Vertical Flux Of Particulate Matter Research Articles

Horizontal and vertical fluxes of particulate matter during wind erosion on arable land in the province La Pampa, Argentina

A detailed analysis of horizontal and vertical particulate matter (PM) fluxes during wind erosion has been done, based on measurements of PM smaller than 10, 2.5, and 1.0 μm, at windward and leeward positions on a measuring field. The three fractions of PM measurement are differently influenced by the increasing wind and shear velocities of the wind. The measured concentrations of the coarser fractions of the fine dust, PM10, and PM2.5, increase with wind and shear velocity, whereas the PM1.0 concentrations show no clear correlation to the shear velocity. The share of PM2.5 on PM10 depends on the measurement height and wind speed and varies between 4 and 12 m/s at the 1 m height ranging from 25% to 7% (average 10%), and at the 4 m height from 39% to 23% (average 30%). Although general relationships between wind speed, PM concentration, and horizontal and vertical fluxes could be found, the contribution of the measuring field was very low, as balances of incoming and outgoing fluxes show. Consequently, the measured PM concentrations are determined from a variety of sources, such as traffic on unpaved roads, cattle drives, tillage operations, and wind erosion, and thus, represent all components of land use and landscape structure in the near and far surroundings of the measuring field. The current results may reflect factors from the landscape scale rather than the influence of field-related variables. The measuring devices used to monitor PM concentrations showed differences of up to 20%, which led to considerable deviations when determining total balances. Differences up to 67% between the calculated fluxes prove the necessity of a previous calibration of the devices used.

Vertical particle fluxes dominate integrated multi‑trophic aquaculture (IMTA) sites: implications for shellfish-finfish synergy

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 9:127-143 (2017) - DOI: https://doi.org/10.3354/aei00218 Vertical particle fluxes dominate integrated multi‑trophic aquaculture (IMTA) sites: implications for shellfish-finfish synergy R. Filgueira1,*, T. Guyondet2, G. K. Reid3,4, J. Grant5, P. J. Cranford6 1Marine Affairs Program, Dalhousie University, 1355 Oxford St., PO Box 15000, Halifax, NS B3H 1R2, Canada 2Department of Fisheries and Oceans, Gulf Fisheries Centre, Science Branch, PO Box 5030, Moncton, NB E1C 9B6, Canada 3Canadian Integrated Multi-Trophic Aquaculture Network (CIMTAN), University of New Brunswick, PO Box 5050, Saint John, NB E2L 4L5, Canada 4Department of Fisheries and Oceans, St. Andrews Biological Station, 531 Brandy Cove Rd., St. Andrews, NB E5B 2L9, Canada 5Department of Oceanography, Dalhousie University, Halifax, NS B3H 4R2, Canada 6Department of Fisheries and Oceans, Bedford Institute of Oceanography, 1 Challenger Dr., Dartmouth, NS B2Y 4A2, Canada *Corresponding author: ramon.filgueira@dal.ca ABSTRACT: Maximizing the mitigation potential of open-water finfish-shellfish integrated multi-trophic aquaculture (IMTA) farms is complex in terms of co-locating the trophic components. Both the dispersal of finfish aquaculture wastes and biological processes are highly influenced by water circulation. Consequently, the evaluation of shellfish-finfish synergy requires a combined study of biological and physical processes, which can be achieved by the implementation and coupling of mathematical models. A highly configurable mathematical model was developed that can be applied at the apparent spatial scale of IMTA sites. The model tracks different components of the seston, including feed wastes, fish faeces, shellfish faeces, natural detritus and phytoplankton. Based on the characterization of these fluxes, a hypothetical IMTA site was used to explore different spatial arrangements for evaluating finfish-shellfish farm mitigation efficiency. The site was modelled following a factorial design, which tested 2 levels of background seston concentrations, 3 farm designs, 2 hydrodynamic conditions and 2 levels of aquaculture intensity. The model predicts that mitigation efficiency is highly dependent on the background environmental conditions, obtaining maximum mitigation under oligotrophic conditions that stimulate shellfish filtration activity. The dominance of vertical fluxes of particulate matter triggered by the high settling velocity of finfish aquaculture wastes suggests that suspended shellfish aquaculture cannot significantly reduce organic loading of the seabed. Consequently, this suggests that waste mitigation at IMTA sites should be best achieved by placing organic extractive species (e.g. deposit feeders) on the seabed directly beneath finfish cages rather than in suspension in the water column. KEY WORDS: Settling velocity · Organic loading · Mitigation · Connectivity · Ecosystem model Full text in pdf format Supplementary material PreviousNextCite this article as: Filgueira R, Guyondet T, Reid GK, Grant J, Cranford PJ (2017) Vertical particle fluxes dominate integrated multi‑trophic aquaculture (IMTA) sites: implications for shellfish-finfish synergy. Aquacult Environ Interact 9:127-143. https://doi.org/10.3354/aei00218 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 9. Online publication date: March 14, 2017 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2017 Inter-Research.

