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We report on mid-summer gross primary production and new production in the Sub-Antarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) south of Tasmania during the SAZ-Sense expedition (January-February 2007). The aim of our study was to assess how well documented regional variability in surface Chl-a biomass translates into variability of primary production and potential export in the area. The selected sites for process studies contrasted in terms of euphotic and mixed layer depths, macro nutrient concentrations and primary production with short term (days) temporal variability of production. Daily euphotic layer integrated gross primary production (from short term 13C incubations) was higher in the SAZ than in the FEZ by about an order of magnitude. Within the SAZ highest production was reached south-west of Tasmania, contrasting with surface ocean Chl-a biomass which was highest in the eastern SAZ. In most cases regenerated production (from 15N-ammonium uptake experiments) was significantly larger than new production (from 15N-nitrate uptake experiments) with f-ratios mostly

For the calibration of any model, measurements are necessary. As measurements are expensive, it is of interest to determine beforehand which kind of samples will provide the maximum of information. Using a criterion related to the Fisher information matrix, it is possible to design a sampling scheme that will enable the most precise model parameter estimates. This approach was applied to a reactive transport model (based on SLIM) of Escherichia coli in the Scheldt Estuary. As this estuary is highly influenced by the tide, it is expected that careful timing of the samples with respect to the tidal cycle will have an effect on the quality of the data. The timing and also the positioning of samples were optimised according to the proposed criterion. In the investigated case studies the precision of the estimated parameters could be improved by up to a factor of ten, confirming the usefulness of this approach to maximize the amount of information that can be retrieved from a fixed number of samples.

Regional and seasonal variability of the Si dissolution: production ratios in the surface ocean have not been well assessed. Here, we propose a new method for determining these rates, using the 30Si-isotopic dilution technique with a high-resolution sector field inductively coupled plasma mass spectrometer (HR-SF-ICP-MS). Relative analytical precision of the isotopic measurement is better than 1%, similar to that obtained by thermal ionization-quadrupole mass spectrometry (TIMS). Accuracy and reproducibility of the isotopic measurements have been checked on artificial and natural solutions by intercomparison between two HR-SF-ICP-MS instruments and one TIMS. Measurements of real Si production and dissolution rates are illustrated for two contrasted situations with an average relative precision of 10%, including one from waters with low Si content (2 µmol L-1), which required an additional purification step by cation exchange chromatography. Si production rate from this later incubation was not significantly different from the one measured by radioactive 32Si. The new method is faster and simpler than TIMS or isotope ratio mass spectrometry (IRMS). Its sensitivity is more than one order of magnitude better than TIMS, and it can cover the whole range of Si concentrations encountered in the ocean.

High densities of mussels of the genus Bathymodiolus are present at hydrothermal vents of the Mid-Atlantic Ridge. It was previously proposed that the chemistry at vent sites would affect their sulphide- and methane-oxidizing endosymbionts’ abundance. In this study, we confirmed the latter assumption using fluorescence in situ hybridization on Bathymodiolus azoricus specimens maintained in a controlled laboratory environment at atmospheric pressure with one, both or none of the chemical substrates. A high level of symbiosis plasticity was observed, methane-oxidizers occupying between 4 and 39% of total bacterial area and both symbionts developing according to the presence or absence of their substrates. Using H13CO−3 in the presence of sulphide, or 13CH4, we monitored carbon assimilation by the endosymbionts and its translocation to symbiont-free mussel tissues. Carbon was incorporated from methane and sulphide-oxidized inorganic carbon at rates 3 to 10 times slower in the host muscle tissue than in the symbiontcontaining gill tissue. Both symbionts thus contribute actively to B. azoricus nutrition and adapt to the availability of their substrates. Further experiments with varying substrate concentrations using the same set-up should provide useful tools to study and even model the effects of changes in hydrothermal fluids on B. azoricus’ chemosynthetic nutrition. ispartof: Biogeosciences vol:5 issue:6 pages:1681-1691 status: published

The VERtical Transport In the Global Ocean (VERTIGO) study examined particle sources and fluxes through the ocean's "twilight zone" (defined here as depths below the euphoric zone to 1000m). Interdisciplinary process studies were conducted at contrasting sites off Hawaii (ALOHA) and in the NW Pacific (K2) during 3-week occupations in 2004 and 2005, respectively. We examine in this overview paper the contrasting physical. chemical and biological settings and how these conditions impact the source characteristics of the sinking material and the transport efficiency through the twilight zone. A major finding in VERTIGO is the considerably lower transfer efficiency (Teff) of particulate organic carbon (POC), POC flux 500/150 m, at ALOHA (20%) vs. K2 (50%). This efficiency is higher in the diatom-dominated setting at K2 where silica-rich particles dominate the flux at the end of a diatom bloom, and where zooplankton and their pellets are larger. At K2, the drawdown of macronutrients is used to assess export and suggests that shallow remineralization above our 150-m trap is significant, especially for N relative to Si. We explore here also surface export ratios (POC flux/primary production) and possible reasons why this ratio is higher at K2, especially during the first trap deployment. When we compare the 500-m fluxes to deep moored traps, both sites lose about half of the sinking POC by > 4000 m, but this comparison is limited in that fluxes at depth may have both a local and distant component. Certainly, the greatest difference in particle flux attenuation is in the mesopelagic, and we highlight other VERTIGO papers that provide a more detailed examination of the particle sources, flux and processes that attenuate the flux of sinking particles. Ultimately, we contend that at least three types of processes need to be considered: heterotrophic degradation of sinking particles, zooplankton migration and surface feeding, and lateral sources of suspended and sinking materials. We have evidence that all of these processes impacted the net attenuation of particle flux vs. depth measured in VERTIGO and would therefore need to be considered and quantified in order to understand the magnitude and efficiency of the ocean's biological pump.

