Organic carbon, biogenic silica and diatom fluxes in the marginal winter sea-ice zone and in the Polar Front Region: interannual variations and differences in composition

https://doi.org/10.1016/S0967-0645(02)00009-7Get rights and content

Abstract

Particle fluxes and composition were examined over 5 years at two mooring sites in the Polar Front Region (site PF: 50°09.S, 5°50.E) and in the marginal winter sea-ice zone (site BO: 54°30.S, 3°20.W) in the eastern Atlantic Sector of the Southern Ocean. Seasonality, interannual variability and the magnitude of total mass fluxes were higher at site BO compared to PF. Five-year averages and standard deviations (1σ) of total mass fluxes were 19.6±18.5 and 24.8±29.9 g m−2 at PF and BO, respectively. Peak fluxes at site BO occurred in January 1995, but the highest peak was measured in February 1991 (almost 1300 mg m−2 d−1) followed by post-bloom sedimentation in March through May. This would imply a time shift of several months between the onset of sea-ice retreat in October and major sedimentation events recorded in January/February with the upper BO traps. At site PF, highest fluxes of about 500 mg m−2 d−1 were found between December and March. Blooms at site BO, influenced by sea ice as indicated by diatom species composition, seem to occur more sporadically (e.g., in 1991 and 1995). Annual diatom fluxes were 11.8×106 and 20×106 valves m−2 during the deployments PF3 (1990) and BO1 (1991), respectively. At PF3, Fragilariopsis kerguelensis (37%) and Thalassionema nitzschioides fo1 (26.5%) dominated diatom flux, while F. kerguelensis (29%) and sea-ice-related algae (40%) were the main contributors to total diatom flux at site BO. During deployment BO1, the bloom collected in February was characterized by a very high molar Si:C of 8.8 that decreased almost continuously during the post-bloom phase, reaching a value of 1 in May. This change, however, was not documented in diatom species composition. We obtained a significant linear increase of biogenic opal with organic carbon fluxes at site PF and a highly significant but exponential relationship at site BO. Higher annual total mass fluxes were recorded at site BO, primarily due to elevated opal and lithogenic fluxes, corresponding to a higher silicate availability in the southern Antarctic Circumpolar Current. In contrast, higher mean organic carbon fluxes were obtained at site PF in accordance with elevated primary production and biomass. We obtained a three-fold higher molar Si:C ratio (5-year mean) for sinking particles collected with the upper BO traps (Si:C=4.0) compared to the PF (Si:C=1.3), consistent with the general pattern of Si and Fe availability. In particular at site BO, the Si:C ratios were usually high, even when accounting for organic carbon decay and biogenic silica (BSi) dissolution in the upper water column. At this study site, the Si:C ratios increased with lithogenic fluxes.

