Magela Floodplain Research Articles

Uranium Exposure to the Tropical Duckweed Lemna aequinoctialis and Pulmonate Snail Amerianna cumingi: Fate and Toxicity

The discharge of catchment-management water from the Ranger uranium (U) mine into Magela Creek upstream of the Ramsar-listed Magela Floodplain in Kakadu National Park is an important part of the mine's water-management system. Because U is one of the primary toxicants associated with this water, a receiving-water trigger value (TV), based on chronic toxicity data from five local native species, was derived for U. To strengthen the data set underpinning the derivation of the TV, the chronic toxicity of U to two additional tropical freshwater species, duckweed Lemna aequinoctialis (96-hour growth rate), and pulmonate gastropod, Amerianna cumingi (96-hour reproduction), was determined. The fate of U within the test systems was an important component of the study because analysis of U concentrations during the snail tests indicated that a substantial proportion of U (approximately 25%) was being lost from the test solutions when integrated during the entire test duration. Analysis of the snails and their food for U indicated that only a small proportion that was lost from solution was being taken up by the snails. Therefore, the majority of U that was lost was considered unavailable to the snails, and thus the exposure concentrations used to calculate the toxicity estimates were adjusted downward. Integrating the loss of U from the L. aequinoctialis test solutions over time showed that only a small proportion (6% to 13%) was lost during the test: Of that, almost half (2-5%) was taken up by the plants (constituting exposure). Uranium was only moderately toxic to L. aequinoctialis, with no observed-effect concentrations, lowest observed-effect concentrations, and inhibition concentrations causing 10% and 50% effects (IC10 and IC50) values of 226, 404, 207, and 1435 microg/l, respectively. A. cumingi was found to be more sensitive to U than L. aequinoctialis, with NOEC, LOEC, IC10, and IC50 values of 60, 61, 15, and 278 microg/l, respectively. The data for these two additional species will be used to revise the current TV for U in Magela Creek.

A tropical sediment toxicity test using the dipteranChironomus crassiforcepsto test metal bio availability with sediment pH change in tropical acid-sulfate sediments

The wetlands of the Magela floodplain of northern Australia, which is the major sink for dissolved metals transported in the Magela Creek system, contain acid-sulfate sediments. The rewetting of oxidized acid-sulfate soil each wet season produces acidic pulses that have the potential to alter the bioavailability of sediment-associated metal contaminants. Acute toxicity tests (72-h mean lethal concentration [LC50]) using the tropical chironomid Chironomus crassiforceps Kieffer showed that copper toxicity decreased from 0.64 mg/L at pH 6 to 2.30 mg/L at pH 4. Uranium toxicity showed a similar trend (36 mg/L at pH 6 and 58 mg/L at pH 4). Sediment toxicity tests developed using C. crassiforceps also showed that both metals were less toxic at the lower sediment pH with pore-water copper toxicity having a lowest-observed-effect concentration of 4.73 mg/L at pH 4 compared to 1.72 mg/L at pH 6. However, a lower pH increased pore-water metal concentrations and overlying water concentrations in bioassays. Hydrogen ion competition on metal receptor sites in C. crassiforceps was proposed to explain the decrease in toxicity in response to increased H+ activity. This study highlights the need to consider site-specific physicochemical conditions before applying generic risk assessment methods.

A tropical sediment toxicity test using the dipteran Chironomus crassiforceps to test metal bioavailability with sediment pH change in tropical acid-sulfate sediments.

The wetlands of the Magela floodplain of northern Australia, which is the major sink for dissolved metals transported in the Magela Creek system, contain acid-sulfate sediments. The rewetting of oxidized acid-sulfate soil each wet season produces acidic pulses that have the potential to alter the bioavailability of sediment-associated metal contaminants. Acute toxicity tests (72-h mean lethal concentration [LC50]) using the tropical chironomid Chironomus crassiforceps Kieffer showed that copper toxicity decreased from 0.64 mg/L at pH 6 to 2.30 mg/L at pH 4. Uranium toxicity showed a similar trend (36 mg/L at pH 6 and 58 mg/L at pH 4). Sediment toxicity tests developed using C. crassiforceps also showed that both metals were less toxic at the lower sediment pH with pore-water copper toxicity having a lowest-observed-effect concentration of 4.73 mg/L at pH 4 compared to 1.72 mg/L at pH 6. However, a lower pH increased pore-water metal concentrations and overlying water concentrations in bioassays. Hydrogen ion competition on metal receptor sites in C. crassiforceps was proposed to explain the decrease in toxicity in response to increased H+ activity. This study highlights the need to consider site-specific physicochemical conditions before applying generic risk assessment methods.

