Tracer-based assessment of the origin and biogeochemical transformation of a cyclonic eddy in the Sargasso Sea (original) (raw)

Abstract

1] Mechanisms of nutrient supply in oligotrophic ocean systems remain inadequately understood and quantified. In the North Atlantic Subtropical Gyre, for example, the observed rates of new production are apparently not balanced by nutrient supply via vertical mixing. Mesoscale eddies have been hypothesized as a mechanism for vertical nutrient pumping into the euphotic zone, but the full range and magnitude of biogeochemical impacts by eddies remain uncertain. We evaluated a cyclonic eddy located near Bermuda for its effect on water column biogeochemistry. In the density range s q 26.1 to 26.7, an eddy core with anomalous salinity, temperature, and biogeochemical properties was observed, suggesting that the eddy was not formed with local water (i.e., not formed of the waters surrounding the eddy at the time of observations), hence complicating efforts to quantify biogeochemical processes in the eddy. We combined conservative hydrographic tracers (density versus potential temperature and salinity) and quasi-conservative biogeochemical tracers (density versus NO, PO, and total organic carbon) to propose the origin of the eddy core water to have been several hundred kilometers to the southeast of the eddy location at sampling. By comparing the observed eddy core's biogeochemical properties with those near the proposed origin, we estimate the net changes in biogeochemical properties that occurred. A conservative estimate of export was 0.5 ± 0.34 mol N m À2 via sinking particles, with export occurring prior to our period of direct observation. Our results suggest that biogeochemical signals induced by mesoscale eddies could survive to be transported over long distances, thus providing a mechanism for lateral fluxes of nutrients and AOU (apparent oxygen utilization). Given that the proposed source area of this eddy is relatively broad, and the eddy-mixing history before our sampling is unknown, uncertainty remains in our assessment of the true biogeochemical impact of mesoscale eddies in the gyre.

Loading...

Loading Preview

Sorry, preview is currently unavailable. You can download the paper by clicking the button above.

References (51)