We monitored the stable nitrogen isotopic composition (d15N) of suspended matter and ammonium in the freshwater stretch of the Scheldt estuary (Belgium) over a full year to investigate for seasonal evolution and possible co-variation between isotopic signatures. The d15N value of ammonium remained rather constant during winter (average = +11.4 pro mille) but increased significantly with the spring and summer bloom, reaching values as high as +70 pro mille. This enrichment of the ammonium pool in 15N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semiclosed system approach we deduced an apparent fractionation factor associated with NH4+ utilization (i.e. combining effects of uptake and nitrification) of 18.4 pro mille (SE = 2.0 pro mille), which is similar to values reported in literature. Observed variations of ammonium d15N could account for about 69% of d15N variation in suspended matter.

The oceanic biological pump drives sequestration of carbon dioxide in the deep sea via sinking particles. Rapid biological consumption and remineralization of carbon in the "twilight zone" (depths between the euphotic zone and 1000 meters) reduce the efficiency of sequestration. By using neutrally buoyant sediment traps to sample this chronically understudied realm, we measured a transfer efficiency of sinking particulate organic carbon between 150 and 500 meters of 20 and 50% at two contrasting sites. This large variability in transfer efficiency is poorly represented in biogeochemical models. If applied globally, this is equivalent to a difference in carbon sequestration of more than 3 petagrams of carbon per year.

We monitored the stable nitrogen isotopic composition (d15N) of suspended matter and ammonium in the freshwater stretch of the Scheldt estuary (Belgium) over a full year to investigate for seasonal evolution and possible co-variation between isotopic signatures. The d15N value of ammonium remained rather constant during winter (average = +11.4‰) but increased significantly with the spring and summer bloom, reaching values as high as +70‰. This enrichment of the ammonium pool in 15N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semi-closed system approach we deduced an apparent fractionation factor associated with NH4+ utilization (i.e. combining effects of uptake and nitrification) of 18.4‰ (SE = 2.0‰), which is similar to values reported in literature. Observed variations of ammonium d15N could account for about 69% of d15N variation in suspended matter.

BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) is a longterm project aiming at applying and developing process-level studies, geochemical proxy tools and numerical tools for assessing and understanding the present-day functioning of the CO2 biological pump in the iron-limited Southern Ocean (S.O.) and for predicting its evolution in response to scenarios of increasing atmospheric CO2. Over the last four years BELCANTO implemented and developed several multi-proxy approaches for assessing nutrient consumption and carbon fluxes, including 234Thdeficit and natural silicon isotopic composition. High quality results were obtained for whole water column δ29Si-silicate and δ29Si-opal, including first ever results for the low silicate Subantarctic region. Results show that the δ29Si signature of diatoms appears to be homogeneous in the mixed layer and between diatom species. The preferential uptake of light isotope by diatoms is well reflected in the vertical distribution of silicate δ29Si, as well as in the upper ocean silicate δ29Si values which increase northwards in parallel with the decrease of silicate concentration. However, the relationship between δ29Si and silicate concentration appears complex and depends on variations in vertical and horizontal supply of silicate. Results based on other proxy tools (Baxs 234Th-deficit, f-ratio & new production) indicate relatively high particulate carbon export and absence of strong mesopelagic mineralization in the Subantarctic Zone, but relatively low export and enhanced mesopelagic mineralization further south in the Polar Front Zone and the southern ACC. Furthermore, remineralization was clearly enhanced during summer as compared to spring. Our observations of 234Th-based carbon export and new production along cross frontal transects also appear to challenge the widely accepted idea of enhanced primary and export production in the Subantarctic Zone and the Polar Frontal Zone, compared to more southern areas. Laboratory-controlled experiments on two widespread phytoplankton species (diatoms-Thalassiosira gravida and Phaeocystis Antarctica) showed an effect of Fe addition on the morphological form, enhancing the presence of the colonial form compared to the free-living cells for Phaeocystis and increasing the appearance of long chains of diatoms vs. free living cells. Under Fe enriched conditions, the maximum photosynthesis and growth rates increased. In addition the quality of the organic matter was modified, enhancing bacterial remineralization. Consequently, Fe addition may impact on the fate of the phytoplankton in the planktonic food web and the resulting efficiency of the biological pump. Accordingly, during the EIFEX large scale iron enrichment experiment, iron addition fosters a diatom bloom which appeared to broke up rather fast and to sink rapidly accordingly to 234Th measurements. This in turn triggered mesopelagic mineralization as evidenced in Baxs measurements. N-uptake experiments with Fe-limited and Fe-replete natural algal communities indicate that the effects of ammonium and iron on f-ratio and new production are not simply cumulative and that the enhancement of the f-ratio due to Fe addition depends on the ambient ammonium concentration. However the relationship between this enhancement and ammonium concentration is at present not fully understood. Our results imply that there is no simple relationship between export production and iron availability. Ammonium appears to counter the effects of iron addition on export production, particularly for HNLC areas such as the Southern Ocean. The meso-scale iron enrichment experiment performed during the EIFEX cruise, clearly showed that iron addition induced a diatom bloom but that the latter did not persist for long and collapsed soon after, thereby inducing a massive export of carbon as witnessed from significant deficits in 234Th activity. This export in turn triggered mesopelagic organic carbon mineralization as evidenced by excess Baxs contents. Measurements of pCO2, scaled using remote sensing data (sea surface temperature, chlorophyll and wind stress) and related CO2 fluxes, suggest that previous budgets of atmospheric CO2 uptake in the S.O. may be overestimated. However, our observations indicate that sea ice cover can act as an additional CO2 sink not taken into account in previous CO2 budgets. This sea-ice CO2 sink function results from physical and biogeochemical processes prevailing within the sea ice itself. Results obtained with SWAMCO-4, an idealized 1-D model of the marine planktonic system calculating C, N, P, Si, Fe cycling within the upper ocean, the export production and air-sea CO2 fluxes, suggest that the sink for atmospheric CO2 will increase in response to the raising atmospheric CO2 concentration. SWAMCO-4 simulations show that the amplitude of the predicted CO2 sink displays large regional and inter-annual variations which are related to local hydrodynamics and the dominant phytoplankton species. This is particularly acute in the Phaeocystis-dominated marginal ice zone of the Ross Sea. There the predicted annual CO2 sink appears positively related to the length of the sea ice cover period. This results from the accumulation of iron within the ice and its sudden release in the water column at the time of ice melting, favoring algal growth. The past and future evolution of the sea ice cover was conducted using the ORCA2- LIM and ECBILT-CLIO 3D ice-ocean models. The influence of the Southern Annular Mode on zonal integrated sea surface temperature and ice concentration seems to be small due to counteracting effects. Over the last 250 years, the annual mean ice coverage decreased in response to both natural and anthropogenic forcing, with the impact of the latter forcing clearly being enhanced over the last 150 years. Nowadays, the decrease of ice cover is more acute in the northern hemisphere as compared to the southern hemisphere, and this difference is due to thermal inertia of the S.O. However, model outputs predict a more abrupt decrease of S.O. sea ice extent in the future, resulting in similar decreases of annual mean sea ice extents in both hemispheres by the end of the century. In parallel, the seasonal amplitude of sea ice extent will increase. Preliminary runs with the coupled ice-ocean-biogeochemical model ORCA-LIMSWAMCO- 4, predict that the efficiency of the S.O. biological pump will be very sensitive to changes in the seasonal amplitude and the mean extent of the ice cover.

Over the past few decades, the radioisotope pair of 238U / 234Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to 234Th. It is focused on the models associated with 234Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C / 234Th ratios (Buesseler et al.), 234Th speciation (Santschi et al.) and present and future applications of 234Th [Waples, J.T., Benitez-Nelson, C.R., Savoye, N., Rutgers van der Loeff, M., Baskaran, M., Gustafsson, Ö., this issue. An Introduction to the application and future use of 234Th in aquatic systems. Marine Chemistry, FATE special issue]. In this paper, we review current 234Th scavenging models and discuss the relative importance of the non-steady state and physical terms associated with the most commonly used model to estimate 234Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1-4 weeks in order to evaluate steady state versus non-steady state estimates of 234Th export, especially during high flux events (> ca. 800 dpm m-2 d-1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g., Equatorial Pacific). These suggestions will greatly enhance the application of 234Th as a tracer of particle dynamics and flux in more complicated regimes.

