Suspended sediment and erosion dynamics in Kugmallit Bay and Beaufort Sea during ice-free conditions

The Mackenzie River is the largest river on the North American side of the Arctic and its huge freshwater and sediment load impacts the Canadian Beaufort Shelf. Huge quantities of sediment and associated organic carbon are transported in the Mackenzie plume into the interior of the Arctic Ocean mainly during the freshet (May to September). Changing climate scenarios portend increased coastal erosion and resuspension that lead to altered river-shelf-slope particle budgets. We measured sedimentation rates, suspended particulate matter (SPM), particle size and settling rates during ice-free conditions in Kugmallit Bay (3–5 m depth). Additionally, measurements of erosion rate, critical shear stress, particle size distribution and resuspension threshold of bottom sediments were examined at four regionally contrasting sites (33–523 m depth) on the Canadian Beaufort Shelf using a new method for assessing sediment erosion. Wind induced resuspension was evidenced by a strong relationship between SPM and wind speed in Kugmallit Bay. Deployment of sediment traps showed decreasing sedimentation rates at sites along an inshore–offshore transect ranging from 5400 to 3700 g m^− 2 day^− 1. Particle settling rates and size distributions measured using a Perspex settling chamber showed strong relationships between equivalent spherical diameter (ESD) and particle settling rates (r² = 0.91). Mean settling rates were 0.72 cm s^− 1 with corresponding ESD values of 0.9 mm. Undisturbed sediment cores were exposed to shear stress in an attempt to compare differences in sediment stability across the shelf during September to October 2003. Shear was generated by vertically oscillating a perforated disc at controlled frequencies corresponding to calibrated shear velocity using a piston grid erosion device. Critical (Type I) erosion thresholds (u) varied between 1.1 and 1.3 cm s^− 1 with no obvious differences in location. Sediments at the deepest site Amundsen Gulf displayed the highest erosion rates (22–54 g m^− 2 min^− 1) with resuspended particle sizes ranging from 100 to 930 µm for all sites. There was no indication of biotic influence on sediment stability, although our cores did not display a fluff layer of unconsolidated sediment. Concurrent studies in the delta and shelf region suggest the importance of a nepheloid layer which transports suspended particles to the slope. Continuous cycles of resuspension, deposition, and horizontal advection may intensify with reduction of sea ice in this region. Our measurements coupled with studies of circulation and cross-shelf exchange allow parameterization and modeling of particle dynamics and carbon fluxes under various climate change scenarios.     

Physical and biological correlates of virus dynamics in the southern Beaufort Sea and Amundsen Gulf

As part of the Canadian Arctic Shelf Exchange Study (CASES), we investigated the spatial and seasonal distributions of viruses in relation to biotic (bacteria, chlorophyll-a (chl a)) and abiotic variables (temperature, salinity and depth). Sampling occurred in the southern Beaufort Sea Shelf in the region of the Amundsen Gulf and Mackenzie Shelf, between November 2003 and August 2004. Bacterial and viral abundances estimated by epifluorescence microscopy (EFM) and flow cytometry (FC) were highly correlated (r² = 0.89 and r² = 0.87, respectively), although estimates by EFM were slightly higher (FC = 1.08 × EFM + 0.12 and FC = 1.07 × EFM + 0.43, respectively). Viral abundances ranged from 0.13 × 10⁶ to 23 × 10⁶ ml^− 1, and in surface waters were ~ 2-fold higher during the spring bloom in May and June and ~ 1.5-fold higher during July and August, relative to winter abundances. These increases were coincident with a ~ 6-fold increase in chl a during spring and a ~ 4-fold increase in bacteria during summer. Surface viral abundances near the Mackenzie River were ~ 2-fold higher than in the Mackenzie Shelf and Amundsen Gulf regions during the peak summer discharge, concomitant with a ~ 5.5-fold increase in chl a (up to 2.4 μg l^− 1) and a ~ 2-fold increase in bacterial abundance (up to 22 × 10⁵ ml^− 1). Using FC, two subgroups of viruses and heterotrophic bacteria were defined. A low SYBR-green fluorescence virus subgroup (V2) representing ~ 71% of the total viral abundance, was linked to the abundance of high nucleic acid fluorescence (HNA) bacteria (a proxy for bacterial activity), which represented 42 to 72% of the bacteria in surface layers. A high SYBR-green fluorescence viral subgroup (V1) was more related to high chl a concentrations that occurred in surface waters during spring and at stations near the Mackenzie River plume during the summer discharge. These results suggest that V1 infect phytoplankton, while most V2 are bacteriophages. On the Beaufort Sea shelf, viral abundance displayed seasonal and spatial variations in conjunction with chl a concentration, bacterial abundance and composition, temperature, salinity and depth. The highly dynamic nature of viral abundance and its correlation with increases in chl a concentration and bacterial abundance implies that viruses are important agents of microbial mortality in Arctic shelf waters.     

