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Marine particles play a central role in controlling the transport, cycling, and inventories of many major elements and trace elements and isotopes throughout the oceans. Studies seeking to elucidate the biogeochemical roles of marine particles often require reliable ways to collect them from the ocean. Here, we review the oceanographic toolbox of techniques and instrumentation that are employed to collect both suspended and sinking particles. With these tools, it is possible to determine both the concentrations and vertical fluxes of important elements and individual particle types. We describe the various methods for quantifying the concentrations of particulate matter with in situ pumps, towed sampling devices, bottle collectors, and large volume capture devices. The uses of various types of flux collection platforms are discussed including surface tethered, neutrally buoyant, and bottom moored devices. We address the issues of sediment trap collection biases and the apparent inconsistencies that can arise due to differences in the temporal and spatial scales sampled by the various methodologies. Special attention is given to collection considerations made for the analysis of trace metals and isotopes, as these methodologies are of high importance to the ongoing GEOTRACES program which seeks to identify the processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean. With the emergence of new particle collection methodologies and the continued reliance on traditional collection methods, it is imperative that we combine these multiple approaches in ways that will help improve their accuracy and precision while enhancing their utility in advancing understanding of the biogeochemical and ecological roles of marine particles.

The traditional separation between primary producers (autotrophs) and consumers (heterotrophs) at the base of the marine food web is being increasingly replaced by the paradigm that mixoplankton, planktonic protists with the nutritional ability to use both phago(hetero)trophy and photo(auto)trophy to access energy are widespread globally. Thus, many 'phytoplankton' eat, while 50% of 'protozooplankton' also perform photosynthesis. Mixotrophy may enhance primary production, biomass transfer to higher trophic levels and the efficiency of the biological pump to sequester atmospheric CO 2 into the deep ocean. Although this view is gaining traction, science lacks a tool to quantify the relative contributions of autotrophy and heterotrophy in planktonic protists. This hinders our understanding of their impacts on carbon cycling within marine pelagic ecosystems. It has been shown that the hydrogen (H) isotopic signature of lipids is uniquely sensitive to heterotrophy relative to autotrophy in plants and bacteria. Here, we explored whether it is also sensitive to the trophic status in protists. The new understanding of H isotope signature of lipid biomarkers suggests it offers great potential as a novel tool for quantifying the prevalence of mixotrophy in diverse marine microorganisms and thus for investigating the implications of the 'mixoplankton' paradigm., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)

Concern over plastic pollution of the marine environment is severe. The mass-imbalance between the plastic litter supplied to and observed in the ocean currently suggests a missing sink. However, here we show that the ocean interior conceals high loads of small-sized plastic debris which can balance and even exceed the estimated plastic inputs into the ocean since 1950. The combined mass of just the three most-littered plastics (polyethylene, polypropylene, and polystyrene) of 32-651 µm size-class suspended in the top 200 m of the Atlantic Ocean is 11.6-21.1 Million Tonnes. Considering that plastics of other sizes and polymer types will be found in the deeper ocean and in the sediments, our results indicate that both inputs and stocks of ocean plastics are much higher than determined previously. It is thus critical to assess these terms across all size categories and polymer groups to determine the fate and danger of plastic contamination.

Suspended particles are major organic carbon substrates for heterotrophic microorganisms in the mesopelagic ocean (100-1000 m). Nonetheless, communities associated with these particles have been overlooked compared with sinking particles, the latter generally considered as main carbon transporters to the deep ocean. This study is the first to differentiate prokaryotic communities associated with suspended and sinking particles, collected with a marine snow catcher at four environmentally distinct stations in the Scotia Sea. Amplicon sequencing of 16S rRNA gene revealed distinct prokaryotic communities associated with the two particle-types in the mixed-layer (0-100 m) and upper-mesopelagic zone (mean dissimilarity 42.5% ± 15.2%). Although common remineralising taxa were present within both particle-types, gammaproteobacterial Pseudomonadales and Vibrionales, and alphaproteobacterial Rhodobacterales were found enriched in sinking particles up to 32-fold, while Flavobacteriales (Bacteroidetes) favoured suspended particles. We propose that this niche-partitioning may be driven by organic matter properties found within both particle-types: K-strategists, specialised in the degradation of complex organic compounds, thrived on semi-labile suspended particles, while generalists r-strategists were adapted to the transient labile organic contents of sinking particles. Differences between the two particle-associated communities were more pronounced in the mesopelagic than in the surface ocean, likely resulting from exchanges between particle-pools enabled by the stronger turbulence., (© 2018 Society for Applied Microbiology and John Wiley & Sons Ltd.)

The biological carbon pump, which transports particulate organic carbon (POC) from the surface to the deep ocean, plays an important role in regulating atmospheric carbon dioxide (CO2) concentrations. We know very little about geographical variability in the remineralization depth of this sinking material and less about what controls such variability. Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization length scale for each site. Combining these results with corresponding data from the North Pacific, we show that the observed variability in attenuation of vertical POC flux can largely be explained by temperature, with shallower remineralization occurring in warmer waters. This is seemingly inconsistent with conclusions drawn from earlier analyses of deep-sea sediment trap and export flux data, which suggest lowest transfer efficiency at high latitudes. However, the two patterns can be reconciled by considering relatively intense remineralization of a labile fraction of material in warm waters, followed by efficient downward transfer of the remaining refractory fraction, while in cold environments, a larger labile fraction undergoes slower remineralization that continues over a longer length scale. Based on the observed relationship, future increases in ocean temperature will likely lead to shallower remineralization of POC and hence reduced storage of CO2 by the ocean.

