The variability in the inherent optical properties along an estuary-coast-ocean continuum in tropical Australia has been studied. The study area, the Fitzroy Estuary and Keppel Bay system, is a shallow coastal environment (depth < 30 m) with highly turbid waters in the estuary and blue oceanic waters in the bay and subject to macrotides. Biogeochemical and inherent optical properties (IOPs) were sampled in the near-surface layer spatially and across the tidal phase during the dry season. These determinations included continuous measurements of spectral absorption, scattering and backscattering coefficients, together with discrete measurements of spectral absorption coefficients of phytoplankton, nonalgal particles and colored dissolved organic matter, and concentrations of phytoplankton pigments and suspended matter. Because of a large variability in the characteristics of the water components on short spatial and temporal scales, we observe a large variability in the associated optical properties. From the estuary to the bay, particle scattering and dissolved absorption decreased by 2 orders of magnitude, and nonalgal particle absorption decreased by 3 orders of magnitude. We also observed a strong variability in particle single scattering albedo and backscattering efficiency (by a factor of 6) and in specific IOPs (IOPs normalized by the relevant constituent concentration) such as suspended matter-specific particle scattering and chlorophyll-specific phytoplankton absorption. Superimposed on this strong spatial variability is the effect of the semidiurnal tide, which affects the spatial distribution of all measured properties. These results emphasize the need for spatially and temporally adjusted algorithms for remote sensing in complex coastal systems.

Geoscience Australia conducted a survey of lakebed (benthic) nutrient fluxes in St Georges Basin, November 2003. The objectives were to: 1. determine the nature of nutrient cycling between the sediment and overlying water; and 2. determine the implications of benthic nutrient fluxes for water quality in the estuary. The relevance to management of this work is that it gives an indication of the susceptibility of the estuary to eutrophication from increased nutrient loads from the catchment. The key findings of the study were: - St Georges Basin was mesotrophic to eutrophic at the time of the survey (spring) based on relatively high respiration rates and O2 demand in the sediments measured by in situ benthic chambers. - Respiration rates were linked to phytoplankton biomass (mainly diatoms) where local fluvial discharge of dissolved nutrients created enhanced primary productivity in the water column, which in turn enhanced mineralisation rates. - St Georges Basin had comparatively low denitrification efficiencies (less than 60%). - St Georges Basin is likely to be prone to eutrophication and may have little tolerance for increases in nutrient and organic matter from the catchment due to the low denitrification efficiencies.

The Fitzoy Estuary is one of several macrotidal estuaries in tropical northern Australia that face ecological change due to agricultural activities in their catchments. The biochemical functioning of such macrotidal estuaries is not well understood in Australia, and there is a pressing need to identify sediment, nutrient and agrochemical pathways, sinks and accumulation rates in these extremely dynamic environments. This is particularly the case in coastal northern Queensland because the impact of water quality changes in rivers resulting from vegetation clearing, changes in land-use and modern agricultural practices are the single greatest threat to the Great Barrier Reef Marine Park. This report includes: 1 Aims and Research questions 2 Study Area 3 Climate and Hydrology 4 Geology 5 Vegetation and land use 6 Methods 7 Sampling strategy 8 Water column observations and samples 9 Bottom sediment properties 10 Core and bottle incubations 11 Data analysis 12 Results 13 Discussion 14 The roll of Keppel Bay in accumulating and redirecting sediment and nutrients from the catchment 15 Sediment biogeochemistry 16 Links between primary production, biogeochemistry and sediment dynamics: A preliminary zonation for Keppel Bay 17 Conclusions

The upper Swan River estuary located in the eastern suburban area of Perth in Western Australia experiences periods of poor water quality in the form high nutrient levels, anoxic bottom water conditions and occasional nuisance algae blooms. It has long been suspected that oxygen uptake and nutrient release from estuarine sediments are major drivers for these poor water quality conditions. Geoscience Australia in conjunction with the Department of Water in Western Australia investigated water quality in the upper Swan River estuary through water and sediment quality studies in October 2006, September 2007 and May 2008. The objectives of these studies were (1) to characterise the distribution of sediments, in particular to identify areas of high nutrient release, (2) to better understand conditions leading to high oxygen consumption and nutrient release, and (3) to determine the influence of the bottom water oxygen status on nutrient release from sediments.