Variations of the abundance and nucleic acid content of heterotrophic bacteria in Beaufort Shelf waters during winter and spring

Depth profiles of heterotrophic bacteria abundance were measured weekly over a 6-month period from December to May in Franklin Bay, a 230 m-deep coastal Arctic Ocean site of the southeastern Beaufort Sea. Total bacteria, low nucleic acid (LNA) and high nucleic acid (HNA) bacteria abundances were measured using flow cytometry after SYBR Green I staining. The HNA bacteria abundance in surface waters started to increase 5–6 weeks after phytoplankton growth resumed in spring, increasing from 1 × 10⁵ to 3 × 10⁵ cells mL^− 1 over an 8-week period, with a net growth rate of 0.018 d^− 1. LNA bacteria response was delayed by more than two months relative to the beginning of the phytoplankton biomass accumulation and had a lower net growth rate of 0.013 d^− 1. The marked increase in bacterial abundance occurred before any significant increase in organic matter input from river discharge (as indicated by the unchanged surface water salinity and DOC concentrations), and in the absence of water temperature increase. The abundance of bacteria below the halocline was relatively high until January (up to 5 × 10⁵ cells mL^− 1) but then decreased to values close to 2 × 10⁵ cells mL^− 1. The three-fold bacterial abundance increase observed in surface waters in spring was mostly due to HNA bacteria, supporting the idea that these cells are the most active.     

Influence of the Mackenzie River plume on the sinking export of particulate material on the shelf

We examined the influence of the Mackenzie River plume on sinking fluxes of particulate organic and inorganic material on the Mackenzie Shelf, Canadian Arctic. Short-term particle interceptor traps were deployed under the halocline at 3 stations across the shelf during fall 2002 and at 3 stations along the shelf edge during summer 2004. During the two sampling periods, the horizontal patterns in sinking fluxes of particulate organic carbon (POC) and chlorophyll a (chl a) paralleled those in chl a biomass within the plume. Highest sinking fluxes of particulate organic material occurred at stations strongly influenced by the river plume (maximum POC sinking fluxes at 25 m of 98 mg C m^− 2 d^− 1 and 197 mg C m^− 2 d^− 1 in 2002 and 2004, respectively). The biogeochemical composition of the sinking material varied seasonally with phytoplankton and fecal pellets contributing considerably to the sinking flux in summer, while amorphous detritus dominated in the fall. Also, the sinking phytoplankton assemblage showed a seasonal succession from a dominance of diatoms in summer to flagellates and dinoflagellates in the fall. The presence of the freshwater diatom Eunotia sp. in the sinking assemblage directly underneath the river plume indicates the contribution of a phytoplankton community carried by the plume to the sinking export of organic material. Yet, increasing chl a and BioSi sinking fluxes with depth indicated an export of phytoplankton from the water column below the river plume during summer and fall. Grazing activity, mostly by copepods, and to a lesser extent by appendicularians, appeared to occur in a well-defined stratum underneath the river plume, particularly during summer. These results show that the Mackenzie River influences the magnitude and composition of the sinking material on the shelf in summer and fall, but does not constitute the only source of material sinking to depth at stations influenced by the river plume.     

