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Edited by Leon P. Zann and David Sutton
Great Barrier Reef Marine Park Authority, Townsville Queensland
Department of the Environment, Sport and Territories, Canberra (1996)
ISBN 0 642 23012 9
Northern Territory Museum of Arts and Sciences
Darwin, Northern Territory
The Northern Territory has an area of 1 347 519 km2 and a mainland coastline of about 7200 km which lies entirely within the monsoon tropics. It is subject to strong south-easterly winds during the dry season (May to October) and north-westerly winds during the wet season (November to April). Rainfall is correlated with latitude (Figure 1) and is generally reliable, although considerable variation in onset and duration of rainfall occurs between years.
The coastline is predominantly macrotidal, with the maximum range of about 8 m occurring in the west and gradually diminishing eastwards to less than 1 m in parts of the Gulf of Carpentaria. Much of the coastline consists of gently sloping muddy shorelines lined by mangroves and washed by shallow, warm, turbid waters. Some rocky reef and sandy foreshores are present, usually on shorelines exposed to strong seasonal winds, and reef coral development is generally poor. Sea temperatures range from 23 to 33oC.
The coast features several large ria systems (wedge-shaped indentations in the coastline), notably Darwin and Bynoe Harbours, and large embayments such as Arnhem, Buckingham and Melville Bays. The hinterland is largely monsoonal lowland, dissected by large rivers with broad seasonal flood plains. The low gradients of these plains and high tidal ranges result in tidal influence up to 100 km upstream. Most of these large tidal rivers and their flood plains show signs of recent ecological change (Woodroffe & Mulrennen 1991).
The population of the Northern Territory is about 157 000 and on the coast, the two largest population centres are Darwin (73 300) and Nhulunbuy (3500). Alyangula on Groote Eylandt has a population of 1200, but other coastal settlements are small (200-300 people), isolated, and their populations are mainly Aboriginal Australian (Gardner 1991). Most of the coastline is therefore largely unpopulated, and remains remote and often inaccessible during the wet season. Access to the coast is further restricted by the designation of 72% of the coastline as aboriginal land (Figure 1), with entry governed by permits issued by Land Councils.
The majority of the population and associated service industries and infrastructure are located in and around Darwin which is the seat of government. Unlike Nhulunbuy or Alyangula where current population levels are entirely dependent on single mining operations (Nabalco and Gemco, respectively), Darwin appears poised for substantial industrial development and population growth. Proximity to Asian markets, improved transport links with the rest of Australia, and the growth of offshore mining for oil, gas and minerals in the Timor and Arafura Seas suggest Darwin will attract large scale projects. Apart from mining and its support industries, aquaculture, agriculture and tourism also offer the potential for growth. The 1991 Darwin Regional Structure Plan identifies substantial industrial development and concomitant population growth as both likely and desirable and suggests the Darwin region will eventually achieve a population of 1 million.
Irrespective of the desirability or likelihood of population growth of that magnitude, current stresses on the coastal marine environment might be expected in proximity to the three largest coastal centres. Darwin is a small city by Australian standards, but its population does have an impact on nearby marine environments. The mining operations at Nhulunbuy and Alyangula might also be expected to have some level of impact upon the marine environment.
The marine environment near each of these population and mining centres is discussed in greater detail separately and the existing and potential impacts are identified. Locations mentioned in the text are shown in Figure 1.
The Nabalco bauxite mine at Nhulunbuy lies inland and the ore is transported by conveyor to the alumina refinery on Gove peninsula. The town of Nhulunbuy lies about 10 km east of the refinery. Gove peninsula runs in a westerly direction along the northern side of a large embayment known as Melville Bay.
Melville Bay is approximately 170 km2 in area, and is fringed by mangroves, with several coarse sandy beaches and some rocky shores. There are small rocky islands and headlands in the northern part of the bay. The water in the Bay is shallow, rarely more than 15 m in depth and mostly less than 10 m. Water temperature data is available for July 1991 (23.9 -26.2oC) and March 1992 (30.6 - 32.6oC). The tidal range is 4 m and on most low spring tides some of the shallow arms of the Bay dry out. Studies of tidal movements and subsequent impact on the distribution of sediments in Melville Bay are presented by Foster et. al. (1968). The substrate over most of the Bay is fine mud and silts, typically anoxic, and seagrasses are present but patchy in distribution. Seagrasses are largely Halophila spp. although several other species are reported.
