A box model for North-East Atlantic coastal waters compared with radioactive tracers

A box model for the estimation of doses to members of the public from discharges of radioactivity into North-East Atlantic coastal waters has been used to predict annual means of radionuclide seawater concentrations which have been compared with measured data of tracer radionuclides, ¹³⁷Cs, ⁹⁹Tc and ¹²⁵Sb. Discharges of these radionuclides into the sea from the nuclear reprocessing plants Sellafield in the UK and La Hague in France have been used as input to the model. Fallout of ¹³⁷Cs from atmospheric nuclear weapons tests and fallout of ¹³⁷Cs from the Chernobyl accident into the Baltic Sea have been taken into account. A large proportion of the measured data was collected during the CEC MAST-900052 project which is concerned with the utilization of radionuclides as tracers for water movements. The data cover the time period 1972–1992. A comparison between predicted and measured concentrations of radionuclides in seawater indicated the need to improve the model with respect to mixing conditions in the English Channel and water transport from the North Sea to the Baltic Sea via the Skagerrak, the Kattegat and the Belt Sea. Improvements were made using the data for ¹³⁷Cs and ⁹⁹Tc and involved adjustments of the mixing conditions (water exchange rates) between adjacent water boxes while keeping the net advective water fluxes of the model unaltered. The results of the modified version of the model have been compared with observed values. The comparison shows a somewhat better agreement for ¹³⁷Cs and ¹²⁵Sb than for ⁹⁹Tc which is believed to be due to the lower reliability of the ⁹⁹Tc discharge data from Sellafield in 1978. An analysis of the predicted-to-observed seawater concentrations indicates an overall predictive accuracy of the model of 0.9 corresponding to a 10% underestimation and a predictive precision of about a factor of 2.5 at a confidence level of 95%. The ¹²⁵Sb data, which are independent of the tuning of the model, support the general conclusions about the model predictions of radionuclide concentrations in seawater.     

Radioactive tracers as a tool in coastal oceanography: An overview of the MAST-52 project

Radionuclides (⁹⁹Tc, ¹²⁵Sb, ⁹⁰Sr, ¹³⁷Cs) discharged from La Hague in France have been used to trace advection and dispersion of water masses in the “European Coastal Current” from the English Channel to the Baltic. Time-series of radionuclide measurements in water samples taken in the English Channel, at the Netherlands coast, in the German North Sea sector and in Danish waters have been compared with reported discharge values. The prospects for using ¹²⁹I measured by accelerator mass spectrometry (AMS) as an oceanographic tracer have been evaluated as positive. The ⁹⁹Tc analytical procedure have been improved and background levels of ⁹⁹Tc, ¹³⁷Cs and ⁹⁰Sr in open North Atlantic seawater without influence from European discharges have been determined. An intercomparison programme have documented the quality of the measured ⁹⁹Tc and ¹²⁵Sb data. Transit times and transfer factors from La Hague to different locations in the study area have been estimated. It is concluded, that 10% of the La Hague discharge is transported through Kattegat and that 1/3 of the inflowing Kattegat bottom water originates from the coastal current. Three fundamentally different numeric models have been further developed under the project. The measured data have then been compared with values simulated by the three models. Models as well as measured data indicate, that a close coastal transport with longer transit times and often higher concentrations than seen in the open water main current is taking place. This coastal transport is important when contaminant transport is monitored. It is concluded, that the collected data gives a unique opportunity to evaluate models on advection and dispersion of coastal water masses and contaminants. The database will be made available as a tool for the evaluation of such models.     

Time series for sea water and seaweed of Tc and Sb originating from releases at La Hague

⁹⁹Tc and ¹²⁵Sb have been released in low level liquid waste at the reprocessing plant at La Hague (Fr). Over a number of years data have been obtained on the concentration of ⁹⁹Tc in sea water and in seaweed from sampling locations close to and at increasing distances north and east of the point of release into the English Channel.In addition such data have been obtained on ¹²⁵Sb for a small number of sampling points. These time series, and the information derived from their intercomparison and from comparison with the release data, are presented in this paper.Time series on ⁹⁹Tc as well as ¹²⁵Sb in sea water and seaweed clearly confirm a considerable decrease in release rates at La Hague over the years 1986–1993. Transit times and transfer factors derived from the series are generally in good agreement with the results of modelling efforts. The data do not provide evidence for a time lag between levels of the radionuclides in sea water and those in seaweed longer than a few months.The very high concentration factors for ⁹⁹Tc in brown seaweeds make these seaweeds very useful for environmental monitoring the radionuclide at low release rates and to trace releases in areas remote from the point of release.     