230Th and 231Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantic transect, and implications for modern and paleoceanographic chemical fluxes

The long-lived uranium decay products 230Th and 231Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230Th and 231Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230Th-normalization and to demonstrate enhanced 231Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231Pa is apparent. The 231Pa/230Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation.

Distribution, Composition, and Vertical Fluxes of Particulate Matter in Bays of Novaya Zemlya Archipelago, Vaigach Island at the End of Summer

An analysis of suspended particulate matter (SPM) and phytoplankton distribution, composition and vertical particle fluxes in Russkaya Gavan’ Bay (Northern Island of the Novaya Zemlya), Bezymyannaya Bay (Southern Island of the Novaya Zemlya), Dolgaya Bay (northwestern part of the Vaigach Island) in comparison with the data from the Svalbard Archipelago is presented. Field studies were carried out by the authors during the 9th expedition of the RV “Professor Logachev” in September 1994, the 11th, 13th, and 14th expeditions of the RV “Akademik Sergey Vavilov” in September-October 1997 and August-September 1998. The data about Spitsbergen fjords are from literature. Our results show that, on the bays of the Barents Sea islands, most SPM stays in the bays (fjords) and only small part of it reaches the open sea. This is due to the hydrodynamic conditions in the bays, the large size of the particles, flocculation, and often to the morphological barriers in the relief at the bay entrances. It is important for ecological purposes to map out migration pathways of the SPM with pollutants from bays to the open sea. Results of our investigation indicate that the western bays of the Novaya Zemlya act as traps for SPM derived from glaciers and coastal abrasion.

Interannual variability in vertical export in the Ross Sea: Magnitude, composition, and environmental correlates

The vertical flux of particulate matter from the surface of the Ross Sea, Antarctica, has been suggested as being large, with substantial seasonal and spatial variations. We conducted a study in which vertical flux was quantified using sediment traps deployed at 200 m and compared to estimates calculated from one-dimensional budgets of nutrients (nitrogen and silicon). Estimates of flux were collected at two locations in the southern Ross Sea from late December to early February during four years: 2001–2002, 2003–2004, 2004–2005, and 2005–2006. Phytoplankton biomass and vertical flux varied substantially seasonally and spatially between the two sites, and among years. The greatest flux was observed in 2001–2002, with a short-term maximum organic carbon flux of 3.13 mmol m −2 d −1, and the summer mean organic carbon flux equal to 0.93 mmol m −2 d −1. In contrast, the mean carbon flux at the same site in 2003–2004 was over an order of magnitude less, averaging 0.19 mmol m −2 d −1, despite the fact that productivity in that year was substantially greater. In 2005–206 the contribution of fecal pellets to flux was smallest among all years, and the pellet contribution ranged from <1 to more than 50% of organic flux. As the moorings also had surface layer fluorometers, the relationship between surface biomass and sediment trap flux was compared. Temporal lags between surface fluorescence and flux at 200 m maxima in 2003–2004 and 2004–2005 ranged from two to six days; however, in 2005–2006 the temporal offset between biomass and flux was much longer, ranging from 11 to 27 days, suggesting that fecal pellet production appeared to increase the coupling between flux and surface production. Estimates of export from the upper 200 m based on one-dimensional nutrient budgets were greater than those recorded by the sediment traps. Nutrient budgets also indicated that siliceous production averaged ca. 40% of the total annual production. The variations observed in the flux of biogenic matter to depth in the Ross Sea are large, appear to reflect different forcing among years, and at present are not adequately understood. However, such variability needs to be both understood and represented in biogeochemical models to accurately assess and predict the effects of climate change on biogeochemical cycles.

