Phillip W. Ford

Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon

Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of 210Pb, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, 228Ra (half-life 5.7 years) and 226Ra (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusion-bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic flux of Ra. The flux of the shortest-lived isotope, 224Ra (half-life 3.7 days), is not significantly enhanced by bioturbation, and its flux is consistent with diffusion-controlled release. However bioturbation enhances the 228Ra flux by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and flux calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of 226Ra is much greater than can be accounted for by the modelled profile, and may be due to slow 226Ra desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on 226Ra pore-water and flux measurements at the time of this study, groundwater flow has an upper limit of 0.3 cm d−1.

Geochemical cycling and speciation of copper in waters and sediments of Macquarie Harbour, Western Tasmania

The factors determining the concentration and speciation of copper in the waters and sediments of Macquarie Harbour, Tasmania were investigated. This harbour is the most extensively copper-contaminated estuarine water body in Australia owing to current and historical inputs of metal-rich waters and sediments from the nearby Mount Lyell copper mine. The dissolved copper concentrations in the harbour water column were highly variable (4–560l gl � 1 ) and displayed a north to south gradient, decreasing with distance from the King River, which carries the inputs from the mine. The most significant process affecting dissolved copper concentrations was the neutralisation of acidic river waters with seawater and the resulting coprecipitation with iron oxyhydroxide flocs. Approximately 60% of the riverine dissolved copper input was removed from solution by this process. Particulate copper concentrations in surficial benthic sediments were high in most regions of the harbour (typically 0.5–1 mg g � 1 ). In the north, sediments were dominated by fine, mine-derived material and showed uniform particulate copper concentrations with depth. Sediment acid-volatile sulphide concentrations were highest (11–142lmol g � 1 ) in the southern harbour and were barely detectable in the northern harbour region (<0.46lmol g � 1 ). A similar north–south gradient of sediment organic carbon concentrations was observed. Very high porewater concentrations of copper (up to 520l gl � 1 ) and iron (200 mg l � 1 ) were found at sites in the northern harbour. The high porewater copper concentrations are believed to result from the oxidation of porewater Fe(II), formation of amorphous iron oxyhydroxide and the associated pH-related dissolution of particulate copper. Calculations indicated a positive flux of dissolved copper from the sediments at sites in the northern harbour. However, in the southern harbour, the high acid volatile sulphide concentrations of the sediments meant that they acted as a sink for dissolved copper, resulting in low porewater copper concentrations (<1–10l gl � 1 ) and a significant copper flux from the overlying water to the sediment. The study illustrates the roles of iron redox chemistry, associated pH gradients, and acid volatile sulphide in controlling copper mobility in contaminated estuarine environments. 2003 Elsevier Science B.V. All rights reserved.

Methane and oxygen dynamics in a shallow floodplain lake: the significance of periodic stratification

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to ∼6  °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l−1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l−1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (∼5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m−3 h−1, respectively. These values correspond to net and gross primary production rates of ∼0.7–1.2 and ∼1.0–1.9 g C m−3 day−1, respectively.

Microphytobenthos Contribution to Nutrient-phytoplankton Dynamics in a Shallow Coastal Lagoon

Nutrient fluxes and primary production were examined in Lake Illawarra (New South Wales, Australia), a shallow (Zmean=1.9 m) coastal lagoon with a surface area of 35 km2, by intensive measurement of dissolved nutrients and oxygen profiles over a 22-h period. Rates of primary production and nutrient uptake were calculated for the microphytobenthos, seagrass beds, macroalgae, and pelagic phytoplankton. Although gross nutrient release rates to the water column and sediment pore waters were potentially high, primary production by microphytobenthos rapidly sequesters the re-mineralized nutrients so that net releases, averaged over times longer than a day, were low. Production in the water column was closely coupled with the relatively low sediment net nutrient release rates and detrital decomposition in the water column. Dissolved inorganic nitrogen and silica concentrations in the water column are drawn down at the beginning of the day. The system did not appear to be light limited so photosynthesis occurs as fast as the nutrients become available to the phytoplankton and microphytobenthos. We conjecture that microphytobenthos are the dominant primary producers and, as has been shown previously, that the nutrient uptake occurs in phase with the various stages of the diatom growth.