Microbial food web responses to light and nutrients beneath the coastal Arctic Ocean sea ice during the winter–spring transition

We measured the abundance and biomass of phototrophic and heterotrophic microbes in the upper mixed layer of the water column in ice-covered Franklin Bay, Beaufort Sea, Canada, from December 2003 to May 2004, and evaluated the influence of light and nutrients on these communities by way of a shipboard enrichment experiment. Bacterial cell concentrations showed no consistent trends throughout the sampling period, averaging (± SD) 2.4 (0.9) × 10⁸ cells L^− 1; integrated bacterial biomass for the upper mixed layer ranged from 1.33 mg C m^− 3 to 3.60 mg C m^− 3. Small cells numerically dominated the heterotrophic protist community in both winter and spring, but in terms of biomass, protists with a diameter > 10 µm generally dominated the standing stocks. Heterotrophic protist biomass integrated over the upper mixed layer ranged from 1.23 mg C m^− 3 to 6.56 mg C m^− 3. Phytoplankton biomass was low and variable, but persisted during the winter period. The standing stock of pigment-containing protists ranged from a minimum value of 0.38 mg C m^− 3 in winter to a maximal value of 6.09 mg C m^− 3 in spring and the most abundant taxa were Micromonas-like cells. These picoprasinophytes began to increase under the ice in February and their population size was positively correlated with surface irradiance. Despite the continuing presence of sea ice, phytoplankton biomass rose by more than an order of magnitude in the upper mixed layer by May. The shipboard experiment in April showed that this phototrophic increase in the community was not responsive to pulsed nutrient enrichment, with all treatments showing a strong growth response to improved irradiance conditions. Molecular (DGGE) and microscopic analyses indicated that most components of the eukaryotic community responded positively to the light treatment. These results show the persistence of a phototrophic inoculum throughout winter darkness, and the strong seasonal response by arctic microbial food webs to sub-ice irradiance in early spring.     

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.     

Picophytoplankton and nanophytoplankton abundance and distribution in the southeastern Beaufort Sea (Mackenzie Shelf and Amundsen Gulf) during Fall 2002

The distribution of picophytoplankton (0.2–2 µm) and nanophytoplankton (2–20 µm) in the Beaufort Sea–Mackenzie Shelf and Amundsen Gulf regions during autumn, 2002 is examined relative to their ambient water mass properties (salinity, temperature and nutrients: nitrate + nitrite, phosphate, and silicate) and to the ratio of variable to maximum fluorescence, Fv/Fm. Total phytoplankton and cell abundances (< 20 µm) were mainly correlated with salinity. Significant differences in picophytoplankton cell numbers were found among waters near the mouth of the Mackenzie River, ice melt waters and the underlying halocline water masses of Pacific origin. Picophytoplankton was the most abundant phytoplankton fraction during the autumnal season, probably reflecting low nitrate concentrations (surface waters average ~ 0.65 µM). The ratio Fv/Fm averaged 0.44, indicating that cells were still physiologically active, even though their concentrations were low (max Chl a = 0.9 mg m^− 3). No significant differences in Fv/Fm were evident in the different water masses, indicating that rate limiting conditions for photosynthesis and growth were uniform across the whole system, which was in a pre-winter stage, and was probably already experiencing light limitation as a result of shortening day lengths.     

