Collaboration is key to Derwent Estuary Health - State of the Derwent Report Card 2023
Dawning of a new day over Hobart city from kunanyi / Mount Wellington.
The 2023 Derwent Estuary digital report card has been released and demonstrates the collaborative efforts of the Derwent Estuary Program (DEP) and its partners to improve estuary health. Several collaborative initiatives, programs and trials have been successful throughout the year.
One of the great results we’ve shared this year is the improvement of water quality at our local swimming sites and the success of our forecasting trial. Identifying and resolving issues requires a partnership approach, and we are seeing the results this year. Four swimming sites have improved, with Howrah Beach (east), Nutgrove Beach (east and west) and Taroona Beach rising from Fair to Good. Further investigation and monitoring will continue to manage stormwater and sources of contamination, particularly at sites with degraded water quality. Our daily forecast trial was successful, which updated the community with timely advice to help people decide where and when they swim during summer. We look forward to establishing a permanent solution in the future based on community feedback.
The Derwent Estuary Report Card provides yearly insights into the health of the Derwent Estuary. Along with monitoring results for swimming beaches, this report also reveals trends in industrial, sewage, and stormwater discharge, heavy metal levels in water and seafood, and actions to clean up the Derwent.
An increase in nutrients discharged from sewage treatment plants to the estuary in 2023 has been driven by above average rainfall, compromised infrastructure and general increases from high density urban areas.
An increase in nutrients has been observed in the middle and upper estuary indicating increased pressure from catchment inputs. However, a highlight of the year was the in-situ analyser project funded by The Ian Potter Foundation. The analysers have been installed in the catchment to better understand nutrient sources and, calculate loads more accurately.
We continue to observe long-term reductions in zinc in the Derwent which is supported by declines in Derwent sediments. Nyrstar Hobart has demonstrated a strong commitment to preventing contaminated groundwater reaching the estuary. The groundwater remediation systems at Nyrstar have extracted over 300 tonnes of zinc from groundwater in the past three years. Despite continued improvements, some deterioration in water quality within New Town Bay has been observed and together we are committed to understanding why this has occurred.
This report card demonstrates the positive impact of our collaborative efforts with our partners across industry, government, science, and community, their commitment to the Program and the resources they’ve provided, and why continued investment is critical to improving the estuary's health.
Other highlights:
- Real-time water quality analyser trial at the Norke Skog Boyer treatment plant has helped optimise nutrient dosing.
- Rice grass has been successfully managed in the Derwent Estuary, and the 2023 survey found no rice grass in previous hotspot locations.
- This year, we joined a collaborative initiative to develop the Ducks of Tasmania booklet. This popular little booklet helps people identify ducks, learn about their diets and antics, and understand how we can all help keep them safe and healthy.
Heavy Metal Levels in Seafood
Oysters and mussels from the Derwent contain high levels of heavy metals, particularly zinc, lead and cadmium. While levels of shellfish have declined since 2003, they are still far more than national food standards. Mercury levels exceed national food standards in several species of Derwent caught fish — particularly black bream — and, to a lesser degree, flathead and trout.
In 2023, there has been no change in current health advice, which is as follows:
- Don’t eat shellfish collected from the Derwent (including Ralphs Bay).
- Don’t eat any bream from the Derwent (including Browns River).
- Limit consumption of other Derwent-caught fish to no more than two meals/week or one meal/week for pregnant and breastfeeding women, women planning to become pregnant and young children.
The information contained in the annual Derwent Report Card is used by all levels of government, industries and the community to gauge the state of the estuary and to inform actions to restore our waterway.
Tuesday 12 November 2024