Distribution and Abundance of Brolgas and Black-necked Storks in the Alligator Rivers Region, Northern Territory

Aerial surveys between 1981 and 1984 were used to identify monthly trends in the abundance of Brolgas Grus rubicundus and Black-necked Storks Ephippiorhynchus asiaticus on five floodplains of the monsoonal, subcoastal Northern Territory. Ground surveys were conducted during the same period on one of the plains, the Magela floodplain. Aerial surveys indicated that Brolgas were significantly more abundant in the dry season than the wet; they congregated particularly on the Cooper floodplain, were of intermediate abundance on the Magela and Nourlangie floodplains, and occurred only sporadically on the East Alligator floodplain and Boggy Plain (a backswamp of the South Alligator River). Ground surveys indicated that aerial surveys under-estimated abundance; based on rough correction factors calculated from ground surveys, peak numbers on the five floodplains were estimated at about 24 000. Blacknecked Storks were seen in low numbers on all floodplains, and did not vary significantly in abundance throughout the year. The total population was estimated at about 1800 birds.

Distribution and abundance of herons, egrets, ibises and spoonbills in the Alligator Rivers Region, Northern Territory

Aerial surveys between 1981 and 1984 were used to identify monthly trends in the abundance of Pacific herons (Ardea pacifica), pied herons (Ardea picata), cattle egrets (Ardeola ibis), great egrets (Egretta alba), little egrets (Egretta garzetta), intermediate egrets (Egretta intermedia), glossy ibis (Plegadis falcinellus), sacred ibis (Threskiornis aethiopica), straw-necked ibis (Threskiornis spinicollis) and royal spoonbills (Platalea regia) on five floodplains of the Alligator Rivers region, 250 km east of Darwin in the monsoonal Northern Territory. Ground surveys were conducted during the same period on one of the floodplains, the Magela. The aerial surveys indicated that the Magela floodplain was inhabited by few of these birds during the wet season (November-March), but that numbers then increased substantially in the dry season. The Nourlangie floodplain and Boggy Plain (a large backswamp of the South Alligator floodplain) showed similar patterns, but the numbers of birds tended to be lower. Birds were generally uncommon on the shallower East Alligator and Cooper floodplains, except for egrets in the wet season. Ground surveys suggested that the birds sought out the persistent swamps that characterise the Magela foodplain in the dry season. Ground surveys also indicated that aerial surveys underestimated densities. On the basis of correction factors calculated from ground surveys, peak numbers on the five floodplains were roughly estimated to be about 4000 pacific herons, 50 000 pied herons, 300 000 egrets (primarily intermediate egrets), 60 000 glossy ibis, 16 000 sacred ibis, 80 000 straw-necked ibis and 35 000 royal spoonbills. Great-billed herons (Ardea sumatrana), white-faced herons (Ardea novaehollandiae), striated herons (Butorides striata), rufous night herons (Nycticorax caledonicus), black bitterns (Dupetor flavicollis) and yellow-billed spoonbills (Platalea flavipes) were recorded sporadically in low numbers. The Alligator Rivers region acted as a dry-season refuge for herons, egrets, ibises and spoonbills because of the unusually persistent fresh waters of the Magela and Nourlangie floodplains and some of the backswamps of the South Alligator, such as Boggy Plain.