Silicon dynamics in the Oder estuary, Baltic Sea

Studies on dissolved silicate (DSi) and biogenic silica (BSi) dynamics were carried out in the Oder estuary, Baltic Sea in 2000–2005. The Oder estuary proved to be an important component of the Oder River–Baltic Sea continuum where very intensive seasonal DSi uptake during spring and autumn, but also BSi regeneration during summer take place. Owing to the regeneration process annual DSi patterns in the river and the estuary distinctly differed; the annual patterns of DSi in the estuary showed two maxima and two minima in contrast to one maximum- and one minimum-pattern in the Oder River. DSi concentrations in the river and in the estuary were highest in winter (200–250 μmol dm^− 3) and lowest (often less than 1 μmol dm^− 3) in spring, concomitant with diatom growth; such low values are known to be limiting for new diatom growth. Secondary DSi summer peaks at the estuary exit exceeded 100 μmol dm^− 3, and these maxima were followed by autumn minima coinciding with the autumn diatom bloom. Seasonal peaks in BSi concentrations (ca. 100 μmol dm^− 3) occurred during the spring diatom bloom in the Oder River. Mass balance calculations of DSi and BSi showed that DSi + BSi import to the estuary over a two year period was 103.2 kt and that can be compared with the DSi export of 98.5 kt. The difference between these numbers gives room for ca. 2.5 kt BSi to be annually exported to the Baltic Sea. Sediment cores studies point to BSi annual accumulation on the level of 2.5 kt BSi. BSi import to the estuary is on the level of ca. 10.5 kt, thus ca. 5 kt of BSi is annually converted into the DSi, increasing the pool of DSi that leaves the system. BSi concentrations being ca. 2 times higher at the estuary entrance than at its exit remain in a good agreement with the DSi and BSi budgeting presented in the paper.     

Past, present and future state of the biogeochemical Si cycle in the Baltic Sea

The Baltic Sea is one of many aquatic ecosystems that show long-term declines in dissolved silicate (DSi) concentrations due to anthropogenic alteration of the biogeochemical Si cycle. Reductions in DSi in aquatic ecosystems have been coupled to hydrological regulation reducing inputs, but also with eutrophication, although the relative significance of both processes remains unknown for the observed reductions in DSi concentrations. Here we combine present and historical data on water column DSi concentrations, together with estimates of present river DSi loads to the Baltic, the load prior to damming together with estimates of the long-term accumulation of BSi in sediments. In addition, a model has been used to evaluate the past, present and future state of the biogeochemical Si cycle in the Baltic Sea. The present day DSi load to the Baltic Sea is 855 ktons y^− 1. Hydrological regulation and eutrophication of inland waters can account for a reduction of 420 ktons y^− 1 less riverine DSi entering the Baltic Sea today. Using published data on basin-wide accumulation rates we estimate that 1074 ktons y^− 1 of biogenic silica (BSi) is accumulating in the sediments, which is 36% higher than earlier estimates from the literature (791 ktons y^− 1). The difference is largely due to the high reported sedimentation rates in the Bothnian Sea and the Bothnian Bay. Using river DSi loads and estimated BSi accumulation, our model was not able to estimate water column DSi concentrations as burial estimates exceeded DSi inputs. The model was then used to estimate the BSi burial from measured DSi concentrations and DSi load. The model estimate for the total burial of BSi in all three basins was 620 ktons y^− 1, 74% less than estimated from sedimentation rates and sediment BSi concentrations. The model predicted 20% less BSi accumulation in the Baltic Proper and 10% less in the Bothnian Bay than estimated, but with significantly less BSi accumulation in the Bothnian Sea by a factor of 3. The model suggests there is an overestimation of basin-wide sedimentation rates in the Bothnian Bay and the Bothnian Sea. In the Baltic Proper, modelling shows that historical DSi concentrations were 2.6 times higher at the turn of the last century (ca. 1900) than at present. Although the DSi decrease has leveled out and at present there are only restricted areas of the Baltic Sea with limiting DSi concentrations, further declines in DSi concentrations will lead to widespread DSi limitation of diatoms with severe implications for the food web.     

Seasonality of picophytoplankton chlorophyll a and biomass in the central Cantabrian Sea, southern Bay of Biscay