Temporal variations of C, N, δ13C, and δ15N in organic matter collected by a sediment trap at Cuenca Alfonso, Bahía de La Paz, SW Gulf of California

To monitor the composition and the vertical flux of particulate matter from the sea surface, a sediment trap was moored in Cuenca Alfonso, Bahía de La Paz, a zone of high productivity in the southwestern Gulf of California. Carbonate-free samples from 2002 to 2005 were analyzed for C org, N, δ 13C, and δ 15N. The results show seasonal and interannual variability, with the δ 13C and δ 15N values larger in spring and summer than in fall and winter. The C:N ratio and δ 13C increased by 1.5 units from 2002 to 2003–2005, suggesting a change in the supply of organic matter and-or the use or preferential degradation of N org. There was no interannual variation in δ 15N. The occasional high δ 15N values suggest that physical mechanisms, such as the shoaling and advection into the bay of 15N-rich subsurface equatorial water, occur over short time periods. The latter is presumed to be related to the periodic development of a significant cyclonic gyre in the southern Gulf.

Vertical flux of particulate matter in an Arctic fjord: the case of lack of the sea-ice cover in Adventfjorden 2006–2007

Seasonal dynamics of suspended minerals, organic matter, particulate, and dissolved organic carbon (DOC), chlorophyll, and their vertical fluxes were studied in a small Arctic fjord (Adventfjorden, Spitsbergen) from November 2006 to October 2007. The fjord was usually covered with fast ice in winter, but remained open throughout the year since 2005. The open-water winter period caused increased wave action and resuspension of organic and mineral particles. The lack of sea-ice in spring accelerated the onset of the productive season. The earlier light signal also caused an earlier appearance of mesozooplanktonic organisms, accompanied by a significant increase of the DOC pool in the water. In the cold period (winter and spring) 47% of the annual organic matter settled on the sampling site, a lot coming from the spring diatom bloom in April. Summer melt (July, August) resulted in turbid and brackish fjord surface water with stratification and increase of both suspended particles and sedimentation, causing 60% of the annual minerals and 53% of the annual organic matter to settle at the sampling site. Increased dissolved organic carbon (DOC) through sloppy feeding on the mixo- and heterotrophic protista by the abundant mesozooplankton indicated intensive secondary production, resulting in a maximum fecal pellets flux of >5 mg C m−2 day−1. A warmer climate with reduced sea-ice cover in fjords will advance the onset of the spring bloom and will also result in a larger input of turbid melt water in summer, restricting the light availability and enhancing flocculation and thereby sedimentation.

Vertical flux of particulate organic matter in a High Arctic fjord: Relative importance of terrestrial and marine sources

