Effects of terrestrial runoff on the water clarity in the Burdekin Region: 10-years of remote sensing data give new insights

Effects of terrestrial runoff on the water clarity in the Burdekin Region: 10-years of remote sensing data give new insights

Spear fishers have known it all along: water on inshore reefs can be pretty murky - especially early in the year, and especially after big floods. But the time scales and processes leading to changes in water clarity have not been scientifically documented to date.

Maintaining water clarity is very important for the health of the Great Barrier Reef. Corals, seagrasses and algae all depend on light, which diminishes if the water gets cloudy. High water clarity is a prerequisite for low seaweed cover, high inshore coral diversity, and high numbers of young corals to settle and replenish coral populations. Water clarity is therefore an important measure that is being used in the GBRMPA Water Quality Guidelines.

NERP researchers from the Australian Institute of Marine Science investigated how GBR water clarity is affected by river runoff. Tropical Ecosystems Hub Project 4.1 is a study based on two independent types of data.

The first study was a collaboration with the Reef Rescue co-funded AIMS Marine Monitoring Program, who maintain turbidity loggers on inshore reefs up and down the whole GBR. The researchers found that water clarity was up to 10-fold higher on inshore reefs away compared to those close to river mouths. They also showed that inshore water clarity was significantly affected by river flow and rainfall, once resuspension by waves and tides were accounted for. On the more river-exposed reefs, water clarity was high towards the end of the dry season but was reduced by over 40% on average at the end of the wet season.

The second study is a collaboration with UQ and JCU, using a 10-year time series (2002-2012) of daily satellite data of water clarity in the Burdekin Region. The researchers found a very strong relationship between annual Burdekin River loads of particulate phosphorus, and mean annual water clarity in the central GBR, an area of about 25,000 km2. Water clarity of the inshore, lagoon and midshelf, i.e. about 12 to 80 km from the coast, rapidly declined with the onset of the river floods (data were again standardised for waves and tides). For the coastal zone that is chronically turbid this relationship was weaker, because sediment on the seafloor is readily available for resuspension all year round. River floods did not affect water clarity in the offshore zone. Throughout the study region, mean annual water clarity was 20% lower in wet compared to dry years. After floods started, water clarity remained reduced by over 30% for about six to eight months, until gradually returning to clearer baseline values.

So, both studies showed that river loads affected GBR water clarity for significant periods of time every year, and especially after big wet seasons. Importantly however, the studies also showed the capacity for GBR water clarity to recover to clear levels within six to eight months after river floods. The research therefore documented that reductions in river loads of nutrients and sediments should measurably improve the GBR water clarity, in particular in inshore, lagoon and midshelf areas. This would provide significant ecosystem benefits, such as increasing coral diversity and less seaweed - and better spear fishing conditions!

Dr. Katharina Fabricius, AIMS

For more information, contact Katharina Fabricius at: k.fabricius@aims.gov.au

Project 4.1: Tracking coastal turbidity over time and demonstrating the effects of river discharge events on regional turbidity in the Great Barrier Reef



Publication Type: 


Search by Project