A comparison of canopy friction and sediment movement between four species of seagrass with reference to their ecology and restoration

Abstract

ABSTRACT: Seagrasses are recognized for their ability to modify currents, promote sediment deposi- tion and provide habitat for marine organisms. As a consequence, they are being considered as a non- structural structural engineering alternative for subtidal erosion control associated with dredging activities. The ability of seagrasses to modify the hydrodynamic environment may also contribute to the observed distribution of fauna in existing and developing meadows. Comparisons of canopy friction and sediment movement for 4 seagrass species were made under 27 combinations of velocity, water depth, and plant density in a flume. Thalassia testudinum, Halodule wrjghtii and Zostera marina, all strap- bladed species, lost friction (through canopy compression) with increasing current velocity at similar rates. The nearly-cylindrical species Syringodium filiforme exhibited no change in friction across the 5 to 30 cm S-' range of treatment velocities. The magnitude of frictional development by the 4 species was ranked T, testudinum > H. wrightii = Z, marina > S. filiforme. Canopy friction exhibited a strong positive relation to the percent of water column occupied by the seagrasses. The effectiveness of the seagrasses in inhibiting sediment movement was ranked as seen for canopy friction. These data confirm that T. testudinum provided the greatest protection of the sediment surface from erosion, H. wrightii and 2. marina provided intermediate levels, and S. filjfome the least protection. Significant alteration of estuarine circulation patterns could result from removal of these seagrass meadows or their creation through transplanting. Because the deposition and distribution of fauna and detritus in a seagrass bed are strongly influenced by the frictional characteristics of the estuarine floor, nursery and refuge value of these seagrass species also may vary directly with their influence on canopy friction and sediment movement. Future studies of seagrass bed fauna could likely explain some of the variation in their distribution through consideration of these hydrodynamic factors. INTRODUCTION