Showing papers on "Amphibolis antarctica published in 2000"

Changes in seagrass cover on Success and Parmelia Banks, Western Australia between 1965 and 1995

Abstract: Changes in seagrass cover on Success and Parmelia Banks, Western Australia between 1965 and 1995 were mapped from aerial photography using changes in the distribution and size of phototonal categories. Aerial photography from 1965, 1972, 1982 and 1995 was used to determine the temporal and spatial changes in seagrass distribution. Aerial photography was rectified using photogrammetric techniques, and manually interpreted at a scale of 1:10 000 to determine the extent of seagrasses and unvegetated sands. The 1995 data were also ground-truthed to determine the seagrass species assemblages. The mapping of seagrass cover using phototones is only feasible for species of seagrasses with a dense leaf canopy. The species that fit this criteria, and that have high cover on Success and Parmelia Banks, are Amphibolis antarctica, Amphibolis griffithii, Posidonia australis, Posidonia sinuosa and Posidonia coriacea. The results of this analysis indicates that the percentage of seagrass cover on Success Bank has increased from 21% (507 ha) in 1965 to 43% (1036 ha) in 1995, whereas on Parmelia Bank the percentage of seagrass cover has remained relatively constant with 46% cover in 1965 (735 ha) and 44% in 1995 (699 ha). The east, central and western regions of Success Bank have all shown an increase in seagrass cover from 1965 to 1995. On Parmelia Bank the seagrass cover on the western region has increased. Whereas, the seagrass cover on the eastern region has decreased. On both Success and Parmelia Banks it appears that the majority of seagrass growth between 1975 and 1995 has been in assemblages that are predominantly single species or mixed species meadows of A. griffithii and P. coriacea. These taxa have previously been considered to be relatively static, not known to colonize over large areas, but this study shows that on Success and Parmelia Banks they are actively colonizing at rates that can be mapped at a scale of 1:10 000.

Dynamics of Plant–Flow Interactions for the Seagrass Amphibolis antarctica: Field Observations and Model Simulations

Abstract: Seagrass canopies influence water flow partly as a consequence of their morphology. Amphibolis antarctica (Labill.) Sonder et Aschers. ex Aschers, an Australian endemic, is different morphologically from more-commonly studied blade-like seagrasses such as Zostera and Thalassia. Field measurements and model predictions were used to characterize water flow within and above an A. antarctica meadow. A series of high resolution three-dimensional velocity measurements were obtained within, above and adjacent to A. antarctica meadows at different heights above the seabed. Field observations on the effect of seagrass canopy on flow show an overall damping effect. Power spectra of the velocity data revealed a reduction in energy from 500 (cm s-1)2 s-1 to 10 (cm s-1)2 s-1 within the canopy. Profiles of kinetic energy were calculated from in situ velocity measurements at 5 cm increments from 10 cm to 80 cm above the seabed, within and above the seagrass canopy. There was an intensification of flow where the canopy structure was densest (approximately 40 cm above the seabed) and slightly above it. The baffling effect of the canopy was most effective 25 cm above the seabed: here the flow was reduced from 50 cm s-1 at free surface to 2-5 cm s-1. A slight increase in flow within the canopy was seen 10 cm above the sediment due to reduced friction exerted by the lower leafless stems of the plants. A high resolution three-dimensional hydrodynamic model was coupled to a ten-layer canopy model for shallow coastal site dimensions. By applying different friction factors to various parts of the plant, mimicking its architecture, water flow was shown to be altered by the plant canopy according to its morphology. The derived computational results were in good agreement with the observed in situ velocity and kinetic energy changes. As a result of this study it is now possible to accurately predict plant-flow interactions determining pollen and particles distribution and dispersal.