Ocean alkalinity enhancement (OAE) is a promising marine carbon dioxide removal (mCDR) method that aims to reduce atmospheric CO2 by increasing the ocean's storage capacity. While global ocean and Earth system models are necessary to track air-sea CO2 equilibration and far-field alkalinity transport on extended timescales, OAE efficiency is strongly influenced by local oceanography and climate. Regional ocean modeling is a powerful tool for capturing the effects of these influences on near-field plume dispersion and CO2 uptake variability. Here, we used a high-resolution, three-dimensional hydrodynamic and biogeochemical model (2 km) to evaluate the effects of deployment location and interannual climate variability on OAE efficiency in Bass Strait, southeast Australia. We simulated the addition of 113.21 Gmol of alkalinity (a theoretical uptake capacity of ~4.2 Mt CO2) over one month, via four infrastructure-constrained pathways: a desalination outfall, a shipping lane, a ferry track, and a series of coastal outfalls. These additions were repeated across three Southern Annular Mode (SAM) phase end-member winters: 2017 (positive SAM–low winds), 2021 (neutral SAM–moderate winds), and 2023 (negative SAM–strong winds). CO2 uptake efficiency variability (mol CO2/mol TA) is primarily influenced by delivery method and location (95.2%) rather than by interannual climate variability (3.9%). At the shelf-break, 70.67% ± 10.53% of added alkalinity is subducted below the mixed layer before equilibration with the atmosphere; this alkalinity is then exported from the Bass Strait region at depth. The subduction and loss of alkalinity from the region before equilibration reduces the realised CO2 uptake and contributes to efficiencies (0.11–0.27) that are lower than those simulated by a global model in the same region (0.31). This mismatch, driven by regionally specific oceanographic processes, has implications for OAE deployment and equilibration timescales in other dynamic shelf environments. To resolve these scale-dependent limitations, we recommend integrated monitoring, reporting, and verification frameworks that combine observational networks, regional models, and global models (with regional model exports as input to global models). This approach is necessary to accurately quantify net carbon removal and to constrain the long-term fate of added alkalinity.

Abstract. We present a new regional coupled atmosphere and ocean model of the Great Barrier Reef (GBR), Australia, called ACCESS-EMS-GBR. The model comprises a regional configuration of the Australian Community Climate and Earth System Simulator (ACCESS) atmospheric model (the United Kingdom Met Office Unified Model), coupled to the Commonwealth Scientific and Industrial Research Organisation Environmental Modelling Suite (EMS) hydrodynamic and biogeochemical model of the GBR. The model includes detailed aerosol and cloud microphysics in the atmosphere, as well as hydrodynamics, biogeochemistry, ecology and coral reef processes in the ocean. When coupled, the models form a valuable tool for simulating interactions between the atmosphere and ocean. The ACCESS-EMS-GBR model was developed to assess the plausibility and implications of marine cloud brightening (MCB) in the GBR, as part of the Reef Restoration and Adaptation Program (RRAP) Cooling and Shading Sub-program (RRAP-CS) but can also be utilised to study various processes including sources of marine aerosol and the impact of aerosol deposition on marine biogeochemistry. Here, we describe the model as configured for the GBR region, including the coupling framework, and evaluate the model skill in simulating measured aerosol and seawater temperature and irradiance. We configure the model aerosol scheme and emission sources to capture observed aerosol properties over the GBR region, with boundary layer nucleation and terrestrial biogenic emissions having a large influence on aerosol number concentration, size distribution and composition. The distribution of sea salt emissions is also expanded to include the soluble Aitken mode; a significant improvement in capability for simulating MCB and possibly for improving global climate model biases.