The distribution of artificial radionuclides in the English Channel, southern North Sea, Skagerrak and Kattegat, 1990–1993

This paper presents the initial results concerning the distribution of artificial radionuclides from research cruises conducted in collaboration by three European institutes as part of an EEC MAST research project. Ten cruises were undertaken covering the English Channel, southern and eastern North Sea, Skagerrak and Kattegat over a period of 2.5 yr. A large number of analyses of four artificial radionuclides (¹³⁷Cs, ⁹⁹Tc, ¹²⁵Sb, ⁹⁰Sr), which behave conservatively in seawater, provided information about the general distribution of water masses and circulation patterns as well as about single transport events in the study area. Controlled liquid releases from the La Hague nuclear fuel reprocessing plant are transported eastwards, forming a characteristic distribution pattern in the Channel and the southern North Sea. This includes a near-coastal “plume” and a distinct boundary between waters contaminated predominantly by La Hague and by Sellafield. Spatial and temporal distributions of radionuclide ratios were used, for the first time, to calculate transit times from the English Channel to the coast of Jutland. The data published herein provide an essential input to the calibration of numerical models simulating water transport processes. The results demonstrate the continuing usefulness of artificial radionuclides as oceanographic tracers, even at the very low concentrations observed at present in north-west European Shelf waters.     

Dynamics of inorganic nutrient species in the Bohai seawaters

Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail.     

Microbial community structure in the marginal ice zone of the Bellingshausen Sea

Phytoplankton, bacteria and microzooplankton were investigated on a transect in the Bellingshausen Sea during the ice melt period in November–December 1992. The transect along the 85°W meridian comprised seven stations that progressed from solid pack-ice (70°S), through melting ice into open water (67°S). The abundance, biomass and taxonomic composition were determined for each component of the microbial community. The phytoplankton was mostly dominated by diatoms, particularly small (<20 μm) species. Diatom abundance ranged from 66 000 cells l⁻¹ under the ice to 410 000 cells l⁻¹ in open water. Phytoplankton biomass varied from <1 to 167 mg C m⁻³, with diatoms comprising 89–95% of the total biomass in open water and autotrophic nanoflagellates comprising 57% under the ice. The standing stocks of autotrophs in the mixed layer ranged from 95 mg C m⁻² under the pack-ice to 9478 mg C m⁻² in open waters. Bacterial abundance in ice-covered and open water stations varied from 1.1 to 5.5×10⁸ cells l⁻¹. Bacterial biomass ranged from 2.4 mg C m⁻³ under pack-ice to an average of 14 mg C m⁻³ in open water. The microzooplankton consisted mainly of aloricate oligotrich ciliates and heterotrophic dinoflagellates and these were most abundant in open waters. Their biomass varied between 0.2 and 54 mg C m⁻³ with a minimum at depth under the ice and maximum in open surface waters. Microheterotrophic standing stocks varied between 396 mg C m⁻² under pack-ice and 3677 mg C m⁻² in the open waters. The standing stocks of the total microbial community increased consistently from 491 mg C m⁻² at the ice station to 13 155 mg C m⁻² in open waters, reflecting the productive response of the community to ice-melt. The composition of the microbial community also shifted markedly from one dominated by heterotrophs (82% of microbial stocks) at the ice station to one dominated by autotrophs (73% of microbial stocks) in the open water. Our estimates suggest that the microbial community comprised >100% of the total particulate organic carbon (POC) under the ice and 62–66% of the measured POC in the open waters.     

