Summer Cruises Research Articles

Comparison between in situ and MODIS-derived spectral reflectances of snow and sea ice in the Amundsen Sea, Antarctica

The spectral albedo and directional reflectance of snow and sea ice were measured on sea ice of various types, including nilas, grey ice, pancake ice, multi-year pack ice, and land-fast ice in the Ross, Amundsen and Bellingshausen seas during a summer cruise in February through March 2000. Measurements were made using a spectroradiometer that has 512 channels in the visible and near-infrared (VNIR) region in which 16 of the 36 bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are covered. Directional reflectance is also retrieved from the MODIS radiometrically calibrated data (Level 1B) concurrently acquired from the first National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) satellite, Terra. The locations of the ground ice stations are identified accurately on the MODIS images, and the spectral albedo and directional reflectance values at the 16 VNIR MODIS bands are extracted for those pixel locations. MODIS-derived reflectance is then corrected for the intervening atmosphere whose parameters are retrieved from the MODIS atmospheric profiles product (MOD07_L2) for the same granule. The corresponding spectral albedo and directional reflectance with the same viewing geometry as MODIS are derived from our ground-based spectroradiometer measurements. Because the footprint of the ground spectroradiometer is much smaller than the pixel sizes of MODIS images, the averaged spectral reflectance and albedo in the vicinity of each ice station are simulated for the corresponding MODIS pixel from the ground spectral measurements by weighting over different surface types (various ice types and open water). An accurate determination of ice concentration is important in deriving ground reflectance of a simulated pixel from in situ measurements. The best agreement between the in situ and MODIS measurements was found when the ground had 10/10 ice concentration (discrepancy range 0.2–11.69%, average 4.8%) or was oneice-type dominant (discrepancy range 0.8–16.9%, average 6.2%). The more homogeneous the ground surface and the less variable the ground topography, the more comparable between the in situ and satellite-derived reflectance is expected.

Simultaneous measurements of remote lidar chlorophyll and surface CO2 distributions in the Ross Sea

A comparison between continuous lidar fluorosensor chlorophyll-a measurements and partial pressure of CO2 (pCO2) in surface water is presented. The data were gathered during the XVI Italian Antarctic expedition in the Western Ross Sea and crossing the Southern Ocean. Anticorrelation between CO2 levels and chlorophyll-a determinations was observed throughout the cruise in the late austral summer 2000/01. The survey revealed the presence of high productivity in polynya regions and close to the ice edges. The lidar data, reported here, are consistent with the results of previous campaigns conducted in the same investigated area. The relationship between pCO2 and chlorophyll-a concentration are discussed in terms of biologically driven phenomena responsible for the pCO2 undersaturation with respect to the atmospheric value during the austral summer.

Carbonate dissolution in the turbid and eutrophic Loire estuary

We measured particulate and dissolved organic carbon (POC and DOC), chlorophyll, oxygen, partial pressure of CO2, pH, total alkalinity (TAlk) and particulate inorganic carbon (PIC) during a late summer cruise in the eutrophic Loire estuary. These parameters reveal an intense min- eralisation of organic matter in the estuarine maximum turbidity zone (MTZ) that results in oxygen deficits (down to 20% of the saturation level) and high CO2 oversaturations (pCO2 up to 2900 µatm). Several facts revealed the occurrence of carbonate dissolution in the Loire MTZ: large amounts of alkalinity were produced in the upper estuary, increasing its transfer to the ocean by 30%; the calcu- lated saturation index showed a net undersaturation for aragonite and a slight undersaturation for calcite in the MTZ; and PIC decreased from 2.1% (% dry weight) in riverine suspension to 0.4% in the MTZ. A stoichiometric approach is used to assess the coupling between aerobic respiration and carbonate dissolution, where apparent oxygen utilisation, excess CO2, TAlk and dissolved inorganic carbon are compared quantitatively. About 20% of the CO2 generated by respiration was involved in carbonate dissolution. The loss of PIC at the river-estuary transition quantitatively corresponds to the amount of authigenic calcite precipitated upstream in the highly eutrophic river. This suggests that CO2 exchange with the atmosphere along the eutrophic river-estuary continuum is buffered by carbonate precipitation in the autotrophic river and its dissolution in the heterotrophic estuary.