The Coral Sea houses expansive coral reefs. Reef health is inextricably linked to water temperatures, which are regulated by the hydrodynamic environment. The ocean current system in the Coral Sea is dominated by jets of the South Equatorial Current (SEC): the North Vanuatu Jet (NVJ), the North Caledonian Jet (NCJ) and the South Caledonian Jet (SCJ). We investigated the projected near-future (2050) changes in the temperature and transport structure of the Coral Sea using the three highest-resolution climate models from the high-resolution coupled modelling experiment (HighResMIP) conducted within the latest iteration of the Coupled Model Intercomparison Project Phase 6 (CMIP6). We found that the HighResMIP models successfully represented the historical temperature and transport structure of the SEC jets and their El Ni & ntilde;o-Southern Oscillation-related variability. Surface ocean warming of 0.78 and 1.12 degrees C was projected in the Coral Sea under 1.5 and 2 degrees C global air temperature warming, respectively. The maximum depth of the warming signal deepened by 30 m per decade, penetrating to 400 m by 2050. This indicated the additional thermal stress that could be experienced by Coral Sea ecosystems. Interestingly, the surface warming was associated with a sub-surface cooling between 400 and 600 m. Decreases in the transports of the NVJ and NCJ and an intensification of the SCJ were also projected in the HighResMIP models. The magnitudes of the changes were relatively small (2 % to 7 % of historical means) and of a similar order to the variability in transport associated with the El Ni & ntilde;o-Southern Oscillation. Our analysis further showed that the transport projections of the NVJ and NCJ varied with depth, where surface intensifications coincided with the areas of greatest warming. These changes could modify western boundary currents and upwelling dynamics on the Great Barrier Reef shelf.

Solar radiation propagating through the water column is scattered and absorbed by optically active constituents in the ocean, in particular phytoplankton, coloured-dissolved organic matter (CDOM), suspended inorganic particulate matter (SPIM) and detritus. These wavelength-dependent processes affect the vertical distribution of heating in the water column and its stratification. The continental shelf north-east of Australia, containing the Great Barrier Reef (GBR), is characterised by highly seasonal and intermittent freshwater inputs leading to large sediment and nutrient discharges that strongly impact the water optical properties. While this complex mixture of optically active constituents is known to affect water clarity and the euphotic zone depth in the river plumes, its impact on the ocean circulation and thermal balance is still unclear at the scale of the GBR. In this study, we use a hydrodynamic-optical-biogeochemical ocean model to investigate the feedback between heat absorption by phytoplankton, CDOM and suspended sediments and ocean dynamics in the GBR region. The results show that the attenuation of the vertical heat flux due to phytoplankton, CDOM and SPIM concentrations is stronger on the continental shelf and dominated by the absorption and scattering from suspended sediments. The presence of absorbing constituents in the water column drives a temperature increase at the surface and a decrease below the mixed layer with stronger stratification and greater heat losses to the atmosphere. Inshore, the ocean heat content increases by up to 1% due to optically active constituents. Offshore, absorption by optically active constituents near the surface is compensated by less absorption underneath the mixed layer resulting in a decrease in the ocean heat content of the top 500 m. We find that considering a spatially- and temporally-variable vertical attenuation of heat due to multiple optically-active components improves hydrodynamic model skill. This study highlights the importance of the impact of water clarity and its spatial variability on hydrodynamic processes.

Carbon Dioxide Removal (CDR) is increasingly recognised as essential for achieving net zero emissions to limit the impacts of climate change. Ocean Alkalinity Enhancement (OAE) presents a potentially scalable marine CDR (mCDR) technique. Here we report on the first OAE field trial in Australia, conducted at a coastal site in Tasmania using continuous addition of aqueous sodium hydroxide (NaOH). The resulting plume of modified seawater was effectively tracked, and changes in surface carbonate chemistry were quantified using a containerised laboratory. At the point of NaOH release, partial pressure of CO$$_2$$ (pCO$$_2$$) decreased by up to 370 $$\mu$$atm with alkalinity increasing by approximately 545 $$\mu$$mol kg$$^{-1}$$. Maximum downstream decreases in pCO$$_2$$ ranged from 22 to 77 $$\mu$$atm, corresponding to signal strengths of < 1 - 5$$\%$$. This small-scale field trial confirmed that the dispersion of a plume of modified seawater occurs rapidly and within meters of the site of addition, and that with appropriate tools, these changes can be measured directly in a coastal ocean location. These results suggest that the deployment of shore-based OAE, in combination with local coastal infrastructure and regional models, have potential as an mCDR approach.