Résumé

Les flux et la composition des particules collectées sur deux sites de mouillage du secteur Est Atlantique de l’Océan Austral situés dans la région du Front Polaire (site PF: 50°09.S, 5°5°.E) et dans la zone marginale de la glace de mer hivernale (site BO: 54°30.S, 3°20.W) ont été étudiés sur une période de cinq ans. La saisonnalité, la variabilité inter-annuelle et l’ordre de grandeur des flux de masse sont plus élevés au site BO qu’au site PF. Les valeurs moyennes sur cinq ans et les écarts-types associés (1σ) sont respectivement de 19.6±18.5 g m−2 et de 24.8±29.9 g m−2 pour les sites PF et BO. Au site BO, des fluctuations importantes du flux sont observées en Janvier 1995 mais les valeurs les plus élevées des flux ont été mesurées en Février 1991 (de l’ordre de 1300 mg m−2j−1) suivies d’une sédimentation post-bloom de Mars à Mai. Ce phénomène suggère un décalage temporel de plusieurs mois entre le retrait de la glace de mer, qui débute en Octobre, et les épisodes de sédimentation importants enregistrés en Janvier/Février dans le piège supérieur du site BO. Au site PF, les flux élevées (de l’ordre de 500 mg m−2 j−1) sont observés entre Décembre et Mars. Les périodes de bloom au site BO, influencées par la glace de mer, comme le montrent les assemblages de diatomées, surviennent sporadiquement (par ex. en 1991 et 1995). Les flux annuels de diatomées mesurés pendant les déploiements de PF3 (1990) et de BOI (1991) sont respectivement de 11.8×106 et de 20×166 valves  m−2. Au site PF3, Fragilariopsis kerguelensis (37%) et Thalassionema nitzschioides fo1 (26.5%) dominent les flux de diatomées, alors qu’au site BO, F. kerguelensis (29%) et les algues associées á la glace de mer (40%) contribuent majoritairement au flux total de diatomées. Pendant le déploiement de BO1, le bloom collecté en Février est caractérisé par un rapport Si:C élevé (8.8). La valeur de ce rapport décroı̂t de façon quasi-continue pendant la période post-bloom, pour atteindre la valeur de 1 en Mai. Cette tendance n’a toutefois pas été enregistrée dans la composition des espèces de diatomées. Au site PF, nous avons observé une augmentation linéaire significative des flux d’opale biogénique avec ceux de carbone organique alors qu’au Site BO, ces deux paramètres montrent une relation exponentielle statistiquement très significative. Les flus de masse totaux annuels les plus élevés ont été collectés au site BO et sont dus principalement aux d’opale et lithogéniques importants, associés á une grande disponibilité en silice dans la zone du courant Circumpolaire Antarctique. Par contre au site PF, les valeurs du flux moyen de carbone organique sont plus élevées qu’au site BO en accord avec une production primaire et une biomasse plus élevées. Le rapport Si:C (moyenné sur cinq ans) est plus élevé d’un facteur 3 pour les particules collectées dans le piège supérieur du site BO (Si:C=4) par rapport á celui observé au site PF (Si:C=1.3). Ce résultat est consistant avec la distribution de la disponibilité en Si et en Fer dans les deux zones. En particular, pour le site BO, les valeurs du rapport Si:C restent exceptionnellement élevées, même lorsque la reminenéralisation du carbone organique et la dissolution de la silice biogénique dans les couches supérieures de la colonne d’eau sont prises en compte. A ce site d’étude, les rapports Si:C augmentent avec les flux lithogéniques.

Introduction

The Southern Ocean plays an important role in the past and present carbon cycle. It is a potential candidate for the drawdown of CO2 during glacial times. It has become increasingly evident that presently it takes up significant amounts of anthropogenic CO2 from the atmosphere (Takahashi et al., 1997). However, climate models show that the Southern Ocean carbon cycle could change in the near future in response to rising CO2, surface water temperature and stratification, which may reduce the capacity to take up CO2 of human origin (Tréguer and Pondaven, 2002). Thus, there is increasing interest to better understand the ecology of the Southern Ocean.

Due to generally low to moderately high chlorophyll concentrations and primary productivity (e.g., from satellite data, Antoine et al., 1996), the Southern Ocean has a high but not fully used macronutrient reservoir (HNLC-region; Martin et al., 1990). Several explanations, for instance, low Fe availability or physical processes (i.e. deep mixing) are presently debated to account for this observation. During glacial times, however, high Fe delivered by dust could have stimulated primary productivity in the Southern Ocean and contribute to the observed decrease in atmospheric PCO2 (Martin et al., 1990). The magnitude of primary productivity and the export of organic carbon in the modern Southern Ocean and its contribution to global estimates is still a matter of considerable debate (Arrigo et al., 1998). According to satellite data from SeaWIFS and CZCS, the Southern Ocean has a high variability of biomass and primary productivity both in space and time. Therefore, the conflicting results obtained for primary productivity from standard sampling techniques may in part be due to the various methods and sampling techniques applied in different seasons and years. A high interannual variability of primary productivity and export might be linked to larger scale oceanic and atmospheric variations such as the Antarctic Circumpolar Waves and the El Niño Southern Oscillation.

