Assessing the impacts of water quality on seagrass extent using satellite time-series analysis

Sediment and seagrass in coastal environments are closely linked, as seagrass beds help to stabilise sediments, cycle nutrients, provide valuable habitat and detrital material for estuarine food webs. Previous monitoring of... [ view full abstract ]

Sediment and seagrass in coastal environments are closely linked, as seagrass beds help to stabilise sediments, cycle nutrients, provide valuable habitat and detrital material for estuarine food webs. Previous monitoring of seagrass in Western Port, Australia, over multiple decades has shown that a significant decline occurred in the 1970’s and 1980’s with a slight recovery in some regions in the late 1990’s. Regions where recovery did not occur were linked to poor water quality. A key issue related to the decline is erosion and sediment transport leading to a reduction of water clarity and light availability for seagrass. This research attempted to investigate the spatial and temporal patterns of turbidity and sediment re-suspension dynamics within the bay with respect to trends in seagrass using archival satellite imagery.
Time series of seagrass coverage and turbidity reconstructed from satellite imagery were compared with 14 years of observed turbidity data from rivers draining to Western Port allowing a better understanding of the importance and persistence of the catchment to receiving water linkage.
Based on the relationship between the satellite derived reflectance and the inherent optical properties of these optically complex waters, semi-empirical algorithms parameterised to provide estimates of bathymetry, substrate composition and water quality information. The application of these methods for multispectral satellite imagery analysis has several limitations, however, the benefit of the underlying hyperspectral models have been found to produce better results compared with more traditional spectral classification methods. Multi-temporal data when combined with the optical modelling approach enables consistent large scale spatio-temporal analysis of seagrass extent and of coastal water quality. These analyses are used in support of catchment sediment load analysis, hydrodynamic modelling and seagrass modelling activities.