There have been a number of reports (Noller 1991; Peerzada et al. 1990b; Peerzada & Dickinson 1989) examining Nabalco's operations and potential environmental impacts on the region. Concerns have included the level of sulphur emissions from the Steam Power Station stacks, the discharge of heated seawater into the Bay, occasional spills of caustic soda into the Bay (Noller 1991), and heavy metal contamination of oysters in Melville Bay.
Peerzada et al. (1990a) found high zinc levels in oysters in Melville Bay at a site close to the bauxite treatment plant. They also found the level of other heavy metals to be higher near Nabalco's bauxite treatment plant. Elevated copper levels in oysters (27.9 ppm wet weight) at one site in Gove Harbour were attributed to the presence of red mud settlement ponds that discharge supernatant liquor straight into the Bay (Peerzada et al. 1990a). This assertion that the elevated level of copper is due to Nabalco's operations in Drimmie Arm is unsubstantiated. Noller (1991) found quite different results to Peerzada et al. (1990a), and suggests that zinc contamination in oysters is probably due to the presence of boats and habitation.
Nabalco has recently undertaken an extensive study (water, sediment, heavy metals, benthic biota) of Melville Bay. Results suggest that although there is no evidence to indicate that effluent from the Nabalco refinery is contributing directly to the heavy metal load in Melville Bay, it is possible that metal levels in the north-western corner of the harbour may be slightly elevated due to discharges from Nabalco (McConchie 1991). Hanley (1993a) reported on the benthic fauna of Melville Bay collected by grab sampler at more than 100 sites during the survey. The results of the survey cannot be discussed here as Nabalco has not yet responded to the information contained within the report.
Apart from the impact of Nabalco operations on the Bay there are several other sources of concern. A barge landing facility and a fuel depot for prawn trawlers are located on the southern side of a low, narrow peninsula dividing Melville Bay from the sea. Further east along this peninsula lies the recreational yacht club in Inverell Bay. The sediments of the small embayments on the southern side of the peninsula are fine mud and silts and are highly anoxic. This is a depositional environment with poor circulation, particularly since the construction of a causeway during World War II has halted movement of seawater through the northern end of Drimmie Arm. The yacht club and small ships facilities are therefore sited in areas where flushing and dilution of effluent is poor. The regulation of activities along this foreshore is virtually nonexistent, and consequently there are small spills and leakages of diesel fuel, paints, and petroleum based products almost daily. Yachts anchored in the eastern part of the bay regularly discharge raw sewage into the bay.
The scale of these problems must be kept in perspective. The population is small, and therefore the volumes of effluent and spillages are also small, and at present there are no contaminants present in concentrations high enough to be of immediate concern. However, the poor circulation in the north-eastern area of Melville Bay suggests that cumulative problems may emerge as fine anoxic sediments act as sinks for contaminants. No information is available on the rate of exchange of seawater between this sheltered area of Melville Bay and the sea north of the peninsula. Observations made during the recent Melville Bay Survey suggest the level of daily exchange is small, and therefore if nutrient loadings to the sheltered north-eastern area increases then eutrophication is possible.
If the population of the region and/or industrial development were to increase substantially then this area of Melville Bay would be increasingly at risk. The current situation in this area requires careful assessment and the development of a management plan.
The township of Alyangula is located on the north-western tip of Groote Eylandt, on the shores of Milner Bay, where manganese ore mined further south at Angurugu is loaded onto bulk carriers. The manganese mine has been operated by Gemco since 1965.