Bathymetry impacts on water exchange modelling through the Danish Straits

Deep and narrow channels in Danish Straits are one of the governing factors for the Baltic–North Sea water and salt exchange. The channels have a depth up to 50 m and a horizontal scale of a few hundred meters. The typical horizontal resolution used in current operational three dimensional Baltic–North Sea models is 1 nautical mile (nm) which can not well resolve these deep channels. In this paper, an alternative method is used to generate the 1 nm resolution bathymetry so that the deep channel is well resolved and at the same time the total water volume is roughly conserved. The impact of the new bathymetry on modelling water and salt transports as well as temperature and salinity structure is assessed by comparing a 3-year model run with the adjusted bathymetry and a control run with the averaged bathymetry. Volume and salt transports through the Great Belt are examined in the two runs. The results show that the model ocean is dominated by a typical two-layer transport (i.e., upper brackish Baltic outflow and lower saltier inflow), and the new bathymetry significantly enhances the two-layer transport. The lighter Baltic outflow is increased by 18% in the upper 10 m and saltier deep inflow is increased by 300% (in comparison with the old bathymetry) below 10 m. The total net transport into the Baltic Sea is increased by 13%. The temperature and salinity structure is also significantly influenced by the bathymetry, especially during inflow events. The stratification is strengthened and the bottom salinity is increased in Danish Straits and adjacent waters. The bathymetry impact is found significant through the entire 3-year model run period, and the signal is propagated to a large area covering the Stopple Channel. Comparison with observations show that such changes are positive improvements to the models. The results suggest that the deep channels in the Danish Straits have to be carefully resolved in order to correctly simulate the Baltic–North Sea water exchange.     

Protist entrapment in newly formed sea ice in the Coastal Arctic Ocean

Protist abundance and taxonomic composition were determined in four development stages of newly formed sea ice (new ice, nilas, young ice and thin first-year ice) and in the underlying surface waters of the Canadian Beaufort Sea from 30 September to 19 November 2003. Pico- and nanoalgae were counted by flow cytometry whereas photosynthetic and heterotrophic protists ≥ 4 µm were identified and counted by inverted microscopy. Protists were always present in sea ice and surface water samples throughout the study period. The most abundant protists in sea ice and surface waters were cells < 4 µm. They were less abundant in sea ice (418–3051 × 10³ cells L^− 1) than in surface waters (1393–5373 × 10³ cells L^− 1). In contrast, larger protists (≥ 4 µm) were more abundant in sea ice (59–821 × 10³ cells L^− 1) than in surface waters (22–256 × 10³ cells L^− 1). These results suggest a selective incorporation of larger cells into sea ice. The ≥ 4 µm protist assemblage was composed of a total number of 73 taxa, including 12 centric diatom species, 7 pennate diatoms, 11 dinoflagellates and 16 flagellates. The taxonomic composition in the early stage of ice formation (i.e., new ice) was very similar to that observed in surface waters and was composed of a mixed population of nanoflagellates (Prasinophyceae and Prymnesiophyceae), diatoms (mainly Chaetoceros species) and dinoflagellates. In older stages of sea ice (i.e., young ice and thin first-year ice), the taxonomic composition became markedly different from that of the surface waters. These older ice samples contained relatively fewer Prasinophyceae and more unidentified nanoflagellates than the younger ice. Diatom resting spores and dinoflagellate cysts were generally more abundant in sea ice than in surface waters. However, further studies are needed to determine the importance of this winter survival strategy in Arctic sea ice. This study clearly shows the selective incorporation of large cells (≥ 4 µm) in newly formed sea ice and the change in the taxonomic composition of protists between sea ice and surface waters as the fall season progresses.     

Biological effects of Mississippi River nitrogen on the northern gulf of Mexico—a review and synthesis

The Mississippi River currently delivers approximately 1.82 Tg N year⁻¹ (1.3×10¹¹ mol N year⁻¹) to the northern Gulf of Mexico. This large input dominates the biological processes of the region. The “new” nitrogen from the river stimulates high levels of phytoplankton production which in turn support high rates of bacterial production, protozoan and metazoan grazing, and fisheries production. A portion of the particulate organic matter produced in the pelagic food web sinks out of the euphotic zone where it contributes to high rates of oxygen consumption in the bottom waters of the inner shelf, resulting in the development of an extensive zone of hypoxia each summer. In spite of the significance of this river system to the coastal ocean of the northern gulf, we do not have an adequate understanding of the inputs, processing and ultimate fates of river nitrogen. Here we review available literature on this important system and propose a conceptual model showing how biological processes evolve in the river plume between the point of discharge and the point where plume waters are fully diluted by mixing with oceanic water.     

