Creating communities and communicating science during COVID-19: From Coast2Coast to Coast2Cast.

The global COVID-19 pandemic has seen extended lockdowns, isolation periods and travel restrictions across many countries around the world since early 2020. Some countries, such as Australia and New Zealand, closed their international borders in early 2020 preventing researchers travelling to other parts of the world. To facilitate the exposure of our students' work, and for them to meet international researchers, as well as foster a sense of coastal community, we started a zoominar series (seminars via Zoom) in April 2020. The Coast2Coast zoominar series had therefore humble origins but we soon discovered that there was an appetite for more widely sharing science across the coastal research disciplines. The Coast2Coast zoominar grew rapidly, attracting researchers from many countries around the world who presented and attended fortnightly online seminars. In just one year and a half we had 38 presentations with roughly 1900 attendees, creating a sense of community and belonging for the researchers involved. In early 2021, two of the co-authors, Giovanni (GC) and Ana (AVC) decided to expand and take this sense of community further creating the Coast2Cast podcast series, where researchers are asked research and non-research questions. In only 7 months, the podcasts have attracted more than 3700 listeners. Importantly, while the main prerequisite was high-quality and impactful research, diversity and inclusion were also a priority in selecting and inviting speakers for the zoominars and guests for the podcast. Importantly, our survey results suggest that there is a place for online events similar to Coast2Coast and Coast2Cast in a pandemic-free future, and that the coastal community involved has greatly benefited from such initiatives.

Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp.

Some of the most profound effects of climate change on ecological communities are due to alterations in species interactions rather than direct physiological effects of changing environmental conditions. Empirical evidence of historical changes in species interactions within climate-impacted communities is, however, rare and difficult to obtain. Here, we demonstrate the recent disappearance of key habitat-forming kelp forests from a warming tropical-temperate transition zone in eastern Australia. Using a 10-y video dataset encompassing a 0.6 °C warming period, we show how herbivory increased as kelp gradually declined and then disappeared. Concurrently, fish communities from sites where kelp was originally abundant but subsequently disappeared became increasingly dominated by tropical herbivores. Feeding assays identified two key tropical/subtropical herbivores that consumed transplanted kelp within hours at these sites. There was also a distinct increase in the abundance of fishes that consume epilithic algae, and much higher bite rates by this group at sites without kelp, suggesting a key role for these fishes in maintaining reefs in kelp-free states by removing kelp recruits. Changes in kelp abundance showed no direct relationship to seawater temperatures over the decade and were also unrelated to other measured abiotic factors (nutrients and storms). Our results show that warming-mediated increases in fish herbivory pose a significant threat to kelp-dominated ecosystems in Australia and, potentially, globally.

Risk classification of low-lying coral reef islands and their exposure to climate threats.

The bio-physical responses of low-lying coral islands to climate change are of concern. These islands exist across a broad range of bio-physical conditions, and vulnerabilities to rising and warming seas, ocean acidification and increased storminess. We propose a risk-based classification that scores 6 island eco-morphometric attributes and 6 bio-physical ocean/climate conditions from recent open-access data, to assign islands with respect to 5 risk classes (Very Low, Low, Moderate, High and Very High). The potential responses of 56 coral islands in Australia's jurisdiction (Coral Sea, NW Shelf and NE Indian Ocean) to climate change is considered with respect to their bio-physical attributes and eco-morphometrics. None of the islands were classed as Very Low risk, while 8 were classed as Low (14.3 %), 34 were Moderate (60.7 %), 11 were High (19.6 %), and 3 were Very High (5.4 %). Islands in the Very High risk class (located on the NW Shelf) are most vulnerable due to their small size (mean 10 Ha), low elevation (mean 2.6 m MSL), angular/elongated shape, unvegetated state, below average pH (mean 8.05), above average rates of sea-level rise (SLR; mean 4.6 mm/yr), isolation from other islands, and frequent tropical storms and marine heatwaves. In contrast, islands in the Low (and Very Low) risk class are less vulnerable due to their large size (mean 127 Ha), high elevation (mean 8.5 m MSL), sub-angular/round shape, vegetated state, near average pH (mean 8.06), near average SLR rates (mean 3.9 mm/yr), proximity to adjacent islands, and infrequent cyclones and marine heatwaves. Our method provides a risk matrix to assess coral island vulnerability to current climate change related risks and supports future research on the impacts of projected climate change scenarios. Findings have implications for communities living on coral islands, associated ecosystem services and coastal States that base their legal maritime zones on these islands.

Improved drag coefficient and settling velocity for carbonate sands.

Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behaviour. A rigorous management of the transport of carbonate sand is essential for the present and future management of sedimentary features in coral reefs such as sandy beaches or reef islands. Here we present a new approach to estimating the drag coefficient of carbonate sands that considers both friction and pressure. Based on our new method, the calculated drag coefficients explain the great variability in settling velocities of carbonate sand observed in nature (from 0.025 m/s to 0.364 m/s in our database). Using our formula, we demonstrate that even small differences in the settling velocity obtained with the new drag coefficient can lead to substantial changes in sediment transport and call for an update of numerical models.

A unified framework for modelling sediment fate from source to sink and its interactions with reef systems over geological times.

Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka before present (BP)); (2) how the fine balance between climate, sea-level, and margin physiography enabled coral reefs to thrive under limited shelf sedimentation rates after ~6 ka BP; and, (3) how since 3 ka BP, with the decrease of accommodation space, reduced of vertical growth led to the lateral extension of reefs consistent with available observational data.