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’ in 2019-20 with score increasing slightly from 53 to 57.
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 water quality declined from ‘good’ to ‘moderate’ with the score decreasing from 61 in 2018-19 to 60.
The lowest scoring indicators were DIN (dissolved inorganic nitrogen) which was graded ‘poor’ with monthly median values frequently not meeting guideline values and chlorophyll a graded ‘moderate’ with monthly median values often not meeting guideline values. FRP (filterable reactive phosphorus) scored 80 and dissolved oxygen (low) scored 64. Both were graded ‘good’ with monthly median values often meeting guideline. Turbidity improved from ‘good’ to ‘very good’ with monthly median values mostly meeting guideline values.
Sampling for pesticides was expanded in 2017-18 and 2018-19 in order to populate the Pesticide Risk Baseline, and dropped back to a more routine sampling regime in 2019-20 which did not include the Barron estuary.
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. For years when pesticides were monitored, pesticide samples were those taken on the Barron River at the Great Barrier Reef Catchment Loads Monitoring Program site 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 updated for 2018-19) and fish barriers (update due for 2020-21). Habitat and hydrology remained ‘moderate’ and the score increased from 45 in 2018-19 to 54.
Mangrove and saltmarsh extent scored 39 (‘poor’) with 31.1% loss from pre-clear to 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 extent scored 23 (‘poor’) with 48% loss from pre-clear to 2017. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2013 and the updated assessment for 2017, however the updated Regional Ecosystem mapping (Version 5) resulted in a 0.3% reduction of the estimated loss of riparian extent from pre-clear which was set at 48.3% for 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 was graded ‘good’ for barrier density, ‘poor’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’).
The Barron estuary score for flows improved from 57 in 2018-19, graded ‘moderate’, to 93, graded ‘very good’. Flows were assessed in the Barron River at Myola and in Freshwater Creek. For the Barron, which represents most of the catchment draining to the estuary, flows retained characteristics similar 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. For Freshwater Creek, the flow measures with low scores were for low flow and high flow.
During previous years with similar rainfall patterns, Freshwater Creek has scored low for these flow measures, for example 2016-17. Freshwater Creek serves as a water supply for the Cairns area with Copperlode Dam and water extraction infrastructure located upstream of the flow assessment site. The alteration of flow from modelled pre-development conditions is likely to be a result of lower rainfall, the impoundment and water extraction activity. The key ecological assets of low flow spawning fish, critical hydraulic habitat, longitudinal connectivity and water quality can be affected by a reduction of low flows, and fisheries production can be affected by a reduction of high flows.
Further information on the flow indicator is available in the full report (Stewart-Koster et al. 2018).
Fish
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.