Onset and persistence of cyanobacterial blooms in a large impounded tropical river, Australia

The dynamic interplay between physical, chemical and biological factors in the development and persistence of cyanobacterial blooms in impounded rivers is an important topic. Over a 3-year study period, variable climatic conditions were recorded in the Fitzroy River, Queensland, Australia, which is a typical, impounded lowland tropical river. Post-flood turbidity reduced the available light in the well-mixed water column to levels insufficient for cyanobacterial growth. Only when the water column stratified and the slowly sinking particles dropped from the surface layer did the ratio of surface mixed layer depth to euphotic depth approach 1, allowing cyanobacterial growth. By the time the light climate became favorable, most of the dissolved nutrients had been scavenged from the water column by settling particles or sequestered by fringing macrophytes and other biogeochemical processes. Cyanobacterial blooms dominated by Cylindrospermopsis raciborskii persisted for several months until the next flood flushed the system. The cyanobacterial species dominating that environment were very small and had high specific phosphorus uptake rates. Their nutrient requirement was met by transfer across the oxycline driven by regular high wind mixing events, entraining nutrient-rich bottom waters. Nutrient fluxes from the sediments into the anoxic bottom layer were sufficient to replace the bottom nutrients lost to the surface layer.

Phosphorus dynamics in Australian lowland rivers

In freshwater systems, phosphorus is adsorbed predominantly to clay within the sediments. Assuming a linear adsorption/desorption isotherm, rapid equilibrium adsorption, and transport by molecular diffusion, estimates are derived for (a) the rates of exchange between the adsorbed phosphorus pool in the sediments and the dissolved pool in the water column and (b) time scales to re-establish equilibrium after a step change in the water column phosphorus concentration. For oxic sediments, the time scale is of the order of tens of days. Anoxic release is much faster;the time scale is tens of minutes. The release of treated sewage at Narrabri abruptly raises the phosphorus concentration in the Namoi River. The concentration only returns to its original level 10–20 km downstream. A sediment adsorptive-uptake model underestimates the downstream phosphorus uptake rates. An alternative model, based on biotic uptake by Cladophora , describes reality better. It treats phosphorus transfer as controlled by physical transport processes and by the phosphorus uptake capacity of the biota. We show also that carp resuspension is faster than diffusion (6 v. 28 days) in restoring phosphorus concentrations in the water column after perturbation by rapid algal drawdown.

Quantifying ecosystem metabolism in the middle reaches of Murrumbidgee River during irrigation flow releases

The relative importance of floodplain carbon inputs and in-stream metabolic processes have not been well quantified in major Australian rivers. We quantified seasonal phytoplankton primary production and net ecosystem production during irrigation flow regimes at four sites each located ∼100 km apart in the middle Murrumbidgee River. During flow periods dominated by storage release, ecosystem gross primary productivity, system respiration and phytoplankton chlorophyll concentrations all increased downstream so that overall net ecosystem metabolism was strongly net heterotrophic upstream and closer to balanced downstream. Phytoplankton production dominated ecosystem production throughout the entire reach and was likely to have been phosphorus limited throughout the study. Additionally, phytoplankton biomass was limited by short residence times at the upstream sites and nitrogen limited downstream in summer, despite an increase in turbidity. Both production and respiration rates were generally lower in winter, as expected, owing to lower temperatures.