Seasonal changes in the abundance and biomass of cyanobacteria (Synechococcus and Prochlorococcus) and picoeukaryotes were studied by flow cytometry in the upper layers of the central Cantabrian Sea continental shelf, from April 2002 to April 2006. The study area displayed the typical hydrographic conditions of temperate coastal zones. A marked seasonality of the relative contribution of prokaryotes and eukaryotes was found. While cyanobacteria were generally more abundant for most of the year (up to 2.4 10⁵ cells mL^− 1), picoeukaryotes dominated the community (up to 10⁴ cells mL^− 1) from February to May. The disappearance of Prochlorococcus from spring through summer is likely related to shifts in the prevailing current regime. The maximum total abundance of picophytoplankton was consistently found in late summer–early autumn. Mean photic-layer picoplanktonic chlorophyll a ranged from 0.06 to 0.53 µg L^− 1 with a relatively high mean contribution to total values (33 ± 2% SE), showing maxima around autumn and minima in spring. Biomass (range 0.58–40.16 mg C m^− 3) was generally dominated by picoeukaryotes (mean ± SE, 4.28 ± 0.27 mg C m^− 3) with an average contribution of cyanobacteria of 30 ± 2%. Different seasonality of pigment and biomass values resulted in a clear temporal pattern of picophytoplanktonic carbon to chlorophyll a ratio, which ranged from 10 (winter) to 140 (summer). This study highlights the important contribution of picoplanktonic chlorophyll a and carbon biomass in this coastal ecosystem.     

Changes in dissolved silicate loads to the Baltic Sea — The effects of lakes and reservoirs

We tested the hypothesis that dissolved silicate (DSi) yields [kg km^− 2 yr^− 1] of 82 major watersheds of the Baltic Sea can be expressed as a function of the hydraulic load (HL) as a measure of water residence time and the total organic carbon (TOC) concentration, both variables potentially increasing the DSi yield. Most boreal rivers fitted a linear regression model using HL as an independent variable to explain the DSi yield. Rivers with high HL, i.e., shortest residence times, showed highest DSi yields up to 2300 kg km^− 2 yr^− 1. This is most likely caused by an excess supply of DSi, i.e., the geochemical sources prevail over biological sinks in these boreal watersheds. The DSi yield for regulated and unregulated larger rivers of the boreal watersheds constituting about 40% of the total water discharge and of the total DSi load to the Baltic Sea, respectively, can be expressed as: DSi yield = 190 + 49.5 HL[m yr^− 1] + 0.346 TOC [µM] (R² = 0.80). Since both HL and TOC concentrations have decreased after damming, the DSi yields have decreased significantly in the regulated boreal watersheds, for the River Luleälven we estimated more than 30%. The larger eutrophic watersheds draining cultivated landscape of the southern catchment of the Baltic Sea and representing about 50% of the annual water discharge to the Baltic Sea, deviated from this pattern and showed lower DSi yields between 60–580 kg km^− 2 yr^− 1. DSi yields showed saturation curve like relationship to HL and it appears that DSi is retained in the watersheds efficiently through biogenic silica (BSi) production and subsequent sedimentation along the entire river network. The relationship between HL and DSi yields for all larger cultivated watersheds was best fitted by a Freundlich isotherm (DSi = 115.7HL¹⁰⁹; R² = 0.73), because once lake and reservoir area exceeds 10% of the watershed area, minimum DSi yields were reached. To estimate an uperturbed DSi yield for the larger eutrophic southeastern watersheds is still difficult, since no unperturbed watersheds for comparison were available. However, a rough estimate indicate that the DSi flux from the cultivated watersheds to the Baltic Sea is nowadays only half the uperturbed flux. Overall, the riverine DSi loads to the Baltic Sea might have dropped with 30–40% during the last century.     

Th-derived particulate organic carbon fluxes in the northern Barents Sea with comparison to drifting sediment trap fluxes

Large-volume sampling of ²³⁴Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. ²³⁴Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m^− 2 d^− 1 in 2003 and 47.1 ± 30.6 mmol C m^− 2 d^− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m^− 2 d^− 1 in 2003 and 17.3 ± 11.4 mmol C m^− 2 d^− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although ²³⁴Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of ²³⁴Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/²³⁴Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of ²³⁴Th may yield relatively high, and possibly inaccurate POC/²³⁴Th ratios. Hence, POC fluxes derived from ²³⁴Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.     