An analysis of the characteristics of chlorophyll in the Sulu Sea

The seasonal and spatial variations of chlorophyll concentrations, Sea Surface Temperature (SST), wind fields and wind-induced Ekman pumping in the Sulu Sea are investigated using a set of new remote sensing measurements from October 1997 to December 2004. The results show the seasonality of chlorophyll, wind fields and SST and reveal the phytoplankton blooming events in the Sulu Sea basin during the northeast monsoon season. In summer, chlorophyll concentrations were relatively low (< 0.2 mg/m³) and distributed uniformly throughout the basin with a narrow belt of high chlorophyll concentrations along the coastal waters, particularly the coasts of Borneo and of the Sulu Archipelago. In winter, chlorophyll concentrations increased (> 0.2 mg/m³) throughout the entire basin, and phytoplankton bloomed southward to the central basin, while chlorophyll concentrations reached high levels (1 mg/m³) in the center of the blooms. One peak was observed during the northeast monsoon season each year. SSTs have significant negative correlations with chlorophyll concentrations; i.e., high and uniformly distributed in summer but lower with an obvious tongue of cold waters southward to the central basin in winter. The seasonal variation of chlorophyll concentrations and SST distribution were associated with the seasonally reversing monsoon. The winter phytoplankton blooming and the tongue of the cold waters were correlated to the vertical upwelling cold and nutrient-rich waters drawn by the northeast wind, with the center of the blooms and the location of cold tongues coinciding with the maximum of the wind speeds and the Ekman pumping velocities.     

Distributions of oxygen and carbon stable isotopes and CFC-12 in the water masses of the Southern Ocean at 30°E from South Africa to Antarctica: results of the CIVA1 cruise

This study presents oceanic distributions of stable isotopes (δ¹⁸O of water and δ¹³C of ΣCO₂) and CFC-12 from samples collected during the CIVA1 cruise (February/March 1993), across the Southern Ocean, along a meridian section at 30°E, from South Africa (44°S) to Antarctica (70°S). The isotopic measurements show important variations between the subantarctic surface waters with low δ¹⁸O–high δ¹³C values and the antarctic surface waters with very low δ¹⁸O–low δ¹³C values. The surface distributions of δ¹³C values follow the major frontal oceanic structures; the vertical distribution shows the progressive upwelling from the subantarctic zone to the antarctic divergence of ¹³C-depleted CO₂ derived from remineralization of organic matter. Along the Antarctic continental shelf, between 2500 and 4000 m, a core of water with δ¹⁸O values close to −0.1‰ is associated with a relative maximum in CFC-12 concentration, although this core is not detected by its temperature and salinity parameters. This water mass, which corresponds to recently formed deep water, may originate from the eastward extension of the Weddell gyre or from bottom waters coming from the East and formed near Prydz Bay.     

Generalized F distribution model with random parameters for estimating property damage cost in maritime accidents

ABSTRACT

This study develops a generalized F distribution model with random parameters to estimate the ship property damage cost in maritime traffic accidents with 10 years’ shipping accident data in the Fujian waters. Model results show that sinking and capsizing can incur the largest property damage cost, followed by collisions, contact, grounding and fire/explosion. There is a smaller ship property damage cost when the ship is moored or docked. The poor visibility has the least impact on the increment of ship property damage cost. Results reveal that the bigger property damage cost is associated with maritime accidents occurring in the Straits/sea areas and under the strong wind/wave condition and nighttime periods. It is also found that the lookout failure exhibits a bigger effect than the operation error. These results are helpful for policy makers to make efficient strategies for reducing property damage cost in maritime accidents. The developed model is useful for insurance companies in determining the appropriate ship insurance rates.     

