Control of the phytoplankton response during the SAGE experiment: A synthesis

The SOLAS Air-Sea Gas Exchange (SAGE) experiment was conducted in Sub-Antarctic waters off the east coast of the South Island of New Zealand in the late summer of 2004. This mesoscale iron enrichment experiment was unique in that chlorophyll a (chl a) and primary productivity were only 2× OUT stations values toward the end of the experiment and this enhancement was due to increased activity of non-diatomaceous species. In addition, this enhancement in activity appeared to occur without a significant build up of particulate organic carbon. Picoeukaryotes (<2 ??m) were the only members of the phytoplankton assemblage that showed a statistically significant increase, a doubling in biomass. To better understand the controls of phytoplankton growth and biomass, we present results from a series of on-deck perturbation experiments conducted during SAGE. Results suggest that the pico-dominated phytoplankton assemblage was only weakly inhibited by iron. Diatoms with high growth rates comprised a small (<1%) fraction of the phytoplankton assemblage, were likely iron limited, and potentially further limited by silicic acid and therefore did not significantly contribute to bloom dynamics. On deck experiments and comparison of SAGE with other iron addition experiments suggested that neither light availability nor deep mixed layers limited phytoplankton growth. Although no substantial increase in grazing rate or specific phytoplankton growth rate was detected, microzooplankton biomass doubled over SAGE as a result of an increase in cell size. The importance of microzooplankton grazing was highlighted by the fact that they were capable of consuming 15-49% of the total phytoplankton production per day. Removal was highest on eukaryotic picophytoplankton production with a mean value of 72% (29-143%). Patch dilution played an important role during SAGE; the mean patch net algal growth:dilution rate, 1.13 (0.4-2.2) was the lowest reported for a mesoscale iron enrichment experiment. Phytoplankton biomass, estimated by chlorophyll a, only accumulated when phytoplankton growth exceeded grazing and when net algal growth exceeded dilution rate. The SAGE results highlight the function of the smallest phytoplankton size fraction described by the ecumenical Iron Hypothesis. Thus, adding iron to HNLC-low silicic acid regions during certain times of the year may simply transfer more carbon through the microbial food web. A primary implication of this study is that any iron-mediated gain in fixed carbon with this set of environmental conditions has a high probability of being recycled in surface waters.     

The planktonic food web of the Bizerte lagoon (south-western Mediterranean) during summer: I. Spatial distribution under different anthropogenic pressures

The structure and the trophic interactions of the planktonic food web were investigated during summer 2004 in a coastal lagoon of south-western Mediterranean Sea. Biomasses of planktonic components as well as bacterial and phytoplankton production and grazing by microzooplankton were quantified at four stations (MA, MB, MJ and R) inside the lagoon. Station MA was impacted by urban discharge, station MB was influenced by industrial activity, station MJ was located in a shellfish farming sector, while station R represented the lagoon central area. Biomasses and production rates of bacteria (7–33 mg C m⁻³; 17.5–35 mg C m⁻³ d⁻¹) and phytoplankton (80–299 mg C m⁻³; 34–210 mg C m⁻³ d⁻¹) showed high values at station MJ, where substantial concentrations of nutrients (NO₃⁻ and Si(OH)₄) were found. Microphytoplankton, which dominated the total algal biomass and production (>82%), were characterized by the proliferation of several chain-forming diatoms. Microzooplankton was mainly composed of dinoflagellates (Torodinium, Protoperidinium and Dinophysis) and aloricate (Lohmaniellea and Strombidium) and tintinnid (Tintinnopsis, Tintinnus, Favella and Eutintinnus) ciliates. Higher biomass of these protozoa (359 mg C m⁻³) was observed at station MB, where large tintinnids were encountered. Mesozooplankton mainly represented by Calanoida (Acartia, Temora, Calanus, Eucalanus, Paracalanus and Centropages) and Cyclopoida (Oithona) copepods, exhibited higher and lower biomasses at stations MA/MJ and MB, respectively. Bacterivory represented only 35% of bacterial production at stations MB and R, but higher fractions (65–70%) were observed at stations MA and MJ. Small heterotrophic flagellates and aloricate ciliates seemed to be the main controllers of bacteria. Pico- and nanophytoplankton represented a significant alternative carbon pool for micrograzers, which grazing represented 67–90% of pico- and nano-algal production in all stations. Microzooplankton has, however, a relatively low impact on microphytoplankton, as ≤45% of microalgal production was consumed in all stations. This implies that an important fraction of diatom production would be channelled by herbivorous meso-grazers to higher consumers at stations MA and MJ where copepods were numerous. Most of the microalgal production would, however, sink particularly at station MB where copepods were scare. These different trophic interactions suggest different food web structures between stations. A multivorous food web seemed to prevail in stations MJ and MA, whereas microbial web was dominant in the other stations.     