Vertical flux of particulate matter was recorded using a moored sediment trap during 2002-03 in the outer region of the 90 km long NE Greenland fjord Young Sound (74°18’N, 20°18’W). Sea ice covered the fjord for c. 9 months during the deployment. At 65 m depth total flux of material was 1420 g dry weight m-2 and annual fluxes of carbonate (g m-2), chlorophyll (mg m-2), particulate organic carbon (POC, g C m-2) and nitrogen (PON, g N m-2), were, 9, 53, 17 and 1.2 respectively. A steep increase in fluxes was observed during the summer thaw when sea ice broke up and water discharge from land began. Within the two months (July and August), &gt;90% of the total annual vertical flux occurred.&#x0D; Isotopic (δ13C &amp; δ15N) analysis of particulate organic material (POM) in the sediment trap, in phytoplankton and in the river material indicated that a maximum of c. 50% of the POM material originated from land. This is supported by the high C:N ratio (by atoms) of up to 22 found in the trapped organic material during the summer thaw as compared with 7–9 during winter and spring, when no discharge from land occurred. Seasonal measurements of the POC discharge from rivers to the outer region of the fjord corresponded to c. 40% of the vertical POC flux measured in the sediment trap, which further indicates a significant terrestrial contribution to the settling material in the outer fjord area.&#x0D; Besides POC, dissolved organic carbon (DOC) is discharged in an equal amount to the fjord from rivers, resulting in a total organic carbon (TOC) input from land to the outer region of Young Sound of 13 g C m-2 yr-1. This corresponds to c. 40% of the net TOC input from the Greenland Sea and to the outer part of the fjord during the ice-free productive period underlining the significant terrestrial contribution to sedimentation in the outer part of the fjord.&#x0D; Mean permanent accumulation rates based on the depth distributions of 210Pb, 137Cs and TOC in sediments at 60 m water depth in the outer fjord area revealed a burial of carbon within the sediment of 7.9 g C m-2 yr-1. In agreement with the sediment trap measurements, δ13C values within the sediment suggest that a substantial amount (c. 40%) of the POC in the sediment was of terrestrial origin. At the same sites, previous studies have reported an annual release of dissolved inorganic carbon (DIC) due to mineralization from the sediment of 12.6 g C m-2 yr-1. The sum of the annual DIC release and the burial within the sediment&#x0D; represents an expected total input to the sediment of 20.5 g C m-2 yr-1and compares well with the vertical flux measurement from the sediment trap of 17.0 g C m-2 yr-1 during the present study.

Vertical flux of particulate matter in Alfonso Basin, La Paz Bay, during 2002.

Every three months during 2002, a 1/8-m2 Technicap PPS-3 time-series sediment trap was moored 50 m above the bottom of Alfonso Basin and provided 7–8 day re...

Contrasts in sedimentation flux below the southern California Current in late 1996 and during the El Niño event of 1997–1998

The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m −2 d −1 between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m −2 d −1. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m −2 d −1 and 3–8 mgC m −2 d −1) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m −2 d −1) and organic carbon (0.2–0.8 mgC m −2 d −1) fluxes. The organics collected in 1996 were predominantly autochthonous ( δ 13 C=−22‰ ; C/N=8). The variations in δ 15N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 ( δ 13 C=−23.6‰ ; 15 N=11.7‰ ; C/N=10.5) indicates degraded material of marine origin. The maximum δ 15N value found (∼14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles.

The vertical flux of particulate matter in the polynya of Terra Nova Bay. Part II. Biological components

Downward fluxes of particulate matter were investigated in the polynya of Terra Nova Bay (western Ross Sea) from February 1995 to December 1997. The main biological components were siliceous phytoplankton (diatoms, silicoflagellates and parmales), abundant faecal pellets of several types and zooplankton (mainly shelled pteropods). Vertical fluxes of particles occurred mainly through diatoms and faecal pellets in the first and second part of the summer, respectively. The highest fluxes were recurrently observed in late summer, when faeces contributed up to 100% of organic carbon. Unusually high fluxes were recorded in winter 1995, when faecal pellets accounted for 84.6% of the organic carbon. Peak fluxes were always driven by the sinking of faecal pellets, that hence appear to be the most efficient vector of export in the polynya of Terra Nova Bay. A major flux component was the pteropod Limacina helicina, which repeatedly sank in high amounts after the growing season. In April–June, L. helicina probably transported biogenic carbon to deep layers as a passive sinker. The inclusion of pteropods in flux estimates resulted in values that were up to 20 (for total mass), 25 (for organic matter) and 48 (for carbonate) times higher than the previously measured fluxes. Fluxes are known to be biased by swimmers, but ultimately attention must be paid to a possible erroneous categorization of some zooplankton as swimmers to avoid severe underestimation of fluxes of total mass (up to 95% in our study), organic matter (up to 96%) and carbonate (up to 100%).

