ALBERT

Description: The persistence of reef building corals is threatened by human-induced environmental change. Maintaining coral reefs into the future requires not only the survival of adults, but also the influx of recruits to promote genetic diversity and retain cover following adult mortality. Few studies examine the linkages among multiple life stages of corals, despite a growing knowledge of carryover effects in other systems. We provide a novel test of coral parental conditioning to ocean acidification (OA) and tracking of offspring for 6 months post-release to better understand parental or developmental priming impacts on the processes of offspring recruitment and growth. Coral planulation was tracked for 3 months following adult exposure to high pCO2 and offspring from the second month were reciprocally exposed to ambient and high pCO2 for an additional 6 months. Offspring of parents exposed to high pCO2 had greater settlement and survivorship immediately following release, retained survivorship benefits during 1 and 6 months of continued exposure, and further displayed growth benefits to at least 1 month post release. Enhanced performance of offspring from parents exposed to high conditions was maintained despite the survivorship in both treatments declining in continued exposure to OA. Conditioning of the adults while they brood their larvae, or developmental acclimation of the larvae inside the adult polyps, may provide a form of hormetic conditioning, or environmental priming that elicits stimulatory effects. Defining mechanisms of positive acclimatization, with potential implications for carry over effects, cross-generational plasticity, and multi-generational plasticity, is critical to better understanding ecological and evolutionary dynamics of corals under regimes of increasing environmental disturbance. Considering environmentally-induced parental or developmental legacies in ecological and evolutionary projections may better account for coral reef response to the chronic stress regimes characteristic of climate change.

Description: While phytoplankton play a key role in ocean biogeochemical cycles, the availability and supply pathways of resources that support their growth remain poorly constrained. Here, we show that the availability of various resources varies over several orders of magnitude throughout the Atlantic Ocean, causing regional contrasts in resource deficiency. Regional variations in the relative availability of nitrogen, phosphorous, silicon, iron, zinc, manganese, cobalt, and cadmium are important and result from the contrasts between winter mixing depths and differences in vertical profiles of the different resources. The winter-time thickening of the mixed layer may replenish or deplete resources via entrainment, depending on the vertical nutrient profile. For nutrients like nitrate, phosphate, and cadmium, entrainment is a consistent source term. While for others, such as manganese and iron, entrainment can reduce ocean resource availability, particularly in subtropical regions. Any future change to the depth of winter-time mixing will cause region-specific changes in relative availability of different resources that may have important ecological consequences.

Description: The ocean's subtropical gyres account for about half of global ocean biological carbon fixation. Their productivity is limited by the rate of supply of essential nutrients to the sunlit surface water, which is continuously depleted by gravitational sinking of organic particles and large-scale, wind-driven downwelling. To address this conundrum of how productivity is sustained in subtropical gyres, we conduct two complementary studies: 1. We conducted a field campaign in in the North Atlantic subtropical gyre, consisting of transects over and off the mid-Atlantic ridge including measurements of turbulent kinetic energy dissipation and nitrate. Diapycnal mixing provides a nutrient supply within the euphotic zone, but a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline. 2. We diagnose an eddy-permitting simulation of ocean physics, biogeochemistry and ecology over the North Pacific subtropical gyre. Near the base of the sunlit zone, small-scale diapycnal mixing and adiabatic excursions of nutrient-rich density surfaces deliver nutrients into the euphotic zone, depleting the layers immediately below. These sub-euphotic layers are replenished by a combination of the remineralization of sinking particles and along-isopycnal eddy stirring. In summary, the nutrient supply to the euphotic zone is achieved via a multi-stage mechanism: a diapycnal transfer of nutrients by small-scale turbulence to the euphotic zone, and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline.

