The export of organic carbon from the surface ocean forms the basis of the biological carbon pump, an important planetary carbon flux. Typically, only a small fraction of primary productivity (PP) is exported (quantified as the export efficiency: export/PP). Here we assemble a global data synthesis to reveal that very high export efficiency occasionally occurs. These events drive an apparent inverse relationship between PP and export efficiency, which is opposite to that typically used in empirical or mechanistic models. At the global scale, we find that low PP, high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low. This implies that a decoupling between PP and upper ocean remineralization processes can result in a large fraction of PP being exported, likely as intact cells or phytoplankton‐based aggregates. As the proportion of PP being exported declines, macrozooplankton and bacterial abundances rise. High export efficiency, high PP regimes also occur infrequently, possibly associated with nonbiologically mediated export of particles. A similar analysis at a biome scale reveals that the factors affecting export efficiency may be different at regional and global scales. Our results imply that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting export efficiency. Further, the existence of low PP, high export efficiency regimes imply that biogeochemical models that parameterize export efficiency as increasing with PP may underestimate export flux during decoupled periods, such as at the start of the spring bloom.

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO 2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications. Keywords: sinking particle fluxes, sinking velocities, carbon content, size, image processing, automated classification, in situ optical particle measurements, biological carbon pump * Different magnifications available. Quoted details are for the magnification that is most suitable for marine snow.

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long term storage of photosynthetically-fixed carbon. This "biological carbon pump" is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~ 200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including significant fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, that Earth System Models disagree on the projected response of the global export flux to climate change, with estimates ranging from -41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritises the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra, and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterisation, and hence improved model parameterization, of export flux, and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. Main text:Biological activity in the upper ocean takes up 50-60 GtC from the atmosphere annually, of which ~ 10% sinks out of the surface ocean 1 . This 'exported' carbon fuels the biological carbon pump and hence plays a central role in storing carbon in the ocean on climatically-relevant timescales 2 . Because of the complexity of the

Recognition of the importance of jellyfish in marine ecosystems is growing. Yet, the biochemical composition of the mucus that jellyfish constantly excrete is poorly characterized. Here we analyzed the macromolecular (proteins, lipids and carbohydrates) and elemental (carbon and nitrogen) composition of the body and mucus of five scyphozoan jellyfish species (Aurelia aurita, Chrysaora fulgida, Chrysaora pacifica, Eupilema inexpectata and Rhizostoma pulmo). We found that the relative contribution of the different macromolecules and elements in the jellyfish body and mucus was similar across all species, with protein being the major component in all samples (81 ± 4% of macromolecules; 3.6 ± 3.1% of dry weight, DW) followed by lipids (13 ± 4% of macromolecules; 0.5 ± 0.4%DW) and carbohydrates (6 ± 3% of macromolecules; 0.3 ± 0.4%DW). The energy content of the jellyfish matter ranged from 0.2 to 3.1 KJ g−1 DW. Carbon and nitrogen content was 3.7 ± 3.0 and 1.0 ± 0.8%DW, respectively. The average ratios of protein:lipid:carbohydrate and carbon:nitrogen for all samples were 14.6:2.3:1 and 3.8:1, respectively. Our study highlights the biochemical similarity between the jellyfish body and mucus and provides convenient and valuable ratios to support the integration of jellyfish into trophic and biogeochemical models.

Often considered detrimental to the environment and human activities, jellyfish blooms are increasing in several coastal regions worldwide. Yet, the overall effect of these outbreaks on ecosystem productivity and structure are not fully understood. Here we provide evidence for a so far unanticipated role of jellyfish in marine nitrogen cycling. Pelagic jellyfish release nitrogen as a metabolic waste product in form of ammonium. Yet, we observed high rates of nitrification (NH4+ → NO3−, 5.7–40.8 nM gWW−1 [wet weight] h−1) associated with the scyphomedusae Aurelia aurita, Chrysaora hysoscella, and Chrysaora pacifica and low rates of incomplete nitrification (NH4+ → NO2−, 1.0–2.8 nM gWW−1 h−1) associated with Chrysaora fulgida, C. hysoscella, and C. pacifica. These observations indicate that microbes living in association with these jellyfish thrive by oxidizing the readily available ammonia to nitrite and nitrate. The four studied species have a large geographic distribution and exhibit frequent population outbreaks. We show that, during such outbreaks, jellyfish‐associated release of nitrogen can provide more than 100% of the nitrogen required for primary production. These findings reveal a so far overlooked pathway when assessing pelagic nitrification rates that might be of particular relevance in nitrogen depleted surface waters and at high jellyfish population densities.

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long term storage of photosynthetically-fixed carbon. This "biological carbon pump" is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~ 200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including significant fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, that Earth System Models disagree on the projected response of the global export flux to climate change, with estimates ranging from -41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritises the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra, and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterisation, and hence improved model parameterization, of export flux, and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. Main text:Biological activity in the upper ocean takes up 50-60 GtC from the atmosphere annually, of which ~ 10% sinks out of the surface ocean 1 . This 'exported' carbon fuels the biological carbon pump and hence plays a central role in storing carbon in the ocean on climatically-relevant timescales 2 . Because of the complexity of the

The export flux of organic carbon from the upper ocean is the starting point of the transfer and long term storage of photosynthetically-fixed carbon in the deep ocean. This "biological carbon pump" is a significant component of the global carbon cycle, reducing atmospheric CO2 levels by ˜50%. Carbon exported out of the upper ocean also fuels the productivity of the mesopelagic zone, including significant fisheries. Despite its importance, export flux is poorly constrained in Earth System Models, with the modelled range in projected future global-mean changes due to climate warming spanning +1.8 to -41%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritises the processes likely to be most important to include in modern-day estimates and future projections of export, as well as identifying the observations and model developments required to achieve more robust characterisation of this important planetary carbon flux. We identify particle fragmentation and zooplankton vertical migration as the mechanisms most likely to substantially influence the magnitude of present-day modelled export flux. Of the processes sufficiently understood to allow implementation in climate models, projections of future export flux and feedbacks to climate are likely to be most sensitive to changes in phytoplankton and particle size spectra, and to temperature-dependent remineralisation. "Known unknown" processes which are not currently represented in models and will have an uncertain impact on future projections include particle stickiness and fish vertical migration. With the advent of new observational technologies, such as biogeochemical-Argo floats and miniaturised camera systems, we will be able to better parameterize models and thus decrease uncertainties in current and future export flux.

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