Isotope parameters (δD, δ<Superscript>18</Superscript>O) and sources of freshwater input to Kara Sea

The isotope characteristics (δD, δ¹⁸О) of Kara Sea water were studied for quantitative estimation of freshwater runoff at stations located along transect from Yamal Peninsula to Blagopoluchiya Bay (Novaya Zemlya). Freshwater samples were studied for glaciers (Rose, Serp i Molot) and for Yenisei and Ob estuaries. As a whole, δD and δ¹⁸O are higher in glaciers than in river waters. isotope composition of estuarial water from Ob River is δD =–131.4 and δ18O =–17.6‰. Estuarial waters of Yenisei River are characterized by compositions close to those of Ob River (–134.4 and–17.7‰), as well as by isotopically “heavier” compositions (–120.7 and–15.8‰). Waters from studied section of Kara Sea can be product of mixing of freshwater (δD =–119.4, δ¹⁸O =–15.5) and seawater (S = 34.9, δD = +1.56, δ¹⁸O = +0.25) with a composition close to that of Barents Sea water. isotope parameters of water vary significantly with salinity in surface layer, and Kara Sea waters are desalinated along entire studied transect due to river runoff. concentration of freshwater is 5–10% in main part of water column, and <5% at a depth of >100 m. maximum contribution of freshwater (>65%) was recorded in surface layer of central part of sea.     

Isotopic,trace element and nutrient characterization of coastal waters from Ubatuba inner shelf area,south-eastern Brazil

Stable isotopes, tritium, radium isotopes, radon, trace elements and nutrients data were collected during two sampling campaigns in the Ubatuba coastal area (south-eastern Brazil) with the aim of investigating submarine groundwater discharge (SGD) in the region. The isotopic composition (δD, δ¹⁸O, ³H) of submarine waters was characterised by significant variability and heavy isotope enrichment. The stable isotopes and tritium data showed good separation of groundwater and seawater groups. The contribution of groundwater in submarine waters varied from a few % to 17%. Spatial distribution of ²²²Rn activity concentration in surface seawater revealed changes between 50 and 200 Bq m⁻³ which were in opposite relationship with observed salinities. Time series measurements of ²²²Rn activity concentration in Flamengo Bay (from 1 to 5 kBq m⁻³), obtained by in situ underwater gamma-spectrometry showed a negative correlation between the ²²²Rn activity concentration and tide/salinity. This may be caused by sea level changes as tide effects induce variations of hydraulic gradients, which increase ²²²Rn concentration during lower sea level, and opposite, during high tides where the ²²²Rn activity concentration is smaller. The estimated SGD fluxes varied during 22–26 November between 8 and 40 cm d⁻¹, with an average value of 21 cm d⁻¹ (the unit is cm³/cm² per day). The radium isotopes and nutrient data showed scattered distributions with offshore distance and salinity, which implies that in a complex coast with many small bays and islands, the area has been influenced by local currents and groundwater–seawater mixing. SGD in the Ubatuba area is fed by coastal contaminated groundwater and re-circulated seawater (with small admixtures of groundwater), which claims for potential environmental concern with implications on the management of freshwater resources in the region.     

An overview of Calanus helgolandicus ecology in European waters

We review current knowledge and understanding of the biology and ecology of the calanoid copepod Calanus helgolandicus in European waters, as well as provide a collaborative synthesis of data from 18 laboratories and 26 sampling stations in areas distributed from the northern North Sea to the Aegean and Levantine Seas. This network of zooplankton time-series stations has enabled us to collect and synthesise seasonal and multi-annual data on abundance, body size, fecundity, hatching success and vertical distribution of C. helgolandicus. An aim was to enable comparison with its congener Calanus finmarchicus, which has been studied intensively as a key component of European and north east Atlantic marine ecosystems. C. finmarchicus is known to over-winter at depth, whereas the life-cycle of C. helgolandicus is less well understood. Overwintering populations of C. helgolandicus have been observed off the Atlantic coast between 400 and 800 m, while in the Mediterranean there is evidence of significant deep-water populations at depths as great as 4200 m. The biogeographical distribution of C. helgolandicus in European coastal waters covers a wide range of habitats, from open ocean to coastal environments, and its contribution to mesozooplankton biomass ranges from 6% to 93%. Highest abundances were recorded in the Adriatic and off the west coast of Spain. C. helgolandicus is generally found in 9-20 °C water, with maximum abundances from 13-17 °C. In contrast, C. finmarchicus is found in cooler water between 0 and 15 °C, with peak abundances from 0 to 9 °C. As water has warmed in the North Atlantic over recent decades, the range of C. helgolandicus and its abundance on the fringes of its expanding range have increased. This review will facilitate development of population models of C. helgolandicus. This will not only help answer remaining questions but will improve our ability to forecast future changes, in response to a warming climate, in the abundance and distribution of this important species.     

