Hdb Env Chem Vol. 5, Part H (2006): 1–49 DOI 10.1007/698_5_022 © Springer-Verlag Berlin Heidelberg 2005 Published online: 8 November 2005 The Tail of Two Rivers in Tasmania: The Derwent and Huon Estuaries Edward C. V. Butler CSIRO Marine Research, GPO Box 1538, 7001 Hobart,Tasmania, Australia [email protected] 1Introduction................................... 2 2 Broad Environment ............................... 3 2.1 HumanInfluence................................ 8 3 River and Estuary Hydrology ......................... 10 3.1 RiversandCatchments............................. 10 3.2 Estuaries..................................... 12 4 Estuarine Chemistry .............................. 15 4.1 SourceWaters.................................. 15 4.1.1Freshwaters................................... 15 4.1.2Seawaters..................................... 17 4.2 HuonEstuary.................................. 18 4.2.1Inputs...................................... 18 4.2.2DissolvedOxygen,OrganicMatterandSuspendedSolids.......... 19 4.2.3Nutrients,ChlorophyllandMicroalgae.................... 21 4.2.4TraceMetalsandOtherElements....................... 27 4.2.5OrganicContaminants............................. 30 4.3 DerwentEstuary................................. 32 4.3.1Inputs...................................... 32 4.3.2DissolvedOxygen,OrganicMatterandSuspendedSolids.......... 32 4.3.3Nutrients,ChlorophyllandMicroalgae.................... 33 4.3.4TraceMetals................................... 36 4.3.5OrganicContaminants............................. 40 5Conclusions................................... 42 References ....................................... 45 Abstract The Derwent and Huon Rivers are two relatively small river systems in the southeast of Tasmania (Australia). They terminate in estuaries that are very similar in structure and function. Historically, runoff from their adjoining catchments has been very dilute, but coloured by dissolved organic matter. Their location in cool, temperate latitudes results in a maritime climate that is changeable, but delivers regular rainfall, and therefore river flow, throughout the year. Marked seasonal cycles in nutrient lev- els and biological activity are seen in neighbouring coastal waters. Discharge from both rivers does not have the same seasonal signature; it is consistently enriched in dissolved organic matter (including nitrogenous forms) and depleted in inorganic nitrogen and phosphorus. Small variations in chemistry of the riverine end-members seem to ensue 2 E.C.V. Butler from intra-catchment differences in geology, soils and vegetation influenced by localised rainfall patterns. Silicon manifestly displays this behaviour in the Huon system. Both the Derwent and Huon estuaries are drowned river valleys, with a strongly stratified (salt-wedge) water column at their head, tending toward partially mixed at their mouth. They are in a microtidal region. The Derwent estuary conforms to a wave- dominated system; the Huon estuary is intermediate between wave-dominated and tide- dominated. Currents are generally weak for both estuaries (≤ 0.2 ms–1), and weaker in subsurface waters. With the arrival of Europeans two centuries ago, the Derwent and Huon estuary de- parted from the same course. The capital city of Hobart established on the western bank of the Derwent estuary, and the catchment was soon modified by agriculture and tree felling, then by damming for irrigation and hydroelectricity generation. During the 20th century, the estuary became more industrialised, and the urban zone around its shores grew to support a population of 190 000, as did the discharges from these activities to the waterway. Development in the Huon catchment was slower and more constrained. Much less of the catchment was taken up with agriculture, and the population has only at- tained 13 000 in scattered small towns. The only intensive industry currently in the Huon catchment is aquaculture, active in the waters of the lower estuary. However, downstream processing of timber from catchment forests is planned to recommence. The consequences of these disparate recent histories are that the conditions of the two estuaries are very different. The Derwent estuary has had heavy inputs of organic matter, suspended solids (wood fibre), nutrients, heavy metals and other toxicants. Of all these, the metals (especially zinc, cadmium, lead and mercury) residing in the estu- arine sediments pose the greatest threat. They contaminate at levels as severe as seen anywhere worldwide. It might be fortuitous that nutrient inputs do not threaten eutroph- ication, because the estuary flushes rapidly (∼ 15 days), and phytoplankton appear to be light-limited in the middle estuary, where most nutrients enter. Organic toxicants cause localised