Turbulence in coastal fronts near the mouths of Block Island and Long Island Sounds

Measurements of turbulence were performed in four frontal locations near the mouths of Block Island Sound (BIS) and Long Island Sound (LIS). These measurements extend from the offshore front associated with BIS and Mid-Atlantic Bight Shelf water, to the onshore fronts near the Montauk Point (MK) headland, and the Connecticut River plume front. The latter feature is closely associated with the major fresh water input to LIS. Turbulent kinetic energy (TKE) dissipation rate, ε, was obtained using shear probes mounted on an autonomous underwater vehicle. Offshore, the BIS estuarine outflow front showed, during spring season and ebb tide, maximum TKE dissipation rate, ε, estimates of order 10^− 5 W/kg, with background values of order 10^− 6 to 10^− 9 W/kg. Edwards et al. [Edwards, C.A., Fake, T.A., and Bogden, P.S., 2004a. Spring–summer frontogenesis at the mouth of Block Island Sound: 1. A numerical investigation into tidal and buoyancy-forced motion. Journal of Geophysical Research 109 (C12021), doi:10.1029/2003JC002132.] model this front as the boundary of a tidally driven, baroclinically adjusted BIS flow around the MK headland eddy. At the entrance to BIS, near MK, two additional fronts are observed, one of which was over sand waves. For the headland site front east of MK, without sand waves, during ebb tide, ε estimates of 10^− 5 to 10^− 6 W/kg were observed. The model shows that this front is at the northern end of an anti-cyclonic headland eddy, and within a region of strong tidal mixing. For the headland site front further northeast over sand waves, maximum ε estimates were of order 10^− 4 W/kg within a background of order 10^− 7–10^− 6 W/kg. From the model, this front is at the northeastern edge of the anti-cyclonic headland eddy and within the tidal mixing zone. For the Connecticut River plume front, a surface trapped plume, during ebb tide, maximum ε estimates of 10^− 5 W/kg were obtained, within a background of 10^− 6 to 10^− 8 W/kg. Of all four fronts, the river plume front has the largest finescale mean-square shear, S² ~ 0.15 s^− 2. All of the frontal locations had local values of the buoyancy Reynolds number indicating strong isotropic turbulence at the dissipation scales. Local values of the Froude number indicated shear instability in all of the fronts.     

Sea ice and the onshore–offshore gradient in pre-winter zooplankton assemblages in southeastern Beaufort Sea

Zooplankton communities were studied in southeastern Beaufort Sea (Arctic Ocean) in September–October 2002. Cluster analysis and non-metric multidimensional scaling revealed three distinct mesozooplankton assemblages. A neritic assemblage occurred on the Mackenzie Shelf and in Franklin Bay, while distinct off-shelf assemblages prevailed in the Cape Bathurst Polynya and on the Beaufort Slope respectively. Over 95% of the mesozooplankton was comprised of eight copepod taxa. Pseudocalanus spp. contributed predominantly to the discrimination of the three assemblages and was the only significant indicator of the Shelf assemblage. Oithona similis, Oncaea borealis, Metridia longa and Calanus hyperboreus were indicators of the Polynya assemblage. Cyclopina sp. and Microcalanus pygmaeus were indicative of the overall off-shelf community (Polynya and Slope assemblages). The importance of omnivores and carnivores increased from the shelf to the polynya and the slope. Station depth and duration of reduced ice conditions during summer (< 50% ice concentration) underpinned the distribution of the assemblages (r² = 0.71 and 0.45 respectively). The abundance of Pseudocalanus spp. was independent of depth and increased with the duration of reduced ice conditions (r_s = 0.438). The abundance of Cyclopina sp., M. pygmaeus and other indicators of the offshore assemblages followed the opposite trend (r_s = − 0.467 and − 0.5 respectively). Under continued climate warming, a reduction of the ice cover will affect the biogeography of mesozooplankton on and around the Mackenzie Shelf, to the potential advantage of Pseudocalanus spp. and other calanoid herbivores.     

