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 1:10000 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% (507ha) in 1965 to 43% (1036ha) in 1995, whereas on Parmelia Bank the percentage of seagrass cover has remained relatively constant with 46% cover in 1965 (735ha) and 44% in 1995 (699ha). 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 1:10000.