State of the Derwent Report Card 2021

Today, approximately 40% of Tasmania’s population — 227,000 people — live around the estuary’s margins. The Derwent is widely used for swimming, boating, fishing, marine transport and industry. Further upstream, the River Derwent supplies about 60% the region’s drinking water and is a major source of hydroelectric power.

A number of environmental issues affect the Derwent estuary, in particular:

• heavy metal contamination
• poor recreational water quality at some bays and beaches
• low oxygen levels in the upper estuary during summer
• elevated nutrient concentrations
• environmental flows and barriers
• introduced marine pests and weeds
• loss of habitats and species
• impacts of climate change, e.g. sea level rise, erosion and habitat loss

Although there have been significant improvements in the treatment of sewage and industrial wastes over the past decade, the Derwent still faces a number of environmental challenges. A strategic and coordinated planning approach across all levels of government, industry and the community is our best hope for a clean and healthy estuary in the future.

The Derwent Estuary Program (DEP) was established in 1999 as a partnership to share science for the benefit of the community, nature and the economy. The program has been successful in bringing together a wide range of stakeholders — firstly to build a common understanding, vision and management framework — and secondly to progressively implement this vision through partnership agreements and practical actions.

The program was initially designed to address environmental quality issues such as industrial and urban water pollution, contaminated sediments, invasive species and loss of estuarine ecosystems. More recently, our scope has broadened to include the catchment and channel influences, as well as education and amenity. Key program areas include environmental monitoring and reporting, coordination of regional activities, stormwater management, heavy metal investigations, wetland, saltmarsh and seagrass restoration, and promotion of walking tracks.

Our program partners

The DEP, a not-for-profit company limited by guarantee, is supported by the Tasmanian Government, six councils that border on the estuary (Brighton, Clarence, Derwent Valley, Glenorchy, Hobart and Kingborough Councils), five business partners (Nyrstar Hobart, Norske Skog Boyer, TasWater, TasPorts and Hydro Tasmania) and research partner Institute for Marine and Antarctic Studies, University of Tasmania. Other project supporters include the CSIRO and NRM South.

A fundamental requirement for effective natural resource management is an on-going and reliable source of environmental data. This principle forms the basis of the DEP’s cooperative monitoring program between the state government, councils, industries and research institutes. Formerly independent monitoring programs are now coordinated so as to provide better information on the estuary as a whole, and to report annually on environmental conditions and trends in the Derwent.

This ‘Report Card’ summarises monitoring data collected by the DEP and our partners, as well as other relevant information collected during 2020 and early 2021, including:

• weekly recreational water quality testing during summer months
• monthly whole-of-estuary water quality monitoring
• surveys of heavy metal levels in fish and shellfish
• biological surveys (rocky reefs, spotted handfish, little penguins)
• weed surveys and control actions (rice grass, karamu)

More detailed information is published in five-yearly State of the Derwent Estuary reports, available on our website.

Industry

Industries have historically been the main source of heavy metal pollution to the Derwent, however loads have declined significantly in recent years.

Contaminated groundwater at Nyrstar is now the largest remaining source, and is being captured and treated using a series of innovative projects. In 2020, over 80 tonnes of zinc and other metals were captured. Elevated recovery of zinc in 2016 was a result of higher flows and a greater volume processed.

	2016	2017	2018	2019	2020
Cadmium	3	2.5	2.3	2	1.7
Zinc	148.9	84.6	85.4	82.8	84.1

Sewage

Sewage treatment plants are the largest source of bioavailable nutrients, followed by the catchment, stormwater and the Norske Skog paper mill.

Effluent reuse turns a waste product into valuable, nutrient-enriched irrigation water, removing nutrients that would otherwise enter the Derwent estuary. In 2020, the volume of sewage effluent reused was similar to the previous year and accounts for approximately 20% of the regional sewage generated.

	2016	2017	2018	2019	2020
Bridge­water	537.612	759.177	668.024	790.373	622.949
Came­ron Bay	31.673	93.731	117.798	121.404	84.382
Rokeby	762.892	730.79	766.29	821.501	855.506
Rosny	1540.692	1225.414	1489.343	1623.25	1305.063
Selfs Point	104.949		12	24	24

Pathogens

Pathogens — usually measured indirectly using faecal indicator bacteria — can be derived from overflows or leaks from the sewerage network, as well as animal faeces associated with stormwater and rural run-off. Urban stormwater accounts for most of the sediment and litter that enters the Derwent, with unmanaged erosion from construction sites a particular concern.