Influence of allochthonous organic matter on bacterioplankton biomass and activity in a eutrophic,sub-tropical estuary

Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates, and growth efficiencies were studied in the Northern region of the Cananéia–Iguape estuarine system, which has recently experienced an intense eutrophication due to anthropogenic causes. Two surveys were carried out during spring and neap tide periods of the dry season of 2005 and the rainy season of 2006. This region receives large freshwater inputs with organic seston and phosphate concentrations that reach as high as 1.0 mg l⁻¹ and 20.0 μM, respectively. Strong decreasing gradients of seston and dissolved inorganic nutrients were observed from the river/estuary boundary to the estuary/coastal interface. Gradients were also observed in phytoplankton and bacterial production rates. The production rates of phytoplankton were 5.6-fold higher (mean 8.5 μg C l⁻¹ h⁻¹) during the dry season. Primary production rates (PP) positively correlated with salinity and euphotic depth, indicating that phytoplankton productivity was light-limited. On the other hand, bacterial biomass (BB) and production rates (BP) were 1.9- and 3.7-fold higher, respectively, during the rainy season, with mean values of up to 40.4 μg C l⁻¹ and 7.9 μg C l⁻¹ h⁻¹, respectively. Despite such a high BP, bacterial abundance remained <2 × 10⁶ cells ml⁻¹, indicating that bacterial production and removal were coupled. Mean specific growth rates ranged between 0.9 and 5.5 d⁻¹. BP was inversely correlated with salinity and positively correlated with temperature, organic matter, exopolymer particles, and particulate-attached bacteria; this last accounted for as much as 89.6% of the total abundance. During the rainy season, BP was generally much higher than PP, and values of BP/PP > 20 were registered during high freshwater input, suggesting that under these conditions, bacterial activity was predominantly supported by allochthonous inputs of organic carbon. In addition, BB probably represented the main pathway for the synthesis of high-quality (low C:N) biomass that may have been available to the heterotrophic components of the plankton food web, particularly nanoheterotrophs.     

Selective uptake of prokaryotic picoplankton by a marine sponge (Callyspongia sp.) within an oligotrophic coastal system

Marine sponges are key players in the transfer of carbon from the pelagic microbial food web into the benthos. Selective uptake of prokaryotic picoplankton (<2 μm) by a demosponge (Callyspongia sp.), and carbon flux through this process, were examined for the first time in the oligotrophic coastal waters of southwestern Australia, where sponge abundance and biodiversity ranks among the highest in the world. Water sampling and flow rate measurements were conducted over five sampling occasions following the InEx method of Yahel et al. (2005), with heterotrophic bacteria and autotrophic Synechococcus cyanobacteria identified and enumerated by flow cytometry. Callyspongia sp. demonstrated high filtration efficiencies, particularly for high DNA (HDNA) bacteria (up to 85.3% in summer 2008) and Synechococcus (up to 91.1% in autumn 2007), however efficiency varied non-uniformly with time and food type (p < 0.01). Overall filtration efficiency for Synechococcus (86.6 ± 6.3%; mean ± s.d.) was always significantly higher (p < 0.05) than for low DNA (LDNA) bacteria (40 ± 17.2%), except during winter 2007 (p = 0.14) when ambient Synechococcus concentrations were lowest. When compared to ambient abundances of the different food types, Callyspongia sp. exhibited consistently negative selectivity for LDNA bacteria and positive selectivity for Synechococcus, while HDNA bacteria was generally a neutral or positive selection. The total carbon removal rate (sum of all prokaryotic picoplankton cells), calculated on a per unit area basis, varied significantly with time (p < 0.01), with lowest rates recorded during the winter (0.5 ± 0.4–0.6 ± 0.8 mg C m⁻² d⁻¹) and highest values recorded in summer (3.5 ± 1.9 mg C m⁻² d⁻¹). These flux estimates quantify the role of a demosponge species in the ultimate fate of prokaryotic picoplankton within the nearshore food webs of southwestern Australia, and support the conclusion that sponges actively select food particles that optimise their nutritional intake.     

