia (Stevens and Hurd, 1997). In contrast, some seagrass studies were unable to detect a kinetic limitation in the assimilation of nutrients in flowing water (i.e. no Uk), instead, assimilation was SD limited up to the maximum velocity tested: 20 cm s-1 for Thalassia tes-tudinum and its epiphytes (Cornelisen and Thomas, 2002) and 34 cm s-1 for Zostera marina (Fonseca and Kenworthy, 1987). This difference may be due in part to experimental conditions. Specifically, studies in which assimilation was only a function of velocity were performed with entire plants rooted in sediment and covered by epiphytes, while the experiments in which assimilation was a function of velocity and enzyme kinetics were done with epiphyte-free leaves, in the laboratory. For further discussion of the role of diffusive boundary layers on photosynthesis, see Larkum et al., Chapter 14.
Mass transfer to seagrass leaves does not only depend on the velocity and SD thickness but also on: (1) the thickness of the periphyton layer (complex of debris, mucus, bacteria, algae, small animals, and sediment particles) on the seagrass leaves (Jones et al., 2000), (2) the reactions occurring within the periphyton layer (Sand-Jensen et al., 1985; Jones et al., 2000; Cornelisen and Thomas, 2002) and (3) the concentration of the molecules in the water column adjacent to the seagrasses-periphyton complex (Sanford and Crawford, 2000). The water interstitial to the periphyton is expected to be static (with the exception of occasional sweep events; Nikora et al., 2002); therefore, SD increases linearly with periphyton thickness (Jones et al., 2000). Consequently, the spatial scale for diffusion of molecules from the water column to the leaf surface is longer and SD-limited conditions are more likely to occur.
The critical SD thickness at Uk has been estimated to be 98 |xm and 280 |xm for periphyton-free leaves of Cymodocea nodosa and Thalassia testudinum, respectively (Koch, 1994), whereas the SD on artificial leaves with periphyton was quantified to be 950 |xm in thickness (Jones et al., 2000). The SD limitation of molecules such as nitrogen, phosphorous and carbon may be further exacerbated by the reactions occurring within the periphyton layer. Epiphytic algae tend to assimilate biologically important molecules before they reach the seagrass surface (Jones et al., 2000; Sanford and Crawford, 2000; Cornelisen and Thomas, 2002), thereby competing for vital nutrients (Sand-Jensen et al., 1985). If the uptake kinetics of epiphytes is more efficient than that of seagrasses, the microalgae could potentially outcompete the seagrasses in the uptake of nutrients (including carbon) from the water col-umn(Sand-Jensenetal., 1985; Beer and Koch, 1996; Cornelisen and Thomas, 2002). According to Fick's first law:
5d where J is the flux of molecules, Cw the concentration in the water column, and Cs the concentration on the seagrass surface. SD limiting conditions become less important as the concentration of nutrients (Cw) in the water column increases (i.e. eu-trophication). Under such eutrophic conditions, uptake is controlled by the kinetics of periphyton and seagrasses (Sanford and Crawford, 2000). As a result, one can hypothesize that as coastal waters become more eutrophic, mass transfer-limitations may become less important to seagrasses, but this is a complex process as the growth of the epiphytes as a function of the nutrient concentration also needs to be taken into account. Additionally, when uptake rates are SD limited, kinetic processes become less important and the uptake rates become a function of the planar area of seagrasses and epiphytes exposed to water flow.
As indicated above, the Stanton number (St), a dimensionless number, can also be used to quantify the efficiency of a seagrass canopy to remove nutrients from the water, as it is the flux of a chemical to a surface divided by its advection past the surface (e.g. Thomas et al., 2000). St can be obtained via direct measurements of nutrient uptake and velocity measurements, or can be calculated.
The calculated values have not always matched the measured values possibly due to the dependence of St on the friction coefficient (Thomas et al.,
2000), a parameter that decreases as the seagrass canopy bends when exposed to increasing velocities (Fonseca and Fisher, 1986). Additionally, the St only parameterizes the transport into the canopy, i.e. it parameterizes the flux across the interface defined by the top of the canopy, not the diffusive sub-layers on individual leaves.
The discussion of fluxes of inorganic nutrients through the DBL so far assumed steady state flows. In nature, the thickness of the 5D tends to fluctuate over time and space (Koch, 1994). Wave-induced oscillatory flows and/or large-scale turbulent eddies tend to disrupt the 5D for short periods of time (fractions of a second) during which the 5D is stripped away and the supply of molecules near the blade surface is replenished (Nikora et al., 2002). If these pulses of enriched water near the seagrass leaf occur on a regular basis such as under wave-dominated conditions, the flux of nutrients to the plant surface is expected to be enhanced (Stevens and Hurd, 1997). As indicated in Section II, little is currently known about the physiological implications of 5D fluctuations on seagrass leaves.
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