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 remained ‘moderate’ and the score has decreased slightly from 57 to 56 since the previous year.
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 declined from ‘good’ in 2017-18 to ‘moderate’ and the score decreased from 65 to 58.
For 2018-19 dissolved oxygen remained the lowest scoring indicator (35) and graded poor, with monthly median values frequently not meeting guideline values. Chlorophyll a remained ‘moderate’, with monthly median values sometimes meeting guideline values.
Nutrients (dissolved inorganic nitrogen (DIN) and filterable reactive phosphorus (FRP)) and turbidity were graded ‘good’, with monthly median values often meeting guideline values.
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 updated for 2018-19) and fish barriers (update due for 2019-20). The habitat and hydrology score increased from 50 in 2017-18 to 55, although the grade remained ‘moderate’.
The mangrove and saltmarsh extent scored 53 (‘poor’) with 20.6% loss from pre-clear to 2017. The riparian extent scored 59 (‘moderate’) with 16.1 per cent loss from pre-clear to 2017. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2013 and the updated assessment for 2017, however the updated Regional Ecosystem mapping (Version 5) resulted in a 1.5% reduction of the estimated loss of riparian extent from pre-clear which was set at 17.6% for 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 improved substantially from poor condition, scoring 31, in 2017-18 to moderate condition, scoring 46. The improvement in estuarine seagrass was largely driven by increases in biomass and area in all three monitored meadows.
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 the 2021 Report Card.