The potential contribution of fecal pellets from large copepods to the flux of biogenic silica and particulate organic carbon in the Antarctic Polar Front region near 170°W

Abstract During a spring and a summer cruise, we determined the amounts of biogenic silica (bSi) and particulate organic carbon (POC) encapsulated into fecal pellets by the community of large copepods in the Antarctic Polar Front region, in order to estimate the potential contribution of sinking pellets to bSi and POC flux from the surface layer. Grazing by large copepods can be an important mechanism for enhancing vertical flux of POC and bSi because particles in the diatom size range are excellent foods for mesozooplankton, bSi is inert in copepod guts, and copepod fecal pellets (which have a chitinous membrane that retards dissolution of pellet contents) have sinking rates of 100s m d−1. Rates of pellet production were measured in deck incubations with natural seawater as food medium. BSi and POC contents of pellets were measured in the laboratory. These data were applied to measured abundances of large copepods and the resultant rates of bSi and POC pellet production were compared to flux measurements made by other investigators. Comparison between pellet production by the community of large copepods and flux at 100 m determined by thorium isotopes indicated that pellet production was equivalent to 22–63% of POC flux and 42–107% of bSi flux during the spring cruise. The potential contribution of pellets to flux at 100 m in the summer was considerably smaller, between 2% and 7% for POC and between 1% and 5% for bSi (with one exception at the southernmost station where there was a diatom bloom). We conclude that, in the spring especially, fecal pellets can contribute significantly to the bSi and POC sinking from the surface layer within the Antarctic Polar Front region. However, sediment traps deployed at a depth of 1000 m during the spring cruise indicated total fluxes of both bSi and POC were less than pellet production in the upper 100 m, indicating consumption or degradation of sinking material is significant between 100 and 1000 m. Fecal pellet production of POC was 4–10 times greater than the flux of POC at 1000 m and pellet production of bSi was 0.4–5 times the flux of bSi at 1000 m. Although pellets can contribute significantly to total flux, a large fraction of pellet material must be remineralized during sinking.

On the evolution of the thermocline and subthermocline eastward currents in the Equatorial Atlantic

Eastward currents in the thermocline and subthermocline layers of the tropical Atlantic Ocean are described using observations obtained during two boreal summer cruises covering the whole equatorial basin. The analysis focuses on the eastward evolution of zonal currents in the thermocline and sub‐thermocline. The Equatorial Undercurrent is not found close to the African coast in boreal summer 2000, maybe because of interannual variability. As observed in the Pacific, the South and North Equatorial Undercurrents are shown to shift poleward from west to east, and the two cruises indicate that the North Equatorial Undercurrent does not enter the Gulf of Guinea. The South and North Intermediate Countercurrents are variable with longitude and, contrarily to the observations in the eastern equatorial Pacific, are not found in the east of the Guinea basin.

Hydrodynamics and the spatial distribution of plankton and TEP in the Gulf of Cadiz (SW Iberian Peninsula)

During a summer cruise in the Gulf of Cadiz area (Southern Iberian Peninsula), an upwelling episode off the eastern coast of Cape Santa Maria was noted. The observed temperature field data suggested an anticyclonic circulation from NW to SE and a velocity of the associated surface currents ranging between 10 and 15 cm s ‐1 was estimated. Research was carried out on the physical‐biological coupling by analysing the influence of these hydrodynamic patterns on the mesoscale distribution of several chemical and biological variables. During the cruise, nitrate concentration, total chlorophyll, chlorophyll (>20 µm), phaeopigments and TEP bulk concentration were measured. The distribution of biological variables appears to be organized by physical forcing. The upwelling area acts as a notable disturbance in the area. A sequence of maxima of different variables classified as bloom phases was observed, arranged in the same direction as dominant currents. A clear frontal structure in radial transects was also apparent. Discussion focusses on the variability of the hydrodynamic patterns observed in the framework of previous observations and the role of physical forcing, moulding the biological distributions and altering their spatial correlations. Special reference is made to TEP, as this variable is scarcely analysed in extensive field studies. The distribution of TEP patches in the area was heterogeneous, which has been discussed in the framework of current theory. Chlorophyll (>20 µm), phaeopigment ratio and TEP concentration maxima had no direct spatial correlation with chlorophyll maxima. The sequence of typical processes in phytoplankton bloom phases and the influence that hydrodynamics has both on environmental suitability for growth and on displacement of patches, could explain the distribution of these biological variables in the area. Subsequent studies on hydrodynamics variability are suggested as a valuable tool for the future understanding of biological processes in the region.