  1. Allen, C. B., J. Kanda, and E. A. Laws (1996), New production and photo- synthetic rates within and outside a cyclonic mesoscale eddy in the North Pacific subtropical gyre, Deep Sea Res., Part I, 43, 917 -936, doi:10.1016/0967-0637(96)00022-2.
  2. Anderson, L. A., and J. L. Sarmiento (1994), Redfield ratios of reminer- alization determined by nutrient data analysis, Global Biogeochem. Cycles, 8, 65 -80, doi:10.1029/93GB03318.
  3. Bates, N. R., and D. A. Hansell (2004), Temporal variability of excess nitrate and nitrogen fixation in the subtropical North Atlantic, Mar. Chem., 84, 225 -241, doi:10.1016/j.marchem.2003.08.003.
  4. Bates, N. R., A. F. Michaels, and A. H. Knap (1996), Seasonal and inter- annual variability of the oceanic carbon dioxide system at the U.S. JGOFS Bermuda Atlantic Time-series Site, Deep Sea Res., Part II, 43, 347 -383, doi:10.1016/0967-0645(95)00093-3.
  5. Broecker, W. S. (1974), ''NO'', a conservative water-mass tracer, Earth Planet. Sci. Lett., 23, 100 -107, doi:10.1016/0012-821X(74)90036-3.
  6. Buesseler, K. O., C. Lamborg, P. Cai, R. Escoube, R. Johnson, S. Pike, P. Masque, D. J. McGillicuddy, and E. Verdeny (2008), Particle fluxes associated with mesoscale eddies in the Sargasso Sea, Deep Sea Res., Part II, 55, 1426 -1444, doi:10.1016/j.dsr2.2008.02.007.
  7. Capone, D. G., J. A. Burns, J. P. Montoya, A. Subramaniam, C. Mahaffey, T. Gunderson, A. F. Michaels, and E. J. Carpenter (2005), Nitrogen fixation by Trichodesmium spp.: An important source of new nitrogen to the tropical and subtropical North Atlantic Ocean, Global Biogeochem. Cycles, 19, GB2024, doi:10.1029/2004GB002331.
  8. Conkright, M. E., R. A. Locarnini, H. E. Garcia, T. D. O'Brien, T. P. Boyer, C. Stephens, and J. I. Antonov (2002), World Ocean Atlas: Objective analyses, data statistics, and figures, CD-ROM documentation, http://odv. awi.de/fileadmin/user_upload/odv/data/WOA01/README.PDF, Natl. Oceanogr. Data Cent., Silver Spring, Md.
  9. Falkowski, P. G., D. Ziemann, Z. Kolber, and P. K. Bienfang (1991), Role of eddy pumping in enhancing primary production in the ocean, Nature, 352, 55 -57, doi:10.1038/352055a0.
  10. Geider, R. J. (1997), Photosynthesis or planktonic respiration?, Nature, 388, 132, doi:10.1038/40536.
  11. Gruber, N., and J. L. Sarmiento (1997), Global patterns of marine nitrogen fixation and denitrification, Global Biogeochem. Cycles, 11, 235 -266, doi:10.1029/97GB00077.
  12. Hansell, D. A., and C. A. Carlson (2001), Biogeochemistry of total organic carbon and nitrogen in the Sargasso Sea: Control by convective overturn, Deep Sea Res., Part II, 48, 1649 -1667, doi:10.1016/S0967-0645(00)00153-3.
  13. Hansell, D. A., N. R. Bates, and D. B. Olson (2004), Excess nitrate and nitrogen fixation in the North Atlantic Ocean, Mar. Chem., 84, 243 -265, doi:10.1016/j.marchem.2003.08.004.
  14. Hansell, D. A., D. Olson, F. Dentener, and L. Zamora (2007), Assessment of excess nitrate development in the subtropical North Atlantic, Mar. Chem., 106, 562 -579, doi:10.1016/j.marchem.2007.06.005.
  15. Harris, R. P., P. Boyd, D. S. Harbour, R. N. Head, R. D. Pingree, and A. J. Pomroy (1997), Physical, chemical and biological features of a cyclonic eddy in the region of 61°10 0 N 19°50 0 W in the North Atlantic, Deep Sea Res., Part I, 44, 1815 -1839, doi:10.1016/S0967-0637(97)00053-8.
  16. Jenkins, W. J. (1988), Nitrate flux into the euphotic zone near Bermuda, Nature, 331, 521 -540, doi:10.1038/331521a0.
  17. Jenkins, W. J., and S. C. Doney (2003), The subtropical nutrient spiral, Global Biogeochem. Cycles, 17(4), 1110, doi:10.1029/2003GB002085.
