Moresby Estuary: Wet Tropics Waterway Health

The area of the Moresby 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 3.3 km north and 2 km south of the river mouth along the coastline and extends approximately 12 km inland. The area includes numerous tributaries that drain into the main river channel.

Land use within the Moresby estuary area is dominated by cropping and pond aquaculture with some residential development. The Moresby locality is on the western fringe of the estuary area. The Port of Mourilyan is located near the mouth of the estuary and exports raw sugar and molasses from a loader, wharf and handling facility.

The Moresby estuary grade has declined from ‘good’ to ‘moderate’ in 2021-22 with the score decreasing from 66 to 60, 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.

WATER QUALITY

Water quality remained ‘good’ with the score decreasing from 76 to 67 since the previous year.

DIN (dissolved inorganic nitrogen) remained ‘good’ with monthly median values often meeting guideline values.

Dissolved oxygen (low) and chlorophyll a declined from ‘good’ to ‘moderate’, with monthly median values sometimes meeting guideline values.

Filterable reactive phosphorus (FRP) and turbidity were graded ‘very good’, with most monthly median values 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 five monitoring sites in the mid-estuary channels.

Pesticides are not monitored in the Moresby estuary therefore a score and grade cannot be provided.

HABITAT AND HYDROLOGY

The habitat and hydrology index is comprised of seagrass, which is updated annually, and three longer-term indicator categories that are updated every four years: mangrove and saltmarsh (extent and shoreline habitat updated in 2021-22), riparian extent (updated in 2021-22) and fish barriers (date of next update to be confirmed). The habitat and hydrology score declined from 58 in 2020-21 to 56 and remained ‘moderate’.

Mangrove and saltmarsh scored 83 and was graded ‘very good’. Shoreline mangrove habitat scored 82 with a grade of ‘very good’ whilst mangrove and saltmarsh extent was also graded ‘very good’ and scored 84 with 4 per cent loss from pre-clear to 2019.

Riparian extent scored 66 (‘good’) with 12 per cent loss from pre-clear to 2019. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2017 and the updated assessment for 2019.

Estuary fish barriers for the Moresby estuary scored 61 (‘good’) and were graded ‘poor’ for barrier density, ‘good’ for percentage of stream length to the first barrier, and there was an absence of low passability barriers (graded ‘very good’).

The seagrass at the Moresby River remained ‘very poor’ condition and the score decreased from 18 in 2020-21 to 0. Seagrass was present at only two of the five monitoring meadows with another meadow disappearing since the previous years. There continues to be an absence of Zostera muelleri, which was one of the key foundation species in the area before widespread loss during 2009-11. The meadows remaining consist of pioneering ephemeral species that can be highly variable over time. Pilot projects which transplanted seagrass fragments have recently been conducted in Mourilyan (Moresby Estuary) by JCU. These trials showed successful survival and expansion of fragments. The successful approach will be applied at a larger scale. Seagrass condition will remain poor in Moresby estuary in the absence of restoration due to the lack of local propagules.

The flow indicator could not be assessed for the Moresby estuary due to the lack of pre-development modelled flow and flow assessment sites, which are required for the analysis.