concerns, but need to be more fully investigated. In contrast, the Huon estuary has been modified, but its environmental quality re- mains high with almost all monitoring results below the Australian guidelines. It serves as a useful baseline, though not pristine, against which the contamination of the Derwent estuary can be evaluated. Nutrients from aquaculture and the lower catchment, causing increased phytoplankton biomass and possibly depleted levels of dissolved oxygen in bot- tom waters, appear to be the only existing challenges for environmental management of the Huon estuary. Keywords Hydrology · Metals · Nutrients · Organic matter · Phytoplankton 1 Introduction The Derwent and Huon Rivers are not large on the world scale, nor even com- pared with rivers in continental Australia. What makes them interesting is that they are examples of rivers that are under-represented in global scientific studies:theyareinanislandintheSouthernHemisphere,inacoldtemperate region, draining catchment soils that are generally poor in nutrients. The two rivers, and their estuaries especially, are similar in many ways [1]. They can be treated as analogues. However, their modifications since the The Tail of Two Rivers in Tasmania: The Derwent and Huon Estuaries 3 arrival of Europeans just 200 years ago have followed different paths. The Derwent estuary has become the centrepiece of the capital city of Tasma- nia, Hobart. Along both its shores is now an urban ribbon, comprising some heavy industry, port facilities and many effluent discharges. The watershed of the Derwent River has been much modified by agriculture and other human activities, and the flow of the river has been modulated and diminished by 16 dams along its course. These changes, and loss of habitat within the estuary it- self, have harmed the natural ecosystem (e.g. the endemic spotted handfish is under threat of extinction) and seemingly favoured the introduction of exotic marine pests that have exacerbated the negative trend. In contrast, the human influence on the Huon River and its estuary has been more moderate. Much of its upper catchment remains as native forest; some tracts are exploited by forestry activities. A patchwork of agriculture – horticulture and livestock grazing – pervades the lower catchment. Human settlement remains sparse with series of small townships along the water- course. A recent development is salmonid, and to a lesser extent shellfish, aquaculture in the marine end of the estuary. As well as critically examining the available physico-chemical data for each estuary and its context in regard to catchment and regional environment, I discuss briefly the possibility of the Huon estuary providing an operational baseline for management of the more polluted Derwent estuary. The ecology of the estuaries, apart from phytoplankton, is not examined here; the reader is referred to Edgar et al. [1], Jordan et al. [2], and the NSR [3] reports. 2 Broad Environment Tasmania has a cool temperate maritime climate dominated by a zonal west- erly wind regime [4, 5]. The westerly winds are present throughout the year, but fluctuate from weekly intervals (with the passage of anticyclones), sea- sonally (strongest in late winter/early spring) and inter-annually. The marine influence results in mild winters and cool summers with a typical daily tem- ◦ perature range of 7 C, but with weather that is very changeable. The island is sufficiently large to have a slight continental effect on climate in its central highlands, much of which form the upper catchment of the Derwent River. Rainfall is broadly highest near the west coast and declines steadily to the east. The rugged topography of many parts of the island creates micro- climates; higher altitude features produce localised rainfall patterns super- imposed on the general trend. In the west of the Huon catchment, annual average rainfall peaks near 3000 cm; it is 500 cm and less in the southeast of the Derwent catchment [6]. Precipitation is generally uniform through- out the year, although toward the west coast it peaks in late winter. In the east, it is slightly higher in autumn and spring, affected by intermittent cy- 4 E.C.V. Butler clonic depressions off the east coast [5]. Atmospheric deposition, wet or dry, throughout Tasmania is typically uncontaminated, because to the west, over the Southern Ocean, is one of the longest fetches anywhere. TheDerwentandHuoncatchments(Fig.1)arelocatedinthesamegeo- logical province characterised by post-Carboniferous cover (Permo-Triassic mudstone, sandstone and shales into which large sheets of Jurassic dolerite have intruded) over pre-Carboniferous basement rocks [7, 8]. Only in their headwaters do they differ, with the Derwent draining highlands of similar geology to its