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Accurate projections of marine particle export production (EP) are crucial for predicting the response of the marine carbon cycle to climate change, yet models show a wide range in both global EP and their responses to climate change. This is, in part, due to EP being the net result of a series of processes, starting with net primary production (NPP) in the sunlit upper ocean, followed by the formation of particulate organic matter and the subsequent sinking and remineralisation of these particles, with each of these processes responding differently to changes in environmental conditions. Here, we compare future projections in EP over the 21st century, generated by four marine ecosystem models under the high emission scenario Representative Concentration Pathways (RCP) 8.5 of the Intergovernmental Panel on Climate Change (IPCC), and determine the processes driving these changes. The models simulate small to modest decreases in global EP between −1 and −12%. Models differ greatly with regard to the drivers causing these changes. Among them, the formation of particles is the most uncertain process with models not agreeing on either magnitude or the direction of change. The removal of the sinking particles by remineralisation is simulated to increase in the low and intermediate latitudes in three models, driven by either warming-induced increases in remineralisation or slower particle sinking, and show insignificant changes in the remaining model. Changes in ecosystem structure, particularly the relative role of diatoms matters as well, as diatoms produce larger and denser particles that sink faster and are partly protected from remineralisation. Also this controlling factor is afflicted with high uncertainties, particularly since the models differ already substantially with regard to both the initial (present-day) distribution of diatoms (between 11–94% in the Southern Ocean) and the diatom contribution to particle formation (0.6–3.8 times higher than their contribution to biomass). As a consequence, changes in diatom concentration are a strong driver for EP changes in some models but of low significance in others. Observational and experimental constraints on ecosystem structure and how the fixed carbon is routed through the ecosystem to produce export production are urgently needed in order to improve current generation ecosystem models and their ability to project future changes., Biogeosciences, 13 (13), ISSN:1726-4170

Global coastlines potentially contain significant amounts of plastic debris, with harmful implications for marine and coastal ecosystems, fisheries and tourism. However, the global amount, distribution and origin of plastic debris on beaches and in coastal waters is currently unknown. Here we analyze beaching and resuspension scenarios using a Lagrangian particle transport model. Throughout the first 5 years after entering the ocean, the model indicates that at least 77% ofpositively buoyant marine plastic debris (PBMPD) released from land-based sources is either beached or floating in coastal waters, assuming no further plastic removal from beaches or the ocean surface. The highest concentrations ofbeached PBMPD are found in Southeast Asia, caused by high plastic inputs from land and limited offshore transport, although the absolute concentrations are generally overestimates compared to field measurements. The modeled distribution on a global scale is only weakly influenced by local variations in resuspension rates due to coastal geomorphology. Furthermore, there are striking differences regarding the origin of the beached plastic debris. In some exclusive economic zones (EEZ), such as the Indonesian Archipelago, plastic originates almost entirely from within the EEZ while in other EEZs, particularly remote islands, almost all beached plastic debris arrives from remote sources. Our results highlight coastlines and coastal waters as important reservoirs ofmarine plastic debris and limited transport ofPBMPD between the coastal zone and the open ocean.

We analyse long-term trends in marine primary and particle export production and their link to marine phytoplankton community composition for the period 1960–2006 using a hindcast simulation of the Biogeochemical Elemental Cycling Model coupled to the ocean component of the Community Climate System Model. In our simulation, global primary and export production decrease significantly over the last 50 yr, by 6.5% and 8% respectively. These changes are associated with an 8.5% decrease in small phytoplankton biomass and 5% decrease in zooplankton biomass. Diatom biomass decreases globally by 3%, but with strong temporal and spatial variability. The strongest decreases in primary and export production occur in the western Pacific, where enhanced stratification leads to stronger nutrient limitation and a decrease in total phytoplankton. The concurrent decrease in diatom fraction and in zooplankton biomass causes a lower export efficiency in this region. Substantial phytoplankton composition changes also occur in the Southern Ocean and North Atlantic, although these are masked in part by a high degree of interannual variability. In these regions, stronger wind stress enhances mixing, reducing the biomass of small phytoplankton, while diatoms profit from higher nutrient inputs and lower grazing pressure. The relative fraction of diatoms correlates positively with the export efficiency (r = 0.8, p < 0.05) in most areas except for the North Pacific and Antarctic Circumpolar Current, where the correlation is negative (r = –0.5, p < 0.05). However, the long-term trends in global export efficiency are ultimately driven by the reduction in small phytoplankton and particularly decreases in coccolithophore biomass. The diagnosed trends point toward a substantial sensitivity of marine primary production and export to climatic variations and trends., Biogeosciences, 10 (11), ISSN:1726-4170

Deep Chlorophyll Maxima (DCMs) are subsurface peaks in chlorophyll-a concentration that may coincide with peaks in phytoplankton abundance and primary productivity. Work on the mechanisms underlying DCM formation has historically focused on phytoplankton physiology (e.g., photoacclimation) and behavior (e.g., taxis). While these mechanisms can drive DCM formation, they do not account for top-down controls such as predation by grazers. Here, we propose a new mechanism for DCM formation: Light-dependent grazing by microzooplankton reduces phytoplankton biomass near the surface but allows accumulation at depth. Using mathematical models informed by grazing studies, we demonstrate that light-dependent grazing is sufficient to drive DCM formation. Further, when acting in concert with other mechanisms, light-dependent grazing deepens the DCM, improving the fit of a global model with observational data. Our findings thus reveal another mechanism by which micro- zooplankton may regulate primary production, and impact our understanding of biogeo- chemical cycling at and above the DCM.

Past model studies have projected a global decrease in marine net primary production (NPP) over the 21st century, but these studies focused on the multi-model mean rather than on the large inter-model differences. Here, we analyze model-simulated changes in NPP for the 21st century under IPCC's high-emission scenario RCP8.5. We use a suite of nine coupled carbon–climate Earth system models with embedded marine ecosystem models and focus on the spread between the different models and the underlying reasons. Globally, NPP decreases in five out of the nine models over the course of the 21st century, while three show no significant trend and one even simulates an increase. The largest model spread occurs in the low latitudes (between 30° S and 30° N), with individual models simulating relative changes between −25 and +40 %. Of the seven models diagnosing a net decrease in NPP in the low latitudes, only three simulate this to be a consequence of the classical interpretation, i.e., a stronger nutrient limitation due to increased stratification leading to reduced phytoplankton growth. In the other four, warming-induced increases in phytoplankton growth outbalance the stronger nutrient limitation. However, temperature-driven increases in grazing and other loss processes cause a net decrease in phytoplankton biomass and reduce NPP despite higher growth rates. One model projects a strong increase in NPP in the low latitudes, caused by an intensification of the microbial loop, while NPP in the remaining model changes by less than 0.5 %. While models consistently project increases NPP in the Southern Ocean, the regional inter-model range is also very substantial. In most models, this increase in NPP is driven by temperature, but it is also modulated by changes in light, macronutrients and iron as well as grazing. Overall, current projections of future changes in global marine NPP are subject to large uncertainties and necessitate a dedicated and sustained effort to improve the models and the concepts and data that guide their development., Biogeosciences, 12 (23), ISSN:1726-4170

Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorlymapped, and most ofthe plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding ofhow plastic debris is transported from coastal and marine sources is crucial to quantifyand close the global inventory ofmarine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport offloating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements ofmarine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales.