Karen Wild-Allen

The exposure of the Great Barrier Reef to ocean acidification.

The Great Barrier Reef (GBR) is founded on reef-building corals. Corals build their exoskeleton with aragonite, but ocean acidification is lowering the aragonite saturation state of seawater (Ωa). The downscaling of ocean acidification projections from global to GBR scales requires the set of regional drivers controlling Ωa to be resolved. Here we use a regional coupled circulation–biogeochemical model and observations to estimate the Ωa experienced by the 3,581 reefs of the GBR, and to apportion the contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ωa variability. We find more detail, and a greater range (1.43), than previously compiled coarse maps of Ωa of the region (0.4), or in observations (1.0). Most of the variability in Ωa is due to processes upstream of the reef in question. As a result, future decline in Ωa is likely to be steeper on the GBR than currently projected by the IPCC assessment report.

New evidence links changing shelf phytoplankton communities to boundary currents in southeast Tasmania

Southern Tasmanian shelf waters are host to the seasonal interplay of Australia’s two poleward boundary currents; the East Australian Current (EAC) and the Leeuwin Current (LC). While the behaviour and properties of the LC remain underexplored, strong research focus has allowed insight into how an intensifying EAC has created greater subtropical influence, leading to changes in the physical and biological oceanography of the region. In this cool temperate setting seven species of dinoflagellates, all in the genus Ceratium, which are more typically associated with warm waters of eastern Australia, were observed. This coincided with the seasonal increase in the EAC’s southward penetration beginning in October. Despite the seasonal peak in EAC activity, temperature-salinity plots, nutrient, chlorophyll a and phytoplankton concentrations all indicate the presence of subantarctic waters on the shelf and in coastal waters in summer. Our results are consistent with the description of the EAC as an erratic, eddy-driven current; this itself allowing the periodic influx of subantarctic waters across the shelf. In winter, temperature-salinity plots and nutrient concentrations indicate that the LC was present in southern shelf waters. In addition to its high nitrate signature, the LC displayed low silicate properties in southern Tasmania. Chlorophyll a concentrations revealed a distinct spring bloom event and an extended, productive summer, typical of temperate and subantarctic systems, respectively. This suggests the region is a transitional state between classic seasonal primary production cycles for temperate and subantarctic waters. This paper links changes in southern Tasmanian microphytoplankton communities to shelf ventilation by the EAC, the LC and subantarctic waters, and provides new insight into the oceanography of the region. Consequently, this study provides an awareness of potential phytoplankton perturbations that may be applied to other coastal cool temperate marine environments.

Development of operational data-assimilating water quality modelling system for South-East Tasmania

With the rapid advances in on-line observing system applications, the paradigm in environmental modelling is shifting from one-off models for specific purposes, to operational models, sequentially assimilating data streams from in situ and remote sensors. Such models can provide products and services to support a wide range of applications, from short-term forecasting to long-term scenarios, and are expected to deliver superior performance much more cost-effectively. In the marine field, this is most advanced for circulation models at large ocean scales. The potential benefit from these advances is even greater in the coastal zone, where human uses, impacts and ecosystem services are concentrated. However, there are substantial challenges to be overcome. Coastal applications typically require biogeochemical, ecological, and ultimately socioeconomic models. These additional models are more complex, with higher uncertainty, and require different approaches to data assimilation and uncertainty analysis. The uncertainties arise from a number of sources including poorly known parameters, structural errors and stochastic forcing. When model realisations are sufficiently fast, Monte Carlo techniques can be used to improve the model performance and assess its quality, otherwise alternative estimation techniques are required. This paper describes the development of an operational, data-assimilating coastal model for SE Tasmania, integrating across hydrodynamics, sediment dynamics and biogeochemistry. Inputs and outputs from the model are expected to be integrated into the regional information system (INFORMD), and to be used directly in multiple management applications, and as input into ecosystem models. A hydrodynamic model, nested inside an operational global model, will be assimilating data from the coastal sensor network and other sources, including remote sensing. The model is based on an operational modelling platform developed by CSIRO through the BlueLink project (ROAM), and will be used to implement and test data-assimilation techniques for coastal models under development in BlueLink. Operational sediment dynamic and biogeochemical models, will be coupled to the hydrodynamic model, either directly or through intermediate transport models. Data-assimilating techniques for these models currently are under development in Computational and Simulation Sciences theme, CSIRO. This paper outlines preliminary results from these developments. A number of candidate techniques including Kalman Filter, Particle Filter and MCMC are discussed. The utility of fast and cheap statistical surrogates of complex models (emulators) for sequential data assimilation is illustrated through the trial application of emulators to one-dimensional sediment/pollutant and 3-d sediment transport models.

