Barron Estuary

WATER QUALITY

Water quality in the Barron estuary declined from ‘good’ to ‘moderate’ with the score decreasing from 70 to 46.

The lowest scoring indicators were Chlorophyll a, which declined from ‘good’ to ‘poor’ and DIN (dissolved inorganic nitrogen) which declined from ‘moderate’ to ‘poor’, with monthly median values for both indicators often not meeting guideline values.

FRP (filterable reactive phosphorus) and dissolved oxygen (low) both declined from ‘good’ to ‘moderate’ with monthly median values sometimes meeting guideline values.

Turbidity declined from ‘very good’ to ‘good’ with monthly median values often 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 and shoreline habitat updated in 2021-22), riparian extent (updated in 2021-22) and fish barriers (updated in 2021-22). Habitat and hydrology remained ‘moderate’ and the score was unchanged on 54.

Mangrove and saltmarsh scored 57 (‘moderate’). Shoreline mangrove habitat scored 73 (‘good’) whilst mangrove and saltmarsh extent scored 42 (‘moderate’) with 29% loss from pre-clear to 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 extent scored 22 (‘poor’) with 48% 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.

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 declined from ‘good’ in the previous assessment to ‘moderate’ and scored 60. It was graded ‘poor’ for barrier density, ‘moderate’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’). The change in score and grade was due to identification of additional barriers from the more comprehensive survey and improved stream mapping which identified additional priority waterways in the estuary zone.

The Barron estuary score for flows remained ‘good’ and the score improved from 75 in 2020-21 to 79. Flows were assessed in the Barron River at Myola and in Freshwater Creek. Flows to the estuary 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. During previous years with less rainfall, Freshwater Creek has scored lower, 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 during drier years 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.

See the detailed reports for further information on the methods used to produce the scores and grades and the detailed results.