Coral bleaching is occurring more frequently as the climate changes, with multiple mass mortality events recently recorded on the Great Barrier Reef. Thermal stress coupled with high irradiance have previously been shown to be primary causes for coral bleaching. Therefore, a reduction in either of these pressures could reduce coral stress and eventual bleaching. Herein, we report the early development of a novel technology capable of reducing the amount of light entering a water body by ~20% in open ocean conditions. This mono-particle “sun shield” consists of an ultra-thin monolayer material and reflective calcium carbonate particles. The monolayer enables spreading of the particles into a thin film across the water surface, with only small amounts of material needed: 7.1 g/m2. A numerical modelling case study of residence times and the build-up of reactive oxygen stress in corals showed that the successful application of a stable film over the Lizard Island reef flat could reduce the reactive oxygen stress to below bleaching levels across approximately 1.5 km2 of reef area. With further development, mono-particle films such as this have the potential to be deployed over at-risk coral reefs at relatively small scales during predicted heatwave conditions, potentially reducing the severity of bleaching on coral reefs.

The gravitational sinking of organic debris from ocean ecosystems is a dominant mechanism of the biological carbon pump (BCP) that regulates the global climate. The fraction of primary production exported downward, the e‐ratio, is an important but poorly constrained BCP metric. In mid‐ and high‐latitude oceans, seasonal and local variations of sinking particle fluxes strongly modulate the e‐ratio. These locally specific e‐ratio variations and their ecological foundations are here encapsulated in the term “export systems” (ES). ES have been partly characterized for a few ocean locations but remain largely ignored over most of the ocean surface. Here, in a fully conceptual approach and with the primary aim to understand rather than to estimate ocean carbon export, we combine biogeochemical (BGC) modeling with satellite observations to map ES at fine spatio‐temporal scales. We identify four plausible ES with distinct e‐ratio seasonalities across mid‐ and high‐latitude oceans. The ES map confirms the outlines of traditional BGC provinces and unveils new boundaries indicating where (and how) the annual relationship between carbon export and production changes markedly. At six sites where ES features can be partially inferred from in situ data, we test our approach and propose key ecological processes driving carbon export. In the light of our findings, a re‐examination of 1,841 field‐based e‐ratios could challenge the conventional wisdom that e‐ratios change strongly with latitude, suggesting a possible seasonal artifact caused by the timing of observations. By deciphering carbon export mechanistically, our conceptual ES map provides timely directions to emergent ocean robotic explorations of the BCP.

Combining ecological field observations and mapping with model outputs is a powerful approach for understanding complex marine and coastal systems.In our study over three wet seasons, we used the eReefs 1 km resolution marine model to simulate the dispersal of diuron, a pesticide commonly found in river catchments that flow into the Great Barrier Reef (GBR) marine environment.By mapping the model outputs together with observed coastal coral reef and seagrass beds, we were able to determine the areas of the GBR, seagrass and coral where diuron plumes exceeded ecologically toxic concentrations (1400, 175 and 60 km2 respectively).This information can guide decision-making processes by producing a more comprehensive understanding of the catchment and ocean dynamics within the region.Moreover, marine model outputs using scenarios of agricultural runoff can be used to make predictions about potential changes such as increased pesticide use and its impact on ecologically sensitive areas.By providing a more complete understanding of complex ecological systems, this approach can help decision-makers make more informed decisions and better manage sediment, nutrient, and pesticide runoff from catchments.In addition, once ecologically vulnerable marine areas have been identified, models such as the eReefs marine model can be used to determine the best field site locations for monitoring and collecting ecological data.For example, models can predict where certain species are likely to occur and where river plumes converge, guiding the selection of field sites.By using models to guide field site selection, researchers can maximize the efficiency and location of their data collection efforts and obtain a more comprehensive understanding of the marine ecosystem.

