Deep Sea Research Part II: Topical Studies in Oceanography
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
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)
- et al.
Spring development of phytoplankton biomass and composition in major water masses of the Atlantic sector of the Southern Ocean
Deep-Sea Research
(1997) - et al.
Export productionseasonality and intermittency, and paleoceanographic implications
Palaeogeography, Palaeoclimatology, Palaeoecology
(1990) - et al.
Silicon dynamics within an intense open-ocean diatom bloom in the pacific sector of the southern ocean
Deep-Sea Research II
(2001) - et al.
The role of a silicate pump in driving production
Deep-Sea Research
(1995) - et al.
Organic carbon fluxes in the Atlantic and Southern Oceanrelationship to primary production compiled from satellite radiometer data
Deep-Sea Research
(2000) - et al.
Particles fluxes and moving fluidsexperience from synchronous trap collections in the Sargasso Sea
Deep-Sea Research
(1992) - et al.
Large scale aperture time-series sediment traps; design, objectives, construction and application
Deep-Sea Research
(1988) - et al.
Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170°W
Deep-Sea Research II
(2000) The abiotically driven biological pump in the ocean and short-term fluctuations in atmospheric CO2 contents
Global and Planetary Change
(1993)- et al.
An automated leaching method for the determination of opal in sediments and particulate matter
Deep-Sea Research I
(1993)
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
Transport of water through the Drake Passage
Deep-Sea Research
Responses of Southern Ocean phytoplankton to the addition of trace metals
Deep-Sea Research
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
Climatic changes and the carbon cycle in the Southern Oceana step forward
Deep-Sea Research
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
Seasonal particle flux in the Bransfield Strait, Antarctica
Deep-Sea Research I
Diatom distribution in Southern Ocean surface sediments (Atlantic sector)implications for paleoenvironmental reconstructions
Palaeogeography, Palaeoclimatology, Palaeoecology
Ocean primary production 2. Estimation at global scale from satellite (Coastal Zone Colour Scanner) chlorophyll
Global Biogeochemical Cycles
Primary production in Southern Ocean waters
Journal of Geophysical Research
Field assessment of sediment trap efficiency under varying flow conditions
Journal of Marine Research
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
Primary productivity and particle fluxes on a transect of the equator at 153 W in the Pacific Ocean
Deep-Sea Research
A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization
Nature
The Si:C:N ratio of marine diatomsinterspecific variability and the effect of some environmental variables
Journal of Phycology
Application of the modern analog technique to marine Antarctic diatomsreconstruction of maximum sea ice extent at the Last Glacial Maximum
Paleoceanography
Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean
Nature
In situ settling speeds of marine snow aggregates below the mixed layerBlack Sea and Gulf of Mexico
Deep-Sea Research
Biogeochemical control and feedbacks on ocean primary production
Science
Light–temperature interactions on the growth of Antarctic diatoms
Polar Biology
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