Catchment

Our understanding of river water quality will be boosted with a three-year trial of real-time water quality analysers in the River Derwent. Funding from The Ian Potter Foundation will allow us to install real-time water quality analysers in six locations across the catchment.

The new analyser technology was originally developed by Prof. Michael Breadmore’s team at the University of Tasmania and engineered for commercialisation by Eco Detection and Grey Innovation. The new analyser technology will measure nutrients as they have the potential to cause undesirable excessive algal growth including toxic algal blooms.

The analysers work in near real-time, meaning they take measurements every 4–6 hours automatically. Data is instantly sent to the internet Cloud where it can be accessed from anywhere, anytime on mobile devices or computers to enable fast and improved decision making.

The first analyser was installed late 2021 at the Turriff Lodge Wastewater Treatment Plant, operated by TasWater. The remaining five analysers will be installed early in 2022. Data summaries will be provided in the 2022 Report Card.

Total rainfall in 2020 (656.2mm) was above average (612.2mm; all records since 1893) as recorded at the Bureau of Meteorology (BOM) weather station, Hobart (Ellerslie Road). All summer months (December–March) in the 2020–21 season experienced above average rainfall, except January 2021.

Month	2020/2021 rainfall (mm)	133 year average (mm)
July 2020	11.6	51.8
August 2020	103.8	54.2
September 2020	35	52.8
October 2020	122	61.9
November 2020	12.2	54.1
December 2020	72.2	56.3
January 2021	28.9	46.9
February 2021	52	39.4
March 2021	59.6	44.7
April 2021	23.8	50
May 2021	12.6	47
June 2021	59.8	53.8

Each summer recreational water quality is monitored at about 42 beaches and bays around the estuary through a collaborative State and Local Government program. Sampling is conducted weekly from December through March at the locations shown in our sampling and discharge points map.

To describe the risk level to swimmers a colour coded system is used based on five years of monitoring data: green indicates ‘Good’, yellow indicates ‘Fair’, and red indicates ‘Poor’ water quality.

Following the 2020–21 season, of the 20 swimming sites sampled, eight are now graded as ‘Good’, which is two less than last year. Seven sites are graded as ‘Fair’, an increase of one from the previous season. One site is graded as ‘Poor’ — there were no ‘Poor’ swimming sites last season.

Of the 22 environmental sites, nine sites are graded as ‘Good’, three less than the previous season. Seven sites are graded as ‘Fair’, three more the previous season. Five sites are graded as ‘Poor’, one more than the previous season.

Rainfall records this season were higher than the long-term summer average for the four BOM weather stations monitored as part of the RWQ program. There were six days throughout the summer with > 10 mm of rain recorded somewhere in the estuary, including some very heavy downpours above 40 mm. However, only a few of these rainfall events occurred within a 24-hour period prior to sampling. Analysis shows that while some results probably were influenced by rainfall, most were not.

The DEP coordinates a whole-of-estuary monitoring program that integrates sampling carried out by the DEP and EPA Tasmania, Nyrstar Hobart, Norske Skog and TasWater. Water quality is monitored each month at 27 sites for indicators such as temperature, salinity, dissolved oxygen, suspended solids, nutrients, organic carbon, chlorophyll a and heavy metals. This information is used to document conditions and trends over time and to provide data for estuarine modelling and process studies.

Seasonally recurrent hypoxia reduces the productivity of 100 hectares of the upper Derwent estuary every summer around Sorell Creek and New Norfolk. Low oxygen levels can result in the release of nutrients and heavy metals from underlying sediments, and can also have adverse impacts on bottom-dwelling organisms and some species of fish.

The hypoxic zone bathymetry is characterised by a sequence of troughs and sills which results in a dynamic relationship between local water quality and river discharge. River flow is a key factor influencing hypoxia dynamics. In winter, sustained high river discharge displaces the hypoxic salt wedge from this area. In summer, with the onset of markedly lower river discharge, the hypoxic zone becomes strongly stratified and during static conditions, oxygen becomes depleted over a period of about 5 days. Temporary oxygen recharge occurs with sustained higher river discharge that is sufficient to displace the salt wedge beyond the easternmost shallow sill that is located just southwest of the motorboat club. Oxygen recharge also occurs when reduced river discharge relieves hydrostatic pressure, allowing oxygenated saltwater to migrate westerly over the sills. This water is saltier than the resident waters so as it spills over the sills and sinks into the hypoxic troughs and mixes the resident waters, resulting in increased salinity and temporary reoxygenation.