It has been challenging to establish the mechanisms that link ecosystem functioning to environmental and resource variation, as well as community structure, composition, and compensatory dynamics. A compelling hypothesis of compensatory dynamics, known as "zero-sum" dynamics, is framed in terms of energy resource and demand units, where there is an inverse link between the number of individuals in a community and the mean individual metabolic rate. However, body size energy distributions that are nonuniform suggest a niche advantage at a particular size class, which suggests a limit to which metabolism can explain community structuring. Since 1989, the composition and structure of abyssal seafloor communities in the northeast Pacific and northeast Atlantic have varied interannually with links to climate and resource variation. Here, for the first time, class and mass-specific individual respiration rates were examined along with resource supply and time series of density and biomass data of the dominant abyssal megafauna, echinoderms. Both sites had inverse relationships between density and mean individual metabolic rate. We found fourfold variation in echinoderm respiration over interannual timescales at both sites, which were linked to shifts in species composition and structure. In the northeastern Pacific, the respiration of mobile surface deposit feeding echinoderms was positively linked to climate-driven particulate organic carbon fluxes with a temporal lag of about one year, respiring - 1-6% of the annual particulate organic carbon flux.

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink.

Climate variation affects surface ocean processes and the production of organic carbon, which ultimately comprises the primary food supply to the deep-sea ecosystems that occupy approximately 60% of the Earth's surface. Warming trends in atmospheric and upper ocean temperatures, attributed to anthropogenic influence, have occurred over the past four decades. Changes in upper ocean temperature influence stratification and can affect the availability of nutrients for phytoplankton production. Global warming has been predicted to intensify stratification and reduce vertical mixing. Research also suggests that such reduced mixing will enhance variability in primary production and carbon export flux to the deep sea. The dependence of deep-sea communities on surface water production has raised important questions about how climate change will affect carbon cycling and deep-ocean ecosystem function. Recently, unprecedented time-series studies conducted over the past two decades in the North Pacific and the North Atlantic at >4,000-m depth have revealed unexpectedly large changes in deep-ocean ecosystems significantly correlated to climate-driven changes in the surface ocean that can impact the global carbon cycle. Climate-driven variation affects oceanic communities from surface waters to the much-overlooked deep sea and will have impacts on the global carbon cycle. Data from these two widely separated areas of the deep ocean provide compelling evidence that changes in climate can readily influence deep-sea processes. However, the limited geographic coverage of these existing time-series studies stresses the importance of developing a more global effort to monitor deep-sea ecosystems under modern conditions of rapidly changing climate.

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north-south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol(-1) was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.

We present the most extensive study to date of globally compiled and analysed weight-specific growth rates in marine epi-pelagic invertebrate metazoan zooplankton. Using specified selection criteria, we analyse growth rates from a variety of zooplanktonic taxa, including both holo- and mero-planktonic forms, from over 110 published studies. Nine principal taxonomic groups are considered, the copepods (number of individual data points (n) = 2,528); crustaceans other than copepods (n = 253); cnidarians (n = 77); ctenophores (n = 27); chaetognaths (n = 87); pteropods (n = 8); polychaetes (n = 12); thaliaceans (n = 88); and larvaceans (n = 91). The copepods are further examined by subdividing them into broadcasters or sac-spawning species, and as nauplii (N1-N6), copepodites (C1-C5) and adults (C6). For each taxonomic group relationships between growth, temperature and body weight are examined using a variety of methods. Weight-specific growth tends to increase with increasing temperature and with decreasing body weight in the crustacean group. Growth does not relate to body weight in the case of chaetognaths and larvaceans, but does increase with temperature. In the cnidarian and ctenophore groups growth does not relate to temperature, but is negatively related to body size. For the thaliceans growth increases with both increasing body weight and temperature. In the entire broadcasting copepod data set, weight-specific growth increases with increasing temperature and decreasing body weight. In sac-spawners, growth increases with increasing temperature, and increases with decreasing body weight at temperatures below 20 degrees C, but decreases with body weight at temperatures above this. Comparison between the different taxa shows important differences and similarities. Our extensive synthesis of data generally confirms that larvaceans, pteropods, cnidarians and ctenophores have rates of weight-specific growth that are typically greater than the copepods, chaetognaths and other crustaceans of similar carbon weight. For the cnidarians, ctenophores and larvaceans growth rates are almost always greater than the general relationship describing copepod growth, and are also at the upper limits or beyond the maximum rates for copepods of a similar weight. For the pteropods, growth rates are generally greater than those of copepods, although the data set was limited to a single carnivorous species in a single study (i.e. Clione limacina). The thaliaceans have the highest growth rates for animals with body weights greater than around 1 mg C ind-1, with rates of up to 2.1 d-1 for Pegea bicaudata. Whilst the larvaceans can achieve rates of 2 d-1 in warm tropical waters (28 degrees C), and as high as > 3 d-1 for < 0.2 mg C individual-1 animals of Oikopleura diocia. These are possibly the highest rates every recorded in epi-pelagic metazoans. Reasons for the differences between taxonomic groups are discussed in relation to intrinsic and extrinsic factors and limitations. The importance of this investigation not only lies in it being the most comprehensive overview of patterns of growth to date, but because the data set highlight the gaps in measurements and current knowledge. We examine the inadequacies in the current data sets, and in the methods being used to measure growth and production. Most of the data are for animals collected from coastal and estuarine waters, and it is clear that for a fuller understanding there is an urgent need for work in the open ocean, and for investigations outside temperate regions. There is also a need to explore the role of food availability, and how food concentrations in incubations, and under food saturation, relate to those experienced in the natural environment.