Benthic Nutrient Cycling and Diagenetic Pathways in the North-western Black Sea

Benthic fluxes of nutrients and metals were measured in the coastal zone of the north-western Black Sea, which is influenced by the Danube and Dniestr rivers. The results from the benthic flux chambers deployed during two EROS 21 cruises in summer 1995 and in spring 1997 yield information on benthic nutrient cycling and diagenetic pathways at the sediment-water interface. This information is discussed in the light of benthic activity as well as pore-water data. The benthic recycling of nutrients varied seasonally due to the availability of oxygen and organic material, and spatially due to river influence. Areas of high benthic fluxes near-shore and of low benthic fluxes offshore on the shelf were distinguished. Nutrients and suspended particulate matter discharged by the Danube is kept nearshore by the coastal current. The oxygen concentrations in the bottom water varied strongly between the two seasons. In summer, the bottom water at the near-shore stations turned anoxic. In spring, the higher oxygen concentrations are related to more intense mixing of the water column due to stormy periods and high river discharge. Highest oxygen concentrations were found on the offshore continental shelf. However, the benthic oxygen consumption rates in spring (13–23 mmol m −2 day −1) were as high as in summer. Areas with highest nutrient concentrations in the overlaying bottom water were found at the Danube delta front and Danube prodelta. On the Danube delta front and the Dniestr mouth, ammonia fluxes were lower in spring (1·1–1·7 mmol m −2 day −1) than in the summer (2·6–4·4 mmol m −2 day −1) due to higher nitrification rates and the lower influx of organic matter. In spring, the concentrations of dissolved iron and manganese in the bottom water were one order of magnitude lower than during the summer. This decrease in flux rates is related to the higher oxygen concentrations in the bottom water in spring. A rough comparison of the river’s nutrient load and the benthic recycling over a certain area showed that the near-shore benthic phosphate and silica recycling account for 50% and 35% of the Danube input in summer, respectively. The fluxes of ammonia from benthic recycling and from the Danube discharge were at the same order of magnitude in both seasons. Nitrogen is introduced by the rivers mostly as nitrate. Benthic recycling is the dominant source of ammonia. The Danube input is phosphate deficient. In brief, benthic nutrient recycling is an important factor in sustaining high productivity of the system.

Observations of oceanic mesoscale features and variability in the Canary Islands area from ERS-1 altimeter data, satellite infrared imagery and hydrographic measurements

The main purposes of this paper are to identify and evaluate the oceanic mesoscale features that appear in the Atlantic Ocean eastern boundary, 20-40° N, 19-9° W, using ERS-1 (1992-1993) satellite altimeter data. The sea surface height anomalies and the eddy kinetic energy fields are calculated. High energy values, between 0.03 and 0.05 m 2 s -2, are observed with the altimeter data in the Canary region mainly in summer. These maximum values are associated with eddies located downstream of Gran Canaria and Tenerife (Canary Islands). Comparison with in situ measurements provided by the hydrographic surveys from a cruise in summer 1993 around the Canary Islands showed a good agreement. Dynamic heights relative to 300 dbar for August and the altimetric heights from ERS-1 data were averaged over 0.125° boxes for the duration of the cruise. The correlation coefficient was 0.7. Negative anomalies of the sea level calculated from ERS-1 between Cape Yubi and Cape Bojador (26.0°-27.5° N) were obtained in August 1993. Possibly these correspond with filaments from the north-west African upwelling coast. Also, the results of the altimetric data were compared with NOAA AVHRR (Advanced Very High Resolution Radiometer) sea surface temperature (SST) satellite images. A cyclonic and an anticyclonic eddy to the south-west of the island of Gran Canaria were identified during the same period in the SST images.

Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia

A multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula was developed in the frame of the Canary Islands Azores Gibraltar Observations Project. Two high-resolution CTD surveys, which included water sampling for chemical (nutrients and dissolved oxygen) and sedimentological analyses, took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The correspondence between the high-salinity Mediterranean Water (MW) layer and the low-nutrient content and relatively high abundance of particles was a general result. Further details of the thermohaline analysis and of the geostrophic computations, especially for the layer of MW, are compared with the results obtained for the chemical properties and the sedimentological characteristics.