The vertical flux of particulate matter in the polynya of Terra Nova Bay. Part I. Chemical constituents

Downward fluxes of particulate matter were investigated in the polynya of Terra Nova Bay (western Ross Sea) from February 1995 to December 1997. Biogenic silica made up the bulk of the sinking matter, reaching a maximum of 85% of the total flux in both the shallow and deep traps. Organic carbon accounted for 2.1 to 23.5% of the sample mass in the shallow trap and 2.8 to 7.2% in the deep trap. Biogenic silica and organic carbon peaks occurred in February–March over the three years and were always coupled in the top trap. Carbonate fluxes showed an uneven temporal trend, were low near the seafloor and contributed up to 69.8% of the material collected by the shallow trap. Vertical fluxes in the polynya showed large seasonal and interannual variability. Differences in primary productivity, ice cover dynamics and current energies accounted for the interannual changes in the flux of materials to the subsurface trap. Advection from sites a few hundred kilometres apart was the most likely reason for the decoupling between shallow and deep fluxes after October 1996. The seasonal pattern of downward fluxes was essentially linked to the seasonal cycle of primary production and spring–summer variations largely resulted from the composition of the phytoplankton assemblage in the overlying water column. The export of phototrophically produced materials to depth was low in late spring–early summer, at the time when Phaeocystis and small flagellates were the predominant autotrophs and the microbial food web prevailed over herbivorous feeding. Enhanced fluxes were linked to the predominance of large diatoms above the trap, in late summer. Temporal dynamics in this polynya were delayed by approximately one month as compared to the southernmost Ross Sea polynya, but showed the same temporal pattern: the time lag between production and accumulation of biogenic materials in the upper water column was a few weeks, while the delay in export from the surface layer relative to the maximum accumulation of biomass was approximately one month.

The abundance, vertical flux, and still-water and apparent sinking rates of marine snow in a shallow coastal water column

In Cape Lookout Bight, NC, measurements were made of marine snow (e.g., aggregates >0.5 mm in diameter) abundance and size, the vertical flux of material through the water column, the vertical flux of marine snow, and the measured still-water sinking rate of marine snow. The abundance and size of marine snow was determined from photographs taken by SCUBA divers. Sampled marine snow were used to determine aggregate dry weight. Marine snow flux and measured still-water sinking rate were made using a time-lapse camera that photographed a sediment trap mounted on a drogue. Flow over the drogue mounted sediment trap was <3 cm s −1. Knowing the flux of marine snow into the trap and the abundance of marine snow in the water column, the sinking rate of marine snow sinking freely in the water column (apparent marine snow sinking rate) was calculated. Throughout the study, marine snow was always present and usually abundant (average 125 l −1). The water-column particulate matter load ranged from 8 to 18 mg l −1. The vertical flux of particulate matter through the water column was large (average 74 g m −2 day −1). The average vertical flux was roughly equal to the sedimentation rate estimated from geochronology studies of sediment deposition in Cape Lookout Bight. More than 90% of the variation in the vertical flux of particulate matter through the water column was due to variations in the flux of marine snow. Variations in the vertical flux of marine snow were, however, not significantly correlated to the abundance of marine snow. Measured sinking rates ranged from around 10 to about 150 m day −1, increased with aggregate size, and varied little between days. The calculated apparent sinking rates of marine snow varied a great deal between days and were not similar to the measured sinking rates. The distribution and orientation of marine snow in the water column photographs appeared to delineate small (mm to cm diameter) eddies. Eddies in a random selection of water column photographs were measured. The upper limit to aggregate size appeared to have been set by the size of small turbulent eddies in the water column. The apparent sinking rate of marine snow, the total vertical flux and the flux of marine snow were all significantly correlated ( r>0.97) to the ratio of aggregate dry weight and eddy diameter. The data suggest that marine snow sinking rates were modified by turbulence in the water column.