The oxygen isotope composition of water masses in the northern North Atlantic

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, δ¹⁸O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their δ¹⁸O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of δ¹⁸O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in δ¹⁸O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of ¹⁸O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in δ¹⁸O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ≈250 km³ is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, δ¹⁸O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, δ¹⁸O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in δ¹⁸O–salinity space, whereas NWABW lies between NEADW and LSW on δ¹⁸O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has δ¹⁸O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.     

Distinctive stable isotope ratios in important zooplankton species in relation to estuarine salinity gradients: Potential tracer of fish migration

To assess the potential of stable isotope ratios as an indicator of fish migration within estuaries, stable isotope ratios in important zooplankton species were analyzed in relation to estuarine salinity gradients. Gut contents from migratory juveniles of the euryhaline marine fish Lateolabrax japonicus were examined along the Chikugo River estuary of the Ariake Sea, which has the most developed estuarine turbidity maximum (ETM) in Japan. Early juveniles in March and April preyed primarily on two copepod species; Sinocalanus sinensis at lower salinities and Acartia omorii at higher salinities. Late juveniles (standard length > 40 mm) at lower salinities preyed exclusively on the mysid Acanthomysis longirostris until July and complementarily on the decapod Acetes japonicus in August. These prey species were collected along the estuary during the spring–summer seasons of 2003 and 2004, and their carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) were evaluated. The δ¹³C values of prey species were distinct from each other and were primarily depleted within and in close proximity to the ETM (salinity < 10); S. sinensis (−26.6‰) < Acanthomysis longirostris (−23.3‰) < Acartia omorii (−21.1‰) < Acetes japonicus (−18.5‰). The overall gradient of δ¹³C with salinity occurred for all prey species and showed minor temporal fluctuations, while it was not directly influenced by the δ¹³C values in particulate organic matter along the estuary. In contrast to δ¹³C, the δ¹⁵N values of prey species did not exhibit any clear relationship with salinity. The present study demonstrated that δ¹³C has the potential for application as a tracer of fish migration into lower salinity areas including the ETM.     

Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions:

1.

Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799-811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 ± 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 ± 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 ± 0.02 Sv of freshwater equatorward.In complete contrast, almost all of the 0.61 ± 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0.1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 ± 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (−0.06 ± 0.04 Sv).This northern-southern hemisphere asymmetry is likely a consequence of the “Drake Passage” effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research I 42(4), 477-500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters .

2.

The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32°S, freshwater import (0.28 ± 0.04 Sv) comes from the Pacific through Bering Strait (0.06 ± 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 ± 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 ± 0.02 Sv): from saline Benguela Current surface water (−0.05 ± 0.01 Sv), fresh AAIW (0.06 ± 0.01 Sv) and fresh AABW/LCDW (0.01 ± 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget.For the Indian Ocean north of 32°S, import of the required 0.37 ± 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 ± 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 ± 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (−0.04 ± 0.03 Sv).The Pacific north of 28°S is essentially neutral with respect to freshwater, −0.04 ± 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (−0.07 ± 0.02 Sv), export to the Indian through the Indonesian Throughflow (−0.17 ± 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (−0.03 ± 0.03 Sv), and import of 0.23 ± 0.04 Sv from the Southern Ocean via the shallow gyre circulation.

3.

Bering Strait’ssmall freshwater transport of <0.1 Sv helps maintains the Atlantic-Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity.