Seasonally, dissolved and particulate metal concentrations in the Scheldt estuary were assessed over a period of 4 years (1995–1998). High quality data were obtained following stringent analytical protocols for each step: sampling, sample treatment, sample storage and analysis. Of the 5 trace metals, Ni showed the most conservative behaviour, while Cd and Cu were clearly transferred from the particulate to the dissolved phase in the middle estuary. A substantial part of the particulate metals entering the estuarine system are lost through sedimentation. General seasonal patterns are the following: lower concentrations in spring and higher ones in winter (sometimes late fall/early winter) for dissolved metals, while in summer a pronounced rise of the longitudinal concentration profile is observed for the particulate metals. A comparison of the trace metal concentrations (dissolved and particulate) at the mouth of the estuary in 1995–1998 with those from 1981 to 1983, reveal reductions between 30 and 58%. Reductions based on direct emission measurements for almost the same period suggest reductions (dissolved + particulate) between 42 and 64%. Biomagnification (BMF) is the accumulation of a compound through the food-chain. It is in our case expressed as the ratio of the metal concentration in the organism (µg g–1, d.w.)/the metal concentration in total suspended matter (µg g–1, d.w.). Almost all BMF-values of Periwinkle, Nereis diversicolor and Macoma balthica (3 bottom organisms in the Scheldt estuary) are negative meaning that these organisms contain less heavy metals than the particulate suspended matter. For all organisms log BMFs for Pb, respectively Ni, are around –1.8, respectively –0.7. For Cd, Periwinkle shows slight enrichment (0.05) and for Cu even more (0.45), while negative values were observed for Nereis diversicolour and Macoma balthica. The latter organisms are more enriched in Zn (–0.09) than Periwinkle (–0.43).

Studies using oxygen isotopes (δ18O) of mollusk shells to determine paleotemperature need to assume water δ18O values, which could severely influence calculated temperatures. We analyzed aragonitic shells of the Butter Clam, Saxidomus giganteus (DeShayes, 1839), to determine the reproducibility of the isotopic signal between individuals and to assess how precisely temperature could be calculated given known salinity and temperature. Furthermore, carbon isotopes are also investigated as an environmental proxy. The abundance of well-preserved S. giganteus shells in archeological and geological deposits in northwestern North America makes them a particularly suitable species for paleoclimate studies. Seasonally resolved stable oxygen isotope profiles in three S. giganteus shells collected from the same site in Puget Sound (Washington, USA) were well correlated (0.77 < R2 < 0.87). Although there were differences up to 0.58‰ in high resolution δ18O profiles of the three shells, the difference between the average δ18O of each shell was less than half of this (0.19‰) and half of what has been reported for between-colony coral variability. δ13C profiles on the other hand were more complex, with shell δ13C being about 2.5‰ lower than expected equilibrium values. However, this roughly conforms to the idea that about 10% of the shell carbon originates from metabolic CO2. Both δ18O and δ13C indicate that S. giganteus do not grow during periods of reduced salinity. Despite the excellent reproducibility of δ18O between shells, and the fact that salinity effects were duly considered, calculated temperature still differed from instrumental temperature. Applying different salinity-δ18O water relationships to average shell δ18O, and considering salinity from the shell collection site and a nearby offshore station resulted in calculated average water temperatures ranging from 1.7 to 6.4 °C warmer than measured. Although we could not determine if S. giganteus precipitate their shells in isotopic equilibrium, we believe that the difficulty in predicting temperature arose from not being able to accurately determine δ18O of the water at the time of shell precipitation. These data highlight the difficulties inherent to using stable isotope profiles of estuarine biogenic carbonates as environmental proxies.

This paper investigates the estimation behavior of six increasingly complex 15N models, for estimating flux rates between phytoplankton and dissolved N pools in aquatic ecosystems. The development of these models over the last 40 years reflects increasing realism in the pools and fluxes that constitute the N cycle. The purpose of this paper is to assess how the model results are influenced by the underlying assumptions. For example, with respect to uptake of 15N by phytoplankton, is anything gained by assuming that regenerated N become isotopically enriched after the introduction of the 15N label, or is it just as accurate to assume that no source other than the initial 15N label contributes to the enrichment signal in phytoplankton? To conduct an objective assessment of the models, we compared them to (1) a set of reference values generated numerically by a process oriented model, and (2) real experimental data. The results show that for a number of 15N models, properties such as accuracy and precision cannot both be optimized under the same conditions, and a compromise must be struck. Oversimplified models risk bias when their underlying assumptions are violated, but overly complex models can misinterpret part of the random noise as relevant processes. Therefore none of the 15N model solutions can a priori be rejected, but each should carefully be assessed with hypothesis testing. A backward regression strategy based on a statistical interpretation of the cost function (sum of the weighted least squares residuals) was used to select optimal solution subsets corresponding to a given data set.

A diffusion method for extracting ammonium from marine, estuarine and fresh waters for 15N/14N isotopic ratio determinations at enriched level was developed and validated. The method is based on the conversion of NH4+ to NH3 gas under alkaline conditions, diffusion of NH3 out of the solution to the headspace, NH3 trapping on an acidified GF/D glass fiber filter, and subsequent 15N/14N isotope ratio determination with mass spectrometry. The diffusion period necessary to extract sufficient N in order to accurately measure the atom% 15N was reduced to less than 15h by bubbling the sample with a carrier gas (Air) at room temperature. The technique uses 250mL sample volume and enables accurate atom% 15N measurements in NH4+ pools as small as 1.25μM. A standard operating procedure for ammonium extraction is given involving method performance criteria, such as accuracy, precision, detection limit, quantification limit and robustness. The efficiency of the NH4+ extraction ranged from 40 to 100%. The quantification limit of the method was estimated at around 0.26μmol% N, for an initial 15N abundance of 1%. The within-laboratory reproducibility amounted to 0.03atom% 15N. It was shown that the recovery rate obtained after extraction of the certified reference material (CRM: IAEA-311) solution falls within the 95% confidence interval of the certified values. By applying the developed method to fortified natural water samples of different conductivities, the atom% 15N determinations were precise and accurate for α=1–5%.

