State of the Derwent Report Card 2023

Introduction

Timtumili minanya is the palawa kani name for the Derwent Estuary. Palawa kani is the reawakened language for Tasmanian Aborigines, revived through extensive linguistic and historical research by the Tasmanian Aboriginal Centre. 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 per cent of Tasmania’s population — 235,000 people — live around the estuary. People enjoy swimming, boating, and fishing throughout the Derwent. It is also important for marine transport and industry. The Derwent supplies about 60 per cent of the region’s drinking water and is a major source of hydroelectric power.

Several 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 many 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 Management and restoration

The Derwent Estuary Program (DEP) was established in 1999 as a partnership to share science and enable informed decisions about the Derwent. The program successfully unites a wide range of stakeholders in building a common understanding, vision, and management framework and implementing this vision through partnership agreements and practical action.

The program was designed to address environmental quality issues such as industrial and urban water pollution, contaminated sediments, invasive species and loss of estuarine ecosystems. Our scope has now broadened to include the catchment influences, education and community enjoyment. 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 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.

1.2 Environmental monitoring and reporting

A fundamental requirement for effective natural resource management is an ongoing 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 2023 and early 2024, including:

• weekly recreational water quality testing during summer months
• monthly whole-of-estuary water quality monitoring
• biological surveys (seagrass, 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.

4.1 River Derwent catchment

The River Derwent is a significant source of contaminants in the estuary. Previous monitoring has indicated that primary sources of nutrients in the catchment are diffuse sources (agriculture, forestry) and point sources (aquaculture). Early warning signs of nutrient stress, including the growth of algae leading to taste and odour issues in Hobart’s water supply and filamentous algal blooms smothering seagrass in the upper estuary, have raised concerns about the water quality of the River Derwent upstream of New Norfolk.

Nutrients in the River Derwent catchment are typically variable, and in the 2023 financial year, loads were generally lower than in the previous financial year (see Fig. 4.3). Total suspended solids declined to below the historic average (see Fig. 4.6). 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. 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 more accurately and understand nutrient dynamics. An overview of this project is provided in River Derwent catchment – water quality section.

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 and the larger catchment inputs carried by the Derwent and Jordan rivers. Other diffuse pollutant sources include air pollution, landfills, aquaculture operations, and ports and marinas waste. Sediments within the estuary may also release pollutants into the overlying waters under certain conditions.

See the point sources map for more details.

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 (see Fig. 4.4). The past two financial years have been characterised by increased loads discharged to the estuary from point sources, particularly STPs. Whilst nutrient loads are greatest in the mid and lower estuary, smaller plants that discharge into poorly mixed waters or nearshore environments may be of comparably greater ecological significance due to the sensitivity of the receiving environment (e.g. seagrass).

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 demand (see Fig. 4.1). During prolonged rainfall events, private water storages reach capacity, and demand for recycled water decreases, resulting in greater discharge. Reduced reuse is also a result of site-specific issues, such as increased discharge at Rosny STP due to ongoing saltwater intrusion.

Norske Skog

Norske Skog Boyer has been operating since 1941, producing newsprint, speciality newsprint and lightweight coated papers. Key contaminants from the site concerning 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 significantly reduced suspended solids, biological oxygen demand and resin acids discharged to the upper estuary. However, an increase in dissolved nutrient discharge was evident in the Norske Skog Boyer combined effluent stream in 2023, as the addition of nutrients is required to sustain the biological secondary treatment process.

Nutrient loads, particularly dissolved nutrients, vary significantly due to challenges in finding the correct nutrient dose to operate the treatment process. The organic load coming into the SETP strongly influences the required nutrient levels.

Nyrstar

The Nyrstar Hobart zinc smelter has been operating since 1917 and has been the main source of heavy metal pollution to the Derwent (see Fig. 4.7). Metal pollution, particularly zinc has declined but is highest in surface waters in New Town Bay and other mid-estuary sites. Concentrations typically decline with distance from the Nyrstar Smelter.