Milner Bay is sheltered during the dry season when prevailing winds are from the south-east, but is exposed to the north-west monsoon. Consequently the bay has a relatively steep floor rising to sandy intertidal flats backed by sand beaches and rocky outcrops. The tidal range in the bay is 2 m, and no creeks of any size discharge into the bay. Rainfall at Alyangula averages 963 mm per year. Some data on water temperatures has recently been collected by Gemco (Klein, pers. comm.). The bay appears to be well flushed by natural tidal and wind generated processes. The National Tide Authority has recently installed a monitoring station at Alyangula.
Johnston (1990) has reported on manganese and other heavy metal levels in marine sediments, seawater and marine organisms (oysters, fish) from samples collected in Milner Bay, a control site on the north side of the island and the two largest rivers on the island. In general, concentrations of heavy metals were found to be within the range of background levels recorded elsewhere in the Northern Territory. The exception was manganese in marine sediments, which was reported at higher than typical levels at several locations.
Alyangula is unlikely to experience any substantial increase in population or industrial activity in the future. The current manganese extraction and shipment operations appear to have had little impact on the marine environment in Milner Bay.
Darwin Harbour or Port Darwin is a large ria system of about 1000 km2 formed by postglacial marine flooding of a dissected plateau. The shoreline of the outer, northern section of the Harbour is predominantly coarse sands and rocky cliffs, and the estuarine shoreline of the inner, southern section of the Harbour is dominated by fine silts and mangroves. Dames and Moore (1985) estimated the volume of seawater in the Harbour at high tide is about 2.5 million m3 and about one third of this amount is exchanged with seawater north of the harbour on every spring tidal cycle.
The three major arms of the harbour receive substantial inputs of fresh water during the wet season but negligible amounts in the dry season. Semidiurnal tides with a maximum range of 8 m ensure rapid mixing of the fresh water (Michie 1987). Salinities in the broad reaches of the harbour range from 30.5 ppt in March to 35.5 ppt in September and records for the upper estuarine sections of the Elizabeth River show a seasonal range of 6-41 ppt. Water temperatures range from 23.4o C in July to 31.0oC in December (Dames & Moore 1985).
Much of the harbour is shallow, under 10 m in depth and exposed at spring low tides. The three arms of the harbour are 10-20 m deep and the main channel of the harbour where it passes through the constriction between East and West Points reaches 36 m depth. The distribution of sediment types in Darwin Harbour has been documented by Michie (1987), but data are incomplete for subtidal areas in Middle and West Arms. A major survey of subtidal sediments and benthic biota is currently in progress and results should be available by December, 1994. The geomorphology and floristics of the intertidal zone of estuarine Darwin Harbour has been described by (Ecosystems 1993; Woodroffe et. al. 1988; Semeniuk 1985). Useful reviews of marine fauna of the harbour can be found in Larson et al (1988), and Hanley (1993b) provides a comprehensive summary of mangrove invertebrate fauna.
The city of Darwin lies on the north-eastern side of the harbour. The majority of environmental impacts on the harbour are concentrated on the area around Darwin city and the East Arm of the harbour. Although Darwin is a capital city it does not have any heavy industry. Some light industrial developments are present, such as slipways, chemical/explosives operations, cement works, fuel storage and a tannery.
Within the Darwin region the major impacts on coastal and marine habitats are the clearing of mangroves and reclamation of intertidal land (Hanley & Couriel 1992). Vehicular and pedestrian traffic on stretches of beach in the northern section of the harbour has also caused coastal dune and vegetation erosion (Kraatz 1992). Other potential problems are the discharge of sewage effluent and stormwater run-off into harbour waters.
The coastline in the Darwin region is almost entirely inaccessible as it consists of broad intertidal mudflats, dense mangroves and saltpans. The large tidal range, the presence of biting midges and mosquitos, and deep, unstable mud are significant barriers to development. However, development is proceeding and large areas of mangrove are being cleared, with landfill in the intertidal zone encouraged for a variety of land use proposals such as port facilities, mooring basins, shipyards and slipways, marinas, housing estates, refineries (oil and gas), aquaculture farms and industrial estates.