Did dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?

An in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125 d⁻¹, with a second estimate from a model of the decline in peak SF₆ concentration yielding a higher lateral dilution rate of 0.16-0.25 d⁻¹. The model was tested on the SOIREE SF₆ dataset and provided a lateral dilution of 0.07 d⁻¹, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF₆ patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3-4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128 d⁻¹. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50-65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3-6 and 10-14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition.     

Temporal changes in community composition of heterotrophic bacteria during in situ iron enrichment in the western subarctic Pacific (SEEDS-II)

Little is known about the effects of iron enrichment in high-nitrate low-chlorophyll (HNLC) waters on the community composition of heterotrophic bacteria, which are crucial to nutrient recycling and microbial food webs. Using denaturing gradient gel electrophoresis (DGGE) of 16S rDNA fragments, we investigated the heterotrophic eubacterial community composition in surface waters during an in situ iron-enrichment experiment (SEEDS-II) in the western subarctic Pacific in the summer of 2004. DGGE fingerprints representing the community composition of eubacteria differed inside and outside the iron-enriched patch. Sequencing of DGGE bands revealed that at least five phylotypes of α-proteobacteria including Roseobacter, Cytophaga-Flavobacteria-Bacteroides (CFB), γ-proteobacteria, and Actinobacteria occurred in almost all samples from the iron-enriched patch. Diatoms did not bloom during SEEDS-II, but the eubacterial composition in the iron-enriched patch was similar to that in diatom blooms observed previously. Although dissolved organic carbon (DOC) accumulation was not detected in surface waters during SEEDS-II, growth of the Roseobacter clade might have been particularly stimulated after iron additions. Two identified phylotypes of CFB were closely related to the genus Saprospira, whose algicidal activity might degrade the phytoplankton assemblages increased by iron enrichment. These results suggest that the responses of heterotrophic bacteria to iron enrichment could differ among phylotypes during SEEDS-II.     

Nitrogen uptake and primary productivity rates in the Mid-Atlantic Bight (MAB)

Nutrient concentrations, primary productivity, and nitrogen uptake rates were measured in coastal waters of the Mid-Atlantic Bight over a two-year period that included measurements from all four seasons. In order to assess carbon productivity and nitrogen demand within the context of the physical environment, the region was divided into three distinct hydrographic regimes: the Chesapeake and Delaware Bay outflow plumes (PL), the southern Mid-Atlantic shelf influenced by the Gulf Stream (SS), and the mid-shelf area to the north of the Chesapeake Bay mouth (MS). Annual areal rates of total nitrogen (N) uptake were similar across all regions (10.9 ± 2.1 mol N m⁻² y⁻¹). However, annual areal rates of net primary productivity were higher in the outflow plume region (43 mol C m⁻² y⁻¹), than along the Mid-Atlantic shelf and in areas influenced by the Gulf Stream (41 and 34 mol C m⁻² y⁻¹, respectively). Rates of net primary productivity were not well correlated with Chl a concentrations and were uncoupled with net N uptake rates. Seasonally averaged annual areal rates of net primary productivity for the Mid-Atlantic Bight measured in this study were higher than those calculated in previous decades and provide important validation information for biogeochemical models and satellite remote sensing algorithms developed for the region.     