Distribution and relation of total bacteria, active bacteria, bacterivory, and volume of organic detritus in Atlantic continental shelf waters off Cape Hatteras NC, USA

Abstract During the Ocean Margins Program, we obtained data on the abundances of bacterioplankton and heterotrophic flagellates, and on rates of bacterivory, across the mid-Atlantic continental shelf off Cape Hatteras, NC, during four spring and summer cruises from 1993 to 1996. Bacterial and grazing parameters were compared for inner, middle, and outer shelf regions. In 1996, we sampled during two seasons: early spring (March) and mid-summer (July), and in addition determined the fractions of in situ bacterioplankton that had visible nucleoids (NV cells), or that had highly active electron transport systems (ETS), i.e. that were positive for reduction of the fluorogenic formazan compound, 5 cyano-2,3 ditolyl tetrazolium chloride (CTC+ cells), as well as the volumetric concentration of organic detrital particles. Detrital volumes and abundances of bacterioplankton and of heterotrophic flagellates, varied by an order of magnitude, and decreased from inshore to offshore shelf regions. In 1996, bacterial abundances and percentages of CTC+ cells were higher across the shelf during the early spring bloom season (March) compared to the post-bloom season (July). In March 1996, percentages of bacterial cells with visible nucleoids varied between 20% and 70%, but showed little change across the shelf; while fractions of total bacteria with highly active ETS were lower and more variable (1–16% CTC+ cells), and on average were twice as high in the inner shelf region compared to the rest of the shelf. Percentages of CTC+ cells were also higher for particle-associated bacteria. There was a strong positive relationship between percent CTC+ cells and volume of organic detrital particles. However, % CTC+ cells and detrital volume were not consistently related to either bulk particulate organic carbon or chlorophyll. Bacterivory, assessed via rate of ingestion of fluorescently labeled bacteria, could remove 2–9% (4–18% accounting for motile cells) of total bacterial stocks per day. If bacterivores selectively grazed CTC+ bacteria, then bacterivory could result in turnover of CTC+ cells at rates on the order of 0.4–1.4 d−1.

Copepod growth and development around South Georgia: relationships with temperature, food and krill

Measurements of egg production rates (EPR) and growth of the early stages of 2 of the Southern Ocean biomass dominant copepods, Calanoides acutus and Rhincalanus gigas, were made over the course of 4 consecutive summer cruises which were carried out in the vicinity of South Georgia. For both species, EPR was found to be weakly but significantly related to chlorophyll a (chl a) although for C. acutus it was below maximum levels recorded in spring. Juvenile mass specific growth rates (g) were found to be body mass, stage and species dependent. Mean g for C. acutus stages CII to CIV decreased from 0.24 to 0.14, and for R. gigas stages CI to CIII from 0.06 to 0.04. Overall, values for both species were within the range predicted by recent global models of copepod growth. Neither stage duration nor g varied systematically with either temperature (mean, 0 to 60 m) or food (chl a, 0 to 60 m). However, carbon mass of nearly all species stages was negatively and sig- nificantly related to silicate levels (mol m -2 , 0 to 60 m), suggesting the positive effect of past produc- tion levels. Ordination of zooplankton species occurrence by station across the survey area indicated that changes in abundance were more pronounced than changes in species composition, and that variation in total copepod abundance was also well explained by silicate levels. Our study indicated that changes in EPR, carbon mass and abundance of copepod populations at South Georgia were all strongly regulated by local primary production. Variation of chlorophyll biomass appeared largely dependent on temperature, rather than grazing pressure exerted by either copepods or krill. Krill at South Georgia were more abundant in colder, silicate replete waters and their presence is presumed to be governed by factors operating at the large scale. In contrast, copepod abundance appeared to differ in response to smaller scale variation in the environment and was linked through silicate to factors determining phytoplankton growth. In turn, chl a concentration was strongly and positively related to habitat temperature. This suggests the importance of the physical environment rather than grazing as ultimate factors controlling phytoplankton biomass in this productive ecosystem.