Many proposed developments have the potential to affect large areas of the intertidal zone adjacent to them. For example, the proposed hazardous and offensive chemical industrial zone on the Middle Arm peninsula would require large scale clearing of mangroves and at least some of the industries will present a threat to the surrounding mangrove flora and fauna in the event of spillages. Housing estates and marinas also have the potential to precipitate widespread destruction of mangroves. After the removal of mangroves on the construction site, there will be pressure from residents of at least some of these developments for removal of surrounding mangroves because of biting insect problems. Guidelines which suggest that no housing construction should take place within 1.6 km (Whelan 1988) of mangroves are consistently ignored.
The mangroves of Darwin Harbour (some 20 000 hectares) are one of the largest single stands of mangroves in the country and as such are nationally important (Hanley 1992a). Any mangrove loss should be accompanied by attempts to maintain productivity levels and a range of all mangrove habitat types. However, no mangrove management plan currently exists, although a management plan to be developed for Darwin Harbour may address the issue of maintenance of productivity. In the meantime, clearing of mangroves and associated landfill continues, with the proposed Darwin South development set to remove some 8-10% of the harbour mangroves if it proceeds (Ecosystems 1993). There is a very real danger that the current mosaic of development will lead to a significant loss of mangrove productivity and habitat similar to that seen elsewhere in Australia.
The majority of sewage from Darwin is treated to secondary stage in oxidation ponds at a number of sites around the city. The two other forms of treatment/disposal are chemical precipitation, at Ludmilla, and an ocean outfall of untreated sewage, at Larrakeyah. All the oxidation ponds are sited adjacent to mangroves into which effluent is released directly. The volume of effluent released varies considerably, with the greatest volumes discharged during the wet season. Several of the smaller ponds may not release any effluent during the dry season because daily evaporation meets or exceeds the volume of water entering the ponds.
The impact of effluent on mangrove flora has been examined by Clough et al. (1983) who suggest that inputs of heavy metals, nutrients and pesticides will not be harmful to mangroves themselves, and that mangroves can act very effectively as sinks, trapping nutrients, heavy metals and pesticides that would otherwise be released into estuarine waters. Saenger et al. (1990) provide supporting evidence for this role with data showing mangroves between the Wynnum refuse tip and Moreton Bay in Queensland have trapped heavy metals, nutrients and pesticides.
Evidence from ambient water quality studies show the harbour waters are high in dissolved oxygen with low nutrient and chlorophyll a concentrations indicating little nutrient enrichment (Wrigley et al. 1990). Clough et al. (1983) presented evidence of nutrient enrichment of mangrove sediments near a sewage effluent discharge in Darwin and suggest that nutrient enrichment may be directly beneficial to mangrove flora. This is a view supported by work at several effluent discharge sites (Hanley & Couriel 1992), although the substantial fresh water input from effluent discharge is also important during the protracted dry season when growth of mangrove flora is usually negligible (Woodroffe et al. 1988).
The impact of sewerage effluent on mangrove fauna has been poorly studied in the Darwin region, although the large tidal range and strong tidal currents are thought to have an ameliorating effect through rapid dilution. However some effluent outfalls are located high enough in the intertidal zone to discharge undiluted effluent for several hours each day directly into mangroves. Hanley and Couriel (1992) compared the number of species of benthic invertebrate fauna in the effluent plume near a treated sewage outfall at Bleesers Creek (Darwin Harbour), with numbers of species from three nearby control sites. The number of species differed significantly between three of the four sites, but was not due to sewage effluent. In addition the species composition of the invertebrate fauna at all four sites was similar and representative of the mangrove fauna usually associated with the mangrove flora at that level on the shore. Presumably, the relatively low volume of effluent discharge has not yet produced conditions which lie outside the range of acceptable conditions for typical mangrove invertebrate fauna. However, any increase in the volume of discharge might produce changes.
The results of this study are not easily extrapolated to the other sewage outfall sites in the Darwin region. Although each outfall discharges into mangroves, the outfalls are located at different heights, carry different effluent volumes and are sited in different mangrove habitats. The largest outfall discharges effluent from Leanyer treatment ponds straight into Buffalo Creek and some evidence suggests effluent is shunted up and down the creek by the ebb and flow of the tides, without significant mixing.
The relatively low levels of industrial infrastructure and maritime activity, and the large, semidiurnal tidal range suggest the low levels of anthropogenic contaminants entering the waters of the harbour are subjected to a rapid and substantial dilution. Ambient water quality surveys of Darwin Harbour have consistently reported low levels of heavy metals, pesticides, PCBs and hydrocarbons and the harbour waters are considered pristine (Dames & Moore 1993; Wrigley et al. 1990; Currey 1988; Peerzada 1988).
Analysis of harbour sediments for concentrations of various heavy metals and other contaminants has never been undertaken in a comprehensive fashion and the available results tend to reflect both the nature of the sediment and its position relative to sources of pollution. In a study of 8 sites around Darwin Harbour, Peerzada (1988), and Peerzada and Ryan (1987) found the highest levels of copper, lead and zinc in sediments around the Darwin wharf precinct. In November 1990 the Conservation Commission of the Northern Territory (CCNT) analysed sediments from three wharves in the Darwin Port area; Fort Hill, Stokes Hill (abandoned) and the Iron Ore Wharf. The study found high levels of trace metals below the Iron Ore Wharf. Copper, lead, zinc, cadmium and arsenic were found to exceed the National Health and Medical Research Council (NH&MRC) limits. All results except that for arsenic from Fort Hill and Stokes Hill were less than NH&MRC limits. Subsequent testing by the CCNT (in January, 1991) of sediments below the Iron Ore Wharf revealed levels in excess of NH&MRC limits for copper, zinc, lead and arsenic. Levels were generally much higher in the upper layers of sediment. The elevated metal levels were all found below or near a conveyor belt used to load metal ores onto cargo vessels. (Warren, pers. comm.). Currey (1988) recorded elevated levels of cadmium near the Stokes Hill wharf which he suggested was the result of localised shipping activity.
By contrast, surveys of heavy metals, hydrocarbons and pesticides from four nearby tidal creeks and a control area on the other side of the harbour has shown low levels of most metals and other contaminants. For chromium and arsenic, concentrations were slightly higher than NH&MRC guideline concentrations for contaminated soils. The spatial pattern of these samples with slightly elevated concentrations does not indicate any source of contamination and probably reflects natural occurrence (Dames & Moore 1993).
Within Darwin Harbour, levels of some metals such as chromium, zinc and arsenic in some sediments appear consistently high. This might be the result of leachate from old dump sites, or the result of recent mining of ore containing high concentrations of zinc in the catchment of the Elizabeth River. A large amount of material was dumped in and near intertidal areas around the harbour after the bombing of Darwin in 1942 and after cyclone Tracey in 1975. The exact location of most dumping grounds is only vaguely known and their contents are unknown. At least some are likely to contain significant amounts of a range of heavy metals. Perhaps some of the elevated levels of various heavy metals recorded in the studies discussed above may be due to leaching from these dumps. High background levels of some heavy metals in sediments may also be due to natural fluvial input from weathered ore bodies in Northern Territory coastal waters.
The results of the few small surveys of metals in sediments suggest some variability in concentrations between sites and sediment types. A recent survey of heavy metals and sediment types at some 200 sites in the harbour should produce a clearer pattern of distribution than is currently available.
Very little data on TBT levels are available. There is currently no regular monitoring of areas adjacent to slipways in Frances Bay (Darwin Harbour) for heavy metal (including TBT) contamination, although spot monitoring of the mud in front of the slipways has revealed TBT contamination (M. Vincent, QDEH, pers. comm.). Analysis of nearby sediments from Sadgroves Creek shows no contamination (Dames & Moore 1993), suggesting the area of impact is small. The use of TBTs on small boats in the Northern Territory is still legal although it is now illegal in most other Australian states.
Very few organisms have been examined for heavy metal and other contaminant levels in Northern Territory waters. The records are patchy, and there is little consistency in the range of contaminants tested. Most of the data are heavy metal concentrations, and these results show that in general, all metals examined were found to be below NH&MRC limits.