Contaminated groundwater at Nyrstar is the largest source and is being captured and treated using a series of innovative projects. Zinc extracted from groundwater increased from an annual average of 80 tonnes (before installation of the 2021 groundwater extraction system) to 104 tonnes in 2022–23. Zinc extraction declined by 16 tonnes from the previous year as an unreliable pump meant an extended downtime for one of the major extraction systems (see Fig. 4.2).

Stormwater

Stormwater starts as rain and eventually makes its way to the River Derwent via gutters, pipes and rivulets. Thirteen major rivulets and over 270 pipes discharge stormwater to the estuary, along with a variety of pollutants including litter, soil, fertilisers, bacteria and hydrocarbons.

Regional stormwater monitoring programs have found that levels of some pollutants regularly exceed national water quality guidelines, and in some cases may result in beach closures after heavy rain. To better understand the contribution of stormwater pollution to the estuary, the DEP are repeating a monitoring program with funding from the Australian Government’s Urban Rivers and Catchments Program. Thirty-one sites will be monitored monthly for 12 months to help us quantify pollutants and prioritise catchments for management.

5.1 Climatic conditions

Rainfall is reviewed annually as it can influence water quality. The 2023 financial year’s annual rainfall (599.8 mm) was close to the long-term average (612 mm; all records since 1893) as recorded at the Bureau of Meteorology (BOM) Hobart weather station (Ellerslie Road).

Monthly rainfall tended to be below average in summer and above average in winter-spring.

The total summer rainfall (December–March inclusive) during the 2022–23 season (170 mm) was slightly below average (187 mm).

5.2 Swimming in the Derwent

The Beach Watch Program is a collaborative effort with local and state governments, which monitors recreational water quality at 38 beaches and bays around the estuary each summer. Sampling is done weekly from December through March at the locations in our sampling and discharge points map.

A colour-coded system based on five years of monitoring data describes the risk level to swimmers: green indicates good water quality, yellow indicates fair water quality, and red indicates poor water quality.

The water quality at most swimming sites was overall better than the last two seasons, largely because it was a dry summer for Tasmania, with summer rainfall down about 16 percent below the long-term average. This season saw only 16 exceedances (enterococci > 140 MPN 100 mL^-1), compared with 22 last summer and 49 the previous season.

At the end of this season, eleven swimming sites were graded as Good, six as Fair, and two as Poor. Four sites improved their rating: Howrah Beach (east), Nutgrove Beach (east and west) and Taroona Beach from Fair to Good.

Four sites received a long-term rating for the first time (now having five years of data). Two of these newly rated sites are Good, Bellerive Beach (east) and Blackmans Bay Beach (north), while Kingston Beach (south) is Fair and Blackmans Bay Beach (south) is Poor.

The water quality at the 19 environmental sites also significantly improved from the previous season. On 24 occasions, enterococci results of over 140 MPN 100 mL^-1 were recorded, compared to 56 times last year. There are six environmental sites with Good long-term ratings, eight Fair, and five Poor. One site, Kangaroo Bay (from Good to Fair) dropped its rating. Following this summer’s sampling, four sites improved their rating, including Victoria Dock, New Town Bay, Marieville Esplanade and Regatta Pavilion, all from Poor to Fair. The mid-river Derwent sampling location continues to be the environment with the consistently best water quality, followed by Montagu Bay, Elwick Bay and now Old Beach and Sullivans Cove.

5.3 Water quality indicators

We coordinate a whole-of-estuary monitoring program that integrates sampling by the DEP and EPA Division, Nyrstar Hobart, Norske Skog and TasWater. Water quality is monitored monthly at 29 sites for indicators such as nutrients, metals, and physicochemical parameters. This information is used to document Derwent Estuary’s health and trends over time.

5.4 Nutrients

Nutrients are essential for plant and algal growth and function. However, elevated nutrients can cause several water quality problems, particularly excessive algae growth. Excessive algae growth does occur at times in the Derwent Estuary. It can have adverse effects, including phytoplankton blooms (microscopic marine algae), smothering of seagrass meadows, and excessive seaweed growth. This can change native habitats, and the low oxygen levels that occur when algae die off can kill fish and affect aesthetic appeal due to foul odours and water discolouration.

Seasonal overview

Nutrient inputs vary seasonally and spatially throughout the Derwent Estuary and we can see strong seasonal patterns. Marine inputs increase in winter when high rainfall carries 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 due to increased productivity. 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.