Stable carbon isotope analysis of tree rings has become a widely used proxy in environmental and palaeoclimatological studies. In those studies, a-cellulose has often been the preferred material because of its singular composition and its immobility in wood. However, cellulose extraction is a time-consuming procedure and since the development of on-line isotope ratio mass spectrometers has become the time-limiting step in the isotopic analysis of wood samples for dendrochemical purposes. In this study we evaluate the necessity of cellulose extraction for isotopic analysis of tree rings in a tropical mangrove tree, Rhizophora mucronata Lam. Comparison between the d13C of extracted a-cellulose and bulk wood material revealed a highly significant linear relationship (d13Cbulk wood = 0.92 (± 0.08) * d13Ca-cellulose - 2.91 (± 2.04); p 13C as compared to bulk wood. The slope of the regression was not significantly different from one (p d13Cbulk wood - d13Ca-cellulose slopes for earlywood and latewood or between the slopes for samples from trees growing in contrasting environmental conditions. These results indicate that bulk wood can be used instead of a-cellulose when measuring stable carbon isotopes in the sapwood of R. mucronata in the context of a dendrochronological investigation.

Belgian citizens face a high background exposure to dioxins and polychlorinated biphenyls (PCBs). During the past 3 years, three episodes of animal feed contamination have added to this exposure. This chapter calculates the body burdens (body burden defined as the concentration of a persistent substance accumulated in a relevant tissue or body fluid) resulting from these exposures and compares the results with measured body burden data in the Belgian population. Geometric mean PCB (sum of seven indicator congeners) background concentrations in Belgian food items range between 16.6 ng/g fat for milk and 72.1 ng/g fat for poultry. Corresponding geometric mean dioxin concentrations range between 0.8 pg TEQ per g fat for pork and 2.4 pg TEQ per g fat for poultry. On top of this, Belgium has faced three animal feed contamination episodes: a major one in 1999, one in 2000, and one in 2002. During the 1999 crisis, a substantial part of the human food chain was contaminated, with PCB concentrations in poultry up to 56,856 ng/g fat and dioxin concentrations up to 2613.4 pg TEQ per g. In 1999, and to a lesser extent in 2001, occasional incidents of isolated contamination also increased concentrations in food items, the contribution of which to body burdens should not be neglected. Based on background exposure levels, the body burdens of Belgian female adolescents and women (ages 50 to 65) have been calculated. Calculated PCB body burdens are 75.5 μ g/kg body weight for female adolescents and 147.0 μ g/kg body weight for adult women, and dioxin body burdens are 3.7 ng TEQ per kg body weight for female adolescents and 7.1 ng TEQ per kg body weight for women. Measured dioxin body burdens are substantially higher and amount to 8.8 ng TEQ per kg body weight for female adolescents and 14.7 ng TEQ per kg body weight for female adults. The difference between calculated and observed values might stem from higher background levels in the past, from episodes of widespread contamination, and from exposure through other routes. Calculated PCB body burdens are 20% higher than the measured ones (57.1 μ g/kg body weight for female adolescents and 121.6 μ g/kg body weight for women). This difference might be explained by assumptions regarding the half-life of PCBs which overestimate the actual half-life. It was estimated that the 1999 crisis resulted in an increase of PCB body burdens by 28% and of dioxin body burdens by 4.3%. As to PCB body burdens, measurements before and after the crisis are concordant with this estimate.

The correlation coefficient is commonly used to evaluate the degree of linear association between two variables. However, it can be shown that a correlation coefficient very close to one might also be obtained for a clearly curved relationship. Other statistical tests, like the Lack-of-fit and Mandel's fitting test appear therefore more suitable for the validation of the linear calibration model. A number of cadmium calibration curves from atomic absorption spectroscopy were assessed for their linearity. All the investigated calibration curves were characterised by a high correlation coefficient (r > 0.997) and low quality coefficient (QC < 5%), but the straight-line model was systematically rejected at the 95% confidence level on the basis of the Lack-of-fit and Mandel's fitting tests. Furthermore, significantly different results were achieved between a linear (LRM) and a quadratic regression (QRM) model in forecasting values for mid-scale calibration standards. The results obtained with the QRM did not differ significantly from the theoretically expected value, while those obtained with the LRM were systematically biased. It was concluded that a straight-line model with a high correlation coefficient, but with a lack of fit, yields significantly less accurate results than its curvilinear alternative. ispartof: DTEW Research Report 0205 pages:1-10 status: published