Bio-optical characteristics of Gerlache and Bransfield Strait waters during an Antarctic summer cruise

Abstract Spatial and temporal variations in bio-optical characteristics of the Gerlache and Bransfield Strait waters were determined during an Antarctic summer cruise (FRUELA) in December 1995 and January 1996. Chlorophyll concentration was estimated using a bio-optical model from natural fluorescence flux over the emission of chlorophyll a, incident irradiance, chlorophyll specific absorption coefficient, and quantum yield of fluorescence. The penetration of both photosynthetic active radiation (PAR) and ultraviolet radiation (UV) depended on the concentration of particulated material. Beam attenuation coefficient at 660 nm (c), a good estimator of the particulate material concentration, ranged from 0.85–2.5 m−1 in the upper mixed layer. The values of particulate beam attenuation and the spectral absorption coefficients suggest that the detritical contribution is relatively low and the package effect is relatively high. Both beam attenuation and vertical attenuation of downward radiation (Kd) were linearly correlated with chlorophyll concentration in the water column. Chlorophyll concentration in the study area varied two orders of magnitude (0.2–23 μg Chl a l−1). During the cruise, the maximal UV-B incident radiation at 305 nm was variable ranging from 0.8 to 9.7 μW cm−2. Great regional differences in penetration of UV-B were observed with Kd(305) values in the upper mixed layer, ranging from 0.01 to 0.56 m−1. On the other hand, the attenuation coefficient of downward PAR radiation (Kd(PAR)) in the upper mixed layer ranged from 0.08 to 0.5 m−1. Kd values of both PAR and UV were higher in the stations of Gerlache Strait, coastal waters of Trinity Peninsula, and in a front system north of Bransfield. The lowest Kd values were found in Bransfield, south of King George and Livingstone islands and were associated to low cell densities and chlorophyll concentrations. The spectral light attenuation between 300 and 800 nm also was analyzed. Blue and red regions of the spectra were drastically attenuated in the surface waters of Gerlache Strait.

Potential impact of the main benthic amphipods on the eastern Weddell Sea shelf ecosystem (Antarctica)

As they represent one of the most diversified taxonomic groups on Antarctic bottoms, amphipods are likely to play a complex role in biogeochemical fluxes that occur within benthic ecosystems. The aim of this paper is to present, using both digestive tract analyses and relative species abundance data, the impact of amphipod feeding on different potential preys of the Weddell Sea. The study is based on data obtained for 29 representative amphipod species collected at 130 stations distributed along the eastern shelf of the Weddell Sea (depth range: 60–2,000 m) during three summer cruises, from 1989 to 1998. Sedimenting plankton particles (10–27%), crustaceans (22–32%) and fish carrion (5–18%) are the main food resources. Other abundant potential preys, such as molluscs or tunicates, do not seem to be consumed. Variations in the proportions of the different preyed food items are observed, mainly related to differences in relative amphipod species composition in samples. Presented results will help in refining ecological models of the prospected area, but also underline the need for accurate and reliable measurements of the feeding rates of Antarctic benthic organisms.

Underwater light field and phytoplankton absorbance in different surface water masses of the Atlantic sector of the Southern Ocean

Spectral water transparency and phytoplankton light absorbance were studied in the Atlantic sector of the Southern Ocean during the Southern Ocean JGOFS ANT XIII/2 cruise in early austral summer 1995/1996. The study area comprised three zones, which differed markedly with respect to their hydrographic and planktological characteristics: the Antarctic Polar Frontal Zone with adiatom bloom, the Antarctic Circumpolar Current outside frontal systems with phytoplankton-poor water and a higher flagellate abundance than in the other two areas, and the marginal ice zone with a Phaeocystis bloom. The influence of phytoplankton on spectral water transparency was assessed by two independent procedures: the pigment-specific beam absorption coefficient, aφ*[λ], at all stations, as estimated by spectroscopy of in vivo light absorption of plankton on glass fibre filters, and the pigment-specific light attenuation, (kc[λ]), as derived by regression analysis of spectral in situ vertical light attenuation coefficients in the sea against concomitant pigment concentrations. Values of aφ*[λ] and vertical profiles of light attenuation by phytoplankton exhibited regional differences that corresponded with the three zones from which samples had been collected. These differences can be related to the specific characteristics of the three zones with respect to cell size distribution, pigment composition and biomass. The observed variations in aφ*[λ] values should be considered when oceanic primary production is to be estimated by biooptical modelling.

