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Supplementary Information:
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Description of lateral and vertical advection and mixing
Eddies can transport nutrients from afar and depth profiles reveal that chemical anomalies
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at depth persist longer in anticyclonic eddies than cyclonic eddies in the ALOHA region (Nolan,
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2008). The mesoscale eddy that we sampled originated northeast of our study area (29°N,
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146°W) where waters are lower in temperature and salinity, and higher in phosphate than waters
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to the south (Lukas, 2001; Church et al., 2008). During the two years of observations, this eddy
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reached a maximum in intensity during the period of our study (Supplementary Figure 3),
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consistent with the strong stirring evident in satellite data (Figure 1).
Evidence of stirring of regional water types by Eddy A is indicated by its clockwise
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wrapping of warm/cool anomalies, and high/low chlorophyll anomalies. Sea surface
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temperatures (SST) also show westward deflection of isotherms parallel to the westward drift of
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ESP during the latter half of the experiment (Figure 1A), indicating the influence of transport
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anomalies induced by the northwest eddy (Eddy B). The pronounced eddy transport anomalies
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and the shear that must have been elevated by opposing flows between them indicate that the
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ESP drift occurred in a highly dynamic zone. Synoptic SST near the mid-point of the ESP drift
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period (Figure 1B) and salinity measurements along the ESP drift track (Figure 1C) confirm this
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and illustrate the high degree of patchiness. Finite-size Lyapunov Exponent (FSLE; Lehahn et
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al., 2007) calculations indicate mixing between Eddies A and B in this region. Also in this
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region (Figure 2D, ESP sample numbers 8-12), waters alternated between elevated in phosphate
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to elevated in nitrogen, with concentrations of nitrite indicating vertical movement of waters into
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the surface layers from near the nitrite maximum (Figure 2D). In this region we see small (0.01-
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0.04 σ-T) inversions in potential density during 9 of the 12 CTD casts, at various depths ranging
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from 10 – 58 m (Supplementary Figure 2), though such inversions are not present in 22 casts
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before or after the apex (encircled in Figure 2A). The first appearance of nitrite in the surface
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layer (0-50m at section distance 90-100km, Figure 2D) is also in the apex region. We sampled a
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range in phosphate concentrations spanning 77% of the range of all summer phosphate
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concentrations measured at Station ALOHA since 1989, as the high phosphate water mass mixed
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with stratified low-nutrient waters (Supplementary Figure 5).
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Drifter behavior
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The quasi-Lagrangian behavior of the drifting ESP is illustrated by comparison of the drift speed
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(Figure 2A) with the speed of water relative to the ESP itself (Figure 2B). Earth-referenced
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velocity of the ESP was determined from the time-series of its GPS position. The ADCP
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measured the velocity of water relative to the ESP, that is, in the frame of reference of the
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drifting ESP. Thus, a low velocity measurement in water near the depth of ESP indicates that the
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platform is effectively moving with the patch of water surrounding it. During the first phase of
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the drift along the western periphery of Eddy A (Figure 1A), relative water speeds were much
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lower than those of the platform moving through the ocean. Zero relative water speed would
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indicate perfect Lagrangian movement, whereas the difference in velocities between the water
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and the platform indicates quasi-Lagrangian observation and sampling. The ESP drift slowed as
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it approached its easternmost extent in the northern periphery of Eddy A, and it remained slow
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during its initial redirection toward Eddy B (Figure 1A, 2B). As it accelerated toward Eddy B,
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ESP-relative currents also increased (Figure 2A,B). Based on the ESP trajectory and along-
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trajectory variability in water properties (Figure 1, 2), we interpret that ESP sampling transited
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through a region of strong eddy-induced stirring and mixing (Figure 1), which is supported by
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calculated FSLE during this time. Measurements made by ESP contextual sensors revealed
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patchiness (Figure 2C) at much finer scales than could be resolved by satellite or ship CTD
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profiling (Figure 1B,C). Patchiness in diazotroph abundance was clearly related to this
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environmental structure (Figure 2D,E) for the northeastward transit of the ESP drift.
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Elevated abundances of diazotrophs in the inter-eddy transition zone.
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The inter-eddy transition zone was both physically and biologically exceptional. On
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September 14 and 15, the ESP drifter was in a region of opposing currents north of Eddy A (ESP
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samples 8-10 in the apex of the drift), and concentrations of unicellular cyanobacteria were
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elevated in this region (Figure 2D). In this region the trends with phosphate deviated from the
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strong correlations observed near Eddy 1 and in 2009-2012 (Figure 4), presumably due to
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complex mixing between opposing currents from each eddy. This is a heterogeneous region
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influenced by vertical advection (see nitrite profile in Figure 2D) and where more complex
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physical dynamics likely drive microbial distributions. Investigations of the effects of vertical
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advection on the resident microbial community are currently underway.
