Eddy-induced reduction of biological production in eastern boundary upwelling systems

Journal name:
Nature Geoscience
Volume:
4,
Pages:
787–792
Year published:
DOI:
doi:10.1038/ngeo1273
Received
Accepted
Published online

Abstract

Eddies and other mesoscale oceanic processes, such as fronts, can enhance biological production in the ocean, according to several open-ocean studies. The effect is thought to be particularly pronounced in low-nutrient environments, where mesoscale processes increase the net upward flux of limiting nutrients. However, eddies have been suggested to suppress production in the highly productive eastern boundary upwelling systems. Here, we examine the relationship between satellite-derived estimates of net primary production, of upwelling strength, and of eddy-kinetic energy—a measure of the intensity of mesoscale activity—in the four most productive eastern boundary upwelling systems. We show that high levels of eddy activity tend to be associated with low levels of biological production, indicative of a suppressive effect. Simulations using eddy-resolving models of two of these upwelling systems support the suggestion that eddies suppress production, and show that the downward export of organic matter is also reduced. According to these simulations, the reduction in production and export results from an eddy-induced transport of nutrients from the nearshore environment to the open ocean. Eddies might have a similar effect on marine productivity in other oceanic systems that are characterized by intense eddy activity, such as the Southern Ocean.

At a glance

Figures

  1. Relationship of observationally based estimates of NPP to upwelling strength and EKE in the four major EBUS.
    Figure 1: Relationship of observationally based estimates of NPP to upwelling strength and EKE in the four major EBUS.

    a, Relationship of EKE-normalized NPP to upwelling strength. b, Relationship of upwelling strength normalized NPP to EKE. NPP was normalized by first fitting it to a multiple linear regression model with upwelling strength and EKE as independent variables, and then using the respective coefficients (shown in the plots as well for the case where data from all EBUS were used) to remove the effect of the other independent variable from the data. The lines represent the projection of the multiple linear regression model to the normalized data.

  2. Modelled impact of eddies on the distribution of primary and export production in the CalCS.
    Figure 2: Modelled impact of eddies on the distribution of primary and export production in the CalCS.

    Maps of model-simulated depth integrated primary production (ac) and of organic carbon export (across 100m) (df), in units of molCm−2yr−1. a,d, Results from the non-eddy simulation (see text for details) colour scales as for b and e, respectively. b,e, Results from the eddy simulation. c,f, Difference between the eddy and the non-eddy cases. Also shown in c and f are the 100km, 500km, and 1,000km offshore regions over which budgets were analysed.

  3. Vertical distribution of modelled density, nitrate, and total organic matter along offshore sections.
    Figure 3: Vertical distribution of modelled density, nitrate, and total organic matter along offshore sections.

    Sections of ac potential density (kg m−3), df, nitrate (mmol m−3), gi, total organic carbon (mmol m−3) with the left column showing the non-eddy case, the middle column the eddy case, and the right column the difference between the eddy and non-eddy cases. The sections represent average conditions over a 100 km swath starting in Monterey Bay (37°N, 121.8°W) and extending to about 33.7°N, 132°W, nearly 1,000km into the open ocean. Colour scales for a,d and g are as for b,e and h, respectively.

  4. Offshore sections illustrating the role of eddies in inducing a lateral loss of total nitrogen (TN) from the CalCS.
    Figure 4: Offshore sections illustrating the role of eddies in inducing a lateral loss of total nitrogen (TN) from the CalCS.

    a, Eddy-induced vertical fluxes ( in nmolm−2s−1 with w′ and TN′ being the time-varying components of the vertical velocity and total nitrogen, respectively). b, Eddy-induced horizontal fluxes ( in μmolm−2s−1, with v′ being the time-varying component of the zonal velocity). White lines in a and b show potential density; black dashed lines indicate negative fluxes. c, Conceptual diagram of the impact of mesoscale eddies on coastal circulation, nitrogen transport, and organic matter production and export. The thick lines indicate total nitrogen transports and the thin lines depict circulation pattern. Shown in blue are the Ekman-driven transports and circulations. The red arrows show the eddy-driven transports and (bolus) velocities. Contour lines denote potential density and green arrows the vertical export of organic matter. The eddy-induced fluxes in a and b were determined by a Reynolds decomposition (see Supplementary Information).

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Author information

Affiliations

  1. Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich 8092, Switzerland

    • Nicolas Gruber,
    • Zouhair Lachkar,
    • Matthias Münnich &
    • Gian-Kasper Plattner
  2. Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, California 90095, USA

    • Hartmut Frenzel &
    • James C. McWilliams
  3. Institut de Recherche pour le Développement, LEGOS, 31400 Toulouse, France

    • Patrick Marchesiello
  4. Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, California 90095, USA

    • James C. McWilliams
  5. Tokyo University of Marine Science and Technology, Tokyo 108-8477, Japan

    • Takeyoshi Nagai
  6. Present address: Climate and Environmental Physics, University of Bern, Bern 3012, Switzerland

    • Gian-Kasper Plattner

Contributions

N.G. designed the study, co-analysed the data, and wrote the paper. Z.L. assisted in the design of the study, performed most of the experiments, and led the data analyses. H.F. and T.N. ran earlier experiments and assisted in the analyses. P.M., J.C.M., G.K.P. and M.M. provided conceptual and theoretical advice with regard to design and analyses. All authors discussed the results and implications and commented on the manuscript at all stages.

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The authors declare no competing financial interests.

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