  18. Koeve, W. (2005), Magnitude of excess carbon sequestration in to the deep ocean and the possible role of TEP, Mar. Ecol. Prog. Ser., 291, 53 -64, doi:10.3354/meps291053.
  19. Law, C. S., A. P. Martin, M. M. Liddicoat, A. J. Watson, K. J. Richards, and E. M. S. Woodward (2001), A Lagrangian SF6 tracer study of an antic- yclonic eddy in the North Atlantic: Patch evolution, vertical mixing and nutrient supply to the mixed layer, Deep Sea Res., Part II, 48, 705 -724, doi:10.1016/S0967-0645(00)00112-0.
  20. Leben, R. R., G. H. Born, and B. R. Engebreth (2002), Operational alti- meter data processing for mesoscale monitoring, Mar. Geod., 25, 3 -18, doi:10.1080/014904102753516697.
  21. Li, Q. P., and D. A. Hansell (2008), Nutrient distribution in baroclinic eddies of the North Atlantic and inferred impacts on biology, Deep Sea Res., Part II, 55, 1291 -1299, doi:10.1016/j.dsr2.2008.01.009.
  22. Li, Q. P., J. Z. Zhang, F. J. Millero, and D. A. Hansell (2005), Continuous colorimetric determination of trace ammonium in seawater with a long- path liquid waveguide capillary cell, Mar. Chem., 96, 73 -85, doi:10.1016/j.marchem.2004.12.001.
  23. Li, Q. P., D. A. Hansell, and J. Z. Zhang (2008), Underway monitoring of nanomolar nitrate plus nitrite and phosphate in oligotrophic seawater, Limnol. Oceanogr. Methods, 6, 319 -326.
  24. Li, Y. H., and T. H. Peng (2002), Latitudinal change of remineralization ratios in the oceans and its implication for nutrient cycles, Global Bio- geochem. Cycles, 16(4), 1130, doi:10.1029/2001GB001828.
  25. Lipschultz, F., N. R. Bates, C. A. Carlson, and D. A. Hansell (2002), New production in the Sargasso Sea: History and current status, Global Bio- geochem. Cycles, 16(1), 1001, doi:10.1029/2000GB001319.
  26. Lukas, R., and F. Santiago-Manddujano (2001), Extreme water mass anom- aly observed in the Hawaii Ocean Time-Series, Geophys. Res. Lett., 28, 2931 -2934, doi:10.1029/2001GL013099.
  27. Martin, A. P., and P. Pondaven (2003), On estimates for the vertical nitrate flux due to eddy pumping, J. Geophys. Res., 108(C11), 3359, doi:10.1029/2003JC001841.
  28. McDonagh, E. L., and K. J. Heywood (1999), The origin of an anomalous ring in the southeast Atlantic, J. Phys. Oceanogr., 29, 2050 -2064, doi:10.1175/1520-0485(1999)029<2050:TOOAAR>2.0.CO;2.
  29. McDowell, S. E. (1986), On the origin of eddies discovered during the POLYMODE local dynamics experiment, J. Phys. Oceanogr., 16, 632 -652, doi:10.1175/1520-0485(1986)016<0632:OTOOED>2.0.CO;2.
  30. McDowell, S. E., and H. T. Rossby (1978), Mediterranean water: An in- tense mesoscale eddy off the Bahamas, Science, 202, 1085 -1087, doi:10.1126/science.202.4372.1085.
  31. McGillicuddy, D. J., A. R. Robinson, D. A. Siegel, H. W. Jannasch, R. Johnson, T. D. Dickey, J. McNeil, A. F. Michaels, and A. H. Knap (1998), Influence of mesoscale eddies on new production in the Sargasso Sea, Nature, 394, 263 -265, doi:10.1038/28367.
  32. McGillicuddy, D. J., et al. (2007), Eddy/wind interactions stimulate extra- ordinary mid-ocean plankton blooms, Science, 316, 1021 -1026, doi:10.1126/science.1136256.
  33. Morel, F. M. M., and N. M. Price (2003), The biogeochemical cycles of trace metals in the oceans, Science, 300, 944 -947, doi:10.1126/ science.1083545.
  34. Ono, S., A. Ennyu, R. G. Najjar, and N. R. Bates (2001), Shallow reminer- alization in the Sargasso Sea estimated from seasonal variations in oxy- gen, dissolved inorganic carbon and nitrate, Deep Sea Res., Part II, 48, 1567 -1582, doi:10.1016/S0967-0645(00)00154-5.