Dissolved organic matter in shelf waters off the Ría de Vigo (NW Iberian upwelling system)

Net in situ production and export of dissolved organic carbon (DOC) and nitrogen (DON) have been studied in shelf waters off the Ría de Vigo (NW Spain), as part of a comprehensive hydrographic survey carried out from September 1994 to September 1995 with a fortnight periodicity. DOC and DON correlated well (r=+0.78), the slope of the regression line being 12.0±0.7 mol-C mol-N⁻¹, about twice the Redfieldian slope of particulate organic matter, 6.5±0.2 mol-C mol-N⁻¹ (r=+0.95). Labile DOC and DON accumulated in the upper 50 m during the upwelling season (March–September), mainly after prolonged periods of wind relaxation, when horizontal flows were reduced. This labile material represented 50% and 35% of the total (dissolved+particulate) organic carbon and nitrogen susceptible of microbial utilisation, which assert the key contribution of dissolved organic matter (DOM) to the export of new primary production in the NW Iberian upwelling system. This surface excess in shelf waters appeared to be formed into the highly productive Ría de Vigo (a large coastal indentation) at net rates of 4.4 μM-C d⁻¹ and 1.3 μM-C d⁻¹ in the inner and outer segments of the embayment respectively, and subsequently exported to the shelf. Once in the shelf, simple dilution with the inert DOM pool of recently upwelled Eastern North Atlantic Central Water (ENACW) occurred. Eventually, the DOM excess produced during the upwelling season is exported to the adjacent open ocean waters by the coastal circulation. Conversely, during the unproductive downwelling season (October–February), the lowest DOC and DON levels were recorded and export was prevented by the characteristic downwelling front associated to the seasonal poleward slope current.     

Seasonal variability of fractionated phytoplankton, biomass and primary production in the Straits of Magellan

The results on the distribution of phytoplankton biomass (expressed as Chla) and primary production (¹⁴C assimilation), during three oceanographic cruises carried out during Austral spring and at the end of the summer and the autumn in the Straits of Magellan, suggest a strong variability of trophic levels for this ecosystem.Seasonal evolution of the biomass concentration goes from the spring maximum of 2.33 μg/l through a sharp decrease, 0.49 μg/l, observed at the end of summer, until the minimum of 0.24 μg/l measured during the autumn.The trophic conditions are dependent on hydrographic, meteo-climatic and geo-morphological characteristics: at the Atlantic entrance and between the two Angosturas the strong mixing of water column limit the development of phytoplankton; at the Western opening and along the Pacific arm the complex exchange mechanisms with the ocean, the glacio-fluvial contribution and the presence of a thermohaline front near the Isla Carlos III influence both biomass and primary production distributions. The maximum values are reached in the Central Zone (Paso Ancho) characterized by high stability of the water column.Primary production ranged from a minimum of 12.3 to a maximum of 125.9 mgC m⁻² h⁻¹. The overall trend seems to be a progressive and simultaneous increase from the Pacific and Atlantic openings to the Central Zone of Paso Ancho where the maximum value was reached. In general, biomass and primary production distributions correspond quite well except for the area of Isla Carlos III where biological and chemico-physical causes tend to limit ¹⁴C assimilation.Contribution of pico-phytoplankton (< 2 μm) to total biomass appears to be time dependent: in the blooms observed during spring a very modest incidence (< 6%) was observed whereas became more (> 50%) during the summer-autumn seasons when total biomass was decreasing.Within the Straits, at the end of summer, the contribution of pico-phytoplankton primary production is 59%, whereas nano and microplankton contribute 39% and 2%, respectively. At the oceanic external stations the photosynthetic activity of the bigger size-fraction (> 2 μm) is predominant (> 50%).These findings support the hypothesis that the pico-phytoplankton ( < 2 μm) is substantially constant, whereas temporal variations are due to the larger (> 10 μm) cells only.     