In vitro simulation of oxic/suboxic diagenesis in an estuarine fluid mud subjected to redox oscillations

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 μmol L⁻¹ h⁻¹, dissolved inorganic carbon (DIC) production of 9–12 μmol L⁻¹ h⁻¹, very low NH₄⁺ and Mn²⁺ concentrations, and constant NO₃⁻ production rates (0.4 - 0.7 μmol L⁻¹ h⁻¹) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6 μmol L⁻¹ h⁻¹ and very fast NO₃⁻ consumption (5.6–6.3 μmol L⁻¹ h⁻¹) and NH₄⁺ production (1.4–1.5 μmol L⁻¹ h⁻¹). The latter rates were positively correlated to NO₃⁻ concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO₃⁻ concentrations and DIC and NH₄⁺ productions of less than 1.3 and 0.1 μmol L⁻¹ h⁻¹, respectively. Mn²⁺ and CH₄ were produced at constant rates (respectively 0.3 and 0.015 μmol L⁻¹ h⁻¹) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36 h following anoxic to oxic switches) showed rapid NH₄⁺ and Mn²⁺ decreases to zero (1.6 and 0.8–2 μmol L⁻¹ h⁻¹, respectively) and very fast NO₃⁻ production (3 μmol L⁻¹ h⁻¹). This NO₃⁻ production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.     

Coastal Mediterranean plankton stimulation dynamics through a dust storm event: An experimental simulation

An enhancement of aeolian inputs to the ocean due to a future increase in aridity in certain parts of the world is predicted from global change. We conducted an experimental simulation to assess the biological response of NW Mediterranean coastal surface waters to an episodic dust addition. On the assumption that planktonic growth was limited by phosphorus, dust effects were compared to those induced by equivalent enrichments of phosphate. The experiment analyzed the dynamics of several parameters during one week: inorganic nutrients, total and fractioned chlorophyll a, bacterial abundance, phytoplankton species composition, abundance of autotrophic and heterotrophic flagellates, particulate organic carbon and particulate organic nitrogen. The maximum addition of dust (0.5 g dust L⁻¹) initiated an increase in bacterial abundance. After 48 h, bacterial numbers decreased due to a peak in heterotrophic flagellates and a significant growth of autotrophic organisms, mainly nanoflagellates but also diatoms, was observed. Conversely, lower inputs of dust (0.05 g dust L⁻¹) and phosphate enrichments (0.5 μmol PO₄³⁻ L⁻¹) only produced increases in phototrophic nanoflagellates. In our experiment, dust triggered bacterial growth, changed phytoplankton dynamics and affected the ratio of autotrophic to heterotrophic biomass, adding to the variability in the sources that affect system dynamics, energy and carbon budgets and ultimately higher trophic levels of the coastal marine food web.     

The fate of production in the central Arctic Ocean - top-down regulation by zooplankton expatriates?

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89-88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50-150 mg C m⁻² d⁻¹ (mean 93 mg C m⁻² d⁻¹, n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03-0.3 mg Chl m⁻³ in the upper 20 m and <0.02 mg Chl m⁻³ below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7-5.3 × 10⁵, mean 4.1 × 10⁵ cells ml⁻¹ in the upper 20 m and 1.3-3.7 × 10⁵, mean 1.9 × 10⁵ cells ml⁻¹ below) and Chl concentrations were closely correlated (r = 0.75). Mineral nutrients (3 μmol NO₃ l⁻¹, 0.45 μmol PO₄ l⁻¹, 4-5 μmol SiO₄ l⁻¹) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30-105 (mean 53) mg C m⁻³ and PON ∼5.4-14.9 (mean 8.2) mg N m⁻³ with no clear vertical trend. The vertical flux of POC in the upper 30-100 m water column was ∼37-92 (mean 55) mg C m⁻² d⁻¹ without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m⁻², mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m⁻²). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m⁻² d⁻¹, being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis, respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.     

