The area of the Johnstone 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 1.5 km north and 2 km south of the river mouth along the coastline and extends approximately 5.4 km inland.
The area includes numerous tributaries that drain into the main river channel. The assessment area of riparian extent extends further up the river than the mangrove and salt marsh communities to the upper tidal limit and includes vegetation types that are more typical of freshwater environments.
Land use within and adjacent to the Johnstone estuary area is dominated by residential and other urban developments and cropping. Urban development is associated with the town of Innisfail which overlays sections of the estuary area.
The Johnstone estuary grade has remained ‘good’ with the score unchanged from the previous year on 65.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
The Johnstone estuary water quality remained ‘good’ with the score increasing from 67 in 2017-18 to 76.
DIN (dissolved inorganic nitrogen) remained the lowest scoring indicator (51) but improved from ‘poor’ in 2017-18 to ‘moderate’, with monthly median values sometimes meeting guideline values.
FRP (filterable reactive phosphorus) scored 79 and improved from ‘moderate’ to ‘good’ and dissolved oxygen remained ‘good’, with monthly median values often meeting guideline values.
Chlorophyll a and turbidity both scored 90 and were graded ‘very good’, with most monthly median values meeting guideline values.
Pesticides remained ‘good’ and the score increased from 61 in 2017-18 to 74, equating to over 97 percent of species protected for the risk assessment metric.
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 five monitoring sites in the mid estuary and a Great Barrier Catchment Loads Monitoring Program site at Coquette Point in the lower estuary. Pesticides monitoring was conducted solely at the Coquette Point site as per the basin assessment.
Habitat and hydrology
The habitat and hydrology index is comprised of flow, 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).
Habitat and hydrology declined from ‘good’ in 2017-18 to ‘moderate’ and the score decreased from 63 to 54 due to the change in the flow indicator score.
Mangrove and saltmarsh extent scored 63 (‘good’) with 13.6% loss from pre-clear to 2017. These results reflect some development within the estuary area. There was no change in extent between the previous assessment for 2013 and the updated assessment for 2017.
Riparian extent is assessed to the upper tidal limit of the river which extends upstream of the estuary area that is characterised by mangroves and saltmarsh. Riparian assessment therefore includes areas that have been cleared for agriculture and other developments. The riparian extent scored 9 (‘very poor’) with 77.3% loss from pre-clear to 2017. There was no change in extent between the previous assessment for 2013 and the updated assessment for 2017.
Estuary fish barriers for the Johnstone estuary are scored 81 (‘very good’) and are graded 'very good’ for barrier density, ‘good’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’).
For flow the Johnstone estuary declined from ‘very good’ in 2017-18 to ‘good’. Flows to the Johnstone estuary retained similar characteristics to modelled pre-development flows in their capacity to support the key ecological assets of water holes, low flow spawning fish, riffle habitats and fisheries production.
The lower score for the Johnstone estuary in 2018-19 compared to the previous two years was likely a result of extreme rainfall pattern since there is little water resource development within the basin.
The fish index is currently under development and will be available in future report cards.