Distribution and abundance of grebes, pelicans, darters, cormorants, rails and terns in the Alligator Rivers Region, Northern Territory

Aerial surveys between 1981 and 1984 were used to identify monthly trends in the abundance of Australian pelicans (Pelecanus conspicillatus), darters (Anhinga melanogaster), little pied cormorants (Phalacrocorax melanoleucos), and whiskered terns (Chlidonias hybrida) on five floodplains of the Alligator Rivers region, 250 km east of Darwin in the monsoonal Northern Territory. Ground surveys were conducted during the same period on one of the floodplains, the Magela plain. The aerial surveys indicated that the Magela floodplain was inhabited by few of these birds during the wet season (November-March), but that numbers then increased substantially in the dry season. The Nourlangie floodplain showed similar patterns, but the numbers of birds tended to be lower. Birds were generally uncommon on the shallower East Alligator and Cooper floodplains. Ground surveys suggested that the birds sought out the persistent swamps that characterise the Magela floodplain in the dry season. Ground surveys also indicated that aerial surveys underestimated densities; on the basis of correction factors calculated from ground surveys, peak numbers on the five floodplains were roughly estimated to be about 2000 darters, 9000 little pied cormorants, 55 000 Australian pelicans and 50 000 whiskered terns. Little black cormorants (Phalacrocorax sulcirostrus) were sometimes abundant, but their sporadic occurrence prevented analysis of seasonal trends. Australasian grebes (Tachybaptus novaehollandiae), great cormorants (Phalacrocorax carbo), pied cormorants (Phalacrocorax varius), buff-banded rails (Rallus philippensis), Baillon's crakes (Porzana pusilla), white-browed crakes (Poliolimnas cinereus), purple swamphens (Porphyrio porphyrio), Eurasian coots (Fulica atra), silver gulls (Larus novaehollandiae), white-winged terns (Chlidonias leucoptera), gull-billed terns (Gelochelidon nilotica) and Caspian terns (Hydroprogne caspia) were recorded in low numbers. The Alligator Rivers region acted as an important dry season refuge because of the unusually persistent fresh waters of the Magela and Nourlangie floodplains and some of the backswamps of the South Alligator, such as Boggy Plain.

Production and major nutrient composition of three grass species on the Magela floodplain, Northern territory, Australia

Seasonal changes in dry weights and nutrient concentrations in three aquatic grasses on the seasonally inundated Magela floodplain in northern Australia were investigated over an 18 month period. The dry weight of the aquatic grass species Pseudoraphis spinescens (R. Br.) Vick., Hymenachne acutigluma (Steudel) Gilliland and Oryza meridionalis Ng varied with water depth on the floodplain. Maximum dry weights (1.67±0.21 kg m −2, 1.41±0.10 kg m −2, 0.51±0.10 kg m −2, respectively) occurred at the end of the wet season when water depth was decreasing. The perennial species P. spinescens and H. acutigluma had two growth periods and an annual productivity of 1.91±0.26 kg m −2 and 2.09±0.38 kg m −2, respectively, compared with 0.51±0.10 kg m −2 for the annual O. meridionalis. Relative to other aquatic and wetland plant species, the former two grasses have high production rates. The major nutrients nitrogen, phosphorus, sulphur, chloride, magnesium, calcium, sodium and potassium were generally present in low concentrations compared with concentrations in other plant species. The annual O. meridionalis had distinct seasonal changes in nutrient concentrations, whereas for P. spinescens and H. acutigluma the changes were not as obvious. An assessment of the nutrient loads that could potentially be turned-over in the detritus originating from these grasses was made. Compared with the annual input to the floodplain of major nutrients in creekwater the grass debris/detritus contained substantially more nitrogen, phosphorus and potassium, less sodium and chloride, and similar amounts of sulphur, magnesium and calcium. Thus, the grasses are an important potential component in the chemical turnover pathways on the floodplain.

Distribution and abundance of magpie geese, Anseranas semipalmata, in the Alligator Rivers Region, Northern Territory