Microbial plankton abundance and heterotrophic activity across the Central Atlantic Ocean

The role of microorganisms in the transfer of carbon of marine systems is very important in open oligotrophic oceans. Here, we analyze the picoplankton structure, the heterotrophic bacterioplankton activity, and the predator-prey relationships between heterotrophic bacteria and nanoflagellates during two large scale cruises in the Central Atlantic Ocean (∼29°N to ∼40°S). Latitud cruises were performed in 1995 between March-April and October-November. During both cruises we crossed the regions of different trophic statuses; where we measured different biological variables both at the surface and at the deep chlorophyll maximum (DCM). The concentration of chlorophyll a varied between 0.1 and 0.8 mg m⁻³, the abundance of heterotrophic bacteria varied between <1.0 × 10⁵ and >1.0 × 10⁶ cells ml⁻¹, and that of heterotrophic nanoflagellates between <100 and >1.0 × 10⁴ cells ml⁻¹. The production of heterotrophic bacteria varied more than three orders of magnitude between <0.01 and 24 μgC L⁻¹ d⁻¹; and the growth rates were in the range <0.01-2.1 d⁻¹. In the Latitud-II cruise, Prochlorococcus ranged between <10³ and >3 × 10⁵ cells ml⁻¹, Synechococcus between <100 and >1.0 × 10⁴ cells ml⁻¹, and picoeukaryotes between <100 and >10⁴ cells ml⁻¹.Two empirical models were used to learn more about the relationship between heterotrophic bacteria and nanoflagellates. Most bacterial production was ingested when this production was low, the heterotrophic nanoflagellates could be controlled by preys during Latitud-I cruise at the DCM, and by predators in the surface and in the Latitud-II cruise. Our results were placed in context with others about the structure and function of auto- and heterotrophic picoplankton and heterotrophic nanoplankton in the Central Atlantic Ocean.     

Trophic transfer of methylmercury and trace elements by tropical estuarine seston and plankton

Methylmercury (MeHg) and trace elements (TE), mercury, selenium, cadmium, lead and copper, were determined in a microbial loop composed by three size classes of autotrophic and heterotrophic microorganism samples, 1.2–70 μm (seston, SPM), 70–290 μm (microplankton) and ≥290 μm (mesoplankton) from five sampling stations within a polluted eutrophic estuary in the Brazilian Southeast coast and one external point under the influence of the bay. TE concentrations were within the range reported for marine microorganisms from uncontaminated locations. Microplankton was primarily composed of proto-zooplankton and diatoms (>90%) while approximately 50% of mesoplankton was composed mainly of copepods. MeHg and TE in samples did not differ among the five sampling stations within the bay. Cd, Pb and Cu in seston were higher in the stations sampled inside Guanabara Bay (0.67 μg Cd g⁻¹, 9.26 μg Pb g⁻¹, 8.03 μg Cu g⁻¹) than in the external one (0.17 μg Cd g⁻¹, 3.98 μg Pb g⁻¹ and 2.09 μg Cu g⁻¹). Hg, MeHg and Se did not differ among the five points within the more eutrophic waters of the estuary and the external sampling station. The trophic transfer of MeHg and Se was observed between trophic levels from prey (seston and microplankton) to predator (mesoplankton). The successive amplification of the ratios of MeHg to Hg with increasing trophic levels from seston (43%), to microplankton (59%) and mesoplankton (77%) indicate that biomagnification may be occurring along the microbial food web. Selenium, that is efficiently accumulated by organisms through trophic transference, was biomagnified along the microbial food web, while Hg, Cd, Pb, Cu did not present the same behavior. Concentrations differed between the three size classes, indicating that MeHg and TE accumulation were size-dependent. MeHg and TE concentrations were not related to the taxonomic groups' composition of the planktonic microorganisms. Results suggest the importance of the role of the trophic level and microorganism size in regulating element transfers. Eutrophication dilution may provide a process-oriented explanation for lower MeHg and TE accumulation by the three size classes of microorganisms collected at the five sampling stations within the bay.     

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.     