Microstructure Observations of Turbulent Mixing in a Partially Mixed Estuary. Part II: Salt Flux and Stress

Abstract Turbulent mixing in the water column was observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall 1995. The focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates (e), and on the tidal and fortnightly variability of these fluxes. In estimating eddy viscosity (Km) and eddy diffusivity (Kρ), the authors follow measurement/modeling techniques of Busch and Osborn, while prescribing a variable flux Richardson number (Rf) dependent upon the gradient Richardson number (Ri). It is argued that a steady-state production–dissipation balance holds in the turbulent kinetic energy budget. All turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle. Subject to complications arising from nontidal flows, the “strongest” mixing occurred during flood on neap tides, and during ebb on spring tides. In the lower part of the water column during floods and spring ebb Km...

Measurements of carbonyl sulfide (COS) in surface seawater and marine air, and estimates of the air‐sea flux from observations during two Atlantic cruises

Carbonyl sulfide (COS) was measured in surface seawater and in marine air during two Atlantic cruises of the R/V Polarstern between Bremerhaven, Germany, and Cape Town, South Africa. The cruises took place in the fall of 1997 and in the summer of 1998. The concentration of COS showed clear diurnal, seasonal, and latitudinal variations, as did its saturation ratio. The concentration of dissolved COS averaged 14.7 pmol L−1 and 18.1 pmol L−1 for the fall and summer cruises, respectively. On most days, seawater was undersaturated in COS during the late night and early morning but was supersaturated during the rest of the day, implying that the ocean can act as both a source and a sink for COS on the same day. The COS content in seawater was correlated significantly with the global radiation, the CH3SH concentration, and the seawater temperature. The air‐sea flux of COS from the open Atlantic Ocean was estimated using exchange coefficients calculated according to Erickson's stability dependent model for air‐sea gas exchange. The largest COS flux into the atmosphere occurred in productive regions (the Benguela Current, the West African upwelling area, and the northeastern Atlantic) during the warmer seasons. A small net oceanic uptake of COS was found in the Benguela Current during the southern winter. The average open ocean fluxes were 13.5 nmol COS m−2 d−1 and 28.6 nmol COS m−2 d−1 for the two cruises, respectively. A global open ocean source of 0.10 Tg COS yr−1 is extrapolated from the measured data. The atmospheric mixing ratio of COS averaged 474±33 and 502±38 pptv for the fall and summer cruises, respectively, and had no significant interhemispheric gradient.

Nitrogen uptake kinetics in the Ross Sea, Antarctica

Abstract The first estimates of uptake kinetic parameters for NH4+, NO3−, and urea in the Ross Sea, Antarctica were measured on three cruises during austral late winter–early spring 1996 (pre-bloom), late spring 1997 (bloom development), and summer 1997 (bloom decline). Nitrogen (N) uptake experiments were conducted with water collected at the 50% light penetration depth using trace-metal clean protocols and 15N tracer techniques. At all sites, ambient NO3− concentrations ranged from 5.8 to 30.5 μg-at N l−1 and silicic acid concentrations were greater than 62.0 μg-at Si l−1. The following trends were observed. First, based on maximum uptake rates (Vmax), apparent N utilization followed the order NO3−>NH4+>urea during the pre-bloom and bloom development cruises. During the summer cruise, as the bloom was declining, the apparent order of utilization was NH4+>NO3−>urea. Second, evidence for possible repression of NO3− uptake by elevated NH4+ concentrations was only observed at one site. Third, the kinetic parameters of NH4+ uptake rates corrected for isotope dilution were compared with the kinetic parameters determined from uncorrected rates. In this comparison, the measure of substrate affinity, α (α=Vmax/Ks) increased by an average of 4.6-fold when rates were corrected for isotope dilution, but values of Vmax remained unchanged. Fourth, using bacterial production data, the magnitude of bacterial N uptake was estimated. Assuming that all bacterial N demands were met with NH4+, the estimated bacterial portion of NH4+ uptake ranged from

Contribution of flow cytometry to estimate picoplankton biomass in estuarine systems