During the survey of metals in sediments near the Darwin Port, two species of sessile fauna were tested (a bivalve, Chama sp. and ascidians) below the Iron Ore wharf and both were below the NH&MRC limit for all metals except lead (Warren pers comm.). Peerzada and Kozllk (1992), Peerzada (1988) and Peerzada and Dickinson (1988) reported on a range of heavy metals concentrations in oysters from sites in Darwin Harbour and concluded that all were fit for consumption as they had heavy metal concentrations well below NH&MRC guidelines. The exception was a sample from Nightcliff, where oysters had lead levels close to or above the maximum recommended level.
Much of the research on heavy metal levels in organisms has concentrated on mangrove-associated molluscs. The mud whelks Telescopium telescopium, Terebralia palustris and Terebralia sulcata are widely eaten by local Aboriginal people, and there is evidence to suggest that some of these organisms may not be fit for consumption. As detritivores, T. telescopium are likely to accumulate some heavy metals, but caution is needed in interpretation of the available data, since no background heavy metal data exist for T. telescopium from Darwin Harbour.
Urban run-off and sewage effluent discharge in populated areas are potential sources of heavy metals. Peerzada et al. (1990a), recorded high lead levels in T. telescopium at Frances Bay (8.14 ppm) and Rapid Creek (8.99 ppm), and `high' concentrations of cadmium (1.26 ppm), zinc (199.47 ppm) and copper (72.05 ppm) in T. telescopium at Ludmilla (Racecourse) Creek,. They remarked that the creek receives treated sewage effluent, but reported no attempt to determine if a gradient in concentrations from the effluent source was present. Recent assessment of heavy metal levels in the potamid molluscs Telescopium telescopium and Terebralia palustris from Ludmilla Creek showed levels of heavy metals were below the limits of concentrations acceptable for human consumption (Hanley 1992b). High cadmium concentrations in T. telescopium were also detected at Elizabeth River, where according to Peerzada et al. (1990a), samples were taken in the vicinity of boat ramps, landfill sites and sewage outlets. However, from the information provided it appears that samples were taken some kilometres distant from some of the proposed sources of pollution.
In the absence of any evidence of a gradient in concentrations of heavy metals between the sample site and the proposed point sources of pollution, or any evidence of the background concentrations of the heavy metals under scrutiny, the attribution of a `high' concentration of any heavy metal to anthropogenic sources is of dubious value. For example, Peerzada and Dickinson (1989) found extremely high concentrations of cadmium in oysters from the Arnhem Land coast, which is sparsely populated and has no anthropogenic source of cadmium. All oysters tested exceeded NH&MRC limits for cadmium. Naturally high concentrations of cadmium have also been reported at Shark Bay, Western Australia (McConchie et al. 1988). Noller and Outridge (1987) reported on the concentrations of heavy metals and arsenic in several species of estuarine shellfish from the mouth of the East Alligator River, and found that cadmium, copper, mercury, lead, and zinc were all below the recommended dietary limits. Arsenic concentrations in several species were at the dietary limits, although it was unlikely all the arsenic was present in inorganic form. The concentrations of selenium in oysters was found to exceed the dietary limit. These studies underline the need for caution in extrapolating the results of localised surveys to other areas of the Northern Territory coastline.
The Northern Territory is in an enviable position. The available evidence supports the view of a largely pristine marine environment. No large scale chronic or acute pollution stresses have been identified. However, for much of the coastline no baseline data exist.
Significant industrial development and population growth is expected in at least the Darwin region during the next few decades. There will be impacts on the marine environment should this occur, and it is imperative that a coordinated program of baseline data collection is implemented during the next decade to allow the development of sound management plans.
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Woodroffe, C.D., Bardsley, K.N., Ward, P.J. & Hanley, J.R. 1988, `Production of mangrove litter in a macrotidal embayment, Darwin Harbour, N.T., Australia', Estuarine Coastal and Shelf Science, vol. 26 (6), pp. 581-599.
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