Description: The effect of the Southern Ocean on global climate change is assessed using Earth system model CIMP6 projections. For an idealised 1% annual rise in atmospheric CO2 , the Southern Ocean plays a broadly comparable role in the global uptake of heat and anthropogenic carbon, accounting for 40+/-5% of heat uptake (based on 12 CMIP6 models) and 42+/-2% of anthropogenic carbon uptake over the global ocean (based on 11 CMIP6 models). In comparison in historical scenarios, the Southern Ocean plays a similar role in the global uptake of anthropogenic carbon, but plays a much more important role in the global uptake of heat. The reason for these differing responses is that in historical scenarios there is a marked reduction in radiative forcing over the northern hemisphere due to aerosol forcing. For the idealised scenario, there are still significant inter-model differences in global and Southern Ocean heat uptake, which are not primarily controlled by ocean overturning, but instead by differences in physical feedbacks, especially cloud feedbacks over the globe and surface albedo feedbacks from sea-ice loss in high latitudes. The ocean carbon response is broadly similar in most models with carbon storage increasing from rising atmospheric CO2, but weakly decreasing from climate change. In summary, while there have been contrasts in the relative importance of the Southern Ocean for the global uptake of heat and anthropogenic carbon, these contrast in heat and carbon uptake are likely to decline as greenhouse gas forcing dominates in the future.

Description: Sub-Antarctic Mode Waters (SAMW), forming in the deep winter mixed layers in the Sub-Antarctic Zone (SAZ) to the north of the Antarctic Circumpolar Current (ACC), connect the ocean thermocline with the atmosphere, contributing to ocean carbon and heat uptake and transporting high-latitude nutrients northward, to fuel primary production at low latitudes. The important climatic role of SAMW is controlled by the rate of fluid subduction from the deep winter mixed layers and the concentration of heat, carbon and nutrients at the end of winter. These concentrations depend on a range of processes, both physical (air-sea exchange, transport of Antarctic waters across the ACC, along ACC advection, eddy fluxes, diapycnal mixing, etc.) and biogeochemical (biological uptake, export and remineralisation), whose relative contributions are very poorly understood. With a Lagrangian particle-tracking experiment in a data-assimilative coupled physico-biogeochemical model of the Southern Ocean (B-SOSE), we assess the origin of the water masses reaching SAMW formation regions and the physico- and biogeochemical transformations occurring along their transport pathways. Our results underline the importance of the advection of subtropical waters along the ACC for the sequestration of heat and anthropogenic carbon and in modulating the fertilization of the low-latitude thermocline.

Description: Hydrothermal vents have been identified as a key source of trace elements supporting ocean ecosystems. The density and extent of the early plume control the pathways connecting the plume to the surface. Hydrothermal vents generate buoyant water which rises, entraining deep water, until reaching neutral buoyancy. The prevailing view is that the plume then spreads horizontally a few kilometres until constrained by rotation. Here we present a unique observational data set from three Hydrothermal systems consisting of: high horizontal resolution, 100m, Tow-Yo sections; and microstructure profiles. These observations show plumes with horizontal extents much less than a rotational constraint implying a process dispersing the plume time scales shorter than rotation. We calculate the Ertel potential vorticity from the observations to reveal regions of low PV with susceptibility to sub-mesoscale instabilities. Diagnostics reveal that the plume is primed for all three forms of instability: convective instability in the rising plume leading to vertical exchange; centrifugal instability in the upper core of neutrally buoyant plume leading to horizontal exchange; and symmetric instability on the flanks of the plume leading to slantwise exchange. We then show strongly enhanced turbulent mixing within the plume compared to typical deep ocean values, 1000 times larger than background. Finally, we apply a linear mixing model which shows that the plume fluid mixes as strongly when at neutral depth as during the rising phase. In combination these results, in contrast to previous views, present a picture of an intensely dynamic plume rapidly transferring hydrothermal tracers throughout the region.

Description: The North Atlantic Oscillation (NAO) index is one of the most common metrics to characterize atmospheric regimes in the northern hemisphere, revealing how different patterns of atmospheric winds and air-sea fluxes over Europe and North America affect the ocean state. We analyse climate model hindcasts to reveal the impacts of the NAO on the North Atlantic subpolar ocean, which exhibits variability on seasonal to decadal timescales. The ocean response to a single winter NAO event is separated into fast and slow responses. The fast response persists over winter-spring seasons, during which wind stress and heat flux anomalies associated with the NAO rapidly modify ocean temperatures via changes in Ekman transport and ocean-atmosphere heat exchanges. The slow response persists for 3-4 years, during which overturning and gyre circulations redistribute opposing-signed surface temperature anomalies created by the NAO. This redistribution modifies east-west temperature contrasts altering the meridional heat transport associated with gyres and changing the strength of the overturning circulation. Hence, the fast and slow responses lead to opposing-signed subpolar temperature anomalies in time from the competing effects of local forcing and horizontal heat convergence.