The Effects of Seagrass (Zostera muelleri) on Boundary-layer Hydrodynamics in Whangapoua Estuary, New Zealand

Abstract

BRYAN, K. R. TAY, H. W, PILDITCH, C. A., LUNDQUIST, C. J. AND HUNT H. L. 2007. The Effects of Seagrass (Zostera muelleri) on Boundary-Layer Hydrodynamics in Whangapoua Estuary, New Zealand. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 668 – 672. Gold Coast, Australia, ISSN 0749.0208 Seagrass beds (Zostera muelleri) form extensive (> 10 Ha) intertidal beds in New Zealand estuaries potentially altering particle and nutrient fluxes to sediments. The effects of seagrass (blade length 5-8 cm, shoot density ~60/0.01m 2 ) on boundary layer dynamics were investigated at a site exposed to both tidal currents (~20 cm/s) and small wind waves (significant height 2-10 cm) over four days providing a variety of wave-current combinations. Flow characteristics were determined using two ADVs: one fixed at 15 cm above the bed and the other at varying heights within and above the canopy. The vertical variation of currents in and at the edge of the patch and on bare sediment during different tidal stages was measured. The inertial dissipation method was used to characterise changes to dissipation rate caused by varying bed character and hydrodynamic conditions. Results show that Z. muelleri altered the shape of the current profile by extending the boundary layer upward. The presence of waves caused substantial differences within the patch, appearing to increase the bed roughness. Dissipation rates outside the patch increased with proximity to the bed, whereas within the patch, they increased with distance from the bed. Dissipation rates were also observed to be greater on the incoming tide, even when differences in mean current speeds between flood and ebb tide were accounted for. Although the sudden transition at the edges of the Z. muelleri canopy caused more variable boundary profiles, the shape of the current profiles at the edge of the patch were clearly different on the ebb and the flood tides.