Biogenic silica (BSi) or opal is the most important constituent within plankton, in sinking matter, and in the sediments below the Antarctic Circumpolar Current (ACC). Most probably, the biological or carbon pump is somehow related to the flux of BSi, which is mainly controlled by the input of silicate into the mixed layer. (‘silicate pump’ model; Dugdale et al., 1995). However, this relationship is not yet fully understood and may vary regionally (Ragueneau et al., 2000). Earlier investigations suggested a strong decoupling of organic carbon and BSi in the Southern Ocean (Nelson et al., 1995), thus limiting the use of BSi for paleoenvironmental reconstructions. Recently, Pondaven et al. (2000) emphasized that the Si:C ratios should be better understood in order to apply BSi for Paleoceanography. Both Takeda (1998) and Hutchins and Bruland (1998) have demonstrated that Fe affects the uptake of silicate and nitrate and the Si:C ratios of diatoms and found that the Si:C ratios are 2–3 times higher under iron limitation. Also Franck et al. (2000) emphasized the potential of both Fe and Si limitation for Si uptake and diatom productivity. In a most recent study, Brzezinski et al. (2002) discussed the role of low Fe to accelerate the onset of Si limitation and reducing the export of organic carbon relative to BSi in the Southern Ocean.

It has become widely accepted that the seasonal retreat of sea ice may induce rapid phytoplankton growth in the meltwater zone (Smith and Nelson, 1986; Sullivan et al., 1988; Moore et al., 2000), thus affecting particle sedimentation. Such meltwater-related blooms may cause a strong seasonal to pulsed sedimentation signal, leading to enhanced export ratios as suggested by Berger and Wefer (1990). Other studies have failed to find so-called ice-edge blooms (de Baar et al., 1995; Smetacek et al., 1997; Bathmann et al., 1997). High biomass standing stocks, primary productivity, and export production in the vicinity of or several 100 km off the ice edge (up to 250 km, Smith and Nelson, 1986) may occur in smaller patches, not always detected by classical and limited sampling techniques in the Southern Ocean. The occurrence of these patches has been suggested to be correlated with the supply of Fe from melting of sea ice (Nolting et al., 1991; Sedwick and DiTullio, 1997), a limiting micronutrient in the Southern Ocean south of the Antarctic Polar Front (APF; Boyd et al., 2000). Under the assumption that during glacial times the Seasonal Ice Zone (SIZ; Tréguer and Jacques, 1992) was expanded (Moore et al., 2000), we might expect that a large ocean area was influenced by particle sedimentation at the retreating ice edge. However, Moore et al. (2000) assumed that due to an expansion of the winter sea-ice area and a summer sea-ice coverage similar to modern conditions, the glacial SIZ was strongly extended. This is based on results from a statistical reconstruction method (Crosta et al., 1998), although questioned by Gersonde and Zielinski (2000). Data from the latter authors for the Atlantic Sector suggest a smaller glacial SIZ compared to the modern one, due to an extended glacial summer sea-ice cover. Besides such discrepancies, the biological processes at the ice edge have not yet been fully investigated and are poorly understood, in particular on time scales longer than days or weeks. Until now, data on seasonal and interannual variability of primary production and export of the Southern Ocean still remain sparse.

In this study we focus on seasonal and interannual variation of particle sedimentation at two mooring sites located in the Atlantic Sector of the Southern Ocean, one located in the Polar Front Region (PFr, site PF) and the other in a zone influenced by sea ice (site BO). Site BO was located more or less close to the marginal winter sea-ice zone (=MWSIZ) during a few months in austral winter and we attribute this site (based on diatom composition) to the boundary region between the Permanently Open Ocean Zone (POOZ) and the SIZ (Tréguer and Jaques, 1992). According to SeaWIFS observations, both study sites can be considered as bloom regions (>1 mg Chla m−3), the former associated with a major hydrographic front (APF), the latter with the receding ice edge, probably delivering also micronutrients such as iron to the spring/summer blooms (e.g., Sedwick and DiTullio, 1997). We study the organic carbon and the BSi fluxes and their relationship in order to better understand the dynamics of the marginal winter sea-ice zone and the PFr. We also provide data on the diatom composition. Finally, we discuss the relationship of particle fluxes to primary productivity, (macro-micro-) nutrient availability, and other environmental variables.

