Particle flux in the oceans: Challenging the steady state assumption

Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption; that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (<40 m d-1). We were able to reproduce this behaviour using a simple model using satellite-derived export rates and literature-derived remineralization rates. We further provide a simple equation to estimate ‘additional’ (or ‘excess’) POC supply to the dark ocean during non-steady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export

The interpretation of particle size, shape, and carbon flux of marine particle images is strongly affected by the choice of particle detection algorithm

In situ imaging of particles in the ocean are rapidly establishing themselves as powerful tools to investigate the ocean carbon cycle, including the role of sinking particles for carbon sequestration via the biological carbon pump. A big challenge when analysing particles in camera images is determining the size of the particle, which is required to calculate carbon content, sinking velocity and flux. A key image processing decision is the algorithm used to decide which part of the image forms the particle and which is the background. However, this critical analysis step is often unmentioned and its effect rarely explored. Here we show that final flux estimates can easily vary by an order of magnitude when selecting different algorithms for a single dataset. We applied a range of static threshold values and 11 different algorithms (seven threshold and four edge detection algorithms) to particle profiles collected by the LISST-Holo system in two contrasting environments. Our results demonstrate that the particle detection method does not only affect estimated particle size but also particle shape. Uncertainties are likely exacerbated when different particle detection methods are mixed, e.g., when datasets from different studies or devices are merged. We conclude that there is a clear need for more transparent method descriptions and justification for particle detection algorithms, as well as for a calibration standard that allows intercomparison between different devices

Variational methods

International audienceThis contribution presents derivative-based methods for local sensitivity analysis, called Variational Sensitivity Analysis (VSA). If one defines an output called the response function, its sensitivity to inputs variations around a nominal value can be studied using derivative (gradient) information. The main issue of VSA is then to provide an efficient way of computing gradients. This contribution first presents the theoretical grounds of VSA: framework and problem statement, tangent and adjoint methods. Then it covers pratical means to compute derivatives, from naive to more sophisticated approaches, discussing their various 2 merits. Finally, applications of VSA are reviewed and some examples are presented, covering various applications fields: oceanography, glaciology, meteorology

Influence of plankton community structure on the sinking velocity of marine aggregates

About 50 Gt of carbon is fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density, where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80–400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms, which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2–2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph-dominated communities may be indicative for food webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35–40%, which we estimate (by one-dimensional modeling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging; both of which are controlled by plankton community structure

In situ particle measurements deemphasize the role of size in governing the sinking velocity of marine particles

Sinking particles are important in delivering carbon to the deep ocean where it may be stored out of contact with the atmosphere. Whilst particle sinking velocities are known to be influenced by a multitude of factors, size-based parameterisations remain common in biogeochemical models and in the methods used to determine particulate fluxes from autonomous platforms. Here we carried out an extensive literature review (62 datasets) into the size-sinking velocity relationship, and find the relationship is much weaker for studies examining particles in situ (median R2 = 0.03) compared with ex situ studies (median R2 = 0.35). This discrepancy may be because particles examined in the laboratory have more uniform properties than those studied in situ. Our review highlights the shortcomings of using a simple relationship between size and sinking velocity to calculate sinking particulate fluxes in the ocean; considering additional particle characteristics will enable more accurate calculations of particulate fluxes

Recent Developments in Helioseismic Analysis Methods and Solar Data Assimilation

MR and AS have received funding from the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013)/ERC Grant Agreement no. 307117

Geographical, seasonal, and depth variation in sinking particle speeds in the North Atlantic

Particle sinking velocity is considered to be a controlling factor for carbon transport to the deep sea and thus carbon sequestration in the oceans. The velocities of the material exported to depth are considered to be high in high-latitude productive systems and low in oligotrophic distributions. We use a recently developed method based on the measurement of the radioactive pair 210Po-210Pb to calculate particle sinking velocities in the temperate and oligotrophic North Atlantic during different bloom stages. Our estimates of average sinking velocities (ASVs) show that slowly sinking particles (&lt;100?m?d?1) contribute significantly to carbon flux at all the locations except in the temperate regions during the bloom. ASVs appear to vary strongly with season, which we propose is caused by changes in the epipelagic community structure. Our results are the first field data to confirm the long-standing theory that particle sinking velocities increase with depth, with increases of up to 90% between 50 and 150?m depth

Size distribution of aggregates across different aquatic systems around Japan shows that stronger aggregates are formed under turbulence

Marine aggregates, composed of various particles, play a crucial role in ocean carbon storage. The overall size distribution of the aggregates (number size spectra) is controlled by the balance between aggregation and disaggregation processes. Turbulence has been proposed to facilitate both aggregation and disaggregation by increasing the collision rate of aggregates or sometimes directly tearing them apart. Predominant processes driven by turbulence typically depend on the level of turbulence—relatively weak turbulence is associated with aggregation while stronger turbulence promotes disaggregation. Aggregate strength also plays a key role, as strongly bonded aggregates can withstand turbulence better, leading to lower disaggregation rates. While the relationship between turbulence and aggregate strength has been studied numerically and experimentally, field measurements remain limited. Here, we compare our number size spectra to turbulence intensity from the field measurements across different environmental settings around Japan to determine the effect of turbulence on aggregate strength. We combined measurements from 10 sites with different environmental settings and observed the flatter slopes (higher net aggregation rate) and shifts in the intersection lengths with an increase of turbulence, while strong turbulence is typically linked with disaggregation. Our findings suggested that stronger aggregates are formed under stronger turbulence and the overall population of strong aggregates also increases with an increase of turbulence intensity. We also compared our number size spectra with three other confounding factors (fluorescence, salinity, and aggregate compositions) to confirm the effects of turbulence are dominant in our aggregate dynamics

