State of the Derwent Report Card 2022

Introduction

The Tasmanian Aboriginal Centre’s (TAC) Language Program has undertaken both linguistics and historical research on the original languages of Tasmania to retrieve and revive Aboriginal language in Tasmania — palawa kani. The name of the River Derwent in palawa kani, is timtumili minanya. Previously, the Aboriginal names for the River Derwent came from G.A. Robinson’s records from the 1830s, where he attempted to give his idea of their sound by dividing words into syllables. The names for the Derwent were noted as: TEETOOMELE MENENNYE, RAY. GHE.PY.ER.REN.NE and NIB.BER.LIN.

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% of 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.

1.1 Environmental monitoring and reporting

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 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 2021, 2022 and early 2023, 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 (seagrass, spotted handfish, little penguins)
• weeds survey and erosion monitoring

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

1.2 Management and restoration

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 action.

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 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), NRM South, EPA Tasmania and research partner Institute for Marine and Antarctic Studies, University of Tasmania. Other project supporters include the CSIRO, The Ian Potter Foundation and EcoDetection.

4.2 Pollution sources

Pollution enters the Derwent Estuary from many sources, commonly referred to as ‘point sources’ and ‘diffuse sources’.

Point sources include sewage treatment plants and large industries, such as the Norske Skog paper mill at Boyer and Nyrstar Hobart zinc smelter at Lutana.

Diffuse sources include stormwater runoff from urban areas as well as the larger catchment inputs carried by the Derwent and Jordan rivers. Other diffuse pollutant sources include air pollution, landfills, aquaculture operations, and wastes associated with ports and marinas. Sediments within the estuary itself may also release pollutants into the overlying waters under certain conditions.

See the point sources map for more details.

River Derwent catchment

The River Derwent is a significant source of contaminants to the estuary. Previous monitoring has indicated that major nutrient sources of nutrients in the catchment are agriculture (in the winter) and aquaculture (in the summer). Early warning signs of nutrient stress including growth of benthic blue-green algae leading to taste and odour issues in Hobart’s main water supply as well as filamentous algal blooms smothering valuable macrophyte communities have raised concerns about the water quality of the River Derwent catchment upstream of New Norfolk.

Loads from the River Derwent catchment are typically variable, however in recent years dissolved nutrient loads have been elevated. River Derwent loads are relatively coarse and are provided as an indicative value of the contribution of contaminant loads to the estuary from the catchment (above Meadowbank). An in situ nutrient analyser project funded by The Ian Potter Foundation has been established in the catchment to better understand nutrient sources, calculate loads and understand nutrient dynamics. An overview of this project is provided in section 5.5.

Sewage treatment plants

Sewage treatment plants (STPs) are the largest source of bioavailable nutrients (nutrients available to living things such as algae), followed by the catchment, stormwater and the Norske Skog paper mill. The 2022 financial year was characterised by an increase in loads discharged to the estuary from point sources, particularly STPs.

Effluent reuse turns a waste product into valuable, nutrient-enriched irrigation water, removing nutrients that would otherwise enter the Derwent Estuary. The last two financial years have seen a significant decline in reuse due to a combination of high rainfall and compromised infrastructure. At the Greenpoint STP (Bridgewater) increased discharge volume and secondary treatment bypass due to heavy rainfall resulted in increased nutrients discharged to the estuary. During prolonged rainfall events, private water storages reach capacity and demand for recycled water decreases resulting in greater discharge.

Norske Skog

Key contaminants originating from the site affecting estuarine water quality are suspended solids, resin acids, organic matter and dissolved nutrients. The commissioning of the biological secondary effluent treatment plant (SETP) in 2008 has been very effective in reducing suspended solids, biological oxygen demand and resin acids. However, increased nutrient and suspended solid discharge was evident in the Norske Skog Boyer combined effluent stream in 2022. This noticeable increase was due to filamentous bacteria outbreaks in the biological secondary treatment plant (SETP) throughout 2021. To better assess and respond to nutrients in the secondary treatment plant a real-time water quality analyser will be installed in 2023 to measure nitrogen including ammonium.

Nyrstar

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 the largest source and is being captured and treated using a series of innovative projects. In the 2022 financial year, zinc extracted from groundwater increased by 50% (120t) due to the recent upgrades of the groundwater extraction system.

5.1 Climatic conditions

Annual rainfall for the financial year (731 mm) was above the long-term average (613mm; all records since 1893) as recorded at the Bureau of Meteorology (BOM) Hobart weather station (Ellerslie Road) (Fig. 5.1). Total spring rainfall (322mm) was the fourth highest on record, well above the long-term average (168.8mm). Summer rainfall (December–March inclusive) during the 2021–22 season (131 mm) was below average (186mm).

5.2 Swimming in the Derwent

Each summer recreational water quality is monitored at about 38 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.

During the 2022–23 Recreational Water Quality (RWQ) season, the water quality at almost all of our 19 swimming sites was significantly better than the last monitoring season, with less than half the number of failures, which occurs when the enterococci results exceed the prescribed trigger level of 140 MPN 100 mL⁻¹ set by the Tasmanian Recreational Water Quality Guidelines 2007 (DoH, 2007), thus requiring retesting. This season saw 22 exceedances, compared with 49 last summer. At the end of this season, five swimming sites were graded as Good, nine sites graded as Fair, one as Poor, with four sites yet to be classified. The sites that improved their rating were Bellerive Beach (west), from Fair to Good, and Howrah Beach (mid), from Poor to Fair. The only site to drop its rating was Hinsby Beach, from Good to Fair. Little Sandy Bay Beach continues to have the consistently best water quality in the program, and Kingston Beach (north) is currently the only site with a Poor rating.