Measured probability distributions of shoreline elevation, swash height (shoreline excursion length) and swash maxima and minima from a wide range of beach types are compared to theoretical probability distributions. The theoretical distributions are based on assumptions that the time series are weakly steady-state, ergodic and a linear random process. Despite the swash process being inherently non-linear, our results indicate that these assumptions are not overly restrictive with respect to modeling exceedence statistics in the upper tail of the probability distribution. The RMS-errors for a range of exceedence level statistics (50, 10, 5, 2, and 1 percent) were restricted to <10 cm (and often <5 cm) for all of the swash variables that were investigated. The results presented here provide the basis for further refinement of coastal inundation modeling as well as stochastic-type morphodynamic modeling of beach response to waves. Further work is required, however, to relate the parameters of swash probability distributions to wave conditions further offshore.

Keppel Bay is a macrotidal environment that represents the interface of the large catchment of the Fitzroy River with the southern GBR continental shelf. In this study, we assessed the distribution of sediments and their depositional characteristics using a combination of sediment sampling, and acoustic (sonar) seabed mapping tools. Using statistical techniques, we classified the seabed sediments of Keppel Bay into five distinct classes, based on sediment grainsize, chemical composition, and modelled seabed hear stress (the influence of waves and tidal currents).

Geoscience Australia's Risk Research Group is using a variety of GIS coverages that span the Fremantle to Hillarys region of the Perth coastal system to assess the vulnerability of the Perth built environment to the potential impact of coastal erosion. Two fundamental questions are asked: whether there is accommodation space in the system that has the potential to act as a sink for eroded sediment, with or without a future sea level rise, and; whether the three-dimensional architecture of the shoreline facies precludes erosion given the current wave and storm climate. Morphological evidence suggest the Garden Island Ridge, up to and including Rottnest Island, has sheltered the coast from prevailing longshore currents. Little sedimentation has occurred in this sector, and consequently there is accommodation space for eroded sediment to be deposited below a level at which it has the capacity to be reworked onto the beach by fair-weather beach building processes. The shoreline geology of the Perth region is dominated by sand and limestone. Shear wave velocities measured through seismic cone penetrometer testing are used in conjunction with natural periods of vibration for the coastal sands to reconstruct the three-dimensional distribution of the erosion-resistant limestone. This reconstruction shows that the upper surface of the limestone is generally above sea level, suggesting the majority of the Perth coastal region is not at risk of significant erosion. At a number of localities, however, the contact between the limestone and the overlying sand is below sea level. These areas are prone to erosion resulting in significant risk to urban development.

The ratio of benthic silicate (SiO4 or DSi1) vs carbon dioxide (CO2) fluxes at 299/341 (87%) sites in 10 Australian estuaries was 0.15 to 0.19 (r2=0.8), indicating diatoms 2,3 as the major type of organic material (OM) being degraded in the sediments . Diatoms contributed 33% of the degradable OM at the remaining 42 (13%) sites. Diatomaceous phytoplankton was thus central to nutrient cycling in these estuaries. Dam-induced DSi depletion in coastal waters, a cause of toxic non-silicious algal blooms is now a global problem1. Our investigation indicated that DSi has another and potentially important global role in nutrient cycling in coastal waterways, also prompting a new DSi-dependent nutrient cycling model. The Si-opal inclusion (frustule) in living diatoms confers a density that causes rapid settling to the sediments4 where denitrification recycles N2 to an atmospheric sink, in this way lowering internal N cycling and preventing eutrophication5. Also, DSi is recycled quantitatively to the water column, participating in repeated cycles of diatom productivity. The silicate-diatom-sediment nexus is thus sustained and the N-loss processes of denitrification and burial are sustained.

Legacy product - no abstract available

Replaced by 78873 Record 2014/003 (A Marshall 29/01/14)