Interaction between hydrodynamics and seagrass canopy structure: Spatially explicit effects on ammonium uptake rates

The hypotheses that (1) different seagrass morphologies may facilitate different nutrient uptake rates under similar hydrodynamic forcing and (2) this effect on nutrient uptake rates is spatially explicit, with the highest uptake rates at edges of patches, where currents and turbulence are highest, were examined under unidirectional flow conditions. Homogeneous patches (2 m long) of two seagrass species (Cymodocea nodosa and Zostera noltii) with contrasting shoot size and density were placed in a race track flume. 15NH z4 uptake and hydrodynamic properties along a gradient from outside to inside the patch were measured at a range of current velocities (0.05 to 0.3 m s21). For each velocity we also determined the height and bending of the canopy. Water velocity affected the ammonium uptake rate of both species. The almost double uptake rates of C. nodosa shoots, compared to those of Z. noltii, were mainly attributed to a twofold difference in the within-canopy water flow (Qc, m3 s21). Spatial patterns in canopy water flow were highly correlated with spatial patterns in NH z4 uptake, thereby explaining the 20% higher uptake rates at the leading edge of both canopies. The correlation between spatial patterns in canopy water flow and ammonium uptake rates underlines the role of canopy and patch configuration in determining the functioning of seagrass landscapes and their associated ecosystem services, such as nitrogen assimilation. Seagrasses inhabit intertidal and subtidal areas throughout the world (Larkum et al. 2006), where they experience variable hydrodynamic forces that interact with their canopies (Koch et al. 2006). In general, velocities decrease within a seagrass canopy, whereas turbulence tends to increase within or at the top of the canopy, compared to turbulence in nonvegetated areas (Gambi et al. 1990; Ackerman and Okubo 1993; Koch and Gust 1999). Smallscale horizontal differences in hydrodynamics within seagrass canopies are also commonly observed, with canopy water flow generally decreasing downstream from the leading edge (Fonseca and Koehl 2006). These hydrodynamic interactions are often influenced by canopy deflection (Abdelrhman 2007), which depends upon characteristics such as shoot density (Gambi et al. 1990; Peterson et al. 2004; Bouma et al. 2005), shoot stiffness (Bouma et al. 2005), the occupied volume (Fonseca et al. 1982; Koch 2001), and patchiness (Folkard 2005). Hydrodynamic conditions can affect the growth and morphometry of seagrass plants, either directly or indirectly (Fonseca and Kenworthy 1987; Schanz and Asmus 2003; Peralta et al. 2005). Indirect effects on growth may be understood by the influence of hydrodynamics on the mass 1 Corresponding author ([email protected]). Acknowledgments We thank Jos van Soelen, Bas Koutstaal, and Louie Haazen for invaluable technical assistance. In addition, we are grateful to Britta Gribsholt, Bart Veuger, Miguel Bernal, Juan Jose Vergara, and Alfredo Izquirdo for helpful discussion. In addition, we thank Josef D. Ackerman and the anonymous reviewers for comments that greatly improved the manuscript. This work and the first author were supported by an EU Marie Curie host fellowship for transfer of knowledge, MTKD-CT2004-509254, and the Spanish national project EVAMARIA, CTM2005-00395/MAR. F.G.B. holds an EU Marie Curie individual fellowship, MEIF-CT-2005-515071. This is publication 4251 of the Netherlands Institute of Ecology (NIOO-KNAW). Limnol. Oceanogr., 53(4), 2008, 1531–1539 E 2008, by the American Society of Limnology and Oceanography, Inc.