J. La Rosa

Dynamics of the downward flux of particles and carbon in the open northwestern Mediterranean Sea

Abstract Downward fluxes and the elemental composition of settling particles at 80, 200 and 1000m depth measured during an 18-month sediment trap experiment are presented, together with associated hydrographic and biological data, in order to assess the relative importance of these factors in controlling particle flux. Mass, carbon and nitrogen fluxes were generally higher during the first half of the year. Mass flux maxima occurred at different times of the year depending on trap depth, whereas the peak carbon flux was measured in late June. Most of the total carbon flux was composed of organic carbon. On an annual basis the total particle flux was 32.9, 40.6 and 8.1 g m −2 year −1 at 80, 200 and 1000 m depth, respectively. The organic carbon flux decreased from 4.8 and 5.0 g m −2 year −1 at 80 and 200 m, respectively to 1.2 g m −2 year −1 at 1000 m, and the corresponding nitrogen flux from 0.7 and 0.8 to 0.2 g m −2 year −1 . Organic carbon and nitrogen content of sinking particles decreased with depth, whereas the carbonate carbon content remained constant. The contribution of zooplankton feces to the carbon flux was highly variable throughout the year, ranging from 1.6 to 62%. Swimmers, including fish, that entered the traps in the upper 200 m were abundant and, on average, were twice the weight of the sinking particle flux. Pelagic production was the main source of particles during the late spring and mid-summer sedimentation pulses; however physical forcing, particularly the winter mixing of the water column, was partially responsible for the high sedimentation during the earlier part of the year. Wind gusts and concomitant vertical mixing appeared responsible for the carbon rich particle flux in summer. Separation of data for the mixed and stratified water column periods clearly indicated that most of the downward flux occured in mixed waterswhereas the organic carbon content was higher in particles sedimenting through stratified waters. The export of organic carbon from the euphotic zone (upper 200 m) represented only 5–7% of the estimated phytoplankton production in this region. Furthermore time-series measurements of POC flux averaged on an annual basis were only 14–42% of indirect estimates of carbon export or “new production” for this region. This discrepancy suggests that a good portion of the total downward carbon flux occurs through advection and/or diffusion of DOC.

Factors affecting methyl and inorganic mercury dynamics in mussels and shrimp

Radiotracer experiments were designed to study the effect of certain factors on the accumulation and loss of methyl and inorganic mercury in mussels (Mytilus galloprovincialis) and benthic shrimp (Lysmata seticaudata). Methyl mercury was accumulated from both food and water to a greater degree than inorganic mercury by both species. There was a tendency for small mussels to concentrate more mercury than larger individuals, but the reason for this difference remains unclear. A trend towards greater mercury accumulation at higher temperatures was noted for mussels, but the temperature effect was relatively small over a 10 Co range between 8° and 19°C. Methyl mercury residues were eliminated by both species more slowly than those of the inorganic form. Loss from mussels was somewhat more rapid at higher temperatures; however, as in the case of accumulation, the effect of temperature was relatively small. Loss rates for small mussels were not significantly different from those for large individuals. Enhanced methyl mercury elimination was noted for the group of mussels maintained in their natural environment. The more rapid metal turnover in these individuals compared with mussels held in the laboratory was attributed to differences in the availability of natural particulate food matter and, hence, subsequent growth of the animals in the two experimental systems. The observed differences underscore the need for caution in predicting in situ flux of metals such as mercury in certain species based solely on data derived from laboratory experiments.

Separation of actinides, cesium and strontium from marine samples using extraction chromatography and sorbents

The development of radioanalytical methods for low level radionuclides in marine environmental samples is presented. In particular, emphasis is placed on the introduction of extraction chromatography (EC) as a tool for improving the quality of results as well as reducing the analysis time. However, the advantageous application of EC often depends on the effective use of suitable preconcentration techniques, such as co-precipitation, to reduce the amount of matrix components which accompany the analytes of interest. On-going investigations in this field relevant to the determination of environmental levels of actinides, 137Cs and 90Sr are discussed.

Analysis of plutonium isotopes in marine samples by radiometric, ICP-MS and AMS techniques

IAEA reference materials (radionuclides in the marine environment) collected in areas affected by nuclear reprocessing plants and nuclear weapons tests have been analysed by semiconductor alpha-spectrometry (SAS), liquid scintillation spectrometry (LSS) and mass spectrometric techniques (high resolution ICP-MS and AMS) with the aim of developing analytical procedures and to study the geochemical behavior of plutonium in the marine environment. The Pu results obtained by SAS, ICP-MS and AMS were in reasonably good agreement (R2 = 0.99). The mean atom ratios of 240Pu/239Pu in IAEA reference materials, IAEA-134, 135 and 381 were (0.212±0.010), (0.211±0.004) and (0.242±0.004), respectively. IAEA-384 (Fangataufa Lagoon Sediment) gave a 240Pu/239Pu mean atom ratio of 0.051±0.001. The results of 241Pu obtained by ICP-MS and LSS also show reasonable agreement (R2 = 0.91). Pu isotopic signatures were useful in tracing Pu origin and in interpreting biogeochemical processes involving Pu in the marine environment.

