Landscape-scale changes in seagrass distribution over time: a case study from Success Bank, Western Australia

Abstract

Seagrasses in temperate Australia persist on sand habitats in shallow coastal environments by recruitment from seedlings and lateral spread of rhizomes from existing meadows. These colonizing processes, combined with seagrass loss from physical disturbance, result in a mosaic of sand and seagrass habitats. Here we describe these changing seagrass landscapes on Success Bank, Western Australia over a 20-year period, using aerial photographs. The 4 ha landscape units (LUs), selected from areas of current Posidonia coriacea Cambridge and Kuo and Amphibolis griffithii (Black) Den Hartog meadows, were analyzed for seagrass cover from aerial photographs from 1972, 1982 and 1993. Two LUs for each species were chosen from three regions (west, central and east) across Success Bank. Changes in landscape features of LUs were then summarized into total area and length of edge to area ratios of seagrass patches and meadows. Seagrass cover in LUs increased by 20,000 to 30,000 m −2 between 1972 and 1993. Such a large increase in seagrasses has not been documented elsewhere in Australia for these seagrass genera. Seagrass expansion was observed as an increase in the number and size of seagrass patches (<50 m 2). A simple model of seagrass colonization, based only on radial extension via rhizome growth, was constructed to test whether such large increases in seagrass cover could be accounted for solely by rhizome elongation. The model fitted observed increases in seagrass cover in some, but not all landscape units. The greatest divergence was in the western region where observed cover was higher than modeled rates. In central and eastern regions the modeled and observed increase in seagrass cover were similar. From aerial photographs, seagrasses have been actively colonizing Success Bank over the last 20 years. These observed changes can be accounted for by published rates of horizontal rhizome elongation for some, but not all, landscape units, suggesting that rhizome elongation is only part of the active seagrass colonization process observed on Success Bank. Future studies should target more accurate assessment of rhizome elongation rates, and colonization by seedlings of P. coriacea and A. griffithii, which were observed in great numbers. Whether the observed increase in seagrass cover is a phenomenon unique to Success Bank and to the seagrass species studied, or more generally applicable to other locations and seagrasses, also requires further study.