Ultrastructure, morphology and flux of microzooplankton faecal pellets in an east Antarctic fjord

Small copepods and protozoa are major contributors to heterotrophic biomass in Antarc- tic waters. They produce small (<300 µm) faecal pellets, the fates of which are largely unknown. We examined the distribution and abundance of microzooplankton and small faecal pellets in Ellis Fjord, east Antarctica. We determined statistical relationships between the abundance of microzooplankton and pellets, and examined pellet morphology and ultrastructure using light and scanning electron microscopy. Our results indicate species-specific differences in the morphology and fate of pellets produced by small copepods: Oithona similis and harpacticoid pellets were retained in upper waters, while Oncaea curvata and Paralabidocera antarctica pellets sank to depth. Protozoan pellets did not sink to depth irrespective of their source or morphology and despite the fact they can be larger than those produced by small copepods. The majority of microzooplankton pellets, composed of phyto- plankton that otherwise may have directly sedimented to depth, was retained in near surface waters and probably recycled and remineralised. Despite producing faecal aggregates, heterotrophic activ- ity of most microzooplankton do not contribute to vertical flux but instead support respiration of mat- ter in upper waters. This may reduce the vertical flux of particulate matter to depth, thereby reduc- ing the capacity of Antarctic waters to act as a carbon sink, with implications for global climate.

The emission and vertical flux of particulate matter &lt;10 μm from a disturbed clay‐crusted surface

Arid and semi‐arid environments are important sources for the atmospheric loading of PM10 (particulate matter &lt;10 μm), although the emission of this material is often limited by surface crusts. This study investigates the emission and vertical flux of PM10 from a clay‐crusted playa, with and without saltating grains to abrade the surface. Using a portable field wind tunnel, it was found that, despite disturbance to the surface, the emission of PM10 decays rapidly without abrasion. Only in the presence of saltating grains was PM10 continuously liberated from the surface, such that the emission rate (the total amount of PM10 emitted from the surface expressed as a horizontal flux) varied linearly with the saltation transport rate. Although the emission of PM10 was found to depend on saltation abrasion, past studies have tended to focus on the relationship between the vertical flux of PM10 (the amount of PM10 being transported vertically through the boundary layer) and the shear velocity. In this study, the vertical flux of PM10 was found to vary with the shear velocity to the power of 2·14. Although the vertical PM10 flux is a proportion of the emission rate (the horizontal flux), no statistically significant relationship was observed between the emission rate and the shear velocity. The disparity of these results is explained by the lack of a consistent relationship between the shear velocity and the saltation transport rate in this supply‐limited environment. This suggests that the observed relationship between the vertical PM10 flux and the shear velocity is a spurious correlation, resulting from the use of shear velocity to calculate the vertical dust flux. It is thus concluded that shear velocity is not an appropriate variable for emission modelling in supply‐limited environments and that improvements in dust emission modelling will only be realized if the abrasion process is the focus of a concerted research effort.

The vertical flux of biogenic and lithogenic material in the Ross Sea: moored sediment trap observations 1996–1998

As part of the USJGOFS-AESOPS program, we deployed an array of moored sediment traps and determined the composition and vertical flux of particulate matter through the water column of the continental shelf environment of the Ross Sea. We deployed two moorings, with two traps each. One mooring was located in the southcentral Ross Sea along the line of AESOPS process studies; the other mooring was located in the northcentral Ross Sea. The flux in the southern Ross Sea was generally double that in the north, and there was a significant increase in flux into the deep traps indicating horizontal transport and possible focusing of particulate matter. The particles are primarily biogenic although lithogenic contributions can increase to over 30% of the flux in the near-bottom trap nearer to the ice shelf. The fluxes collected by the traps were generally low but within the range of previous observations. Several unique observations stand out from these data: (1) The export of organic matter to 200 m during the spring and summer was extremely low, consistent with observations that the ongoing phytoplankton bloom was being stored or recycled in the upper-water column; (2) A major pulse of biogenic silica export occurred in the fall, just as ice was reforming. The correlation of this event with lithogenous particles, excess Fe, and with the early breakout of katabatic winds suggests that this event might be related to Fe fertilization of diatom production and export; (3) The largest flux of organic matter during 1996–1997 was carried by the pteropod Limacina helicina and was observed at both moorings under the ice in the late fall, just after the diatom pulse. Although it is difficult to estimate the proportion of these organisms that were swimmers contaminating the flux, we believe that a significant fraction settled into the traps and may represent the die-out of this herbivore population after the suspended POM concentration had dropped to very low levels. Fluxes to the upper traps were much lower than estimates of export flux made by other methods during synoptic ship studies. These differences are discussed and the fluxes are compared with preliminary data from the ROAVERRS trap program, which was simultaneously deployed in the Ross Sea.