Section snippets

Material and methods

Particle fluxes were determined using large-aperture time-series sediment traps mostly of the Kiel-type (20 cups, 0.5 m2 aperture; Kremling et al., 1996). For a few deployments, Honjo MARK V (1.17 m2) and MARK VI (0.5 m2) traps were used (for description see Honjo and Doherty, 1988), both having comparable aspect ratios to the Kiel-type trap. Sampling cups were poisoned with HgCl2 prior to and after deployment. Pure NaCl was added to the cups filled with filtered seawater to increase the salinity

Oceanographic and biological setting

In Table 2 the main characteristics of the two study sites are summarized. Both sites were located in the ACC regime, which is characterized by high eastward transport of water masses down through the entire water column. Site PF was located close to the APF (Fig. 1), considered to be a broad meandering zone (Moore et al., 1999a). According to the definition of Tréguer and Jacques (1992) and Smetacek et al. (1997), our site PF was located within the PFr. Site BO in the southern Antarctic

Current speeds and trapping efficiencies

Current speeds close to the traps are crucial on determining trapping efficiencies (Gust et al., 1992). Reid and Nowlin (1971) measured current speeds in the ACC subsurface layers to about 1000 m depth of >10–20 cm s−1, which may be quite critical for sampling efficiency. Current speeds were measured with Aanderaa current meters (RCM 5,8) beneath the traps (E. Fahrbach, pers. comm., AWI Bremerhaven). The results, some of which are published in Walter et al. (2001), indicate values between 1–8 cm s−1

Seasonal occurrence of blooms and composition of particles

Total mass fluxes shown in Fig. 2, Fig. 3 were plotted against an annual cycle (for sampling data see Table 1). At site PF, total mass flux reached values around 500 mg m−2 d−1 during austral spring and summer, but there was still some sedimentation in austral fall (Fig. 2). Only in austral winter (July–September), sedimentation was almost negligible at both depth levels. Therefore, seasonality in the PFr is not so clearly expressed compared to other high-productivity sites in the Atlantic Sector

Summary and conclusions

We have examined particle and diatom fluxes and important elemental ratios in the PFr and in marginal winter sea-ice zone (MWSIZ) in the eastern Atlantic Sector of the Southern Ocean. Our main findings, also summarized in Table 5, are as follows:

(1) On a seasonal basis, almost three-fold higher total fluxes were found at site BO. This was mostly due to higher BSi and lithogenic contributions. However, higher organic carbon fluxes were obtained for the PFr, in good agreement with available

Acknowledgements

We thank the crew of RV Polarstern for their assistance and the help for the development of the moorings. We also acknowledge the friendly help and cooperation of E. Fahrbach, G. Rohardt, A. Wisotzki (all at Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven) and G. Ruhland (Geoscience Department, Bremen). We are also indebted to M. Klann for laboratory work. G. Fischer would like to thank O. Romero for helpful comments. The authors also thank the reviewers for helpful

References (62)

  • O Ragueneau

    A review of the Si cycle in the modern oceanrecent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

    Global and Planetary Change

    (2000)
  • J.L Reid et al.

    Transport of water through the Drake Passage

    Deep-Sea Research

    (1971)
  • R Scharek et al.

    Responses of Southern Ocean phytoplankton to the addition of trace metals

    Deep-Sea Research

    (1997)
  • V Smetacek et al.

    Ecology and biogeochemistry of the Antarctic Circumpolar Current during austral springa summary of Southern Ocean JGOFS cruise ANT X/6 of R.V. Polarstern.

    Deep-Sea Research

    (1997)
  • P Tréguer et al.

    Climatic changes and the carbon cycle in the Southern Oceana step forward

    Deep-Sea Research

    (2002)
  • H.J Walter et al.

    Shallow vs. deep-water scavenging of 231Pa and 230Th in radionuclide enriched waters of the Atlantic sector of the Southern Ocean

    Deep-Sea Research I

    (2001)
  • G Wefer et al.

    Seasonal particle flux in the Bransfield Strait, Antarctica

    Deep-Sea Research I

    (1988)
  • U Zielinski et al.

    Diatom distribution in Southern Ocean surface sediments (Atlantic sector)implications for paleoenvironmental reconstructions

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (1997)
  • D Antoine et al.

    Ocean primary production 2. Estimation at global scale from satellite (Coastal Zone Colour Scanner) chlorophyll

    Global Biogeochemical Cycles

    (1996)
  • K.R Arrigo et al.

    Primary production in Southern Ocean waters

    Journal of Geophysical Research

    (1998)
  • E.T Baker et al.

    Field assessment of sediment trap efficiency under varying flow conditions

    Journal of Marine Research

    (1988)
  • U Bathmann et al.

    Short-term variations in particulate matter sedimentation off Kapp Norvegia, Weddell Sea, Antarcticarelation to water mass advection, ice cover, plankton biomass and feeding activity

    Polar Biology

    (1991)
  • R.R Betzer

    Primary productivity and particle fluxes on a transect of the equator at 153 W in the Pacific Ocean

    Deep-Sea Research

    (1984)
  • P.W Boyd

    A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

    Nature

    (2000)
  • M.A Brzezinski

    The Si:C:N ratio of marine diatomsinterspecific variability and the effect of some environmental variables

    Journal of Phycology

    (1985)
  • Brzezinski, M.D., Dickson, M.-L., Nelson, D.M., Sambretto, R., 2002. Ratio of Si, C and N uptake by microplankton in...
  • X Crosta et al.

    Application of the modern analog technique to marine Antarctic diatomsreconstruction of maximum sea ice extent at the Last Glacial Maximum

    Paleoceanography

    (1998)
  • H.J.W de Baar

    Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean

    Nature

    (1995)
  • A.R Dierks et al.

    In situ settling speeds of marine snow aggregates below the mixed layerBlack Sea and Gulf of Mexico

    Deep-Sea Research

    (1997)
  • P.G Falkowski et al.

    Biogeochemical control and feedbacks on ocean primary production

    Science

    (1998)
  • M Fiala et al.

    Light–temperature interactions on the growth of Antarctic diatoms

    Polar Biology

    (1990)
  • Cited by (67)

    • Diatom species fluxes in the seasonally ice-covered Antarctic Zone: New data from offshore Prydz Bay and comparison with other regions from the eastern Antarctic and western Pacific sectors of the Southern Ocean

      2019, Deep-Sea Research Part II: Topical Studies in Oceanography
      Citation Excerpt :

      A detailed description of the diatom species analysis of the 1400 m time-series trap samples is provided in Section 3.2. In the second part of the paper, biogeochemical and diatom species flux data from Station PZB-1 are compared with already published datasets from four sediment trap deployments (Table 1) in distinct settings of the AZ of the Southern Ocean (Fischer et al., 2002; Grigorov et al., 2014; Rigual-Hernández et al., 2015a). Next, we summarize the field experiments and the environmental conditions at each of the stations compared.

    • The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

      2016, Palaeogeography, Palaeoclimatology, Palaeoecology
      Citation Excerpt :

      For these reasons, understanding the processes that diatom assemblages undergo from their initial production in the surface layer until their eventual preservation in the sediments is of critical importance to assess the information preserved in the sedimentary record and to determine the role of diatoms in the biological pump and cycling of silica. Analysis of the diatom assemblages captured by the sediment traps at different depths and in the surface sediments can be used to monitor the changes that diatom assemblages and the labile components of the flux undergo from their initial production in the upper water column until their preservation in the sediments (e.g. Abelmann and Gersonde, 1991; Honjo et al., 2000; Fischer et al., 2002; Romero and Armand, 2010; Grigorov et al., 2014). This coupled approach can help us to understand the processes occurring in the water column and the functioning of the biological pump.

    View all citing articles on Scopus
    View full text