There are several seagrass modifiers that relate to the structure of the seagrass habitats. These are primarily based on the density of the blades and overall spatial distribution of the seagrass. The key categories are Seagrass, Patchy seagrass and Sparse seagrass.
Over the 5-10 m depth range, beyond the effective range of detection from aerial photographs, the division of seagrass beds into various density grade areas is dependent on interpretation of the sounder recording and visual assessment from video transects. As the sounder signal and video images only sample an area directly underneath the vessel, only a series of lines through the surveyed area can be mapped with certainty. Because of this, field transects over seagrass habitats are generally conducted at closer intervals than other habitat types. This scale of mapping is complemented with finer resolution assessment of seagrass and algal biomass through underwater visual transects.
The dominant seagrass type mapped in the Bruny Bioregion was Heterozostera tasmanica. Another common but minor species, Halophila australis, often occurred in conjunction with Heterozostera.. In some of the very shallow waters (mostly intertidal) the seagrass species Zostera muelleri is occasionally present.
The classification "patchy seagrass" represents areas where patch size varies from less than 1 m up to 20 m in linear extent. The patches generally consist of dense seagrass.
This classification usually applies to seagrass that occurs in waters exposed to significant swell and wind exposure, or in deeper water. Whilst the density of the shoots of the seagrass (primarily Zostera tasmanica) is low, the beds can cover extensive areas. The beds often have associated hard sand acoustic signals indicating the possibility of other biotic elements, such as shells. In sparse seagrass areas, the substrate beneath the seagrass is easily visible, often consisting of more than 70% cover.