     

Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical North Pacific waters

Isotopic analyses of nitrate by the denitrifier method, and indeed by many other analytical methods, do not discriminate between nitrate and nitrite. For samples containing both chemical species, accurate isotopic analysis of nitrate requires either removal of nitrite or independent isotopic analysis of nitrite and subtraction of its contribution to the mixed isotopic signal. This study evaluates the application of a variety of available analytical approaches to the isotopic analysis of mixed nitrate and nitrite solutions, with the goal of producing accurate coupled isotopic analyses of both nitrate and nitrite. These methods are tested on mixtures of standard solutions of nitrate and nitrite, and then applied to the coupled δ¹⁵N and δ¹⁸O analyses of nitrate and nitrite in waters of the Eastern Tropical North Pacific (ETNP). Results from standard mixtures show that even for extreme values of nitrate and nitrite δ¹⁵N and δ¹⁸O, both nitrite removal by ascorbate and nitrite isotopic analysis and subtraction from the mixed isotopic signal yield nitrate δ¹⁵N and δ¹⁸O values that are close to the expected values. Application of these analyses to samples from the ETNP yielded δ¹⁵N_NO3 and δ¹⁸O_NO3 values as high as 21‰ vs. AIR and 19‰ vs. VSMOW, respectively. Conversely, very low δ¹⁵N values were observed in nitrite, with values ranging from − 7.2 to − 18.5‰ vs. AIR. Removal of nitrite from ETNP samples thus revealed differences of up to 5‰ between NO₃^- and NO₂^- + NO₃^- for both δ¹⁵N and δ¹⁸O. Moreover, the δ¹⁵N offset between co-occurring nitrate and nitrite is greater than expected from the action of denitrification alone and may provide a unique constraint on the processes involved in the cycling of nitrite in and around oxygen deficient zones. Finally, subtraction of the nitrite δ¹⁵N and δ¹⁸O from ETNP samples allows the extension of the Δ(15,18) tracer into suboxic regions containing nitrite. The magnitude and distribution of Δ(15,18) in these samples suggests an important role for nitrite reoxidation in nitrate isotope variations.     

Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007

Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and δ¹⁸O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and δ¹⁸O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO^*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO^*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin.Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30–50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift.The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32–34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30–32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.     

Distributions of Calanus spp. and other mesozooplankton in the Labrador Sea in relation to hydrography in spring and summer (1995-2000)

We collected mesozooplankton samples in the upper 100 m in spring or early summer each year between 1995 and 2000 along a section from Hamilton Bank (Labrador) to Cape Desolation (Greenland), and along additional sections in spring 1997 and early summer 1995. The North Atlantic waters of the central basin were characterised by the presence of the copepods Calanus finmarchicus, Euchaeta norvegica and Scolecithrocella minor and euphausiids. Calanus glacialis, Calanus hyperboreus and Pseudocalanus spp. were associated with the Arctic waters over the shelves. Amongst the other enumerated groups larvaceans were concentrated over the shelves and around the margins. Amphipods, pteropods and the copepods Oithona spp. and Oncaea spp. showed no definable relationships with water masses or bathymetry, while the diel migrant ostracods and chaetognaths were confined to deep water. Metrida longa, also a strong diel migrant, and Microcalanus spp., a mainly deep water species and possible diel migrant, were both sometimes quite abundant on the shelves as well as in the central basin, consistent with their likely Arctic origins.Analysis of community structure along the section across the Labrador Sea indicated that stations could be grouped into five different zones corresponding to: the Labrador Shelf; the Labrador Slope; the western and central Labrador Sea; the eastern Labrador Sea and Greenland Slope; and, the Greenland Shelf. The boundaries between zones varied spatially between years, but community composition was relatively consistent within a given zone and a given season (spring versus early summer). The relationship between community composition and water masses was not entirely straightforward. For example, Labrador Shelf water was generally confined to the shelf, but in spring 2000 when it also dominated the adjacent slope zone, the community in the Labrador Slope zone was similar to those found in other years. Conversely, in spring 1997, when Arctic organisms were unusually abundant in the Labrador Slope zone, there was no increased contribution of shelf water. In addition, North Atlantic organisms were often found on the shelves when no slope or central basin water was present.Although other organisms were sometimes very abundant, the mesozooplankton preserved dry weight biomass was dominated everywhere by the three species of Calanus, which together always accounted for ≥70%. One species, C. finmarchicus, comprised >60% of the total mesozooplankton biomass and >80% of the abundance of large copepods in spring and summer throughout the central Labrador Sea. In western and central regions of the central basin average C. finmarchicus biomass was ca 4 g dry weight m⁻² and average abundance, ca 17?000 m⁻² over both seasons. Highest levels (ca 7 g dry weight m⁻², >100?000 m⁻²) occurred in the northern Labrador Sea in spring and in eastern and southwest regions in early summer. C. hyperboreus contributed ca 20% of the total mesozooplankton biomass in the central basin in spring and <5% in early summer, while C. glacialis accounted for <1%. Over the shelves, C. hyperboreus contributed a maximum of 54% and 3.6 g dry weight m⁻², and C. glacialis, a maximum of 29% and 1 g dry weight m⁻², to the total mesozooplankton biomass.     

