2 resultados para Snow mechanics

em Plymouth Marine Science Electronic Archive (PlyMSEA)


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Assessment of the quality of the marine environment forms an important part of the new 1992 OSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic that was ratified and entered into force on 25 March 1998. In the ministerial statement at the signing of the Convention it was agreed that the first assessment (Quality Status Report, QSR) for all Convention waters should be produced for the year 2000. To oversee this charge a new Environmental Assessment and Monitoring Committee (ASMO) was established and a junior group under this committee, to implement necessary actions, the Assessment Co-ordination Group (ACG). Because of the wide geographical diversity and varying levels of information available in different parts of the Convention area it was decided to produce five regional reports for: I The Arctic; II The North Sea; III The Celtic seas; IV The Bay of Biscay and Iberian Coast; V The Wider Atlantic, which will be synthesised in a holistic QSR for the year 2000. The report for the North Sea will largely be an update of QSR 1993 and forms the third cycle of a developing management system for the North Sea. This paper will present the procedures that have been adopted to implement the QSRs, and outlines the guidelines that have been developed for their structure, format, design and publication.

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During a 25 d Lagrangian study in May and June 1990 in the Northeast Atlantic Ocean, marine snow aggregates were collected using a novel water bottle, and the composition was determined microscopically. The aggregates contained a characteristic signature of a matrix of bacteria, cyanobacteria and autotrophic picoplankton with inter alia inclusions of the tintiniid Dictyocysta elegans and large pennate diatoms. The concentration of bacteria and cyanobacteria was much greater on the aggregates than when free-living by factors of 100 to 6000 and 3000 to 2 500 000, respectively, depending on depth. Various species of crustacean plankton and micronekton were collected, and the faecal pellets produced after capture were examined. These often contained the marine snow signature, indicating that these organisms had been consuming marine snow. In some cases, marine snow material appeared to dominate the diet. This implies a food-chain short cut wherby material, normally too small to be consumed by the mesozooplankton, and considered to constitute the diet of the microplankton can become part of the diet of organisms higher in the food-chain. The micronekton was dominated by the amphipod Themisto compressa, whose pellets also contained the marine snow signature. Shipboard incubation experiments with this species indicated that (1) it does consume marine snow, and (2) its gut-passage time is sufficiently long for material it has eaten in the upper water to be defecated at its day-time depth of several hundred meters. Plankton and micronekton were collected with nets to examine their vertical distribution and diel migration and to put into context the significance of the flux of material in the guts of migrants. “Gut flux” for the T. compressa population was calculated to be up to 2% of the flux measured simultaneously by drifting sediment traps and <5% when all migrants are considered. The in situ abundance and distribution of marine snow aggregates (>0.6 mm) was examined photographically. A sharp concentration peak was usually encountered in the depth range 40 to 80 m which was not associated with peaks of in situ fluorescence or attenuation but was just below or at the base of the upper mixed layer. The feeding behaviour of zooplankton and nekton may influence these concentration gradients to a considerable extent, and hence affect the flux due to passive settling of marine snow aggregates.