Pattern of seagrass distribution in the Great Barrier Reef World Heritage Area
Seagrasses in waters deeper than 15m in the Great Barrier Reef World Heritage Area adjacent to the Queensland coast were surveyed using a camera and dredge towed for 4-6 minutes at 1,429 sites spanning from 10ºS to 25ºS and from inshore out to the reef edge up to 120nm from the coast. At each site seagrass presence, species and biomass were recorded together with depth, sediment, sechii, algae and epibenthos, and proximity to reefs. Seagrasses in this region extend down to 60 m water depths and are difficult to map other than by generating distributions from point source data. Statistical modelling of the seagrass distribution suggests 40,000 km^2 of the bottom has a probability of some seagrass being present. There is strong spatial variability driven in part by the constraint of the Great Barrier Reef¿s long, thin shape and physical processes associated with the land and ocean. We map the four main species, Halophila ovalis, H. spinulosa, H. decipiens, and H. tricostata. Distributions of H. ovalis and H. spinulosa show strong depth and sediment effects, whereas H. decipiens, and H. tricostata are only weakly correlated with environmental variables but show strong spatial patterns. Distributions of all species are correlated most closely with depth and the proportion of medium sized sediment.
As part of the Reef Atlas project (now the eAtlas) the seagrass observations were interpolated over the whole GBR by Glenn De'ath using Generalized Additive Models with a Quasibinomial fit. This produced a gridded version of the dataset and is available as a KML and ASCII grid file.
Data Units: Probability of occurrence (0 to 1).