AbstractAerial surveys from 1981 to 1984 were used to identify monthly changes in the abundance of magpie geese on Jive floodplains in the Alligator Rivers Region of the monsoonal Northern Territory, and ground surveys were conducted during the same period on one of the plains to provide more detailed distributional information. The aerial surveys showed that the Magela floodplain was inhabited by few geese during the wet season (November‐March), but that numbers then increased to an estimated average peak of 80 000 in the late dry season. The Nourlangie floodplain and Boggy Plain (a large backswamp of the South Alligator floodplain) exhibited a similar pattern, except that the peaks occurred 2–3 months before the end of the dry season and comprised many more geese (an estimated average of 350 000 birds). In contrast, geese were uncommon on the East Alligator floodplain except during the wet season, and densities and numbers were lower than on the three previous plains. The Cooper floodplain was occupied intermittently, and numbers and densities were always relatively low. Geese appeared to return from their breeding localities to the floodplains of the Alligator Rivers Region progressively during the dry season, concentrating first on the extensive Eleocharis swamps of the Nourlangie floodplain and then waiting out the remainder of the dry on the substantial permanent waters of the Magela floodplain and other nearby wetlands. Ground surveys on the Magela floodplain suggested that geese were highly mobile, apparently seeking suitable nesting habitat in the late wet season, and then a sequence of feeding areas during the dry season. Aerial surveys underestimated densities; on the basis of ground surveys, average peak numbers on the Magela plain were calculated to be 500 000. We estimate that the Alligator Rivers Region supported an average of about 1.6 million geese in the dry season, but very many fewer during the wet season.

Distribution and Abundance of Ducks in the Alligator Rivers Region, Northern Territory

Aerial surveys between 1981 and 1984 were used to identify monthly trends in the abundance of wandering whistling-duck Dendrocygna arcuata, plumed whistling-duck D. eytoni, radjah shelduck Tadorna radjah, Pacific black duck Anas superciliosa, and grey teal A. gibberifrons on five floodplains of the Alligator Rivers region, 250 km east of Darwin in the monsoonal north of the Northern Territory. Ground surveys were conducted during the same period on one of the floodplains, the Magela plain, to provide more detailed information. The Magela floodplain was inhabited by few ducks during the wet season (November to March), but numbers then increased to dramatic peaks in the late dry season. The Nourlangie floodplain and Boggy Plain (a large backswamp of the South Alligator floodplain) showed similar patterns, but the numbers of ducks were usually fewer. Ducks were uncommon on the shallower East Alligator and Cooper floodplains except for relatively brief periods in the wet season. The ground surveys suggested that ducks sought out the persistent swamps that characterise the Magela floodplain in the dry season. Ground surveys also indicated that aerial surveys underestimated densities; on the basis of correction factors calculated from the ground surveys, peak numbers on the five floodplains were roughly estimated to be 400 000 wandering whistling-ducks, 70 000 plumed whistling-ducks, 20 000 radjah shelducks, 50 000 Pacific black ducks, and 50 000 grey teal. Pink-eared ducks Malacorhynchus membranaceus and hardhead Aythya australis were recorded sporadically in low numbers. The Alligator Rivers region acted as a dry season refuge for large concentrations of ducks because of the atypical persistence of freshwaters on the Magela and Nourlangie floodplains and some of the backswamps of the South Alligator, such as Boggy Plain. The large aggregations appear to be unique in Australia.

A water budget model for the tropical Magela flood plain

A water budget model is presented for the tropical Magela flood plain, which is located downstream from the Ranger uranium mine in the Northern Territory, Australia. Using input data on rainfall and water discharge from Magela Creek, the model provides daily estimates of the volume of surface water on the flood plain and the rate of discharge at the outlet of the flood plain. Annual variations in the relationship between gauge height and rate of discharge at the outlet are interpreted in terms of a correlation between early wet-season rainfall and seed germination on the flood plain. The daily volume of water on the flood plain can be used to estimate the dilution and subsequent evapoconcentration of any waterborne contaminants that may be transported by Magela Creek from the mine site downstream to the flood plain. The model was validated by testing its ability to predict the measured daily water depth at the outlet channel; comparison of prediction with 12 years' data produced an average error of about 16%.

A Transition from Mangrove Forest to Freshwater Wetland in the Monsoon Tropics of Australia

The coastal freshwater wetlands of western Arnhem Land in the monsoon tropics of the Northern Territory of Australia are subject to new and increasing pressures from a range of land uses. Wetland history is needed as a basis for management decisions. Radiocarbon dates and pollen analyses of samples from fifty surveyed sites on the Magela floodplain show mangrove vegetation encroaching as sea level rose from about 8000 BP to 6000 BP. The extensive Rhizophora forest established at that time lasted until about 3000 BP, when the sediments built up above the upper tidal limit for these mangroves. Avicennia and other mangrove genera became more abundant in the subsequent transition phase. The floodplain has been a freshwater wetland since about 1300 BP. High resolu- tion pollen analyses of contiguous 1 cm samples through the transition at two sites show parallel sequences of vegetation changes. Large-scale spatial and temporal stability of the two mutually exclusive ecosystems, mangrove forest and freshwater wetland, contains considerable small- scale variation. Maximum diversity and variability occurred during the time of least environmental stability in the transition phase. Vegetation change was discontinuous, with each major shift followed by variation around a new mean. A rise in sea level of 0.5-1.0 m could destroy the present freshwater wetland and allow some mangroves to return to the Magela.

