Large-scale seagrass dieback in northern Spencer Gulf, South Australia

A major dieback of seagrass occurred in South Australia where 12,717 ha of intertidal and shallow subtidal seagrasses were lost along the north eastern coast of Spencer Gulf. This was a rapid decline, occurring toward the end of summer in January or early February of 1993. The extent and location of the dieback was mapped from aerial photographs taken before (1987) and after the event (1994). Eight habitat categories were represented including sand, dieback (severe and moderate) and seagrass (dense, intermediate and sparse). Of the total area of loss, the majority (8269 ha) was classified as severe dieback. It is significant that most of the dieback was previously dense seagrass (7523 ha) compared with the smaller areas of sparse (1044 ha) and intermediate (1600 ha) seagrass which were subsequently identified as dieback. Presence of seagrass remnants and data from previous surveys indicated that subtidal Amphibolis antarctica and intertidal Zostera spp were the main species that died back. The pattern of the dieback, restricted to shallow subtidal and intertidal areas, in combination with extreme conditions associated with a hot El Niño summer, strongly suggest the loss resulted from environmental causes. Anthropogenic factors are unlikely as the sources of pollution along this sparsely populated coast cannot account for such a wide geographic impact or the pattern of loss.