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 easter n 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 aft er the event (1994). Eight habitat categories were represented including sa nd, dieback (severe and moderate) and seagrass (dense, intermediate and spa rse). Of the total area of loss, the majority (8269 ha) was classified as s evere dieback. It is significant that most of the dieback was previously de nse seagrass (7523 ha) compared with the smaller areas of sparse (1044 ha) and intermediate (1600 ha) seagrass which were subsequently identified as d ieback. Presence of seagrass remnants and data from previous surveys indica ted that subtidal Amphibolis antarctica and intertidal Zostera spp were the main species that died back. The pattern of the dieback, restricted to sha llow subtidal and intertidal areas, in combination with extreme conditions associated with a hot El Nino summer, strongly suggest the loss resulted fr om 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. (C) 2000 Elsevier Science B. V. All rights reserved.