Gas transfer velocities of CO2 and CH4 in a tropical reservoir and its river downstream

We have measured simultaneously the methane (CH₄) and carbon dioxide (CO₂) surface concentrations and water–air fluxes by floating chambers (FC) in the Petit-Saut Reservoir (French Guiana) and its tidal river (Sinnamary River) downstream of the dam, during the two field experiments in wet (May 2003) and dry season (December 2003). The eddy covariance (EC) technique was also used for CO₂ fluxes on the lake. The comparison of fluxes obtained by FC and EC showed little discrepancies mainly due to differences in measurements durations which resulted in different average wind speeds. When comparing the gas transfer velocity (k₆₀₀) for a given wind speed, both methods gave similar results. On the lake and excluding rainy events, we obtained an exponential relationship between k₆₀₀ and U₁₀, with a significant intercept at 1.7 cm h^− 1, probably due to thermal effects. Gas transfer velocity was also positively related to rainfall rates reaching 26.5 cm h⁻¹ for a rainfall rate of 36 mm h^− 1. During a 24-h experiment in dry season, rainfall accounted for as much as 25% of the k₆₀₀. In the river downstream of the dam, k₆₀₀ values were 3 to 4 times higher than on the lake, and followed a linear relationship with U₁₀.     

Organic carbon budget for the Gulf of Bothnia

We calculated input of organic carbon to the unproductive, brackish water basin of the Gulf of Bothnia from rivers, point sources and the atmosphere. We also calculated the net exchange of organic carbon between the Gulf of Bothnia and the adjacent marine system, the Baltic Proper. We compared the input with sinks for organic carbon; permanent incorporation in sediments and mineralization and subsequent evasion of CO₂ to the atmosphere. The major fluxes were riverine input (1500 Gg C year^− 1), exchange with the Baltic Proper (depending on which of several possible DOC concentration differences between the basins that was used in the calculation, the flux varied between an outflow of 466 and an input of 950 Gg C year^− 1), sediment burial (1100 Gg C year^− 1) and evasion to the atmosphere (3610 Gg C year^− 1). The largest single net flux was the emission of CO₂ to the atmosphere, mainly caused by bacterial mineralization of organic carbon. Input and output did not match in our budget which we ascribe uncertainties in the calculation of the exchange of organic carbon between the Gulf of Bothnia and the Baltic Proper, and the fact that CO₂ emission, which in our calculation represented 1 year (2002) may have been overestimated in comparison with long-term means. We conclude that net heterotrophy of the Gulf of Bothnia was due to input of organic carbon from both the catchment and from the Baltic Proper and that the future degree of net heterotrophy will be sensible to both catchment export of organic carbon and to the ongoing eutrophication of the Baltic Proper.     

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.     

Analysis of the hydrophysical structure of the Sea of Azov in the period of the bottom anoxia development

The hydrophysical and hydrochemical structure of the Sea of Azov, with developed bottom anoxia, was studied during the RV “Akvanavt” cruise from July 31 to August 03, 2001. The anoxic zone with a thickness from 0.5 to 4 m above the bottom was found in all deep regions of the Sea. Concentrations of hydrochemical parameters were similar to the pronounced anoxic conditions (about 90 mmol m^− 3 of hydrogen sulfide, 17 mmol m^− 3 of ammonia, 6 mmol m^− 3 of phosphate, 7 mmol m^− 3 of total manganese). The hydrophysical structure was characterized by the uniform distribution of temperature in the upper 6–7 m mixed layer (UML). Below this a thin (0.4–0.8 m) thermocline layer was observed, just above the anoxic waters. Formation of this phenomenon was connected with that summer weather conditions. Intensive rains led to increased influx of river waters in June. That resulted in large input of allochtonous organic matter (OM) and inorganic nutrients; the latter were consumed on the additional autochthonous organic matter production. In July the weather was characterized by a significant rise in the daily averaged air temperature and large oscillations of temperature during the day. In this period a wind of constant direction was absent, but wind bursts were observed. The completed analyses showed that the formation of such a structure could be connected with the following factors: (i) positive growth trends of the daily averaged temperature and the daily oscillations of temperature, (ii) presence of wind bursts. The joint action of these factors resulted in the formation of the UML. The amplitude of wind bursts determined the depth of UML, and the value of trend determined the value of the temperature change in the thermocline. An initial presence of bottom halocline (caused by the Black Sea water influx to the bottom of the Sea of Azov) prevented the heating of the bottom layer and therefore led to an increase of vertical gradient of temperature in the thermocline. The spatial distribution of the turbulent exchange coefficient confirmed the existence of a “stagnation” area located above the anoxia zone, which is also, apparently, the reason for its occurrence.     