A freshwater species wintering in a brackish environment: Habitat selection and diet of Slavonian grebes in the southern Baltic Sea

After the breeding season, Slavonian grebes (Podiceps auritus) leave their freshwater breeding habitats and migrate to wintering grounds in marine or brackish waters. The most important wintering area in northwestern Europe is located in the southern Baltic Sea, with the largest concentrations in the offshore area of the Pommeranian Bight. Analysis of ship-based surveys revealed that the habitat selection of Slavonian grebes in this brackish area is significantly influenced by water depth and bottom sediment type. The grebes prefer shallow waters of 4–14 m depth and occur only over sandy sediments. While the diving depths of endothermic animals is limited due to energetic constraints and thermoregulation, sediment type is regarded to be a proxy for food choice. The diet of Slavonian grebes in the Pomeranian Bight consists mainly of demersal gobies (Gobiidae) that frequently occur over sandy bottom substrates.     

Stable isotope composition of modern bryozoan skeletal carbonate from the Otago Shelf,New Zealand

The oxygen (δ¹⁸O) and carbon (δ¹³C) isotope ratios of 10 species of living Bryozoa collected from the Otago Shelf, New Zealand were analysed to assess the extent to which isotopic equilibrium (relative to inorganic equilibrium isotope fractionation) is attained during the precipitation of skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibrium with respect to environmental conditions, two species (Celleporina grandis and Hippomonavella flexuosa) yield δ¹⁸O_calcite values which indicate significant disequilibrium oxygen isotope fractionation during calcification. Sufficient data are available from one species (C. grandis) to demonstrate that disequilibrium is probably related to kinetic factors associated with diffusion‐controlled transport of HCO₃‐ to the site of calcite precipitation. Carbon isotope signatures indicate significant departures from inorganic isotope equilibrium in all but one bryozoan species (Hippomenella vellicata). Although greater uncertainties are associated with estimates of the isotopic composition of total dissolved inorganic carbon (δ¹³C_SDIC), the data suggest that two factors—kinetic fractionation and incorporation of respiratory CO₂—are important in controlling carbon isotope disequilibrium. Where bryozoan species exhibit evidence for disequilibrium in both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is likely that kinetic factors are primarily responsible for observed departures from carbon isotope equilibrium. In contrast, the probable explanation for those species which display evidence for carbon isotope disequilibrium only, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO₂. Differences between the carbon isotope composition of skeletal elements from the same species and co‐existing species living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO₂ are utilised during calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metabolic, and environmental factors on the carbon isotopic composition of bryozoan skeletal carbonate.     

Effects of sea ice formation and diapycnal mixing on the Okhotsk Sea intermediate water clarified with oxygen isotopes

Processes relating to the formation of dense shelf water and intermediate water in the Okhotsk Sea were studied by examining oxygen isotope ratios (δ¹⁸O), salinity, and temperature. The salinity and δ¹⁸O of the cold dense shelf water on the northern continental shelf showed peculiar relationship. The relationship indicates that 3% of the mixed-layer water, having salinity of 32.6, froze and the remaining 97% became dense shelf water of salinities of more than 33.2 (σ_θ>26.7) during the sea ice formation. The salinity–δ¹⁸O relationship also shows that 20% of the Okhotsk Sea Intermediate Water at the σ_θ=26.8 level was derived from the dense shelf water. The remaining 80% came from the Western Subarctic Pacific water modified by diapycnal mixing of water affected by the surface cooling and freshening within the Okhotsk Sea. The mixing with dense shelf water contributes to only 26% of the temperature difference or 8% of the salinity difference between the original Pacific water and the Okhotsk Sea Intermediate Water at σ_θ=26.8. This result suggests that the cold and less saline properties of the Okhotsk Sea Intermediate Water are produced mainly by diapycnal mixing, rather than by mixing of the Pacific water with the dense shelf water.     