  35. Oschlies, A. (2002), Can eddies make ocean deserts bloom?, Global Bio- geochem. Cycles, 16(4), 1106, doi:10.1029/2001GB001830.
  36. Oschlies, A., and V. Garcon (1998), Eddy-induced enhancement of primary production in a model of the North Atlantic Ocean, Nature, 394, 266 - 269, doi:10.1038/28373.
  37. Prater, M. D., and T. Rossby (1999), An alternative hypothesis for the origin of the ''Mediterranean'' salt lens observed off the Bahamas in the fall of 1976, J. Phys. Oceanogr., 29, 2103 -2109, doi:10.1175/ 1520-0485(1999)029<2103:AAHFTO>2.0.CO;2.
  38. Prospero, J. M., K. Barrett, T. Church, F. Dentener, R. A. Duce, J. N. Galloway, H. I. I. Levy, J. Moody, and P. Quinn (1996), Atmospheric deposition of nutrients to the North Atlantic basin, Biogeochemistry, 35, 27 -73, doi:10.1007/BF02179824.
  39. Redfield, A. C., B. H. Ketchum, and F. A. Richards (1963), The influence of organisms on the composition of seawater, in The Sea, Ideas and Observations on Progress in the Study of the Sea, vol. 2, edited by M. N. Hill, pp. 26 -77, Wiley-Interscience, New York.
  40. Richardson, P. L. (1993), A census of eddies observed in North Atlantic SOFAR float data, Prog. Oceanogr., 31, 1 -50, doi:10.1016/0079- 6611(93)90022-6.
  41. Schmitz, W. J., and P. L. Richardson (1991), On the sources of the Florida Current, Deep Sea Res., Part A, 38, 389 -409.
  42. Seki, M. P., J. J. Polovina, R. E. Brainard, R. R. Bidigare, C. L. Leonard, and D. G. Foley (2001), Biological enhancement at cyclonic eddies tracked with GEOS thermal imagery in Hawaiian waters, Geophys. Res. Lett., 28, 1583 -1586, doi:10.1029/2000GL012439.
  43. Siegel, D. A., D. J. McGillicuddy, and E. A. Fields (1999), Mesoscale eddies, satellite altimetry, and new production in the Sargasso Sea, J. Geophys. Res., 104, 13,359 -13,379, doi:10.1029/1999JC900051.
  44. Smythe-Wright, E., A. L. Gordon, P. Chapman, and M. S. Jones (1996), CFC-113 shows Brazil Eddy crossing the South Atlantic to the Agulhas Retroflection region, J. Geophys. Res., 101, 885 -895, doi:10.1029/ 95JC02751.
  45. Sweeney, E. N., D. J. McGillicuddy, and K. O. Buesseler (2003), Biogeo- chemical impacts due to mesoscale eddy activity in the Sargasso Sea as measured at the Bermuda Atlantic Time Series (BATS) site, Deep Sea Res., Part II, 50, 3017 -3039, doi:10.1016/j.dsr2.2003.07.008.
  46. Takahashi, T., W. S. Broecker, and S. Langer (1985), Redfield ratio based on chemical data from isopycnal surfaces, J. Geophys. Res., 90, 6907 - 6924, doi:10.1029/JC090iC04p06907.
  47. Vaillancourt, R. D., J. Marra, M. P. Seki, M. L. Parsons, and R. R. Bidigare (2003), Impact of a cyclonic eddy on phytoplankton community structure and photosynthetic competency in the subtropical North Pacific Ocean, Deep Sea Res., Part I, 50, 829 -847, doi:10.1016/S0967-0637(03)00059-1.
  48. Williams, P. J., and N. W. Jenkinson (1982), A transportable microproces- sor-controlled precise Winkler titration suitable for field station and ship- board use, Limnol. Oceanogr., 27, 576 -584.
  49. Williams, R. G., and M. J. Follows (2003), Physical transport of nutrients and the maintenance of biological production, in Ocean Biogeochemistry, edited by M. Fasham, pp. 19 -49, Springer, New York.
  50. ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ À N. R. Bates and R. J. Johnson, Bermuda Biological Station for Research Inc., 17 Biological Lane, St. George's GE 01, Bermuda. D. A. Hansell, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA.
  51. Q. P. Li, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA. (qian@coast.ucsd.edu) D. J. McGillicuddy Jr., Woods Hole Oceanographic Institution, 266