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Anticyclonic eddies promote nutrification of the near-surface ocean in this region through
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stimulation of N2-fixing cyanobacteria (15). However, physical and chemical changes within
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eddies have a differential impact on each species of N2-fixing cyanobacteria: while abundances
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of unicellular cyanobacteria (Atelocyanobacterium and Crocosphaera) and Trichodesmium were
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elevated in higher phosphate waters, Trichodesmium populations, which have also been
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associated with anticyclonic eddies in the Atlantic (Davis and McGillicuddy, 2006), tended to be
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less abundant where phosphate concentrations were elevated (Figure 4). This might be expected
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if this population depleted phosphate concentrations locally (Hashihama et al., 2009) or used
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organic phosphorus as a P source (White et al., 2010; Dyhrman et al., 2006).
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Supplementary Figures
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Supplementary Figure 1. Cruise track with ESP and ship CTD sampling locations superimposed
onto sea surface height anomalies. Red “ESP” sample numbers correspond with sample numbers
indicated in Figure 2D, top panel.
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Supplementary Figure 2. Temperature/Salinity plots of station CTD casts from the BioLINCS
cruise. Colors represent phosphate concentrations. (A) Three groups of casts can be seen here:
high phosphate, low salinity in surface waters (purple/pink at left), low phosphate, high salinity
with a typical mixed layer (grey at right) and intermediate casts representing the mixing of these
water masses. Phosphate data are from 5, 25, 45, 75, 100, 125, 150, 175m depth top to bottom.
Numbers at top depict ESP station numbers in Figure 1C. (B) Cast from Station 1: here there is
instability (constant density over depth) within the shallow photic zone between 25 and 45m
depth (approaching the 23.2 σ density isopycnal). (C) Instability at the apex of the transit (circled
in Figure 2A) is exhibited by density inversions of 0.02-0.04 σ-T at several depths (arrows) for 4
CTD casts at the northeast corner of the ESP transit. Purple casts are examples from more typical
mixed layers, from before (low potential density) and after (high potential density) the apex of
the drift, for comparison.
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Supplementary Figure 3. Eddy track before and during the BioLINCS cruise, with the sea
surface height field during BioLINCS included for orientation. The size of Eddy A is depicted
over time by the size of the red circle, showing that we sampled during a peak in SLA.
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Supplementary Figure 4: Satellite altimetry for the 10 highest phosphate concentration samples
over the summers. 8 of the 10 samples were taken on days of positive sea surface height
anomaly at Station ALOHA (black dot).
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Supplementary Figure 5. HOT Phosphate data. (A) Rank order of phosphate concentrations
sampled at Station ALOHA in the spring vs. summer since 1989. All samples are included in this
figure. Maximum and minimum phosphate concentrations in the summer are higher and lower,
respectively, than spring maximums and minimums. This larger range of phosphate
concentrations has implications for diazotroph heterogeneity over the summer months. (B)
Phosphate vs. salinity for the HOT dataset. The vertical line is the 80nM phosphate threshold
and the horizontal line shows the average salinity concentration in the summer at ALOHA. All
high phosphate samples are from anticyclonic eddies except for the circle symbol (from a
cyclonic eddy). The outlined symbols are samples without satellite-derived SLA data.
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Supplementary Figure 6. Log of Atelocyanobacterium, Crocosphaera and Trichodesmium
abundances vs. salinity for the BioLINCS cruise. Squares with thin borders indicate samples
from the northeastward transit within Eddy A, and squares with thicker borders are from samples
from the inter-eddy transition zone (the westward transit). Samples from the westward transit do
not correlate with the conservative salinity as well as those from Eddy A.
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Supplementary Table 1. Quantitative PCR reaction details for each qPCR assay addressed in the
text. “MFB” signifies the ESP’s Microfluidic Block for qPCR. “N/A” in this category indicates
gene targets that were not quantified using the ESP’s MFB.
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Supplementary Figure 7. Synechococcus phnD qPCR assay specificity tests. The minimum
concentrations quantified for this time series are given in each plot. In order to change target
quantifications, the non-target sequences must be present at an abundance that results in an
equivalent or lesser Ct than the minimum target concentration in that sample. Five non-target
standards were run against every assay and those with the highest cross-reactivity are shown
here. For this study, the specificities of these assays are accurate enough for reliable target
quantifications. Maximum log copies of each cluster are: Clade II, cluster 1: 4.09; Clade II,
cluster 2: 3.51, Clade III: 2.66.
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