The area of the Barron 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 10.5 km north and 4.5 km south of the river mouth along the coastline and extends approximately 5.5 km inland. The area includes several tributaries that drain into the Barron River and coastal streams that drain directly into the sea.
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 Barron estuary area is dominated by urban development. The northern beach suburbs of Machans Beach, Yorkeys Knob and Holloways beach, and the Cairns suburbs of Aeroglen, Cairns North and Whitfield overlay the estuary area.
Industries within the estuary area include the Cairns international and domestic airport, quarries, aquaculture and sugarcane.
In 2018 the grade for the Barron estuary has remained 'Moderate' and the score has increased from 46 to 55 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
The Barron estuary had the most substantial change of all the estuaries, improving from ‘moderate’ to ‘good’ in 2015-16.
The improved water quality was due to lower concentrations of chlorophyll a in the Barron estuary - the score increased from 6 (‘very poor’) for the previous year to 60 (moderate). The score for dissolved oxygen decreased from 90 (‘very good’) in 2015-16 to 76 (good). All the other indicator scores were similar to the previous year, and grades were unchanged.
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 channel of the Barron River.
Habitat and hydrology
The habitat and hydrology index is comprised of three longer-term indicator categories that are updated every four years: mangrove and saltmarsh extent (update due for 2017-18), riparian extent (update due for 2017-18) and fish barriers (update due for 2019-20).
The habitat and hydrology index has increased from 41 in 2015-16 to 45 (both years graded ‘moderate’). This change is due to the addition of the flow indicator.
The mangrove and saltmarsh extent scored 39.9 (‘poor’) with 31.1% loss from pre-clear to 2013. 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. The riparian extent scored 22.7 (‘poor’) with 48.3% loss from pre-clear to 2013.
These results reflect the high level of development within the estuary area. The low mangrove and saltmarsh extent and riparian extent scores for the Barron are mostly the result of historic extensive clearing activity. More recently mangrove communities in the Barron estuary have been effectively managed to ensure there has been no major new clearing and to allow for some revegetation.
For estuary fish barriers, the Barron estuary scored 61 (‘good’) and is graded ‘good’ for barrier density, ‘poor’ for percentage of stream length to the first barrier, and has an absence of low passability barriers (graded ‘very good’).
The Barron estuary scored 59 for flows and was graded as ‘moderate’ during 2016-17. Flows were assessed in the Barron River at Myola and in Freshwater Creek. The Barron scored the lowest for flow of all three estuaries for which flow was assessed. The low score was attributable to Freshwater Creek which had flows substantially altered from predevelopment conditions and included changes in the duration and the frequency of medium flows and the duration of low flows.
Such changes to flow can adversely affect riffle habitats, macrophyte beds, low flow spawning fish, critical hydraulic habitat, longitudinal connectivity and water quality (Stewart-Koster et al. 2018). The altered flows are likely to result from the water resource development of Freshwater Creek which serves as a water supply for the Cairns area, with Copperlode Dam and water extraction infrastructure located upstream of the flow assessment site.
Further information on the flow indicator is available in the full report (Stewart-Koster et al. 2018).
The fish index is currently under development and will be available in future report cards.
References:
Stewart-Koster, B., Bofu Yu, B., Balcombe, S., Kennard, M., Marsh, N. 2018 Development of Report Card flow Indicators for the Mackay-Whitsunday and Wet Tropics regions. Australian Rivers Institute, Griffith University and Truii Pty Ltd. Brisbane.