Pteropods in Southern Ocean ecosystems

To date, little research has been carried out on pelagic gastropod molluscs (pteropods) in Southern Ocean ecosystems. However, recent predictions are that, due to acidification resulting from a business as usual approach to CO₂ emissions (IS92a), Southern Ocean surface waters may begin to become uninhabitable for aragonite shelled thecosome pteropods by 2050. To gain insight into the potential impact that this would have on Southern Ocean ecosystems, we have here synthesized available data on pteropod distributions and densities, assessed current knowledge of pteropod ecology, and highlighted knowledge gaps and directions for future research on this zooplankton group.Six species of pteropod are typical of the Southern Ocean south of the Sub-Tropical Convergence, including the four Thecosomes Limacina helicina antarctica, Limacina retroversa australis, Clio pyramidata, and Clio piatkowskii, and two Gymnosomes Clione limacina antarctica and Spongiobranchaea australis. Limacina retroversa australis dominated pteropod densities north of the Polar Front (PF), averaging 60 ind m⁻³ (max = 800 ind m⁻³) and 11% of total zooplankton at the Prince Edward Islands. South of the PF L. helicina antarctica predominated, averaging 165 ind m⁻³ (max = 2681 ind m⁻³) and up to >35% of total zooplankton at South Georgia, and up to 1397 ind m⁻³ and 63% of total zooplankton in the Ross Sea. Combined pteropods contributed <5% to total zooplankton in the Lazarev Sea, but 15% (max = 93%) to macrozooplankton in the East Antarctic. In addition to regional density distributions we have synthesized data on vertical distributions, seasonal cycles, and inter-annual density variation.Trophically, gymnosome are specialist predators on thecosomes, while thecosomes are considered predominantly herbivorous, capturing food with a mucous web. The ingestion rates of L. retroversa australis are in the upper range for sub-Antarctic mesozooplankton (31.2-4196.9 ng pig ind⁻¹ d⁻¹), while those of L. helicina antarctica and C. pyramidata are in the upper range for all Southern Ocean zooplankton, in the latter species reaching 27,757 ng pig ind⁻¹ d⁻¹ and >40% of community grazing impact. Further research is required to quantify diet selectivity, the effect of phytoplankton composition on growth and reproductive success, and the role of carnivory in thecosomes.Life histories are a significant knowledge gap for Southern Ocean pteropods, a single study having been completed for L. retroversa australis, making population studies a priority for this group. Pteropods appear to be important in biogeochemical cycling, thecosome shells contributing >50% to carbonate flux in the deep ocean south of the PF. Pteropods may also contribute significantly to organic carbon flux through the production of fast sinking faecal pellets and mucous flocs, and rapid sinking of dead animals ballasted by their aragonite shells. Quantification of these contributions requires data on mucous web production rates, egestion rates, assimilation efficiencies, metabolic rates, and faecal pellet morphology for application to sediment trap studies.Based on the available data, pteropods are regionally significant components of the Southern Ocean pelagic ecosystem. However, there is an urgent need for focused research on this group in order to quantify how a decline in pteropod densities may impact on Southern Ocean ecosystems.     

Assessment of carbon- and fluorescence-based primary productivity in Ariake Bay,southwestern Japan

A comparative account of primary productivity (PP), in the characteristically turbid and highly dynamic waters of Ariake Bay, measured by ¹³C uptake and fast repetition rate fluorometer (FRRF) was conducted to ensure compatibility between the two methods. Estimates from both methods depicted strong linearity for both short-term (r² > 0.90) and daily (r² = 0.42–0.93) measurements, except in the near-surface (∼0 m) layer. ¹³C-based short-term (1 h; in situ) PP estimates showed similar magnitudes and trend with the instantaneous PP measured by FRRF concurrently. Whereas, unlike short-term measurements, the daily PP estimates from both methods showed large difference, with FRRF-based time integrated daily PP resulting in 1.09–1.82 times higher than the carbon-based daily (24 h; simulated in situ) PP. This difference between daily PP estimates was mainly due to: (1) the temporal variation of water column chlorophyll a (Chl a) because of frequent moving of water mass, and (2) the dissimilarity in ambient light field conditions between the two methods. Results revealed that considering the above two environmental factors invariable over a daylength, fairly close approximation of daily PP, compared to ¹³C-based daily PP, could be obtained from FRRF. Hence, FRRF-based daily PP can be considered as more realistic in this highly dynamic water body like Ariake Bay where water column parameters are subjected to strong temporal variation. The relationship between Chl a-specific photosynthetic rate (P^B) and the corresponding photosynthetically active radiation (PAR) in the water column (PAR–P^B relationship) was found to be linear for FRRF and curvilinear for ¹³C-based measurements in the near-surface layer, for the same intensities of incident PAR, and this is thought to be the primary basis for the higher difference in PP estimates at the near-surface layer. Considering the minor variations in FRRF-based time series of PAR–P^B relationships, a combined and/or instantaneous PAR–P^B relationship in combination with incubation Chl a and light field condition was used to obtain fairly close estimates of daily water column integrated PP from FRRF.     