Action plans for a better protection of the North Sea were presented at the successive North Sea Conferences. These plans included far-reaching undertakings to reduce aqueous discharges and atmospheric emissions of substances such as trace metals that are toxic, persistent and liable to bioaccumulate. For cadmium, lead and mercury the reduction targets were achieved, but an additional effort is necessary for copper and zinc. It is interesting to compare, for the various trace metals, the above mentioned emission reduction percentages with the immission reduction levels observed in the Scheldt estuary over the same period. The Scheldt estuary is not only a receiving water body: evacuation of pollutants to the sea, storage in the bottom sediments and depuration via efflux to the atmosphere all contribute to a reduction of the metal burden in the water column. All in- and outgoing flows have been put together in a box-model, and a mass balance for each box has been calculated. Only in 5 boxes on 30, the imbalance was larger than 20% with a highest value of 29%. For mercury, also the food-chain transfer in the Scheldt, including bioconcentration and bioaccumulation factors, has been studied. Despite the fact that Hg levels in the Scheldt estuary are much higher than in the North Sea, it appears that Hg concentrations in fish are even slightly lower in the Scheldt. A more striking observation is the very low MMHg fraction in Scheldt fish (average 57%), compared to that in North Sea fish (average 95%).

25 pages, 8 tables, 12 figures, As part of the OMEX I project, nutrient determinations were made on 17 cruises in the region of the Goban Spur and La Chapelle Bank between 46 and 51°N, in all seasons of the year, between 1993–1995. Over this period no change was detectable in the structure of the water masses below the deep winter mixed layer. The N : P (dissolved nitrate-to-phosphate) ratio changed from 16 at 100-m depth to less than 15 at 3300-m depth. At intermediate depths nutrient and oxygen data indicate the presence of Mediterranean Outflow water overlying Labrador Sea Water at its most eastern extension. Estimated maximum levels of production in the spring bloom are the total N-limited new primary production equivalent between 24 and 41 gC m−2, the equivalent maximum diatom production is 11 gC m−2. Measurements during the spring bloom suggest a conversion factor of 1 μM nitrate to 1 μg l−1 chlorophyll, at the shelf break, which is consistent with other recent measurements in European shelf seawaters. Sediment trap data suggest that 80% (5.4 g m−2) of the opal produced in the spring bloom dissolved before reaching the sediment trap at 600 m. A comparison of the winter and summer profiles for dissolved silicon suggests a similar dissolution of 9±3 g opal m−2 above 300-m depth. Measurements of dissolved organic carbon (DOC) in September 1994 show an enrichment of 7 μM-C above the seasonal thermocline relative to the winter values (52±4 μM). In winter dissolved organic nitrogen represents 40% of the pool of total dissolved nitrogen. There is no consistent evidence of an increase in the concentration of DON during summer. Measurements of nitrate in surface waters in January 1994 show that concentrations off-shelf vary with the temperature of the water and are related to the depth of winter mixing. Mixing in surface waters is discontinuous at the shelf break, demonstrating the degree to which exchange across the shelf break is limited even in winter. OMEX winter measurements of nitrate concentrations can be used to estimate the flow of water across the shelf break that would be required to maintain the nitrogen balance in the North Sea at a steady state. The estimate is 0.6 Sv (1 Sv=106 m3 s−1), which is similar to an earlier estimate of a total flow of 1.7 Sv based on salt budgets (cf. Huthnance, Deutsche Hydrographische Zeitschrift, 49 (1997) 153), This work was partly supported by the EU through the MAST programme, contract MAS3-CT96-0056 (Ocean Margin EXchange—OMEX)

This paper reviews the data obtained in the OMEX I Project on biological production in the surface waters of the Celtic Sea shelf break. The study focused on two regions— the Goban Spur and La Chapelle Bank. Satellite images of the Celtic Sea frequently show a region of cooler water at the shelf break, which results in the mixing of cooler, nutrient-rich waters to the sea surface. To examine the hypothesis that the Celtic Sea shelf break might be a region of enhanced production and sedimentation, observations were made at five regions. These were four sites along a transect of the Goban Spur, from the Celtic Sea shelf (water depth -2 a-1, with cells -2 a-1; the f-ratio is generally -2 a-1. The sum of microzooplankton and mesozooplankton grazing and the carbon demands of bacteria were significantly lower than primary production from November through May, but heterotrophic processes were quantitatively greater than phytoplankton production from July to October. The data suggest that up to 62 g C m-2 a-1 of primary production was not grazed by micro- or mesozooplankton in the surface mixed layer, or utilised directly by bacteria. Depending on the region, up to 38% of the primary production at the Celtic Sea margin was apparently not grazed in the surface mixed layer and would be available for heterotrophic organisms in mid-water and the benthos. The estimated respiration of the heterotrophic community of the surface mixed layer estimated also suggested that between 37% and 60% of the carbon fixed by photosynthesis in the euphotic zone was not remineralised in the surface mixed layer.Data from satellite remote sensing are used in conjunction with the experimental data to extend the seasonal coverage of the observations made in OMEX I. The archive of the coastal zone color scanner provides mean monthly values of chlorophyll concentration, and these agree well with the seasonal variation of “green colour” of the CPR survey. Primary production has been estimated from the satellite-derived chlorophyll concentrations for the period April–September and is calculated to be 90 g C m-2 for the 6-month period; the estimated production for the same period from in situ experiments suggests that primary production was ca. 116 g C m-2. Nitrate concentrations in the surface water were correlated with sea-surface temperature, and this relationship was applied to temperature measurements from the advanced very high resolution radiometer sensor to estimate the potential nitrate concentrations over the region. The f-ratio was related to nitrate concentration by a simple hyperbolic function (r2=0.73). which was applied to the images of potential nitrate concentration for the region to estimate new production based on satellite data. For the period April through September, new production was calculated to be 46 g C m-2 from satellite estimates of temperature, nitrate, and f-ratio, which compares favourably with the estimated new production of 57 g C m-2 by direct measurement.

