Anthropogenic impacts are typically detrimental to tropical coral reefs, but the effect of increasing environmental stress and variability on the size structure of coral communities remains poorly understood. This limits our ability to effectively conserve coral reef ecosystems because size specific dynamics are rarely incorporated. Our aim is to quantify variation in the size structure of coral populations across 20 sites along a tropical-to-subtropical environmental gradient on the east coast of Australia (~23°S to 30°S), to determine how size structure changes with a gradient of sea surface temperature, turbidity, productivity and light levels. We use two approaches: 1) linear regression with summary statistics (such as median size) as response variables, a method frequently favoured by ecologists; and 2) compositional functional regression, a novel method using entire size-frequency distributions as response variables. We then predict coral population size structure with increasing environmental stress and variability. Together, we find fewer but larger coral colonies in marginal reefs than in tropical reefs, where environmental conditions are more variable and stressful for tropical corals. Our model predicts that coral populations may become gradually dominated by larger colonies (> 148 cm2) with increasing environmental stress. Fewer but bigger corals suggest low survival of smaller corals, slow growth, and / or poor recruitment. This finding is concerning for the future of coral reefs as it implies populations may have low recovery potential from disturbances. We highlight the importance of continuously monitoring changes to population structure over biogeographic scales.

The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river-fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within a fjord, in Bute Inlet (Canada). We show that 60 ± 10 % of the OC supplied by the two river sources, is buried across the fjord surficial (2 m) sediment. The sand-dominated submarine channel and its terminal lobe contain 63 ± 14 % of the annual terrestrial OC burial in the fjord. In contrast, the muddy overbank and distal flat basin settings contain the remaining 37 ± 14 %. OC in the channel, lobe and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least three times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long-term (>100 year) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient in storing OC supplied by rivers in their near-surface deposits.

Sediments composed of mixed cohesive clay and non-cohesive sand are widespread in a range of aquatic environments. The dynamics of ripples in mixed sand–clay substrates have been studied under pure current and pure wave conditions. However, the effect of cohesive clay on ripple development under combined currents and waves has not been examined, even though combined flows are common in estuaries, particularly during storms. Based on a series of large flume experiments, we identified robust inverse relationships between initial bed clay content, C0, and wave–current ripple growth rates. The experimental results also revealed two distinct types of equilibrium combined–flow ripples on mixed sand–clay beds: (a) large asymmetrical ripples with dimensions and plan geometries comparable to clean-sand counterparts for C0 ≤ 10.6%; and (b) small, flat ripples for C0 > 11%. The increase in bed cohesion contributed to this discontinuity, expressed most clearly in a sharp reduction in equilibrium ripple height, and thus a significant reduction in bed roughness, which implies that the performance of existing ripple predictors can be improved by the incorporation of this physical cohesive effect. For C0 ≤ 10.6%, strong clay winnowing efficiency under combined flows resulted in the formation of equilibrium clean-sand ripples and clay loss at depths far below the ripple base. In natural environments, this ‘deep cleaning’ of bed clay may cause a concurrent sudden release of a large amount of pollutants during storms, leading to a sudden reduction in post-storm resistance to erosion of mixed sand–clay substrates.

