The area of the Trinity Inlet estuary reporting zone can be defined by the pre-cleared extent of mangrove and saltmarsh remnant vegetation which is characterised by species that require periodic inundation of sea water.
This area extends approximately 14 km inland south of the inlet’s mouth and is approximately 8 km in width. It includes numerous tributaries that drain into Trinity Inlet.
Land use within and adjacent to the Trinity estuary area includes residential, industrial and commercial developments associated with the central business district of Cairns.
Trinity inlet includes the Port of Cairns which is a multi-purpose port that caters for bulk cargo cruise shipping, fishing vessels and reef passenger vessels. Land use adjacent to the smaller upstream tributaries is dominated by cropping (sugarcane) with some residential development.
The Trinity Inlet estuary grade has declined from ‘good’ in 2016-17 to ‘moderate’ in the current reporting period, with the score decreasing from 64 to 57.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
Trinity Inlet estuary water quality remained ‘good’ and the score decreased to 65 in 2017-18 from 78 in the previous year. The lowest scoring indicator for 2017-18 was dissolved oxygen (37) and graded poor, chlorophyll a was graded ‘moderate’, nutrients (dissolved inorganic nitrogen and filterable reactive phosphorus) were graded ‘good’ and turbidity was graded ‘very good’.
Water quality grades are based on aquatic ecosystem guidelines for protection of estuarine waters, and are not based on load reduction targets for the marine environment. The results are derived from a transect of eight monitoring sites in the channel of Trinity Inlet.
Pesticides are not monitored in the Trinity Inlet estuary therefore a score and grade cannot be provided.
Habitat and hydrology
The habitat and hydrology index is comprised of seagrass, which is assessed annually, and three longer-term indicator categories that are updated every four years: mangrove and saltmarsh extent, riparian extent (both due for updates in 2018-19) and fish barriers (update due for 2019-20). The habitat and hydrology score remained unchanged, scoring 50 and was graded ‘moderate’
The mangrove and saltmarsh extent scored 53 (‘poor’) with 20.6% loss from pre-clear to 2013. The riparian extent scored 57 (‘moderate’) with 17.6 per cent loss from pre-clear to 2013. These results reflect the high level of development within the estuary area of Trinity Inlet. The low mangrove and saltmarsh extent scores for Trinity Inlet are mostly the result of historic extensive clearing activity. More recently mangrove communities in the Trinity Inlet estuary have been effectively managed to ensure there has been no major new clearing and to allow for some revegetation.
Estuary fish barriers for Trinity Inlet are scored 61 (‘good’) and are graded 'moderate’ for barrier density, ‘moderate’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’).
The seagrass at Trinity Inlet remained in poor condition, scoring 31. This was largely due to reduced meadow biomass in two meadows, and limited recovery of the third meadow following complete meadow loss in 2016-2017.
The flow indicator could not be assessed for the Trinity Inlet estuary due to the lack of pre-development modelled flow, which is required for the analysis.
The fish index is currently under development and will be available in future report cards.