Three main rocky reef classes have been identified in the SeaMap Tasmania habitat classifications. These classes include high profile reef, medium profile reef and low profile reef. An additional category defined as patchy reef is baed on the spatial structuring that results in distinct patches of reef between areas of unconsolidated substrate.
High profile reef is used to characterise the consolidated seabed when the vertical profile changes by up to 4 m across a linear distance of 10 m. It is usually coincident with steep underwater cliffs, large boulder fields or areas of very high rugosity.
Medium profile reef characterises vertical changes in profile of the consolidated seabed from 1 -4m.
Low profile reef is used to characterise the consolidated seabed where there is very little change in relif (< 1m).
Patchy reef is used to characterise the seabed where the unconsolidated habitat consists of reef elements, such as boulders and rocks which intermittently outcrop from unconsolidated sediments, principally sand. This class is common on the seaward side of coastal reef areas.
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.
Unvegetated substrates consist of four main classifications: sand, hard sand, silty sand and silt..
Sand is a commonly encountered unconsolidated substrate but is most common in high to semi-exposed environments. This category represents the coarser end of a scale of sediments from silt to sand. Sand is generally characterised by a distinct second echo on the sounder trace from single beam acoustic sampling.
Hard sand refers to unconsolidated substrate containing elements that confound the single beam acoustic sounder output causing the signal to appear either harder or rougher than would be expected from sand. There are several factors that lead to a substrate being classified as hard. These are defined as modifiers in the habitat classification and includes large grain size, shell matter (either whole shells or shell grit) or biological material.
Examples of modifiers that are included in this hard sand category include: Physical: Coarse sand/gravel, compacted sand, rippled sand, shell or shell grit in sediment; Biological: Burrows, seawhips, holothurians
Hard sand is common in and around seagrass beds indicating the possible presence of rhizoidal mats of the seagrass and associated organisms in the substrata. It is also common on the seaward side of reefs indicating the presence of shells, detritus and organisms whose origin is dependent on the nearby reefs. The extensive areas of shelly substrate in high current areas in the D'Entrecasteaux Channel were characterised by a hard sand signature. On more exposed shores sand is harder closer to shore, due to coarser grade sands produced by the sorting action of waves.
Silty sand is common in low exposure and sheltered waters. It broadly incorporates any sediment with a significant proportion of coarse 'sand' particles and 'fine' silt particles. Silty sand is characterised by a less distinct echo on the single beam sounder trace.