On the distribution of dissolved lead in the Loire estuary and the North Biscay continental shelf, France

The dissolved lead was studied in the whole salinity gradient of the system composed of the Loire estuary and the North Biscay continental shelf. About 130 samples were collected in winter 2001 and spring 2002 during Nutrigas and Gasprod campaigns (Programme PNEC-Golfe de Gascogne, RV Thalassa) and metal measurements were conducted on board by Potentiometric Stripping Analysis. In the Loire estuary, levels of dissolved lead ranged from 0.15 to 0.24 nM and from 0.04 to 0.26 nM in winter and spring, respectively. Compared to the concentrations reported in 1987 and 1990 (0.4–1.7 nM; Boutier, B., Chiffoleau, J.F., Auger, D., Truquet, I., 1993. Influence of the Loire river on dissolved lead and cadmium concentrations in coastal waters of Brittany. Estuar. Coast. Shelf S., 36:133–143, Estuarine, Coastal and Shelf Science 36, 133–143) our study indicated much lower values. The fall in concentration in the estuary could be attributed to the stopping of activity of Octel, a big manufacturer of tetra alkyl lead. Discharge in dissolved metal to the continental shelf by the Loire river was assessed as 7.5 and 1.9 kg day^− 1 for winter and spring, respectively. On the continental shelf, levels of dissolved lead varied within 0.06 and 0.27 nM in winter (0.15 ± 0.06 nM, sd = 1.96, n = 49), whereas concentrations measured in spring were in the range 0.06–0.17 nM (0.09 ± 0.03 nM, sd = 1.96, n = 60). This difference in metal concentration was related to the amounts of rainfall that have fallen over the continental shelf: estimations of inputs by this way (74 and 32 kg day^− 1 in winter and spring, respectively) appeared to be significantly higher than inputs from the Loire river (7.5 and 1.9 kg day^− 1 in winter and spring, respectively). The distributions of dissolved metal in the surface waters highlighted the role of suspended particular matter (SPM) for a rapid “trapping” of lead near the mouth of the estuary. The vertical distributions showed, in the stratified area, a biological transfer of lead between winter and spring from surface waters to the halocline.     

Pore water phosphate and ammonia below the permanent halocline in the south-eastern Baltic Sea and their benthic fluxes under anoxic conditions

In this paper the results of a study on the distribution of pore water phosphates and ammonia, and their fluxes under anoxic condition in a deep (> 70 m) accumulation-type bottom of the south-eastern Baltic Sea, namely in the Gdańsk Deep and the adjacent areas, are presented. All measurements were taken during the growth period, i.e. in September 2000, April 2001 and June 2002. Benthic phosphate and ammonia fluxes were estimated using Fick's First Law. Phosphate and ammonia concentrations ranged from 7.5 to 266.3 μmol dm^− 3 and from 53.6 to 1248.3 μmol dm^− 3, respectively. The values recorded in the central part of the Gdańsk Deep were lower than those found both on its slopes and on the SW slope of the Gotland Deep. The lowest phosphate contents were typical of the Oblique Sill which separates the Gdańsk and Gotland Deeps.In 1993–2002, as a result of anoxia the sediments in the Gdańsk Deep released about 5.1 × 10³ t P and 22.8 × 10³ t N. These loads supplied on average 1.5% and 0.9% of phytoplankton's demand for P and N, respectively. In comparison to the total external load of nutrients discharged to the Gulf of Gdańsk (i.e. 8.79 × 10³ t year^− 1 P_tot and 130.79 × 10³ t year^− 1 N_tot; [Witek, Z., Humborg, Ch., Savchuk, O., Grelowski, A. and Łysiak-Pastuszak, E., 2003. Nitrogen and phosphorus budgets of the Gulf of Gdańsk (Baltic Sea). Est. Coast. Shelf Sci., 57:239–248.]), the return flux of P and N from the anoxic sediments to the water column in the Gdańsk Deep was a minor source of these elements.