Spatial variability of epifaunal communities from artificial habitat: Shipwrecks in the Southern Bight of the North Sea

We investigated the cover, community structure and abiotic environment of nine shipwrecks lying at increasing distance from the Belgian coast. Results indicated that all shipwrecks were strongly dominated by cnidarians in terms of biomass and by amphipods in terms of abundances. Based on their epifaunal composition, three groups of shipwrecks could be determined. Metridium senile dominated a species poor community of the coastal sites. On the same sites, a Tubularia larynx community with a more species-rich assemblage was also developing. The T. larynx community had a lower biomass value (102 g AFDW m⁻²) and significantly lower species richness compared to the other sites. The coastal sites were characterized by periodic salinity decreases, large seasonal temperature fluctuation, high total suspended matter load and reduced current velocity. Channel water masses influence the offshore sites causing a more stable temperature and salinity environment, less turbid waters and high current speed. Tubularia indivisa dominated this community, with an average biomass of 229 g AFDW m⁻². Intermediate sites were also dominated by T. indivisa, but a higher biomass (424 g AFDW m⁻²) was observed. They showed intermediate results for the abiotic parameters and fast current velocities. Hypotheses for the observed variation in community structures are discussed in the light of the abiotic characterization of the shipwrecks.     

Variability in the fractionation of stable isotopes during degradation of two intertidal red algae

Macroalgae contribute to intertidal food webs primarily as detritus, with unclear implications for food web studies using stable isotope analysis. We examined differences in the thallus parts of two South African rhodophytes (Gelidium pristoides and Hypnea spicifera) and changes in overall δ¹³C, δ¹⁵N signatures and C:N ratios during degradation in both the field and laboratory. We hypothesized that both degrading macroalgal tissue and macroalgal-derived suspended particulate material (SPM) would show negligible changes in δ¹³C, but enriched δ¹⁵N signatures and lower C:N ratios relative to healthy plants. Only C:N laboratory ratios conformed to predictions, with both species of macroalgae showing decomposition related changes in δ¹³C and significant depletions in δ¹⁵N in both the field and laboratory. In the laboratory, algal tissue and SPM from each species behaved similarly (though some effects were non-significant) but with differing strengths. Gelidium pristoides δ¹³C increased and C:N ratios decreased over time in tissue and SPM; δ¹⁵N became depleted only in SPM. Hypnea spicifera, δ¹³C, δ¹⁵N and C:N ratios all decreased during degradation in both SPM and algae.     

Past,present and future nutrient loads of the North Sea: Causes and consequences

Comprehensive, aggregate nutrient budgets were established for two compartments of the North Sea, the shallow coastal and deeper open regions, and for three different periods, representing pre-eutrophication (∼1950), eutrophication (∼1990) and contemporary (∼2000) phases. The aim was to quantify the major budget components, to identify their sources of variability, to specify the anthropogenic components, and to draw implications for past and future policy. For all three periods, open North Sea budgets were dominated (75%) by fluxes from and to the North-East Atlantic; sediment exchange was of secondary importance (18%). For the coastal North Sea, fluxes during the eutrophication period were dominated by sediment exchange (49% of all inputs), followed by exchange with the open sea (21%), and anthropogenic inputs (19%). Between 1950 and 1990, N-loading of coastal waters increased by a factor of 1.62 and P-loading by 1.45. These loads declined after 1990. Interannual variability in Atlantic inflow was found to correspond to a variability of 11% in nutrient load to the open North Sea. Area-specific external loads of both the open and coastal North Sea were below Vollenweider-type critical loads when expressed relative to depth and flushing. External area-specific load of the coastal North Sea has declined since 1990 from 1.8 to about 1.4 g P m⁻² y⁻¹ in 2000, which is close to the estimate of 1.3 for 1950. N-load declined less, leading to an increase in N/P ratio.     