Geographical patterns

Nutrient concentrations are typically low in the upper estuary, though nutrient concentrations close to the floor of the estuary (bottom waters) are often elevated during summer when dissolved oxygen is low (see Fig. 5.2). Nutrients are most significant in the mid-estuary, with a higher density of point and diffuse sources. This is most evident in poorly flushed bays in the mid-estuary, such as Prince of Wales Bay, where nutrient levels are the greatest. Nutrient concentrations are typically low in the lower estuary, particularly in surface waters.

Fig. 5.2 Spatial variation in nitrogen for surface and bottom waters

Spatial variation in nitrogen for surface and bottom waters collected from Derwent Estuary sites in the 2023 financial year. The limit of the boxes represents the 20th and 80th percentiles. Functional zones are coloured coded where upper estuary is light blue, mid-estuary is green, mid-estuary bays are yellow, lower estuary sites are dark blue, and Ralphs Bay is purple. The red dashes mark each zone’s DRAFT Derwent Estuary default guideline values (DGVs).

Changes over time

Upper Derwent nitrogen levels have gradually increased over the last 15 years (see Fig. 5.3). Declines in water quality have coincided with signals of nutrient stress in the upper estuary in 2015, including taste and odour issues in the Hobart drinking water supply and algal blooms smothering seagrass meadows. Over the last two years, there has been a notable increase in nitrogen in the upper estuary, which has coincided with increased loads from local point sources (Norske Skog Boyer, Turriff Lodge STP and Green Point STP) and increased River Derwent catchment loads.

Fig. 5.3 Monthly total ammonia nitrogen (TAN) concentrations (µg/L) at Prince of Wales Bay monitoring site

Ambient nutrient concentrations are typically highest in the mid-estuary, where the density of STPs is greatest (see Fig. 5.2). Recent data has shown that previously reported zone-wide declines (mid-estuary) in dissolved nitrogen likely reflected a change in the process rather than an improvement in effluent quality.

Over the past two years, upward trends in water quality parameters have been observed throughout the estuary. This trend is most apparent in surface waters, particularly upper estuary sites, though the signal is clear throughout the estuary. This trend has likely been driven by increased discharges from point sources (particularly STPs) and diffuse sources (stormwater contributions are unquantified) as a result of high annual rainfall during this period.

5.5 Estuary metal contamination

Heavy metals persist in the environment and can become toxic to marine life at elevated concentrations. Some metals also bioaccumulate to high levels in fish and shellfish — a risk for human consumers. Metals in Derwent Estuary sediments are among the highest in Australia. Derwent sediments are fine-grained and organic-rich and significantly exceed national sediment quality guidelines for zinc, copper, mercury, lead, cadmium and arsenic.

Zinc concentrations have continued to decline throughout the monitoring program in Derwent Estuary waters. Declines have occurred in bottom and surface waters and are most pronounced in the mid-estuary. These declines are consistent with sediment core work, indicating a continued decline in metals in Derwent sediments since peak production in the 1970s. Observed declines are likely a result of proactive site remediation and improved management practices, combined with gradual burial of contaminated sediment with clean fill.

As reported for nutrient indicators, zinc concentrations in surface water have also gradually increased in recent years. These increases have been most notable at New Town Bay. This has occurred despite increased zinc recovery from groundwater and no change in effluent discharge. This trend will be further investigated in 2024.

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).

Seagrasses

Brackish water aquatic macrophytes, referred to here as seagrass beds, cover an area of over 6 km² of the mid-upper Derwent Estuary and are mostly continuous and dense (see Fig. 6.1). This highly productive system supports many essential ecosystem services such as providing habitat, nutrient cycling, water column filtering, bank stabilisation and blue carbon storage. At times (spring/summer), these beds can be covered by a growth of dense epiphytic algae, which can result in the decline of the beds. In 2011, a seagrass survey found the upper Derwent seagrasses were in good condition, with less than 10% of seagrasses covered by algae. However, algal smothering was observed in the summer of 2015–16, where up to 90% of beds were covered with epiphytic algae, prompting further, regular monitoring.

