Historical seagrass mapping in Port Phillip Bay, Australia

Seagrass beds are highly productive ecosystems and a decline in this habitat has become a global concern in recent decades. This study mapped seagrass at three sites in Port Phillip Bay between 1939 and 2011 and reviewed possible influences on seagrass cover changes. Historical aerial photographs from multiple sources were digitally scanned and orthorectified. Automated image processing techniques incorporating an unsupervised classification combined with minor editing in a GIS were applied to map seagrass cover and analyse variations in the size and distribution of seagrass beds. Large declines in seagrass cover were observed at all three sites after 1998. In contrast to other world-wide observations, these recent declines were preceded by a period of sustained seagrass expansion between the 1960s and 1990s and lower levels of seagrass cover were observed in the 1930s/40s. The recent and earlier low levels of seagrass cover coincided with extended droughts characterised by large reductions in nutrient inputs to the Bay. However, recent declines were not consistent across the Bay with three other sites remaining relatively stable during this period. The sites with large declines are all subject to longshore drift and changes in nearshore sediment transport driven by variations in weather patterns coinciding with extended periods of drought may be important influences on seagrass cover at these locations.