The annual silica cycle of the North Pacific subtropical gyre

Silica cycling in the upper 175 m of the North Pacific Subtropical Gyre was examined over a two year period (January 2008-December 2009) at the Hawaii Ocean Time-series (HOT) station ALOHA. Silicic acid concentrations in surface waters ranged from 0.6 to 1.6 ??M, exhibiting no clear seasonal trends. Biogenic silica concentrations and silica production rates increased by an order of magnitude each summer following stratification of the upper 50 m reaching values of 157 nmol Si L⁻¹ and 81 nmol Si L⁻¹ d⁻¹, in 2008 and 2009, respectively. Sea surface height anomalies together with analyses of variability in isothermal surfaces at 150-175 m indicated that the summer periods of elevated biogenic silica were associated with anticyclonic mesoscale features during both years. Lithogenic silica concentrations increased in the spring during the known period of maximum atmospheric dust concentrations with maximum values of 36 nmol Si L⁻¹ in the upper 10 m. Dust deposition would enhance levels of dissolved iron in surface waters, but there was no response of diatom biomass or silica production to increases in near-surface ocean lithogenic silica concentrations suggesting iron sufficiency of diatom silica production rates.Low ambient silicic acid concentrations restricted silica production rates to an average of 43% of maximum potential rates. Si sufficiency only occurred during the summer period when diatom biomass was elevated suggesting that bloom diatoms are adapted to exploit low silicic acid concentrations. Annual silica production at HOT is estimated to be 63 mmol Si m⁻² a⁻¹ with summer blooms contributing 29% of the annual total. Diatoms are estimated to account for 3-7% of total phytoplankton primary productivity, but 9-20% of organic carbon export confirming past suggestions that diatoms are relatively minor contributors to primary productivity and autotrophic biomass, but important contributors to new and export production in oligotrophic open-ocean ecosystems.Annual silica production at HOT is nearly 4-fold lower than estimates at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from the 1990s, but annual silica export at the base of the euphotic zone is similar between the two gyres indicating very different balances between silica production and its loss in surface waters. On a relative basis, BATS is a more productive system with respect to silica, where biogenic silica is recycled with high efficiency in surface waters; in contrast the NPSG is a lower productivity system with respect to silica, but where lower recycling efficiency leads to a much higher fraction of new silica production. The two gyres show contrasting long-term trends in diatom biomass as biogenic silica concentrations at HOT have been increasing since 1997, but they have been decreasing at BATS suggesting very different forcing of decadal trends in the contribution of diatoms in carbon cycling between these gyres. Combining the data from both gyres indicates that globally subtropical gyres produce 13 Tmol Si a⁻¹, which is only 51% of previous estimates reducing the contribution of subtropical gyres to 5-7% of global annual marine silica production.     

Sedimentary iron inputs stimulate seagrass (Posidonia oceanica) population growth in carbonate sediments

The relationship between sedimentary Fe inputs and net seagrass population growth across a range of Posidonia oceanica meadows growing in carbonate Mediterranean sediments (Balearic Islands, Spain; SE Iberian Peninsula, Spain; Limassol, Cyprus; Sounion, Greece) was examined using comparative analysis. Sedimentary Fe inputs were measured using benthic sediment traps and the net population growth of P. oceanica meadows was assessed using direct census of tagged plants. The meadows examined ranged from meadows undergoing a severe decline to expanding meadows (specific net population growth, from −0.14 yr⁻¹ to 0.05 yr⁻¹). Similarly, Fe inputs to the meadows ranged almost an order of magnitude across meadows (8.6–69.1 mg Fe m⁻² d⁻¹). There was a significant, positive relationship between sedimentary iron inputs and seagrass net population growth, accounting for 36% of the variability in population growth across meadows. The relationship obtained suggested that seagrass meadows receiving Fe inputs below 43 mg Fe m⁻² d⁻¹ are vulnerable and in risk of decline, confirming the pivotal role of Fe in the control of growth and the stability of seagrass meadows in carbonate sediments.     