Quantification of the impacts of finfish aquaculture and bioremediation capacity of integrated multi-trophic aquaculture using a 3D estuary model

Reduced water quality is a potential outcome from intensive finfish aquaculture. Integrated multi-trophic aquaculture (IMTA) can mitigate the negative effects of dissolved nutrients emanating from fish farms by harvesting species that extract nutrients grown at adjacent sites. In this study, a coupled 3D hydrodynamic, sediment, and biogeochemical model was used to simulate an idealized temperate test estuary. A macroalgal-based IMTA model was applied within the estuarine model, to examine the spatial pattern of phytoplankton production arising from increasing levels of finfish aquaculture and the capacity of Macrocystis pyrifera to bioremediate the impacts of nutrification. Through increasing fish farm waste loads of dissolved inorganic nitrogen (DIN), the water quality in the estuarine model was forced into a “poor water quality” classification as determined by annual mean concentration of chlorophyll. Primary production was greatest in the northern section of the estuary due to circular water motion set up by a region of freshwater influence (ROFI). A nonlinear increase in annual phytoplankton biomass was simulated (under elevated finfish loads) due to the occurrence of an additional autumn phytoplankton bloom under elevated fish farm nutrient loads. IMTA scenario results demonstrated a strong spatial variability in the capacity of M. pyrifera-based IMTA to reduce water column chlorophyll concentration. Siting macroalgae farms next to those finfish farms situated in areas of high natural phytoplankton production resulted in a “good water quality” classification for the whole system. This demonstration of the use of IMTA to improve system wide water quality is valuable for regional planners and managers as it provides an analysis and quantification of a method to achieve estuarine health and economic benefit.

Simulated fate of catchment-derived sediment on the Great Barrier Reef shelf

Numerical experiments using a 3D model of fine sediment transport in the Great Barrier Reef region indicate deposition of the bulk mass of catchment sediments from river plumes within a few tens of kilometres from river mouths. A very fine fraction of easily resuspended catchment sediment has a capacity to propagate over much greater distances reaching out into the mid-shelf and outer-shelf regions. The model suggests such particles, instrumental to the development of low density flocs in the marine environment, can play a critical role in altering optical properties of water masses over the shelf during wet years. The mid-term (4 year) impact of Great Barrier Reef catchments on the probability of suspended sediment concentration exceeding the ecologically significant trigger value of 2 mg/L is confined to inshore regions adjacent to river mouth locations.

Autonomous profiling glider observations in Storm Bay

Glider data collected along a sawtooth section crossing Storm Bay in south east Tasmania, show high spatial and temporal variability in temperature, salinity, fluorescence and dissolved oxygen . A Slocum glider was used to repeat 6 transects from the mouth of the Derwent Estuary to the continental shelf in autumn 2009. The instrument was deployed for 25 days, travelled 974 km and dived to within 10m of the bottom (50–160m) providing high resolution (5 second sampling interval) profiles of water properties. Data were quality controlled by comparison with calibrated CTD casts and bottle samples at the beginning and end of the deployment. This very high resolution dataset is the first of its kind for the region and provides a unique insight into the autumnal water column dynamics of Storm Bay.