ICP-sector field mass spectrometry analysis of plutonium isotopes: recognizing and resolving potential interferences

Despite the resulting sacrifices in sensitivity, the analysis of plutonium isotopes by ICP-sector field mass spectrometry (SFMS) in higher resolution mode can be beneficial in resolving non-hydride polyatomic interferences. Prepared samples are normally first analyzed in low-resolution mode for maximum sensitivity. The acquired actinide peaks are then assessed for the presence of a polyatomic interference, evidenced by a measurable negative shift in mass position of the isotope of interest. If, based on this assessment, a polyatomic interference is suspected, medium-resolution mode is employed for unequivocal resolution of the analyte peak. Examples of Pu isotope data acquired in both low-resolution and medium-resolution modes are presented.

Recent developments in radiometric and mass spectrometry methods for marine radioactivity measurements

The most important recent developments in radiometric techniques have been the operation of high efficiency HPGe detectors with anticosmic or antiCompton shielding often placed underground, ship-board measurements of 234Th using gamma-spectrometry or beta-counting and underwater gamma-spectrometry. In mass spectrometry techniques, the availability of high resolution ICP-MS and applications of AMS for the analysis of long-lived radionuclides have opened doors for investigations which previously required too large samples, or were not possible because of lack of sensitivity. These developments are illustrated by several examples of marine radioactivity studies, which include contaminated sites (e.g., Fangataufa Atoll) and open ocean studies (e.g., the Atlantic Ocean) as well as analyses of IAEA reference materials. A comparison of Pu results obtained by alpha-spectrometry, ICP-MS and AMS has shown that a reasonably good agreement has been obtained between all three methods.

Determination of 239Pu/240Pu isotopic ratio by high-resolution alpha-particle spectrometry using the ADAM program

A novel analysis program to unfold alpha-particle energy spectra was introduced, demonstrated and validated using radiochemically processed test sources, which contained different amounts of (239)Pu and (240)Pu. A high-resolution alpha spectrometer was used for data acquisition. The software known as ADAM unfolds the spectra using nuclide-specific decay data as a constraint. The peaks can have different shapes and the software can also cope with the coincidences between alpha particles and electrons/photons. In the present paper, the (239)Pu/(240)Pu activity ratios from alpha spectrometry agreed, within the stated uncertainties, with the reference values. Number of counts in the (239,240)Pu peak group must be larger than 100 to obtain reliable values when using semiconductor detector of energy resolution FWHM=10.6 keV.

Results of an international comparison for the determination of radionuclide activity in bilberry material.

Radioactivity found in wild food products has assumed greater importance when assessing the total exposure of the population. For this reason, IRMM has been developing a reference material for the activity concentration of three radionuclides in bilberry samples. In order to characterise this new material, a CCRI(II) supplementary comparison was organised. The difficulties encountered in this comparison are discussed, in particular the efficiency calibration for volume sources of gamma-ray emitters, and comparison reference values for (137)Cs and (40)K are calculated.

Effect of different radiotracer labelling techniques on radionuclide excretion from marine organisms

Experiments were designed to assess the effect of different techniques of radiotracer labelling on subsequent radioisotope excretion rates in marine crustaceans. A small amphipod (Gammarus locusta) that accumulated 65Zn under a close approximation of natural conditions excreted the radiotracer during a 3-month period at a markedly different rate than those of comparable amphipods labelled with 65Zn in the laboratory via different combinations of radioactive food and seawater. Shrimp (Lysmata seticauda) administered 65Zn by three different methods in the laboratory displayed different excretion kinetics during the first 2 months of loss. The difference between excretion rates most likely was a reflection of the degree to which the various zinc pools within the shrimp had equilibrated with the radiotracer. During the next several months all 65Zn loss rates were quite similar, probably indicating that radiotracer excretion was taking place from similar zinc pools within the shrimp. The importance of adequate radiotracer labelling techniques as well as proper design of subsequent excretion experiments, used to gain reliable information on flux rates of the corresponding stable metal, is discussed. It is concluded that laboratory radiotracer experiments which are intended to supply information on actual situations in the sea require careful design and execution.

Interannual Variation in Transuranic Flux at the Vertex Time-Series Station in the Northeast Pacific and Its Relationship to Biological Activity

From October 1986 through May 1988, seven cruises took place at approximately three-month intervals at the VERTEX time-series station (33°N, 139°W) in the North Pacific. Data on plutonium (239+240Pu) and, in some cases, 241Am in unfiltered sea water, suspended particulate matter, sediment trap material, and zooplankton and their fecal pellets were collected over a 2000 m water column in order to examine temporal variations in particulate-associated transuranic flux and its relationship to biological activity in the upper water layers. Some seasonality in 239+240Pu flux was noted with the highest fluxes corresponding to the lowest mass fluxes below 400 m in summer through early winter (possibly due to scavenging by fine particles), and the lowest 239+240Pu fluxes corresponding to the highest mass fluxes above 150 m at the same time of year (due apparently to minimal scavenging in the euphotic zone). Little apparent difference was noted in the relationship between 239+240Pu flux and mass flux during late winter and spring at all depths, suggesting minimal scavenging at depth and generally small mass fluxes out of the euphotic zone. Salps which feed on tiny particles have relatively high concentrations of 239+240Pu in their fecal pellets while copepods which feed more selectively on larger particles have lower transuranic concentrations in their pellets.