Spatial and temporal variability of size-fractionated biomass and primary production in the Ross Sea (Antarctica) during austral spring and summer

The results of a study on the spatial and temporal dynamics of size-fractionated biomass and production of phytoplankton in the Ross Sea during the austral spring and summer are reported. The spring cruise took place in the offshore Ross Sea from 14 November to 14 December 1994. Sampling was carried out on a transect of 27 stations distributed from 76.5 to 72.0°S along 175°E, and covered the three main Antarctic environments of the polynya open waters, the marginal ice zone and the pack ice area. Three subsystems were identified. The subsystem of the polynya was characterised by the predominance of the micro- and nano-planktonic fractions, chlorophyll (Chl a) concentrations from 69.6 to 164.7 mg m⁻² and production rates from 0.68 to 1.14 g C m⁻² day⁻¹. The second subsystem, the marginal ice zone, showed a relative increase of the micro-planktonic fraction, high biomass levels (from 99.64 to 220 mg Chl m⁻²) and production rates from 0.99 to 2.7 g C m⁻² day⁻¹. The subsystem of the pack ice area had a phytoplankton community dominated by the pico-planktonic fraction and showed low biomasses (from 19.4 to 37.7 mg Chl m⁻²) and production rates (0.28 to 0.60 g C m⁻² day⁻¹). Selective grazing by krill is considered an important factor in determining the size structure of the phytoplankton communities. The summer study consisted of a time series carried out in inshore waters of Terra Nova Bay from 12 January to 8 February 1990. In a well stabilised water column and with high levels of PAR always available, the primary production rates of a community dominated by micro-plankton varied from 0.52 to 1.2 g C m⁻² day⁻¹ (average 0.84). A high P/B ratio, up to 3, and a remarkably elevated mean phaeopigment (Phaeo)/Chl a ratio of 2.4 indicated an active removal of biomass by grazing, confirmed by the presence of faecal pellets in quantities reaching 6000 m⁻³ in the upper 50 m. The peculiarities of the inshore versus offshore environments in terms of community size structure, production processes and their implications as regards the food web are discussed.     

Heterogeneous archaeal communities in the particle-rich environment of an arctic shelf ecosystem

We evaluated the phylogenetic diversity of particle-associated and free-living archaeal assemblages from the Mackenzie River and Beaufort Sea in the western Canadian Arctic. The physico-chemical characteristics of the water separated the sampling sites into three groups: riverine, coastal and marine water, which had strikingly different archaeal communities. The riverine water was characterised by the presence of Euryarchaeota mainly belonging to the LDS and RC-V clusters. The coastal water was also dominated by Euryarchaeota but they were mostly affiliated to Group II.a. The marine waters contained most exclusively Crenarchaeota belonging to the Marine Group I.1a. The results suggest that Euryarchaeota in the coastal surface layer are associated with particle-rich waters, while Crenarchaeota are more characteristic of Arctic Ocean waters that have been less influenced by riverine inputs. The particle-associated communities were similar to the free-living ones at the riverine and marine sites but differed from each other at the coastal site in terms of the presence or absence of some taxonomic groups in one of the fractions, or differences in the proportion of the phylogenetic groups. However, there was no specific archaeal group that was exclusively restricted to the free-living or particle fraction, and the diversity of the particle-associated archaeal assemblages did not significantly differ from the diversity of the free-living communities.     

Phytoplanktonic nutrient utilisation and nutrient signature in 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.     

Pb and Pu in the Northeast Water Polynya, Greenland: particle dynamics and sediment mixing rates

Distributions of the radionuclides ²¹⁰Pb and ^239,240Pu in sediment cores from the Northeast Water Polynya, Greenland, showed that these nuclides reached depths of 5–15 cm by particle mixing and sediment accumulation. End-member average values of the particle mixing coefficient and sediment accumulation rate were 0.13 cm² y⁻¹ and 0.06 cm y⁻¹, obtained from the ²¹⁰Pb profiles by assuming that each process is dominant relative to the other. Both ²¹⁰Pb and ^239,240Pu were measured on four cores; using the Pu data to constrain mixing rates produced corrected sediment accumulation rates that were 20–80% of the values calculated by neglecting mixing. Organic carbon burial in the polynya sediments was ≤0.4 mmol m⁻² d⁻¹, based on measured POC values at depth in the sediments and sediment accumulation rates corrected for mixing. This value is about 1% of the independently measured POC flux leaving the euphotic zone and compares with benthic carbon remineralization rates of 7% calculated by others from O₂ uptake in the sediments.The inventories of excess ²¹⁰Pb in the sediments ranged from 6 to 28 dpm cm⁻². Relative to the atmospheric input of ²¹⁰Pb and in situ production from decay of ²²⁶Ra, approximately 5 dpm cm⁻² of ²¹⁰Pb was being removed from the water column. The difference between the removal from the water column and sediment inventories suggests a net import of ²¹⁰Pb to the polynya. This may occur by input of dissolved ²¹⁰Pb from offshore waters or by input of ²¹⁰Pb carried by sea ice. Particulate matter in land-derived fast ice adjacent to the polynya contained 330 ± 14 dpm of excess ²¹⁰Pb g⁻¹. If particles transported in sea ice are comparable to those extracted from fast ice, then sea ice transport into the polynya followed by melting may be an important source of excess ²¹⁰Pb to the area. Fast ice also may contribute ²¹⁰Pb if portions break off and melt within the polynya, as occurred in 1993.     