A preliminary investigation of the fish food web in the Gironde estuary,France, using dietary and stable isotope analyses

Carbon and Nitrogen stable isotopes and stomach contents analyses were used to investigate an estuarine fish food web and identify the contribution of these two methods to the knowledge and understanding of the food web's structure and its functioning. The nine most abundant fish species during the warm period in the Gironde estuary (southwest France, Europe) are examined. Observation of the stomach contents reflects a variety of feeding modes between fish species that consume a diverse assortment of prey, with limited dietary overlap. Nevertheless, when regarding the whole fish community, few prey species dominate the stomach contents. Nitrogen isotope ratios indicate a high intraspecific variability inducing an interspecific covering of the signatures. However, a tendency to δ¹⁵N enrichment according to the trophic position of the species studied was observed. Fish assemblages show a trend towards enrichment of their carbon isotopic signatures from the upper estuary (−20.8 ± 1.8‰) towards the lower estuary (−18.3 ± 1.6‰). But whatever the capture zone considered, most of the individual δ¹³C values for each fish analysed are comprised between −22 and −16‰. Only few specimens, belonging to migratory amphihaline species, have significantly lighter values.     

Influence of river discharge on plankton metabolic rates in the tropical monsoon driven Godavari estuary,India

To examine the influence of river discharge on plankton metabolic balance in a monsoon driven tropical estuary, daily variations in physico-chemical and nutrients characteristics were studied over a period of 15 months (September 2007 to November 2008) at a fixed location (Yanam) in the Godavari estuary, India. River discharge was at its peak during July to September with a sharp decrease in the middle of December and complete cessation thereafter. Significant amount of dissolved inorganic nitrogen (DIN, of 22–26 μmol l⁻¹) and dissolved inorganic phosphate (DIP, of 3–4 μmol l⁻¹) along with suspended materials (0.2–0.5 g l⁻¹) were found at the study region during the peak discharge period. A net heterotrophy with low gross primary production (GPP) occurred during the peak discharge period. The Chlorophyll a (Chl a) varied between 4 and 18 mg m⁻³ that reached maximum levels when river discharge and suspended loads decreased by >75% compared to that during peak period. High productivity was sustained for about one and half months during October to November when net community production (NCP) turned from net heterotrophy to autotrophy in the photic zone. Rapid decrease in nutrients (DIN and DIP by ∼15 and 1.4 μmol l⁻¹, respectively) was observed during the peak Chl a period of two weeks. Chl a in the post monsoon (October–November) was negatively related to river discharge. Another peak in Chl a in January to February was associated with higher nutrient concentrations and high DIN:DIP ratios suggest possible external supply of nitrogen into the system. The mean photic zone productivity to respiration ratio (P:R) was 2.38 ± 0.24 for the entire study period (September 2007–November 2008). Nevertheless, the ratio of GPP to the entire water column respiration was only 0.14 ± 0.02 revealing that primary production was not enough to support water column heterotrophic activity. The excess carbon demand by the heterotrophs could be met from the allochthonous inputs of mainly terrestrial origin. Assuming that the entire phytoplankton produced organic material was utilized, the additional terrestrial organic carbon supported the total bacterial activity (97–99%) during peak discharge period and 40–75% during dry period. Therefore, large amount of terrestrial organic carbon is getting decomposed in the Godavari estuarine system.     

Balance Between Autotrophic and Heterotrophic Components and Processes in Microbenthic Communities of Sandy Sediments: A Field Study