Studies of the dissolved and sediment composition of global rivers provide crucial insights into the relationship between climate, weathering, and landscape dynamics. The chemical composition of suspended riverine sediment constrains contemporary erosion and chemical weathering, and is critical to the interpretation of sedimentary records, such as continental shelf deposits. Here we present suspended sediment flux and chemical composition data from the Irrawaddy (Ayeyarwady) and Salween (Thanlwin) rivers in Myanmar, from samples collected in the wet and dry seasons in 2017-2018. Analyses of major element, 87/86Sr, and εNd composition are combined with a multi-step leaching protocol to determine the ccompositions of silicate, carbonate, and iron oxide components in the bulk sediment. Sources of the organic matter are determined by analyses of carbon and nitrogen isotopes. We show that, as in other large rivers, suspended sediment concentration and composition in the river channel is depth-dependent due to hydrodynamic sorting. Depth profile sampling is therefore required for the complete characterization of the sediment flux and composition in these rivers. Having accounted for hydrodynamic sorting, we demonstrate that there are key differences in the composition of the Irrawaddy and the Salween rivers at the mouth. We further show how the composition of the Irrawaddy sediments evolves along a downstream transect from the northern headwaters to the delta, due to inputs from tributaries draining distinct lithologies and the continued weathering of sediments transported through the floodplain. Finally, we discuss the implications of our findings for the interpretation of offshore sedimentary deposits and for quantifying the regional chemical weathering and carbon budgets.

Turbidity currents transport vast quantities of sediment across the seafloor and form the largest sediment accumulations on Earth. These flows pose a hazard to strategically important seafloor infrastructure and are important agents for the transport of organic carbon and nutrients that support deep-sea ecosystems. Therefore, it is important to quantify the scale of these flows, the amount of sediment they transport, and the evolution of their discharge over time and space along their flow path. Two modes of flow evolution have been proposed based on experimental and numerical models. The first is termed ignition, where flows entrain seafloor sediment, becoming more voluminous and powerful and increasing their discharge. In the second mode of evolution, called dissipation, sediment falls out of suspension, so flows decelerate and lose discharge. Thus far, field-scale turbidity currents have only been measured at a handful of sites worldwide, and never in detail at multiple locations along their full course. Therefore, it has not yet been possible to determine when, where, and why flows diverge into these two modes in the deep sea, or how flow discharge varies. The ambitious multi-institution Coordinated Canyon Experiment measured turbidity currents at seven instrumented moorings along the Monterey Canyon, offshore California. Fifteen flows were recorded, including the fastest events yet measured at high resolution (>8 m/s). This remarkable dataset provides the first opportunity to quantify down-channel sediment and flow discharge evolution of turbidity currents in the deep sea. To understand whether flows ignite or dissipate, we derive total and sediment discharges for each of the flows at all seven mooring locations down the canyon. Discharges are calculated from measured velocities, and sediment concentrations are derived using a novel inversion method. We observe two distinct flow modes, as most flows rapidly dissipated in the upper reaches of the canyon, while three ran out for the full 50 km array length. We then explore why only these three flows ignited and discuss the implications for canyon and channel capacity and evolution.

The majority of marine plastic pollution originates from land-based sources with the dominant transport agent being riverine. Despite the widespread recognition that rivers dominate the global flux of plastics to the ocean, there is a key knowledge gap regarding the nature of the flux, the behaviour of microplastics (<5mm) in transport and its pathways from rivers into the ocean. To predict transport, fate and biological interactions of microplastics in aquatic environments at a global scale, the factors that control these processes must be identified and understood. Currently, there remains a large knowledge gap around prediction of microplastic transport in rivers, especially in regards to how biofilm formation influence particle settling velocities. This prevents progress in understanding microplastic fate and hotspot formation, as well as curtailing the evolution of effective mitigation and policy measures. A settling experiment was therefore undertaken to understand how different factors, including salinity, suspended sediment concentration and biofilm formation influence microplastic particle settling velocity. The results presented herein explore the role of biofilms on the generation of microplastic flocs and the impact on buoyancy and settling velocities. Five different polymers were tested and compared including fragments and fibres. Settling velocities were then combined with observed flow velocity data from the Mekong River, one of the top global contributors to marine plastic pollution, allowing predictions of areas of microplastic fallout and hotspots. The results highlight potential areas of highest ecological risk related to the dispersal and distribution of microplastics across the river-delta-coast system including the Tonle Sap Lake. Future work involves supporting predicted hotspots with aligned fieldwork from the Mekong River that details the particulate flux and transport of microplastic, throughout the vertical velocity profile.