The water quality at the 19 environmental sites was similar to the previous season. On 56 occasions, enterococci results over 140 MPN 100 mL⁻¹ were recorded, compared to 51 times last year. There are now seven environmental sites with Good long-term ratings, two as Fair, and nine as Poor. Two sites dropped their ratings, i.e. Prince of Wales Bay (from Good to Fair) and Victoria Dock (from Fair to Poor). No sites improved their rating following this summer’s sampling. The Mid-river Derwent sampling location continues to be the environmental site with the consistently best water quality, followed by Montagu Bay and now Elwick Bay.

Overall, it was a dry summer for western Tasmania with closer to average rainfall in the east, including in the Derwent Estuary where the first half of December was very wet followed by almost no rain until late January and largely only two rain periods in February and March. While rainfall is a common driver of pollution at our swimming sites, it is difficult to draw definite conclusions between rainfall and enterococci results. Many of this summer’s environmental site failures appear linked with rain, whereas the swimming site failures probably less so.

5.3 Water quality indicators

The DEP coordinates a whole-of-estuary monitoring program that integrates sampling carried out by the DEP and EPA, Nyrstar Hobart, Norske Skog and TasWater. Water quality is monitored each month at 31 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.

5.4 Nutrients

Nutrients are essential for plant and algal growth and function. Plants and algae readily take up nutrients from their environment and their growth can be limited by low nutrient availability. A number of water quality problems are caused by elevated nutrients, particularly the excessive growth of algae. Excessive algal growth does occur at times in the Derwent Estuary and can have adverse effects including blooms of phytoplankton (microscopic marine algae), excessive growth of filamentous algae which may smother seagrass, excessive seaweed growth which can alter native habitats, low oxygen levels when algae die off which can kill fish and diminished aesthetic appeal due to foul odours and discolouration of the water.

The major sources of nutrients to the Derwent include the ocean, River Derwent catchment, STPs, the Boyer paper mill and urban run-off (see contaminant load section). These inputs vary from year to year and throughout the Derwent Estuary.

Seasonal and geographical patterns

Strong seasonal patterns are evident for dissolved nutrients in the Derwent. River Derwent and marine inputs are greatest in winter when high rainfall transports nutrients from the catchment to the estuary, and nutrient rich sub-antarctic waters intrude into the lower estuary. In summer, nutrient levels in surface waters are typically lower. These seasonal patterns are augmented by year-round inputs from sewerage treatment plants and industry sources which result in elevated nutrient levels, particularly in the mid-estuary.

General geographical patterns for nutrients in the Derwent Estuary include:

• Nitrogen and phosphorus values increase downstream towards the estuary mouth;
• Nutrient levels are greater at depth than in surface waters, except for the upper estuary; and
• Dissolved nutrient concentrations are highest in the mid-estuary and upper-estuary bottom waters.

Changes over time

In the upper Derwent Estuary dissolved nutrient levels have increased significantly over the last 15 years. These increases have generally coincided with increased loads from local point sources in recent years (Norske Skog Boyer, Turriff Lodge STP and Green Point STP) and increased River Derwent catchment loads. This increase has also coincided with signals of nutrient stress in the upper estuary in 2015 including taste and odour issues in the Hobart drinking water supply, algal blooms smothering seagrass meadows, and fish kills (juvenile barracouta) due to low dissolved oxygen. To better understand nutrient supply and dynamics from the Derwent River catchment, the DEP commenced a three year project to trial in situ analysers at seven locations in the Derwent River catchment (see River Derwent Catchment – Water Quality).

In the mid-estuary where nutrients are typically greatest, dissolved nutrients have declined significantly. These declines have been observed throughout the mid-estuary and are related to significant improvements of local STP processes, particularly at Prince of Wales Bay (POWB) (Fig. 5.2; Fig. 5.3). However, nutrient levels in the mid-estuary increased in 2022, likely as a result of increased loads discharged from Derwent STPs (see contaminant load section for more information).

Fig. 5.2 Monthly total ammonia nitrogen (TAN) concentrations (µg/L) at Prince of Wales Bay monitoring site

5.6 Estuary metal contamination

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.

Ambient water quality data show a decline in heavy metal levels at most monitoring sites in the estuary, aligning with the most recent survey of metals in estuary sediments.

Please see State of the Derwent Estuary – 2020 Update for more information.

6.1 Estuarine habitat and species

Surveys of the Derwent Estuary indicate that unvegetated, soft-bottom habitats are by far the most abundant habitats in the estuary (86%), followed by seagrass and macrophytes (7%; primarily in the upper estuary), tidal sandflats (6%; primarily in Ralphs Bay) and rocky reefs (1%; primarily in the lower estuary).

6.2 Heavy metals in seafood

Oysters and mussels from the Derwent contain high levels of heavy metals, particularly zinc, lead and cadmium. While levels have declined since 2003 including in the period between 2016–2020, in some areas (i.e. above the Tasman Bridge), they are still far in excess of 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.

6.3 Marine pests, weeds and disease

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) — a highly invasive intertidal weed — has been successfully managed in the Derwent Estuary through annual surveys and control actions. The area of infestation had been reduced from two hectares in 1995 to now possibly zero. No rice grass has been observed since 2016. Monitoring continues.

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 involves the Derwent Catchment Project, Department of State Growth, Parks and Wildlife Service, NRM South, DEP, Crown Land Services, 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.