Fig 1.
Schematic of isopycnal deformations and vertical transport processes in two eddy types depicted for the southern hemisphere.
Shown are a cyclonic (clockwise ‘CW’ spin) and an anticyclonic mode-water eddy (counterclockwise ‘CCW’ spin). Both seasonal (dotted lines; σ1–3) and main pycnoclines (dotted line; σ4) are illustrated. At the surface, cyclonic and anticyclonic mode-water eddies cause a negative and positive sea surface height anomaly, respectively. Vertical transport processes in the eddy center can vary in direction and magnitude [20], here, we illustrate the direction of vertical transport in the eddy center derived from eddy-wind interaction Ekman flow. In the center of anticyclonic mode-water eddies, upwelling stimulates chlorophyll accumulation (green), in addition, inward swirling currents concentrate chlorophyll in the eddy center from surrounding waters, also known as eddy entrapment [20]. In contrast, cyclonic eddies distribute chlorophyll downwards through the eddy center. In both eddy types submesoscale vertical transport is expected to be enhanced along either side of the density front (i.e. along the tilted isopycnals). This area coincides with an increase in isopycnal spacing (red line) and the eddy horizontal velocity, which can be used to differentiate the eddy periphery from the eddy center. Submesoscale processes drive two-way vertical transport. A net upward transport of nutrients into the euphotic zone stimulates chlorophyll production, while subduction can act to re-distribute chlorophyll over a greater depth, and can cause chlorophyll pockets to form below the surface mixed layer. Submesoscale vertical velocities at the eddy periphery exceed velocities at the center, as represented by the length and thickness of the vertical arrows. This figure represents a synthesis of principles discussed by Mahadevan et al., [40]; Omand et al., [32]; and McGillicuddy [20].
Fig 2.
Distribution of anammox activity across eddies A, B and C in the ETSP region.
(A) Sea surface height altimetry (SSHA) of sampled eddies A, B and C during the M90 cruise, November 22nd, 2012. The approximate locations of sampled stations within the eddy are shown, please note that the eddies propagated westward over the sampling period. Stations sampled for nutrients only (open circles) and nutrients plus nitrogen loss rates (open triangles with station numbers) are indicated. The red and blue dotted lines indicate transects sampled across eddies A, B and C, note that three defined transects were performed across eddy A (T1a/b in blue and T2 in red). Transects shown in panels B-D represent red dotted lines, whereas additional transects T1a (blue) are shown in S2–S3 Figs. (B) Horizontal velocity depth profiles are adapted from Stramma et al., [37]. Isopycnal contours are indicated by black lines, while reference isopycnals 25.4 and 26.0 kg m-3 are highlighted by black dotted lines. (C, D) Indicate volumetric and depth-integrated anammox rates for 15N-NH4+ experiments. Error bars for depth-integrated anammox rates represent the standard error. BD indicates below the limit of detection. Stations numbered in red (B0, B1, C0, and A0) were sampled in or near the eddy center while stations with black numbers (C3, C2, C1, and A1) were sampled on the eddy periphery, identified according to eddy-induced horizontal velocities, density fronts, and SSHA, shown in panels a and b. Note that the center of eddy C, based on SSHA and supported by horizontal velocities, is at -16.25°N, -80.38°E 14 km northeast of station C0. Note that data from station C2 is not included in the transect profiles shown in panel B (indicated by (C2)), but is shown in panels C, D. The coastal upwelling station is indicated by ‘A2’. The vertical black dotted lines in panels C and D indicate the edge of the respective eddies.
Fig 3.
Relationship between isopycnal spacing and chlorophyll.
(A) Correlation of isopycnal spacing versus longitude for eddies A (green), B (blue), and C (red), alongside coastal upwelling stations (orange) and offshore stations extending past eddy B (grey). (B) Correlation of isopycnal spacing versus depth-integrated chlorophyll. Chlorophyll at all stations was depth-integrated down to 300 m depth, except for coastal stations which were depth-integrated down to 200 m. Dotted linear regression lines indicate eddy specific trends (RC = eddy C, RB = eddy B and Roff = offshore). Pearson correlation values are indicated in each panel (p-values).
Fig 4.
Widespread distribution of mesoscale eddies and the heterogeneity of anammox rates in the offshore ETSP region.
Aerial sea surface height during the M90 (November 22nd, 2012) and the M77-4 (February 5th, 2009) research cruises. Eddies A, B and C shown in Fig 2A are highlighted by the dashed box in the left panel. Overlaid are depth integrated anammox rates (mmol N m-2 d-1) from 15N-incubation experiments from this study (left panel), and rates from the M77-3 and M77-4 research expeditions (right panel) [13]. Anammox rates are depth integrated over the OMZ at a cutoff of 20 μM oxygen.