We present barium data for sediment traps deployed in a northeast Atlantic margin environment (Bay of Biscay). Fluxes of excess barium were measured with the objective of calculating carbon export production rates from the surface mixed layer and thus contribute to the understanding of organic carbon transport in a margin environment. Therefore, it was necessary to properly understand the different processes that affected the barium fluxes in this margin environment. Seasonal variability of POC/Ba flux ratios and decrease of barium solubilisation in the trap cups with increasing depth in the water column probably indicate that the efficiency of barite formation in the organic micro-environment varies with season and that the process is relatively slow and not yet completed in the upper 600 m of water column. Thus barite presence in biogenic aggregates will significantly depend on water column transit time of these aggregates. Furthermore, it was observed that significant lateral input of excess-Ba can occur, probably associated with residual currents leaving the margin. This advected excess-Ba affected especially the recorded fluxes in the deeper traps (>1000 m) of the outer slope region. We have attempted to correct for this advected excess-Ba component, using Th (reported by others for the same samples) as an indicator of enhanced lateral flux and assigning a characteristic Ba/Th ratio to advected material. Using transfer functions relating excess-Ba flux with export production characteristic of margin areas, observed Ba fluxes indicate an export production between 7 and 18 g C m-2 yr-1. Such values are 3–7 times lower than estimates based on N-nutrient uptake and nutrient mass balances, but larger and more realistic than is obtained when a transfer function characteristic of open ocean systems is applied. The discrepancy between export production estimates based on excess-Ba fluxes and nutrient uptake could be resolved if part of the carbon is exported as dissolved organic matter. Results suggest that margin systems function differently from open ocean systems, and therefore Ba-proxy rationales developed for open ocean sites might not be applicable in margin areas.

Cu, Zn, Cd and Pb have been analysed in suspended matter and water samples from the Scheldt estuary, collected during five cruises between May 1982 and May 1983. In order to evaluate the overall metal behaviour in the estuary, continuous longitudinal profiles of the total, the particulate and the dissolved metal concentrations are discussed in relation to various specific physico-chemical parameters. By means of correlation and multi-regression techniques, several estuarine processes could be identified which dominate and/or control the behaviour of the individual trace metals in three distinct areas of the Scheldt estuary. The behaviour of the four selected metals was found to show some common features, but is besides this also characterised by many individual and specific properties. The total metal concentrations seem to co-vary well with the turbidity in the maximum turbidity zone between salinities 2 and 10 psu. In the downstream area a dilution profile is observed for the total metal concentrations of Cu and Cd as well as for the dissolved concentrations of all metals. Dissolved metal production rates are high for Cu and Cd, but much lower for Zn and Pb, confirming the different longitudinal dissolved metal profiles. For Cu and Cd these dissolved metal profiles are characterised by a broad mobilisation area, covering the whole middle estuary. Pb on the other hand, shows a more confined mobilisation area, and for Zn no such area was found. During winter the dissolved metal production rates seem to be controlled, predominantly, by desorption processes. During summer dissolved metal production is correlated mainly to the dissolved oxygen content, suggesting the domination of redox processes solely or in combination with desorption processes. In some cases, co-existent with nearly zero oxygen concentrations, removal of dissolved metals was observed, possibly reflecting precipitation reactions. Longitudinal particulate metal profiles all show a negative deviation versus the dilution profile. Cu and Cd provide the higher deviations, characterised by a narrow enrichment zone preceding a broad mobilisation area. Finally, all particulate metal profiles appeared to correlate well with POC, when expressed in volumetric units.