A infância representada no livro escolar: tipos de textos e estereótipos sociais

Esta investigação inscreve-se dominantemente no âmbito dos estudos da criança e ancora a sua área específica nas questões linguísticas, assumindo os pressupostos teóricos e metodológicos da análise do discurso (de tradição francófona), considerando também contributos de áreas conexas. Analisa textos de manuais/livros escolares usados no sistema de ensino português, pretendendo tornar visíveis as imagens que estes constroem das crianças e da infância, bem como os recursos linguísticos mais relevantes associados a tal construção. Procura desvendar a representatividade das tipologias textuais nos manuais e de que forma estas contribuem para a construção das referidas imagens. Conclui que as crianças dos textos são seres sem voz e sem poder, consumidoras, mas não coprodutoras de cultura, com os textos inculcando valores conformadores da manutenção da ordem social estabelecida, sendo a forma como as tipologias textuais estão organizadas contribuidora para se alcançar esse propósito.This research forms primarily part of the child studies and is based on its specific field of linguistic issues, assuming the theoretical and methodological assumptions of the speech analysis (of French-speaking tradition), having also regard the inputs of related fields. It analyses texts from handbooks/textbooks used on the Portuguese educational system, intending to reveal pictures that these books construct about children and childhood, as well as the most relevant linguistic resources associated with such construction. This paper aims at unravelling the representation of the text typologies in the handbooks and how these can contribute for building of such pictures. From this, one can conclude that children from the texts are voiceless and have no power, they are consumers but not co-producers of culture, with these texts instilling conservative values for maintaining the established social order, but also that the way texts typologies are organized to contribute for reaching that propose.Esta investigación se ensere en el ámbito de los estudios del niño y más precisamente en la área de las cuestiones lingüísticas, asumiendo los presupuestos teóricos e metodológicos de la análisis del discurso (de tradición francófona), considerando también aportes de las áreas conexas. Analiza textos de manuales/libros escolares usados en el sistema de enseñanza portugués, pretendiendo tornar visibles las imágenes que este construyen de los niños y de la infancia, bien como los recursos lingüísticos más relevantes asociados a esa construcción. Busca desvendar la representatividad de las tipologías textuales en los manuales y de qué forma contribuyen para la construcción de las referidas imágenes. Concluye que los niños de los textos son seres sin voz y sin poder, consumidores, pero no coproductores de cultura y que los textos inculcan valores conformadores de manutención del orden social establecido, mas también que la forma como las tipologías textuales están organizadas contribuye para alcanzar ese propósito.CIEC - Centro de Investigação em Estudos da Criança, IE, UMinho (UI 317 da FCT), Portugal. Fundos Nacionais através da FCT (Fundação para a Ciência e a Tecnologia) e cofinanciado pelo Fundo Europeu de Desenvolvimento Regional (FEDER) através do COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) no âmbito do CIEC (Centro de Investigação em Estudos da Criança, da Universidade do Minho) com a referência POCI-01-0145-FEDER-007562info:eu-repo/semantics/publishedVersio

Seasonal variation of zooplankton community structure and trophic position in the Celtic Sea: A stable isotope and biovolume spectrum approach

Zooplankton on continental shelves represent an important intermediary in the transfer of energy and matter from phytoplankton to the wider ecosystem. Their taxonomic composition and trophic interactions with phytoplanktonvaryinspaceandtime, andinterpreting theimplicationsofthis constantlyevolvinglandscaperemainsamajorchallenge.Herewecombineplanktontaxonomicdatawiththeanalysisofbiovolumespectraand stableisotopestoprovideinsightsintothetrophicinteractionsthatoccurinashelfseaecosystem(CelticSea) across the spring-summer-autumn transition. Biovolume spectra captured the seasonal development of the zooplankton community well, both in terms of total biomass and trophic positioning, and matched trophic positionsestimatedbystableisotopeanalysis.InearlyApril,largemicroplankton(63–200µm)occupiedhigher trophic positions than mesozooplankton (>200µm), likely reflecting the predominance of nanoplankton (2–20µm) that were not readily available to mesozooplanktongrazers. Biomass and number of trophic levels increasedduringthespringbloomaselevatedprimaryproductionallowedforahigherabundanceofpredatory species.DuringJuly,theplanktonassemblageoccupiedrelativelyhightrophicpositions,indicatingimportant links to the microbial loop and the recycling of organic matter. The strong correlation between biomass and communitytrophiclevelacrossthestudysuggeststhattheCelticSeaisarelativelyenclosedandpredominantly energy-limited ecosystem. The progression of the zooplankton biomass and community structure within the centralshelfregionwasdifferenttothatattheshelf-break,potentiallyreflectingincreasedpredatorycontrolof copepodsby macrozooplanktonandpelagicfishesattheshelfbreak.Wesuggestthatthecombinationofsize spectra and stable isotope techniques are highly complementary and useful for interpreting the seasonal progressionoftrophicinteractionsintheplankton