Johnstone Estuary: Wet Tropics Waterway Health, North Queensland

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’ in 2021-22 although the score decreased from 70 to 61.

The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.

WATER QUALITY

Water quality also remained ‘good’ with the score decreasing from 76 in 2020-21 to 67.

DIN (dissolved inorganic nitrogen) remained the lowest scoring indicator (37) and was graded ‘poor’, with monthly median values often not meeting guideline values.

FRP (filterable reactive phosphorus) scored 48 and declined from ‘good’ to ‘moderate’ with monthly median values sometimes meeting guideline values.

Dissolved oxygen (low) declined from ‘very good’ to ‘good’ with monthly median values sometimes meeting guideline values.

Chlorophyll a and turbidity were graded ‘very good’, meaning for both indicators most monthly median values met guideline values.

Pesticides remained ‘good’ and the score decreased from 75 in 2020-21 to 69, equating to over 96 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 three monitoring sites in the mid estuary and a Great Barrier Catchment Loads Monitoring Program site at Coquette Point in the lower estuary. Pesticide 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 2021-20) and fish barriers (date of next update to be confirmed). Habitat and hydrology remained ‘good’ and the score increased from 62 in 2020-21 to 63.

Mangrove and saltmarsh extent scored 63 (‘good’) with 13.6% loss from pre-clear to 2019. These results reflect some development within the estuary area. There was no change in extent between the previous assessment for 2017 and the updated assessment for 2019.

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. Riparian extent scored 9 (‘very poor’) with 77 per cent loss from pre-clear to 2019. There was no change in extent between the previous assessment for 2017 and the updated assessment for 2019.

Estuary fish barriers for the Johnstone estuary scored 81 (‘very good’) and were 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’ to ‘good’ with the score decreasing from 98 in 2020-21 to 71. Flows to the Johnstone estuary retained similar characteristics to modelled predevelopment flows in their capacity to support the key ecological assets of water holes, low flow spawning fish, riffle habitats and fisheries production.