Magela Creek system, Northern Australia. II. Material budget for the floodplain

A materials budget was estimated for the Magela Creek system during the 1982-83 wet season. This tropical system in northern Australia consists largely of a well-defined creek (Magela Creek contributes approximately 50% of the total inflow to the floodplain) flowing into an extensive wetlands area and then into the East Alligator River. Intensive sampling of creek water, rainfall and water flowing from the system provided the data base for the budget calculations. The annual transport of both dissolved and particulate matter by Magela Creek (area 600 km2) is very low, even when compared with other low-relief tropical systems. The annual load transported during 1982-83 was 1260 t (21 kg ha-1) of dissolved salts and 2330 t (39 kg ha-1) of particulate matter. Rainfall appeared to contribute all the sodium, potassium and chloride, and part of the calcium (c. 30%) and magnesium (25%) transported during the 1982-83 wet season by Magela Creek. Most of the manganese (c. 60%) (and probably iron) was contributed from weathering processes occurring in the catchment. Only small amounts of the trace metals copper, lead, zinc and uranium were transported by the creek. During the 1982-83 wet season, more trace metals were contributed in rainfall than transported from the catchment by the creek. However, this is probably atypical and resulted from dust particles that had entered the atmosphere in greater numbers due to the extended dry season. The vast bulk of the nutrients (total P 93%, NO3- N 86%, NH4+ N 98%) added to the catchment by rainfall was removed by the catchment, probably via uptake by the vegetation. Consequently, the creek transported only very small amounts of nutrients to the floodplain. An input-output budget for the Magela floodplain was calculated. The uncertainty in the net amounts deposited or released from the floodplain was estimated using a new quantitative method developed for this purpose. The uncertainties in the net values estimated were high, ranging from around 30% for bicarbonate to 500% for uranium. These data suggest that the Magela floodplain is a net source of the major ions (sodium, potassium, calcium, magnesium, chloride, sulfate and bicarbonate) and also of iron, and a net sink for suspended solids, nutrients (total phosphorus, nitrate and ammonia) and manganese. The floodplain also appears to be a net sink for the trace metals copper, lead, zinc and uranium.

Changes in Melaleuca forest density on the Magela floodplain, Northern Territory, between 1950 and 1975

AbstractThe stability of the Magela floodplain prior to the development of uranium mining at Ranger, in the Magela catchment, was estimated by comparing aerial photographs taken in 1950 and 1975, using the density of Melaleuca swamp forest as an index of change. The 1975 photographs were interpreted in the light of a ground survey carried out in the dry season of 1976. There was no increase in the area occupied by Melaleuca trees, indicating that infilling of perennial swamp and plant succession were not significant agents of change in this period. Thirty‐eight per cent of the forested area suffered a significant decrease in tree density: fire, wind and buffalo are among the probable causes, and these hazards should be taken into account in monitoring the effects of the uranium mining.

Vegetation and stream pattern as indicators of water movement on the Magela floodplain, Northern Territory

Magela Creek is a hydrologically complex waterway which drains Australia's two largest uranium deposits, carries a pastoral and tourist industry and has been proposed for inclusion in a national park. In this study, the vegetation and stream patterns of the floodplain are described and are used as indicators of surface water movement through the system. The Magela floodplain was found to be a freshwater lagoon, enclosed at its outlet by, the floodplain of the East Alligator River; the vegetation types are direct indicators of water depth within the lagoon. The capacity of the lagoon is approximately 60 × 106 m3, the average daily input during the wet season is approximately 3% of the lagoon volume and during the dry season the lagoon dries out to approximately 5 × 106 m3 of perennial swamp. The lagoon is maintained by the East Alligator River and so its hydrology must not be studied in isolation but in relation to the larger system to which it belongs.