Impact of particle aggregation on vertical fluxes of organic matter

Particles sinking out of the euphotic zone are important vehicles of carbon export from the surface ocean. Most of the particles produce heavier aggregates by coagulating with each other before they sink. We implemented an aggregation model into the biogeochemical model of Regional Oceanic Modelling System (ROMS) to simulate the distribution of particles in the water column and their downward transport in the Northwest African upwelling region. Accompanying settling chamber, sediment trap and particle camera measurements provide data for model validation. In situ aggregate settling velocities measured by the settling chamber were around 55 m d−1. Aggregate sizes recorded by the particle camera hardly exceeded 1 mm. The model is based on a continuous size spectrum of aggregates, characterised by the prognostic aggregate mass and aggregate number concentration. Phytoplankton and detritus make up the aggregation pool, which has an averaged, prognostic and size dependent sinking. Model experiments were performed with dense and porous approximations of aggregates with varying maximum aggregate size and stickiness as well as with the inclusion of a disaggregation term. Similar surface productivity in all experiments has been generated in order to find the best combination of parameters that produce measured deep water fluxes. Although the experiments failed to represent surface particle number spectra, in the deep water some of them gave very similar slope and spectrum range as the particle camera observations. Particle fluxes at the mesotrophic sediment trap site off Cape Blanc (CB) have been successfully reproduced by the porous experiment with disaggregation term when particle remineralisation rate was 0.2 d−1. The aggregationdisaggregation model improves the prediction capability of the original biogeochemical model significantly by giving much better estimates of fluxes for both upper and lower trap. The results also point to the need for more studies to enhance our knowledge on particle decay and its variation and to the role that stickiness play in the distribution of vertical fluxes

Offshore advection of particles within the Cape Blanc filament, Mauritania: Results from observational and modelling studies

This article will review major features of the 'giant' Cape Blanc filament off Mauritania with regard to the transport of chlorophyll and organic carbon from the shelf to the open ocean. Within the filament, chlorophyll is transported about 400 km offshore. Modelled particle distributions along a zonal transect at 21 degrees N showed that particles with a sinking velocity of 5 m d(-1) are advected offshore by up to 600 km in subsurface particle clouds generally located between 400 m and 800 m water depth, forming an Intermediate Nepheloid Layer (INL). It corresponds to the depth of the oxygen minimum zone. Heavier particles with a sinking velocity of 30 m d(-1) are transported from the shelf within the Bottom Layer (BL) of more than 1000 m thickness, largely following the topography of the bottom slope. The particles advected within the BL contribute to the enhanced winter-spring mass fluxes collected at the open-ocean mesotrophic sediment trap site CB-13 (similar to 200 nm offshore), due to a long distance advection in deeper waters. The lateral contribution to the deep sediment trap in winter-spring is estimated to be 63% and 72% for organic carbon and total mass, respectively, whereas the lateral input for both components on an annual basis is estimated to be in the order of 15%. Biogenic opal increases almost fivefold from the upper to the lower mesotrophic CB-13 trap, also pointing to an additional source for biogenic silica from eutrophic coastal waters. Blooms obviously sink in smaller, probably mesoscale-sized patches with variable settling rates, depending on the type of aggregated particles and their ballast content. Generally, particle sinking rates are exceptionally high off NW Africa. Very high chlorophyll values and a large size of the Cape Blanc filament in 1998-1999 are also documented in enhanced total mass and organic carbon fluxes. An increasing trend in satellite chlorophyll concentrations and the size of the Cape Blanc filament between 1997 and 2008 as observed for other coastal upwelling areas is not documente

Migration and wintering patterns of a central European population of Common Cranes Grus grus

7 paginas, 1 figura y 1 tableAims To describe migration routes and phenology, and the interannual fidelity to staging and wintering sites. Methods A total of 93 cranes were colour-banded, and 67 of them radiotagged, at their breeding territories in northern Germany and later located at their wintering areas in Spain. Results After a migratory trip lasting 3–28 days, most cranes arrived at Gallocanta in northeastern Spain, where they staged for 1–44 days. Some families stayed there the whole winter, but most continued to southwestern Iberia, where they dispersed over at least 13 wintering areas. Site fidelity was more marked in adult pairs than immatures, half of which used different areas in their second and third winters from those used by their parents. Conclusions Most German cranes wintered in southwestern Spain, with smaller numbers in France. Some immatures remained in France as second- or third-year birds, after having spent their first winter in Spain with their parents, whereas none of them shifted southwards. This suggests that immatures have probably contributed more than adult pairs to the northward shift in the winter range observed during the last decadesWe thank H. Dirks, T. Fichtner, V. Günter, S. Röper and W. Mewes for their help during crane capture and radiotagging, and K. Peter for preparing the figure. Funds for transmitters and fieldwork were provided by Kranichschutz Deutschland (Naturschutzbund Deutschland, Umweltstiftung WWF Deutschland, Lufthansa Umweltförderung) and Umweltministerium Mecklenburg-Vorpommern (Abt. Naturschutz). Facilities for travelling between Spain and Germany were kindly provided by Lufthansa. Aerial location of Common Cranes was possible thanks to the generous collaboration of the Spanish Air Forces and the staff of the Getafe Air Base. We thank 42 Group and its pilots, as well as the maintenance staff, who installed the antennae on the aircraft.Peer reviewe

In-situ sinking speed measurements of marine snow aggregates acquired with a settling chamber mounted to the Cherokee ROV

Marine snow plays a key role in the global carbon cycle because it transfers huge amounts of carbon dioxide (around 1-2 Gt per year) from the ocean surface to the deep-sea, thus removing it from the global system. It is of major field of study for several decades to quantify the amount of particulate matter settling through the water column. A central parameter for ocean mass flux estimates is the settling velocity of larger particles. Most of the few available datasets have been acquired by Scuba divers but are they are limited to a diving depth of a few tenth of meters. Particle settling speeds for the deeper water column may be estimated with the help of sediment trap recordings, having the disadvantage to integrate settling speeds over a long period of time and for the entire particle population settling through the water column. In situ sinking speed measurements of individual aggregates however, are rare and difficult to obtain. We present results from a settling chamber constructed for in situ sinking speed measurements of marine snow. The settling chamber was mounted to the MARUM Cherokee ROV during RV Poseidon Cruise 365 in 2008 off Cape Blanc, Mauritania. It was constructed in consideration of a similar device used by the MBARI ROV Ventana. It is a simple plexiglas box which can be opened and closed to allow an infinite number of measurements with little disturbance inside. A collimated light source illuminates a defined sample volume in which aggregates can be observed after the box has been closed. We sampled a total of 51 aggregates at four depth levels, between 50 m and 400 m water depth. The depths were chosen after collecting a vertical particle profile acquired by a deep-sea still image camera system before the deployment of the ROV. Sinking speeds ranged from 10 m d-1 to 287 m d-1 with a mean value of 57 m d-1. No clear relationship between the size of the particles and their sinking speed was found. Furthermore we - could not observe increasing particle sinking speeds with increasing water depth as found by other authors. This underlines the complexity of such studies and implies more deployments during upcoming cruises and comparison of in situ measurements with additional methods in the future

Gas emissions at the continental margin west off Svalbard: mapping, sampling, and quantification

We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m water depth. Flares from this depth have gained significant attention in the scientific community in recent years because they may be caused by bottom-water warming-induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m water depth, which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the Forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost.Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m water depth as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average ?13C = ?55.7‰ V-PDB).Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single-beam echosounder data, bubble stream mapping using a ROV-mounted horizontally looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow for a large range of uncertainty due to our method (9 to 118 × 106 mol yr?1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociatio