Ian T. Webster

Ten steps applied to development and evaluation of process-based biogeochemical models of estuaries

The procedures involved in model development may be set out as a ten step process, beginning with defining the purpose of the model and ending with evaluation of the appropriateness and utility of the completed model. This process, recently outlined by Jakeman et al. [Jakeman, A.J., Letcher, R.A., Norton, J.P., 2006. Ten iterative steps in development and evaluation of environmental models. Environmental Modelling and Software 21, 602-614], is often iterative as model development is a continuous process that refines and improves the intended capacity of the model. Here, the ten steps of model development are critiqued and applied using a process-based biogeochemical model of aquatic systems, with examples from two case studies: a model of phytoplankton succession and nutrient concentrations in the Swan-Canning Estuary (Western Australia) and a model of sediment and nutrient transport and transformation in the Fitzroy Estuary and Keppel Bay (Queensland).

Modelling the effect of salinity on radium desorption from sediments

Abstract The desorption of the four naturally occurring radium isotopes 223 Ra, 224 Ra, 226 Ra, and 228 Ra from estuarine sediments is investigated. These isotopes are created within sediments by the radioactive decay of insoluble thorium parents. Due to competition from other ions for the occupation of adsorption sites on the sediment grains, radium desorption is a function of salinity. A model is developed to describe radium desorption as it is affected by salinity, grain size, and sediment concentration. It is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters. One of these parameters is the concentration of radium available for exchange on the sediment; the other depends on major ion and radium exchange coefficients. Desorption experiments performed on sediments from the bed of the Bega River, Australia, are used to validate the model and to evaluate the parameters needed for model application.

Anthropogenic impacts on the ecosystems of coastal lagoons: modelling fundamental biogeochemical processes and management implications

This paper presents a biogeochemical model of a coastal lagoon intended to be representative of lagoons occurring along the south-east and south-west coasts of Australia. Many of these lagoons are threatened by increased nutrient loads because of land use change, by alterations to their freshwater inflows and by modification to their tidal flushing regimens. The model simulates the biogeochemical response of the lagoon to nutrient (nitrogen) loading and includes nutrient transformation processes in the sediments, as well as in the water column. The paper focuses on the response of primary producers to increasing and decreasing nutrient loads and how the response is altered by changes in the flushing rate of the lagoon with the sea. In common with lakes, the modelled lagoon exhibits alternative stable states representing macrophyte or phytoplankton dominance depending on nutrient loading and history. A third state representing severe degradation occurs when denitrification shuts down. A characteristic of Australian coastal lagoon systems is that, due to highly sporadic rainfall patterns, nutrient inflows are dominated by intermittent extreme events. The modelling demonstrates that such a loading regimen may be expected to generally increase the vulnerability of the lagoon to increasing nutrient loads. The results of the analysis presented are pertinent to several questions raised by coastal managers, such as what are the expected benefits of improving flushing by dredging and what are the consequences of altering the timing and magnitudes of the loads reaching the lagoons?

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.

Horizontal mixing of Great Barrier Reef waters: offshore diffusivity determined from radium isotope distribution

The Great Barrier Reef (GBR), northern Australia, is the largest coral reef system in the world and provides habitat for highly diverse tropical marine ecosystems. Mixing in the coastal waters of the GBR is an important parameter influencing the health of these ecosystems. We have used the distribution of the four naturally occurring radium isotopes to determine the rate of mixing of nearshore waters of the central part of the GBR lagoon with water from the Coral Sea. The observed radium distribution is modeled using a one-dimensional diffusion model. The model improves on previous radium offshore mixing models by incorporating the benthic flux of radium diffusing across the sediment-water interface and offshore changes in water column depth. We find that the inner lagoon diffusivity (<20 km offshore) is best estimated using the short-lived isotopes 224Ra and 223Ra. The concordance of K x estimated using the two different isotopes and the apparent consistency between measured riverine inflows to the lagoon and inflows inferred from the modeled salinity distribution provide confidence in the results. The mean value of K x for the inner lagoon region of the southern central zone between latitudes 15.8°S and 19.0°S (265 ± 36 m2 s−1) is more than twice that in the northern central zone (14.3°S to 15.8°S). This difference likely reflects the different reef matrix density in the two zones. The distribution of the longer-lived isotope 228Ra indicates more rapid mixing in the middle and outer lagoon. These results indicate that central GBR water within 20 km of coast is flushed with outer lagoon water on a timescale of 18–45 days, with the flushing time increasing northward.

Pore water sampling with sediment peepers

Abstract The use of in situ equilibrium dialysis samplers (peepers) for the collection of sediment pore waters for trace metal analysis is reviewed. Optimum peeper designs, construction and preparation procedures are described. Field deployment and sampling protocols are outlined with their utility illustrated by dissolved metal profiles obtained from field studies.

An analysis of primary production in the Daly River, a relatively unimpacted tropical river in northern Australia

In this paper, the dynamics of primary production in the Daly River in tropical Australia are investigated. We used the diurnal-curve method for both oxygen and pH to calculate photosynthesis and respiration rates as indicators of whole-river productivity. The Daly River has maximum discharges during the summer, monsoonal season. Flow during the dry season is maintained by groundwater discharge via springs. The study investigated how primary production and respiration evolve during the period of low flow in the river (April–November). The relationship between primary production and the availability of light and nutrients enabled the role of these factors to be assessed in a clear, oligotrophic tropical river. The measured rate of photosynthesis was broadly consistent with the estimated mass of chlorophyll associated with the main primary producers in the river (phytoplankton, epibenthic algae, macroalgae, macrophytes). A significant result of the analysis is that during the time that plant biomass re-established after recession of the flows, net primary production proved to be ~4% of the rate of photosynthesis. This result and the observed low-nutrient concentrations in the river suggest a tight coupling between photosynthetic fixation of carbon and the microbial degradation of photosynthetic products comprising plant material and exudates.

Rotational dispersion in porous media due to fluctuating flows

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.

The use of mechanistic descriptions of algal growth and zooplankton grazing in an estuarine eutrophication model

A simple model of estuarine eutrophication is built on biomechanical (or mechanistic) descriptions of a number of the key ecological processes in estuaries. Mechanistically described processes include the nutrient uptake and light capture of planktonic and benthic autotrophs, and the encounter rates of planktonic predators and prey. Other more complex processes, such as sediment biogeochemistry, detrital processes and phosphate dynamics, are modelled using empirical descriptions from the Port Phillip Bay Environmental Study (PPBES) ecological model. A comparison is made between the mechanistically determined rates of ecological processes and the analogous empirically determined rates in the PPBES ecological model. The rates generally agree, with a few significant exceptions. Model simulations were run at a range of estuarine depths and nutrient loads, with outputs presented as the annually averaged biomass of autotrophs. The simulations followed a simple conceptual model of eutrophication, suggesting a simple biomechanical understanding of estuarine processes can provide a predictive tool for ecological processes in a wide range of estuarine ecosystems.

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.