Fitzroy River Basin, Queensland, Australia. I. Identification of Sediment Sources in Impoundments and Flood Events

Environmental Context. The Fitzroy River Basin is a major contributor to the loads of suspended sediment and nutrients reaching coastal areas in the southern Great Barrier Reef. Cost-effective investment in improved land, vegetation, and water management to lower these loads requires an understanding of the sources and movement of sediments within the basin. This multidisciplinary geochemical and modelling study provides for the first time a quantitative estimate of sediment sources and spatial and hydrology-related variation within the Fitzroy River Basin. Abstract. An integrated geochemical, modelling, and reconnaissance soil sampling approach has been used to identify the sources of sediment in the Fitzroy River Basin (FRB). The composition of sediment in weirs and dams within the FRB indicate that in the southern and central FRB the Dawson River contributes only a small basaltic component and the inputs are dominated by soils from the Surat and Bowen Basins. Rivers from the central FRB carry variable amounts of basaltic soils. In contrast, basaltic soils constitute the majority of sediment transported during flood events. Surat Basin soils form a minor component of flood events with little contribution from soils of the Bowen Basin despite it constituting the majority of the area of the central FRB. Soils from the Thomson Fold Belt constitute a substantial proportion of the sediment transported by, and retained in, impoundments in the central FRB and also dominate sediment delivered from the western FRB. This study will inform cost-effective investment by government to target remedial actions to reduce sediment and nutrient loads within the FRB that may be ultimately transported via the Fitzroy River Estuary to the southern Great Barrier Reef.

Fitzroy River, Queensland, Australia. II. Identification of Sources of Estuary Bottom Sediments

Environmental Context. The Fitzroy River Basin constitutes a major source of suspended sediment and nutrient fluxes to the southern Great Barrier Reef. Improved land management practices to ameliorate these catchment loads require an understanding of the sediment sources and dynamics. This multidisciplinary geochemical and modelling study provides for the first time a quantitative estimate of sediment sources delivered to, and their degree of retention in, the Fitzroy River Estuary. Abstract. Sources of sediment deposited in the Fitzroy River Estuary (FRE) have been identified and quantified using an integrated geochemical, modelling and reconnaissance soil sampling approach. A companion paper (this volume) identifies the major sources of sediments in impoundments on the major river systems and sediment sampled from flood events in the Fitzroy River Basin (FRB). Sediment within the FRE may display distinct longitudinal variation with little basaltic material retained. Sediments derived from the Bowen Basin, which occupies the greatest portion of the FRB, and from the Surat Basin display the greatest longitudinal variation. All FRB soils have a similar total phosphorus (P) concentration. Thus, in considering P export from the catchment it is the total sediment flux which is of major importance, rather than the relative proportions of individual catchment soils. This research provides crucial new regional scale information on the sediment sources deposited within the FRE.

Delineation of sediment sources to a coastal wetland in the Great Barrier Reef catchment: influence of climate variability and land clearing since European arrival

Environmental context. Undisturbed sediments provide a record to past events in a catchment. In this study we examine changes in sources of sediment and their variation over the past century due to changes in climate and extensive modification of the catchment after European settlement. We also highlight how multiple lines of forensic evidence acquired from the sediments can be used to reconstruct catchment history over a range of timescales. Abstract. Enhanced delivery of sediment and nutrients to the Great Barrier Reef has the potential to profoundly influence ecological processes in this natural icon. Within the Fitzroy River Basin (FRB) of north-eastern Australia, natural impoundments such as Crescent Lagoon provide an invaluable archive of accumulated sediment that can be dated using multiple techniques to reconstruct the history of sediment export. During the last century, net rates of accumulation of sediment remain similar; however, large variations in sediment sources are apparent. A major sedimentary and geochemical discontinuity is present between ~45 to 29 years before present. Within this time interval a redox front is preserved corresponding to a change in organic matter influx; C3 plant detritus derived from the onset of broadscale agriculture within the FRB provided an assimilable carbon source resulting in more reducing conditions within the sediments. Statistical correlations demonstrate a notable correspondence between some sediment fractions supporting the notion of a short-lived disturbance to the sedimentation regime in the 1960–70s.