Inorganic carbon fluxes through the boundaries of the Greenland Sea Basin based on in situ observations and water transport estimates

A carbon budget for the exchange of total dissolved inorganic carbon C_T between the Greenland Sea and the surrounding seas has been constructed for winter and summer situations. An extensive data set of C_T collected over the years 1994–1997 within the European Sub-polar Ocean Programmes (ESOP1 and ESOP2) are used for the budget calculation. Based on these data, mean values of C_T in eight different boxes representing the inflow and outflow of water through the boundaries of the Greenland Sea Basin are estimated. The obtained values are then combined with simulated water transports taken from the ESOP2 version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The fluxes of inorganic carbon are presented for three layers; a surface mixed layer, an intermediate layer and a deep layer, and the imbalance in the fluxes are attributed to air–sea exchange, biological fixation of inorganic carbon, and sedimentation. The main influx of carbon is found in the surface and the deep layers in the Fram Strait, and in the surface waters of direct Atlantic origin, whereas the main outflux is found in the surface layer over the Jan Mayen Fracture Zone and the Knipovich Ridge, transporting carbon into the Atlantic Ocean via the Denmark Strait and towards the Arctic Ocean via the Norwegian Sea, respectively. The flux calculation indicates that there is a net transport of carbon out of the Greenland Sea during wintertime. In the absence of biological activity, this imbalance is attributed to air sea exchange, and requires an oceanic uptake of CO₂ of 0.024±0.006 Gt C yr⁻¹. The flux calculations from the summer period are complicated by biological fixation of inorganic carbon, and show that data on organic carbon is required in order to estimate the air–sea exchange in the area.     

Fluxes of biogenic carbon in the Southern Ocean: roles of large microphagous zooplankton

The Southern Ocean is an extreme environment, where waters are permanently cold, a seasonal ice cover extends over large areas, and the solar energy available for photosynthesis is severely restricted, either by vertical mixing to considerable depths or, especially south of the Antarctic Circle, by prolonged seasonal periods of low or no irradiance. Such conditions would normally lead to low productivity and a water column dominated by recycling processes involving microbial components of pelagic communities but this does not seem to be the case in the Southern Ocean, where there is efficient export to large apex predators and deep waters. This paper investigates the role of large microphagous zooplankton (salps, krill, and some large copepods) in the partitioning of biogenic carbon among the pools of short- and long-lived organic carbon and sequestered biogenic carbon. Large microphagous zooplankton are able to ingest microbial-sized particles and thus repackage small, non-sinking particles into both metazoan biomass and large, rapidly sinking faeces. Given the wide spatio-temporal extent of microbial trophic pathways in the Southern Ocean, large zooplankton that are omnivorous or able to ingest small food particles have a competitive advantage over herbivorous zooplankton. Krill efficiently transfer carbon to a wide array of apex predators and their faecal pellets are exported to depth during occasional brief sedimentation episodes in spring time. Salps may be a significant link towards some fish (directly) and other apex predators (indirectly) and, at some locations (especially in offshore waters) and time, they may account for most of the downward flux of biogenic carbon. Large copepods are a trophic link towards fish and at least one whale species, and their grazing activity generally impedes the export of organic particles to depth. As a result, biogenic carbon is channelled mainly towards apex predators and episodically into the deep ocean. Without these original interactions, Antarctic waters might well be dominated by microbial components and recycling processes instead of active export from the generally small primary producers towards large apex predators.