Pesticides from urban and agricultural runoff have been detected at concentrations above current water quality guidelines in the Great Barrier Reef (GBR) marine environment. We quantify the load of the pesticide diuron entering GBR waters using the GBR-Dynamic SedNet catchment model. After comparison of simulated distributions with observations at 11 monitoring sites we determined a half-life of diuron in GBR marine waters of 40 days. We followed diuron dispersal in the GBR (2016-2018) using the 1 km resolution eReefs marine model. The highest diuron concentrations in GBR waters occurred in the Mackay-Whitsunday region with a spike in January and March 2017, associated with 126 and 118 kg d-1 diuron loads from Plane Creek and the O'Connell River respectively. We quantify areas of GBR waters exposed to potentially ecotoxic concentrations of diuron. Between 2016 and 2018, 400 km2 and 1400 km2 of the GBR were exposed to concentrations exceeding ecosystem threshold values of 0.43 and 0.075 μg L-1 respectively. Using observed mapped coral and seagrass habitat, 175 km2 of seagrass beds and 50 km2 of coral habitats had peak diuron concentrations above 0.075 μg L-1 during this period. While the highest concentrations are localised to river plumes and inshore environments, non-zero diuron concentrations extend along the Queensland coast. These simulations provide new knowledge for the understanding of pesticide dispersal and management-use in GBR catchments and the design of in-water monitoring systems.

Macroalgae are an important component of coral reef ecosystems. We identified spatial patterns, environmental drivers and long-term trends of total cover of upright fleshy and calcareous coral reef inhabiting macroalgae in the Great Barrier Reef. The spatial study comprised of one-off surveys of 1257 sites (latitude 11-24°S, coastal to offshore, 0-18 m depth), while the temporal trends analysis was based on 26 years of long-term monitoring data from 93 reefs. Environmental predictors were obtained from in situ data and from the coupled hydrodynamic-biochemical model eReefs. Macroalgae dominated the benthos (≥50% cover) on at least one site of 40.4% of surveyed inshore reefs. Spatially, macroalgal cover increased steeply towards the coast, with latitude away from the equator, and towards shallow (≤3 m) depth. Environmental conditions associated with macroalgal dominance were: high tidal range, wave exposure and irradiance, and low aragonite saturation state, Secchi depth, total alkalinity and temperature. Evidence of space competition between macroalgal cover and hard coral cover was restricted to shallow inshore sites. Temporally, macroalgal cover on inshore and mid-shelf reefs showed some fluctuations, but unlike hard corals they showed no systematic trends. Our extensive empirical data may serve to parameterize ecosystem models, and to refine reef condition indices based on macroalgal data for Pacific coral reefs.

In times of rapid change and rising human pressures on marine systems, information about the future state of the ocean can provide decision-makers with time to avoid adverse impacts and maximise opportunities. An ecological forecast predicts changes in ecosystems and its components due to environmental forcing such as climate variability and change, extreme weather conditions, pollution, or habitat change. Here, we summarise examples from several sectors and a range of locations. We describe the need, approach, forecast performance, delivery system, and end user uptake. This examination shows that near-term ecological forecasts are needed by end users, decisions are being made based on forecasts, and there is an urgent need to develop operational information systems to support sustainable ocean management. An operational information system is critical for connecting to decision makers and providing an enduring approach to forecasting and proactive decision making. These operational systems require significant investment and ongoing maintenance but are key to delivering ecological forecasts for societal benefits. Iterative forecasting practices could provide continuous improvement by incorporating evaluation and feedback to overcome the limitations of the imperfect model and incomplete observations to achieve better forecast outcomes and accuracy.

Improvements in physiological understanding, observational capabilities, and computational power have led modelers of phytoplankton in natural systems to represent more phytoplankton classes with better differentiated characteristics. In this chapter, we describe the key phytoplankton processes that are used to differentiate plankton, including phytoplankton absorption, photoadaptation, and elemental stoichiometry ratios. We then briefly survey model applications, in which resolving phytoplankton diversity matter are important: ocean-color, harmful algal blooms, carbon export to the deep ocean, as well as climate change-induced shifts in planktonic communities. Researchers are finding significant benefits from improved representation of phytoplankton types, so it is likely this work will continue to develop in the future.

