Seasonal variability in sediment distribution along an exposure gradient in a seagrass meadow in Shoalwater Bay, Western Australia

Seagrasses, by blocking water flow as it passes through the leafy canopy, are believed to have a significant impact on sediment dynamics, and the formation of sand and mud banks in some areas. The role of seagrasses in sedimentation processes is poorly understood in high-energy environments, such as those found in south-Western Australia. Studies of sediment size fraction distribution were conducted over a 14-month period within a Posidonia sinuosa meadow, at an exposed and a sheltered site, to investigate the role of seagrass canopies on sediment dynamics. Sediment size analyses, obtained by sieving sediment samples, showed a difference between exposed and sheltered sites, as well as seasonal influences on the data. A three-way ANOVA run on the data indicates a summer and winter pattern superimposed over the sheltered and exposed pattern. This suggests that during the calmer conditions experienced in summer there was an increased proportion of finer grain sizes at the sheltered site, while in winter the grain sizes tended to become coarser, more closely matching the pattern observed at the exposed site. These results suggest that reduced water motion at the sheltered site during summer permitted finer sediment grain sizes to settle out, while increased water motion during winter increased the proportion of coarse grain sizes. At the exposed site this seasonal difference was not observed. It therefore appears that the P. sinuosa canopy reduces flow through a dense meadow, but this effect appears to be modified by overall wave energy, observed to operate seasonally.