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.
The Barron estuary grade has remained ‘moderate’ and the score has decreased slightly to 54 in 2017-18 from 55 in 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.
The Barron estuary water quality remained ‘good’ with the score increasing to 66 from 64 in 2016-17. The lowest scoring indicator was chlorophyll a (38) and graded moderate. Nutrients (dissolved inorganic nitrogen and filterable phosphorus) were ‘moderate’ whilst dissolved oxygen and turbidity were both ‘very good’.
Pesticides were graded “very good” and scored 87, equating to 99.4 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. Except for pesticides the results were derived from a transect of five monitoring sites in the channel of the Barron River. Pesticides were monitored on the Barron River at the same Great Barrier Reef Catchment Loads Monitoring Program site as used for 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 due for updates in 2018-19) and fish barriers (update due for 2019-20). The habitat and hydrology score decreased from 45 in 2016-17 to 43 in the current period, due to the change in the flow indicator score, and was graded ‘moderate’.
The mangrove and saltmarsh extent scored 39 (‘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 (‘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 49 for flows and was graded as ‘moderate’ during 2017-18. 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 substantially altered flows from predevelopment conditions at Myola on the Barron River and for Freshwater Creek. Flow alteration included changes in the duration of low, medium and high flows and the proportion of flow during dry months. Such changes to flow can adversely affect riffle habitats, macrophyte beds, low flow spawning fish, critical hydraulic habitat, longitudinal connectivity, water quality and fisheries production (Stewart-Koster et al. 2018).
The altered flows are likely to result from the water resource developments of Tinaroo Falls Dam on the Barron River and Copperlode Dam on Freshwater Creek, both of which serve as major water supplies and are located upstream of the flow assessment sites.
The fish index is currently under development and will be available in future report cards.
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.