The Great Barrier Reef (GBR) is a globally significant coral reef system supporting productive and diverse ecosystems. The GBR is under increasing threat from climate change and local anthropogenic stressors, with its general condition degrading over recent decades. In response to this, a number of techniques have been proposed to offset or ameliorate environmental changes. In this study, we use a coupled hydrodynamic-biogeochemical model of the GBR and surrounding ocean to simulate artificial ocean alkalinisation (AOA) as a means to reverse the impact of global ocean acidification on GBR reefs. Our results demonstrate that a continuous release of 90 000 t of alkalinity every 3 d over one year along the entire length of the GBR, following the Gladstone-Weipa bulk carrier route, increases the mean aragonite saturation state ( Ωar ) across the GBR’s 3860 reefs by 0.05. This change offsets just over 4 years (∼4.2) of ocean acidification under the present rate of anthropogenic carbon emissions. The injection raises Ωar in the 250 reefs closest to the route by ⩾0.15 , reversing further projected Ocean Acidification. Following cessation of alkalinity injection Ωar returns to the value of the waters in the absence of AOA over a 6 month period, primarily due to transport of additional alkalinity into the Coral Sea. Significantly, our study provides for the first time a model of AOA applied along existing shipping infrastructure that has been used to investigate shelf scale impacts. Thus, amelioration of decades of OA on the GBR is feasible using existing infrastructure, but is likely to be extremely expensive, include as yet unquantified risks, and would need to be undertaken continuously until such time, probably centuries in the future, when atmospheric CO2 concentrations have returned to today’s values.

Water quality of the Great Barrier Reef (GBR) is determined by a range of natural and anthropogenic drivers that are resolved in the eReefs coupled hydrodynamic - biogeochemical marine model forced by a process-based catchment model, GBR Dynamic SedNet. Model simulations presented here quantify the impact of anthropogenic catchment loads of sediments and nutrients on a range of marine water quality variables. Simulations of 2011-2018 show that reduction of anthropogenic catchment loads results in improved water quality, especially within river plumes. Within the 16 resolved river plumes, anthropogenic loads increased chlorophyll concentration by 0.10 (0.02-0.25) mg Chl m-3. Reductions of anthropogenic loads following proposed Reef 2050 Water Quality Improvement Plan targets reduced chlorophyll concentration in the plumes by 0.04 (0.01-0.10) mg Chl m-3. Our simulations demonstrate the impact of anthropogenic loads on GBR water quality and quantify the benefits of improved catchment management.

Recently, corals on the Great Barrier (GBR) have suffered mass bleaching. The link between ocean warming and coral bleaching is understood to be due to temperature-dependence of complex physiological processes in the coral host and algal symbiont. Here we use a coupled catchment-hydrodynamic-biogeochemical model, with detailed zooxanthellae photophysiology including photoadaptation, photoacclimation and reactive oxygen build-up, to investigate whether natural and anthropogenic catchment loads impact on coral bleaching on the GBR. For the wet season of 2017, simulations show the cross-shelf water quality gradient, driven by both natural and anthropogenic loads, generated a contrasting zooxanthellae physiological state on inshore versus mid-shelf reefs. The relatively small catchment flows and loads delivered during 2017, however, generated small river plumes with limited impact on water quality. Simulations show the removal of the anthropogenic fraction of the catchment loads delivered in 2017 would have had a negligible impact on bleaching rates.