Dissolved oxygen levels were uncharacteristically high in 2020. Unlike previous years where hypoxia was severe throughout the summer, dissolved oxygen levels, for the most part, remained high throughout the year. These results are likely a due to above average rainfall in 2020 and resultant high flows in the River Derwent.

Month	Dissolved oxygen (% saturation)
July 2020	99.6
August 2020	101.8
September 2020	102.7
October 2020	98.1
November 2020	23.4
December 2020	96
January 2021	11
February 2021	1.6
March 2021	8.6
April 2021	92.3
May 2021	95.5
June 2021	97.3

Levels of heavy metals in Derwent estuary sediments are among the highest in Australia. Derwent sediments tend to be fine-grained and organic-rich and significantly exceed national sediment quality guidelines for zinc, copper, mercury, lead, cadmium and arsenic.

A University of Tasmania Honours study revisited metal levels in sediment, in addition to nutrient levels, with eight sediment cores collected over the summer of 2019–2020. Core U2 was taken from a site adjacent to the zinc smelter, and zinc concentrations in the surface sediment have decreased to just 13% of the recorded historical maximum at this location (2927 mg/ kg at 0.5 cm depth compared to 21840 mg/kg at 38.5 cm depth). The zinc and lead concentrations are not back to background levels at any middle estuary site, but the steady decreases demonstrate the successful reduction in metal contaminated effluent into the estuary since the 1970s.

Zinc concentrations in sediment core G2 (mid-estuary) versus sediment depth and age. Initial zinc concentration increase above background levels around 1910, and zinc concentration peaks around 1970.

Middle depth (cm)	Zn (ppm)	Year (CFCS model)
0.5	1600	2019
1.5	1550	2017
2.5	1920	2015
3.5	2100	2013
4.5	2160	2012
7.5	2370	2006
9.5	2650	2002
15	2810	1990
21	3220	1977
31	814	1954
41	278	1926
51	95.2	1892
63	82.2	1829

Stevens, H., Chase, Z., Zawadzki, A., Wong, H., and Proemse, B.: Reconstructing the History of Nutrient Loads and Sources in the Derwent Estuary, Tasmania, Australia, using Isotopic Fingerprinting Techniques. Estuaries and Coasts 44, 2236–2249 (2021). https://doi.org/10.1007/s12237-021-00919-0

The Derwent estuary is extensively colonised by introduced marine species. At least 79 invasive species have been recorded, including four species of particular national concern: northern Pacific seastar, European green crab, Japanese seaweed, and European clam. A number of other species (e.g. New Zealand half crab, New Zealand seastar, and New Zealand screw shell) also pose a significant threat to the ecology of the estuary.

Rice grass (Spartina anglica) — an invasive intertidal weed — has been successfully managed in the Derwent estuary through annual surveys and control actions. The area of infestation has been reduced from large “swards of over 100 m²” in 1995 to finding no plants in 2009 and 2010. Small patches have been observed since then in the middle estuary region, but none since spring 2016. Due to its invasive nature, the surveying for rice grass will continue. The rice grass survey also provides the opportunity to survey and inform land managers of other weeds and litter infesting the mid-estuary. A Derwent region weeds group has been established by the DEP with the aim of identifying emerging weed threats, sharing knowledge and resources.

Karamu (Coprosma robusta) is an evergreen shrub originating from New Zealand that is a declared weed under the Tasmanian Weed Management Act 1999, requiring landholders to remove it from their property. The upper Derwent estuary had previously been identified as the largest infestation in Tasmania. While much has been eradicated over the past five years, a large dense infestation remains around New Norfolk. Active and ongoing management of Karamu, led by the Derwent Catchment Project and a new management plan, includes participation from Department of State Growth, Parks and Wildlife Service, NRM South, DEP, Property Services Tasmania, and the Derwent Valley Council. In 2021 additional funding from the Weed Action Fund’s Large Grant was provided to the Derwent Catchment Project to help tackle the core infestation at New Norfolk that is proving particularly dense and difficult to access.