Variability and downward fluxes of particulate matter in the Otranto Strait area

The space-time distribution and vertical fluxes of particulate matter in the Otranto channel area have been studied. A three-layered structure was observed: (i) a surface layer with abundance of suspended matter, characterised by the presence of two water masses (the Adriatic Surface Water and the Ionian Surface Water); (ii) an intermediate layer (Levantine Intermediate Water) with very low particle content; and (iii) a bottom nepheloid layer. In winter, the particulate matter concentration at the surface revealed an east–west gradient that was not evident in summer, when the water exchange through the strait was reduced and local recirculation phenomena prevailed. Sediment traps and current meters were moored on the Italian shelf and on the shelf slope. Total vertical flux values of particulate matter, which were by one-tenth lower in the slope area with respect to the shelf, were in good agreement with the distribution of suspended matter in the water column observed during the seasonal cruises. Flux values measured at the shelf station appear to be related both to biological and advective processes, whereas the main source in the slope area appears to be the vertical transfer of biogenic material.

Vertical fluxes of particulate matter in Aarhus Bight, Denmark

Sediment traps, transmissometer readings and sediment core-dating have been used to describe sediment dynamics within a shallow micro-tidal coastal environment, characterized by intense resuspension. A method to separate the downward fluxes of primary settling matter and resuspended sediments is proposed. It is shown that these fluxes can be separated during periods when a marked pycnocline exists in the water column, preventing resuspended sediment from reaching the sediment traps above the pycnocline. At such times it is possible to make quantitative calculations of the flux of primary settling matter at the sediment-water interface. Good agreement between particulate fluxes calculated on the basis of sediment trap measurements and accumulation rates determined from Pb-210 core dating have been obtained in this study, supporting the idea of using sediment trap measurements to calculate sediment accumulation at the bottom.

Do upper-ocean sediment traps provide an accurate record of particle flux?

SEDIMENT traps are widely used to measure the vertical flux of particulate matter in the oceans. In the upper ocean, sediment traps have been used to determine the extent to which CO2 Axed by primary producers is exported as particulate organic carbon1–3. In addition, the observed decrease of particle flux with depth has been used to predict regeneration rates of organic matter and associated elements3. Over seasonal or annual timescales, the import of limiting nutrients into the upper ocean (new production) should be balanced by particle export4,5. Given the importance of accurately determining the sinking particle flux, it has been suggested that 234Th might be used to 'calibrate' shallow-trap fluxes6. Here I present a re-evaluation of existing 234Th data which indicates that trap-derived and model-derived 234Th particle fluxes can differ by a factor of ±3–10, suggesting that shallow traps may not provide an accurate measure of particle fluxes.

Sinking rates of cast exoskeletons of Antarctic krill ( Euphausia superba Dana) and their role in the vertical flux of particulate matter and fluoride in the Southern Ocean

The dry weight of cast exoskeletons of Euphausia superba was found to be log-linearly related to the dry weight of the post-moult animal, a relationship similar to that obtained for other species of euphausiids. The moult of this species represented an average of 7.5% of the post-moult dry weight of the animal, which is twice the estimate of other authors. The moults sank at rates ranging from 0.06 to 1.18 cm s −1 and the sinking rate was a function of the moult weight and of its surface area. Freshly shed exoskeletons had a mean carbon to nitrogen ratio of 5.6 and a mean organic content of 73.2% of the high dry weight. Fluoride levels in the cast exoskeleton were high (mean 5.44 ppt), and the fluoride was shown to leach out following ecdysis. The rate of fluoride loss was most rapid in the first 2 days following moulting and showed markedly in the following 3 days. The effect of these processes on the transport of organic matter and fluoride from the Antarctic surface waters is discussed.