Picoplankton (plankton ≤3 μm) biomass was determined by flow cytometry in three European estuarine systems (Krka Estuary in Croatia, Rhone Delta in France, and Lena Delta and Laptev Sea in Russia). The size of natural phytoplankton groups was obtained by a calibration curve, with different picoplankton's strains (from 1.6 to 3.4 μm), measured by a Coulter counter (size) and a flow cytometer (light-scattering). Two natural groups of picoplankton were identified by flow cytometry in the three systems: Synechococcus sp and picoeukaryotes. Picoplankton cells abundance ranged between: 2800 and 42 000, 5000 and 37 000, 1000 and 50 000 cells ml−1 in the Krka estuary, in the Rhone delta and in the Lena-Laptev system, respectively. In the Krka estuary, picoplankton biomass ranges between 11 and 68 μgC l−1. It can make up as much as 88% of the total photosynthetic plankton population and 50% of total organic particulate carbon. Picoplankton biomass was greater in the summer than in the autumn. At the halocline layer this biomass can attempt ca. 390 μgC l−1during the summer cruise. In the Rhone delta, a lower picoplankton biomass (6–39 μgC l−1) was observed at the end of the winter. These biomass represented between 0.4 and 22% of the particulate organic carbon, which could reach 71% of the total photosynthetic plankton biomass at the marine station. In the Lena-Laptev system, picoplankton biomass varied between 6 and 56 μgC l−1 in surface waters. Picoplankton biomass decreased with depth, but picoeukaryotes were still observed in deep samples (20, 30 m) in the Laptev Sea, showing a considerable autotrophic activity in spite of low temperatures (0–1 °C). Although the widely dispersed estuary geographic distribution and their different estuarine characteristics, the data point out that these small organisms can also play an important role in the transfer of organic carbon from rivers to oceans and that flow cytometry can be able to detect these small cells in turbid systems.

Transport processes in the Kara Sea

Field data from late summer cruises in 1993–1996 and results from a rotating laboratory model study are used to infer transport routes for contaminants in the Kara Sea on the western Siberian coast. The data show a circulation pattern that differs significantly from published concepts and contains a richer structure. Both the inflowing coastal water from the west and the modified Atlantic Water from the Barents Sea and the Arctic Ocean in the north affect the water properties of the East Novaya Zemlya Trough through mixing, intrusion, and possible transformation during winter convection. Much of the current structure can be explained as a result of topographical steering of the throughflows. Weak summer winds of variable direction affect only the upper layer, whereas persistent strong winds may affect long‐term circulation patterns and transports. The main outflow routes are north to the Santa Anna Trough and northeast to the Voronin Trough. The outflowing surface water and ice to the north are expected to join the inflowing Polar Water from the Arctic Ocean that forms the Persey Current into the Barents Sea. Deeper outflows are expected to follow the continental slope to the east in the Arctic Ocean. The laboratory model has been validated to earlier current measurements to the north and south of Novaya Zemlya and shows several of the features observed in the newer field measurements. This suggests that the dynamic similitude and boundary forcing of the model are good. Two notable exceptions are the westward transport of drifting buoys to the north of Novaya Zemlya and the late winter hydrography to the north of the Ob and Yenisey estuaries. Both are probably due to persistent winds. The conclusions from the measured and modeled currents are supported by measurements of 137Cs in the seawater and sediments.

Microstructure Observations of Turbulent Mixing in a Partially Mixed Estuary. Part I: Dissipation Rate

Variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall of 1995. Variability patterns of stratification, shear, Richardson number (Ri), and turbulent dissipation rates (ϵ) were similar to those of earlier observations, with strong turbulence in the weakly stratified bottom layer, weak turbulence in the halocline during neap tides, and low Ri and high ϵ spanning the water column during spring ebbs. Depth-integrated turbulent dissipation rates ∫ ρϵ dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy. Alternatively, a suitable scaling for ∫ ρϵ dz is also provided by the product of bottom shear stress and depth-average velocity τbυ, a relationship that fails at slack tide, however. At heights above bottom of z ≲ 0.3 m, and again excepting slack tides, observed ϵ were highly correlated with a law-of-the-wall dissipation rate ϵb = u3∗/(κz). Ratios ϵ/ϵb were close to 1. Here, the friction velocity is u∗, and von Kármán’s constant is κ. Most profiles of the normalized dissipation rate ϵ/ϵb showed a weak increase with z in the lowest 1.5 m, a departure from law of the wall scaling attributed to stable stratification. Such deviations from the law of the wall were smallest during spring floods when the near-bottom stratification was weak or unstable. Turbulence in the stratified water column well above the bottom appeared to be locally generated by shear instability even though ϵb and ϵ were correlated throughout the water column. During spring ebbs, ϵ exceeded ϵb by almost an order of magnitude at z ≳ 2 m.