Many inputs are likely to impact seagrass bed conditions in the mid-upper estuary. Increased turbidity (total suspended sediments) from catchment runoff reduces light reaching seagrass, particularly in winter. Epiphytic algal smothering events also influence low-light penetration. Algal growth is linked to increased primary productivity in the water column as a result of diffuse nutrient enrichment from industry, sewage treatment plants or STPs, and the catchment.

Surveys were recorded at four seagrass beds at the boundary of the upper and mid-estuary zones (mid-upper estuary). Thirty sites at each meadow were monitored from 2015 to 2023 using a GoPro mounted to a photo quadrat, and the images were scored for percentage cover.

Fig. 6.1 Seagrass survey location map

A review of seagrass data from 2015 to 2023 showed that:

• Seagrass health varied between sites, with Granton Banks typically having the greatest cover.
• At the regional scale (all sites), conditions were greatest in 2018 and 2023.
• Seagrass beds are resilient and can recover from increased stressors and even dieback.
• Jordan River has improved significantly in recent years.
• Murphys Flat tends to be the first site impacted and a good indicator site for pressure from algal bloom events.

Fig. 6.2 Seagrass health at the mid-upper estuary seagrass beds

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Seagrass images collected at the mid-upper estuary seagrass beds during the 2023 financial year. Murphys Flat, Dromedary Marsh, Granton Bank and Jordan River on three sampling dates (September 2022, January 2023 and June 2023).

Little Penguins in the Derwent Estuary

We manage monthly monitoring of Little Penguin colonies within the Derwent Estuary in collaboration with the Penguin Advisory Group (PAG), local councils, UTAS scientists and volunteers. This routine monitoring remains the most rigorous long-term data set of Little Penguins nesting in Tasmania. It is an important data set utilised by researchers and helps land managers (including local councils) continuously improve conditions for Little Penguin that supports breeding success.

The population numbers reported below are estimates from the number of observed occupied nests, i.e. nests with either eggs, chicks or adult birds, multiplied by two to signify a breeding pair of Little Penguins while also avoiding counting the same chicks twice. The population estimates do not include any data from the eastern shore of the Derwent and other sites around the estuary. Therefore, the reported figures underestimate the actual estuary Little Penguin population size.

For the 2022–23 financial year, the estimated population of Little Penguins at monitored colonies within the Derwent Estuary was 252. This estimate is much lower than the previous financial year’s estimate of 537. The lower estimate is due to data collection at the largest colony only occurring in September 2022, October 2022 and October 2023. The plot below shows the significant increase in population estimates for these months relative to the other months where this colony was not monitored. No monitoring was conducted in March 2023; hence, there are no population estimates. This highlights the issue of monitor resourcing and availability, which has recently been an issue for the PAG. The PAG is working to increase the volunteer base to assist with this routine monitoring program.

Eggs were observed at the colonies in late winter and early spring, with the highest number of eggs across the colonies observed in September 2022 and October 2023. Chicks were seen at the colonies almost all year round in the 2023 calendar year, with all months where monitoring occurred recording chick observations except for January. This shows that breeding occurs across all seasons at the Derwent Estuary Little Penguin colonies.

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 in 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.

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 national concern: northern Pacific seastar, European green crab, Japanese seaweed, and European clam. Several 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 — an invasive intertidal weed — has been successfully managed in the Derwent Estuary through annual surveys and control actions. The area of infestation was reduced from two hectares in 1995 to zero in 2009 and 2010. Surveys in 2015 found several small patches in the middle estuary region; however, since spring 2016, no rice grass has been observed in the Derwent Estuary. Previous ‘hotspot’ locations are monitored annually, but the 2023 survey found no rice grass.

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.

We are involved in the active and ongoing management of Karamu in collaboration with the Derwent Catchment Project, Department of State Growth, Parks and Wildlife Service, NRM South, 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, which is particularly dense and challenging to access. In this reporting period treatment has been focused on follow-up control, mainly targeting new saplings, with less than 10 established plants found. Future work will target blackberry and willow to reduce the habitat for the karamu to grow on (willow trunks) and ‘clean’ sites to minimise the effort of looking for karamu and treating it.