Phytoplankton nitrogen uptake is studied in relation to the biomass and structure of phytoplankton community in the Atlantic and Indian sectors of the Southern Ocean. Two scenarios of seasonal evolution of uptake regime and phytoplankton community structure are described. The first scenario includes the Marginal Ice Zone areas of the Weddell Sea and adjacent areas where a predominantly nitrate based, diatom dominated assemblage, thriving in a stable water column at the beginning of the season was transformed into a mainly ammonium based, flagellate dominated assemblage, towards the end of the season. The change in phytoplankton community structure was caused by selective grazing by large grazers and reduced stability of the water column and the shift in uptake regime was due to increased ammonium availability and changes in community structure. In the second scenario, in the Coastal and Continental Shelf Zone (CCSZ) and Open Oceanic Zone (OOZ) of the Indian sector, a shift in uptake regime occurred without a big change in phytoplankton community structure. These areas were sampled late in the growth season and were characterized by prolonged water column stability, less grazing pressure on large diatoms and high ammonium availability. Diatoms dominated the assemblage and about 80% phytoplankton biomass was on the >10 µm size fraction. Unlike the first scenario, diatoms were largely based on ammonium. Thus, in areas of persistent water column stability and less selective grazing pressure, a shift in uptake regime can occur without change in community structure. The dominance of diatoms under regenerated production provides a physiological evidence for the excess net removal of silicate over nitrate occurring in certain provinces of the Southern Ocean.

The separation in Southern Ocean provinces of silicate excess at nitrate exhaustion and of nitrate excess at silicate exhaustion was already introduced by Kamykowski and Zentara (Kamykowski, D., Zentara, S.J., 1985. Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser. 26, 47–59; and Kamykowski, D., Zentara, S.J., 1989. Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser. 58, 101–111) and our investigations of the silicate to nitrate uptake ratios confirm the earlier distinction. Oligotrophic antarctic waters mainly exhibit proportionally higher silicate removal what induces a potential for nitrate excess. The nitrogen uptake regime of such areas is characterised by low absolute as well as specific nitrate uptake rates throughout. Maximal values did not exceed 0.15 µM d-1 and 0.005 h-1, respectively. Corresponding f-ratios ranged from 0.39 to 0.86. This scenario contrasts strikingly to the more fertile ice edge areas. They showed a drastic but short vernal increase in nitrate uptake. Absolute uptake rates reached a maximum value of 2.18 µM d-1 whereas the maximal specific uptake rate was 0.063 h-1. In addition to an optimal physical environment for bloom development, accumulation of ammonium stimulated nitrate uptake in a direct or indirect way. Since ammonium build-up in surface waters traces enhanced remineralisation, release of other essential compounds during degradation of organic matter might have been the main trigger. This peak nitrate utilisation during early spring led to the observed potential for silicate excess. With increasing seasonal maturity the nitrate uptake became inhibited by the presence of enhanced ammonium availability (up to 8% of the inorganic nitrogen pool), however, and after a short period of intensive nitrate consumption the uptake rates drop to very low levels, which are comparable to the ones observed in the area of nitrate excess at silicate exhaustion.

Previous in vivo investigations have shown that glutathione reductase is one of the sites of action of the dithiocarbamate fungicide tetramethylthiuram disulphide (thiram) in the yeast Saccharomyces cerevisiae. The inactivation of glutathione reductase by thiram has now been demonstrated in vitro. This inactivation was time-dependent and occurred only with the enzyme in the reduced state and in the absence of glutathione. Since the turnover rate of the enzyme with thiram as a substrate was significantly higher than the rate of enzyme inactivation, it was suggested that more than one enzyme-inhibitor complex was involved in the reaction. Arguments supporting a covalent modification of glutathione reductase were further obtained by experiments carried out with [14C]thiram and gel filtration. A kinetic scheme for the inactivation is proposed and the relevance of the in vitro data to previous in vivo studies is discussed taking into consideration current concepts of glutathione reductase inactivation by affinity reagents.

From 1989 to 1992, five cruises were conducted in an area covering the English Channel, the Celtic Sea and the Gulf of Biscay. During these campaigns, suspended matter was collected by different techniques in order to analyze the spatial and vertical distribution of several particulate trace elements in relation to the global biogeochemical cycle of carbon. It appears that horizontal distribution varies strongly from one element to another and is mainly influenced either by anthropogenic and terrigenous inputs or by scavenging processes. On the other hand, the vertical distribution seems mainly linked to the primary production. In parallel to these trace element measurements, experiments were carried out to estimate the transfer rates of these elements from the dissolved to the particulate phase using radioactive tracers. The results clearly indicate that biological activity plays an important role in these transfers.

Since the role of continental shelves and coastal areas in the global biochemical carbon are nowadays a leading concern in oceanographic research programmes, this study aims to define the specific effect of nutrients on the onset and development of the phytoplankton bloom at the ocean margins. Comparison experiments of the carbon, nitrogen and phosphorus assimilation rates were carried out from 1989 to 1991 in the English Channel and the Gulf of Biscay. New production were obtained from the traditional approach using f-ratio values, i.e. the ratios of directly measured nitrate to carbon uptakes and/or the ratios of nitrate to nitrate plus ammonium uptakes. Values of f agree well on the shelf, slope and deep basin regions of the Gulf of Biscay but are most likely overestimated in the coastal zone of the English Channel. The C/N/P uptake ratios exhibit, furthermore, great spatial variability that can reflect variations in the nutrient status of phytoplankton during its growth.