2008年夏季加拿大海盆永久冰区的淡水累积现象

A combination of δ~(18)O and salinity data was employed to explore the freshwater balance in the Canada Basin in summer 2008.The Arctic river water and Pacific river water were quantitatively distinguished by using different saline end-members.The fractions of total river water,including the Arctic and Pacific river water,were high in the upper 50 m and decreased with depth as well as increasing latitude.In contrast,the fraction of Pacific river water increased gradually with depth but decreased toward north.The inventory of total river water in the Canada Basin was higher than other arctic seas,indicating that Canada Basin was a main storage region for river water in the Arctic Ocean.The fraction of Arctic river water was higher than Pacific river water in the upper 50 m while the opposite was true below 50 m.As a result,the inventories of Pacific river water were higher than those of Arctic river water,demonstrating that the Pacific inflow through the Bering Strait is the main source of freshwater in the Canada Basin.Both the river water and sea-ice melted water in the permanent ice zone were more abundant than those in the region with sea-ice just melted.The fractions of total river water,Arctic river water,Pacific river water increased northward to the north of 82°N,indicating an additional source of river water in the permanent ice zone of the northern Canada Basin.A possible reason for the extra river water in the permanent ice zone is the lateral advection of shelf waters by the Trans-Polar Drift.The penetration depth of sea-ice melted waters was less than 30 m in the southern Canada Basin,while it extended to 125 m in the northern Canada Basin.The inventory of seaice melted water suggested that sea-ice melted waters were also accumulated in the permanent ice zone,attributing to the trap of earlier melted waters in the permanent ice zone via the Beaufort Gyre.     

δ15N of PON and Nitrate as a Clue to the Origin and Transformation of Nitrogen in the Subarctic North Pacific and Its Marginal Sea

Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The ¹⁵N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The ¹⁵N and ¹³C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N₂ fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both ¹⁵N-NO₃ ^– and N* variation in seawater. This model was applied to the observed data set of ¹⁵N-NO₃ ^– and N* in the North Pacific water and estimated the ¹⁵N-NO₃ ^– of primordial nitrate in the North Pacific deep water as 4.8. The North Pacific intermediate water for all stations showed similar ¹⁵N-NO₃ ^– and N* values of 6 and –3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m.     

Tracing suspended organic nitrogen from the Yangtze River catchment into the East China Sea

Total suspended matter was collected along the Yangtze River (Changjiang) and in the East China Sea in April to May and in September 2003, respectively, to study origin and fate of particulate organic nitrogen. Concentrations of particulate organic carbon (POC), nitrogen (PN) and hydrolyzable particulate amino acids (PAA; d- and l-enantiomers) were higher in the Yangtze Estuary than in the river and decreased offshore towards the shelf edge. In the coastal area, higher values of PAA were observed in the surface layer than in the bottom water. Stable carbon isotope ratios (δ¹³C) of POC increased from − 24.4‰ in the river to values around − 21‰ on the East China Sea Shelf. Dominant amino acids were aspartic acid + aspartine (Asx), glutamic acid + glutamine (Glx), glycine, alanine and serine. The proportions of Asx, Glx and isoleucine were higher in the marine than in the riverine samples contrary to the distributions of glycine, alanine, threonine and arginine. The proportions of d-amino acids were highest in the riverine suspended organic matter (6% of PAA) decreasing towards the shelf edge (1.5% of PAA). d-arginine, not reported in natural aquatic samples so far, was the most abundant d-amino acid in the river. The amino acid composition of the particulate organic matter (POM) in the Yangtze River indicates an advanced stage of degradation of POM. Highly degraded organic matter from soils is probably a main source of POM in the Yangtze River, but the relatively high δ¹³C values and low C/N ratios (7.7 ± 1.6) also indicate contribution from anthropogenic sources. The degraded riverine material was a dominant organic matter source in the estuary, where aquatic primary production had only a small overall contribution. In the East China Sea, gradual settling of riverine organic matter and the addition of fresher phytoplankton impacted the amino acid composition and δ¹³C values, and on the outer shelf relatively fresh phytoplankton-derived organic matter dominated.