Influence of waves on the three-dimensional distribution of plastic in the ocean.

The world's oceans are facing plastic pollution, 80 % of which of terrestrial origin flowing from the mismanaged waste of coastal populations and from river discharge. To study the fate of this pollution, the three-dimensional trajectories of neutral plastic particles continuously released for 24 years according to realistic source scenarios are computed using currents from a global ocean-wave coupled model at 14∘ resolution and from a reference ocean-only model. These Lagrangian simulations show that neutral particles accumulate at the surface in the subtropical convergence zones from where they penetrate to about 250 m depth and strongly disperse over 40∘ of latitude. About 5.3 % of the particles remain at the surface with the wave-coupled model currents, whereas only 2 % for the uncoupled model, with some modulation in the location of the convergence zones. Increased surface retention results from upward vertical velocities induced by widespread divergence of waves-induced Stokes transport in the surface layers.

Biogeochemical feedbacks associated with the response of micronutrient recycling by zooplankton to climate change.

Recycling by zooplankton has emerged as an important process driving the cycling of essential micronutrients in the upper ocean. Resupply of nutrients by upper ocean recycling is itself controlled by multiple biotic and abiotic factors. Although the response of these drivers to climate change will shape future recycling rates and their stoichiometry, their magnitude and variability are unaddressed by climate change projections, which means potentially important feedbacks on surface biogeochemistry are neglected. Here, we assess the impacts of climate change under the high emissions RCP8.5 scenario on the recycling of the essential micronutrients Fe, Zn, Cu, Co and Mn and quantify the regional control by zooplankton food quality, prey quantity, sea surface temperature and zooplankton biomass. A statistical assessment of our model results reveals that the variability in recycling fluxes across all micronutrients is mainly driven by the variability of zooplankton and prey biomass. In contrast, the variability in micronutrient recycling stoichiometry and its response to climate change are more complex and is regulated by zooplankton food quality and prey quantity. Regionally, the relative influence of each driver on recycling changes in our model by the end of the 21st century. Temperature becomes an important driving factor in the polar regions while the expansion of oligotrophic regions leads to the importance of food quality increase for low and mid-latitudes. These responses lead to novel feedbacks that can amplify the response of surface ocean biogeochemistry and alter nutrient deficiency regimes.

Insights Into the Major Processes Driving the Global Distribution of Copper in the Ocean From a Global Model.

Copper (Cu) is an unusual micronutrient as it can limit primary production but can also become toxic for growth and cellular functioning under high concentrations. Cu also displays an atypical linear profile, which will modulate its availability to marine microbes across the ocean. Multiple chemical forms of Cu coexist in seawater as dissolved species and understanding the main processes shaping the Cu biogeochemical cycling is hampered by key knowledge gaps. For instance, the drivers of its specific linear profile in seawater are unknown, and the bioavailable form of Cu for marine phytoplankton is debated. Here we developed a global 3-D biogeochemical model of oceanic Cu within the NEMO/PISCES global model, which represents the global distribution of dissolved copper well. Using our model, we find that reversible scavenging of Cu by organic particles drives the dissolved Cu vertical profile and its distribution in the deep ocean. The low modeled inorganic copper (Cu') in the surface ocean means that Cu' cannot maintain phytoplankton cellular copper requirements within observed ranges. The global budget of oceanic Cu from our model suggests that its residence time may be shorter than previously estimated and provides a global perspective on Cu cycling and the main drivers of Cu biogeochemistry in different regions. Cu scavenging within particle microenvironments and uptake by denitrifying bacteria could be a significant component of Cu cycling in oxygen minimum zones.