The Great Barrier Reef (GBR) is of immense biological, cultural and economic importance, but has also rapidly degraded over the last 30 years. Improved spatial information on reef geomorphic zonation and benthic cover type (including coral type) is critical to support scientific work to understand how the GBR is changing, and to support resource management decisions that enable conservation of the reef and its essential ecosystem services. Yet, no comprehensive maps exist that detail the geomorphic zonation or benthic cover for the GBR's ~ 3000 reefs. This study presents three new types of shallow reef maps for 237 reefs in the central Cairns Management Region of the GBR Marine Park (GBRMP), explores how the detailed habitat maps created compared to current maps and posits how the new maps may support and refine current critical key science outputs and management challenges.Geomorphic Zonation,Benthic CoverandCoral Typehabitat maps were created using a unique combined object-based image analysis and ecological modelling approach that incorporated satellite imagery, limited field data and key reef physical attributes (depth, slope, waves) using a previously peer-reviewed mapping approach developed for the Capricorn Bunker Group reefs, Southern GBR. The mapping approach was consistent and repeatable, suggesting applicability to mapping all 3000 reefs in the GBRMP. Compared to existing maps that only outline each reef, the increase in detail provided by these new habitat maps enabled discrete characterisation of each reef's geomorphology and benthic composition. With the new habitat maps, areas within each reef can be identified as either coral habitat or not coral habitat. This has not been possible previously. As such, a model of coral ecological and biophysical processes that depends on bottom reflectance of sand and coral areas can be fine-tuned. Similarly, for reef restoration, nursery structures are commonly placed in non-coral habitats, and/or coral larvae are dispersed in areas of known coral habitat. The new habitat maps presented permit more accurate identification of these areas such that restoration projects can be targeted more effectively. These findings confirm the need to now apply this mapping approach to the full extent of the GBR.

Coral bleaching driven by ocean warming is one of the most visible ecological impacts of climate change and perhaps the greatest threat to the persistence of reefs in the coming decades. In the absence of returning atmospheric greenhouse gas concentrations to those compatible with ocean temperatures below the mass coral bleaching temperature thresholds, the most straightforward means to reduce thermal-stress induced bleaching is to cool water at the seabed. The feasibility of reducing the seabed temperature through cool-water injections is considered first by analysing the feasibility of doing so on 19 reefs with differing physical environments using a simple residence time metric in 200 m resolution hydrodynamic model configurations. We then concentrate on the reefs around Lizard Island, the most promising candidate of the 19 locations, and develop a 40 m hydrodynamic model to investigate the effect of the injection of cool water at differing volumetric rates. Injecting 27°C seawater at a rate of 5 m3 s-1 at 4 sites in early 2017 cooled 97 ha of the reef by 0.15°C or more. The power required to pump 5 m3 s-1 through a set of pipes over a distance of 3 km from a nearby channel is ∼466 kW. This power applied at 4 sites for 3 months achieves a 2 Degree Heating Weeks (DHWs) reduction on 97 ha of reef. A more precise energy costing will require further expert engineering design of the pumping equipment and energy sources. Even for the most physically favourable reefs, cool-water transported through pipes and injected at a reef site is energy expensive and cannot be scaled up to any meaningful fraction of the 3,100 reefs of the GBR. Should priority be given to reducing thermal stress on one or a few high value reefs, this paper provides a framework to identify the most promising sites.

Seawater acidification from increasing CO2 is often enhanced in coastal waters due to elevated nutrients and sedimentation. Our understanding of the effects of ocean and coastal acidification on present-day ecosystems is limited. Here we use data from three independent large-scale reef monitoring programs to assess coral reef responses associated with changes in mean aragonite saturation state (Ωar ) in the Great Barrier Reef World Heritage Area (GBR). Spatial declines in mean Ωar are associated with monotonic declines in crustose coralline algae (up to 3.1-fold) and coral juvenile densities (1.3-fold), while non-calcifying macroalgae greatly increase (up to 3.2-fold), additionally to their natural changes across and along the GBR. These three key groups of organisms are important proxies for coral reef health. Our data suggest a tipping point at Ωar 3.5-3.6 for these coral reef health indicators. Suspended sediments acted as an additive stressor. The latter suggests that effective water quality management to reduce suspended sediments might locally and temporarily reduce the pressure from ocean acidification on these organisms.