The microscopic community of a microtidal sandy sediment on the Swedish west coast was studiedin situat two depths (0·5 and 4 m) on four occasions (January, April, August and October). Biomass of microalgae, bacteria, ciliates and meiofauna, as well as primary and bacterial productivity, were quantified. Meiofaunal grazing on algae and bacteria was measured simultaneously by radiolabelling intact sediment cores. Autotrophic biomass dominated the microbial community at both depths and on all sampling occasions, accounting for 47–87% of the microbial biomass. Meiofauna contributed 10–47%, while bacteria and ciliates together made up less than 6%. The microflora was dominated by attached (epipsammic) diatoms, but occasional ‘ blooms ’ of motile species occurred. Vital cells of planktonic diatoms contributed to benthic algal biomass in spring. Primary productivity exceeded bacterial productivity in April and August at both depths, while the balance was reversed in October and January. Meiofauna grazed between 2 and 12% of the algal biomass per day, and between 0·3 and 37% of the bacterial biomass. Almost an order of magnitude more algal (17–138 mg C m⁻²) than bacterial (0·1–33 mg C m⁻²) carbon was grazed daily. At the shallow site, primary productivity always exceeded grazing rates on algae, whereas at the deeper site, grazing exceeded primary productivity in October and January. Bacterial productivity exceeded grazing at both depths on all four occasions. Thus, meiofaunal grazing seasonally controlled microalgal, but not bacterial, biomass. These results suggest that, during summer, only a minor fraction (<10%) of the daily microbenthic primary production appears to enter the ‘ small food web ’ through meiofauna. During spring and autumn, however, a much larger fraction (≈30–60%) of primary production may pass through meiofauna. During winter, meiofaunal grazing is a less important link in the shallow zone, but at sublittoral depths, algal productivity may be limiting, and meiofauna depend on other food sources, such as bacteria and detritus.     

Seasonal food web structures and sympagic-pelagic coupling in the European Arctic revealed by stable isotopes and a two-source food web model

We simultaneously followed stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in a two-source food web model to determine trophic levels and the relative importance of open water- and ice-associated food sources (phytoplankton vs. ice algae) in the lower marine food web in the European Arctic during four seasons. The model is based upon extensive seasonal data from 1995 to 2001.Phytoplankton, represented by samples of particulate organic matter from open water (Pelagic-POM) and ice algae, represented by samples from the underside of the ice (Ice-POM), were isotopically different. Ice-POM was generally dominated by the typical ice diatoms Nitzschia frigida and Melosira arctica and was more enriched than Pelagic-POM in ¹³C (δ¹³C = −20‰ vs. −24‰), but less enriched in ¹⁵N (δ¹⁵N = 1.8‰ vs. 4.0‰). However, when dominated by pelagic algae, Ice-POM was enriched in ¹³C and ¹⁵N similarly to Pelagic-POM.The derived trophic enrichment factors for δ¹⁵N (Δ_N = 3.4‰) and δ¹³C (Δ_C = 0.6‰) were similar in both pelagic and sympagic (ice-associated) systems, although the Δ_C for the sympagic system was variable.Trophic level (TL) range for zooplankton (TL = 1.8-3.8) was similar to that of ice fauna (TL = 1.9-3.7), but ice amphipods were generally less enriched in δ¹⁵N than zooplankton, reflecting lower δ¹⁵N in Ice-POM compared to Pelagic-POM. For bulk zooplankton, TLs and carbon sources changed little seasonally, but the proportion of herbivores was higher during May-September than in October and March. Overall, we found that the primary carbon source for zooplankton was Pelagic-POM (mean 74%), but depending on species, season and TL, substantial carbon (up to 50%) was supplied from the sympagic system. For bulk ice fauna, no major changes were found in TLs or carbon sources from summer to autumn. The primary carbon source for ice fauna was Ice-POM (mean 67%), although ice fauna with TL > 3 (adult Onisimus nanseni and juvenile polar cod) primarily utilized a pelagic food source.     

Dissolved organic carbon in the southern Baltic Sea: Quantification of factors affecting its distribution

During a cruise of r/v ‘Oceania’ in May 2006, seven vertical dissolved organic carbon (DOC) concentration profiles were produced against a background of CTD, chlorophyll a (chl a) and phaeopigment concentration profiles. The results indicate distinct vertical and spatial DOC fluctuations, ranging from 248 ± 7 μmol C dm⁻³ at 70 m depth at the westernmost station G/06 to 398 ± 5 μmol C dm⁻³ at 5 m depth at station A/06 in the western Gulf of Gdańsk. DOC concentrations were the highest at 10 m depth, where phytoplankton activity was relatively intensive, as reflected by the active chl a concentration distribution. DOC concentrations decreased towards the sea bottom.