Biological and physical forcings of late summer chlorophyll blooms at
30°N in the oligotrophic Pacific

Cara Wilson^a^, Tracy A. Villareal^b^, Nikolai Maximenko^c^,\
Steven J. Bograd^a^ , Joseph P. Montoya^d^, and Caimee A.
Schoenbaechler^b^

^a^NOAA/NMFS/SWFSC Environmental Research Division, 1352 Lighthouse
Ave., Pacific Grove, CA 93950

^b^Marine Science Institute, The University of Texas at Austin, 750
Channel View Dr., Port Aransas, Texas 78373

^c^International Pacific Research Center, SOEST, University of Hawaii,
Honolulu, HI, 96822

^d^School of Biology, Georgia Institute of Technology, Atlanta, GA 30332

Corresponding author: Cara Wilson

<cara.wilson@noaa.gov>, Tel: (1) 831-648-5337, Fax: (1) 831-648-8440

Submitted to a special volume of *Journal of Marine Systems*

"Physical-Biological Interactions in the Upper Ocean"

Revised version, September 15, 2005

**Abstract**

Large chlorophyll blooms have been observed with ocean color satellite
data in late summer near 30°N, 135°-155°W in the oligotrophic
subtropical Pacific. Although blooms do not develop every year, their
timing and location are consistent when they do appear. Their biological
and physical forcings are unknown. Here we examine biological and
physical data from a number of cruises along 30°N in the eastern Pacific
between 1980-2004, focusing on biological data from a cruise that
fortuitously sampled a small chlorophyll bloom in July 2002. The
climatological bloom location is characterized by elevated levels of
*Rhizosolenia* diatom mats as well as endosymbiotic and unicellular
nitrogen fixation. We conclude that nitrogen fixation and the vertical
migration of *Rhizosolenia* diatom mats below the nutricline are likely
mechanisms for the supply of new nitrogen to fuel the chlorophyll blooms
and that both mechanisms could be at work. The blooms occur in the
eastern gyre of the North Pacific, a region characterized by converging
weak surface currents that create an environment favorable for
accumulation of positively buoyant particles such as *Rhizosolenia*
mats. These blooms could have important ecosystem consequences by
impacting local carbon fluxes and serving as an aggregation point for
pelagic animals.

**Keywords:**

chlorophyll blooms, diatoms, nitrogen fixation, satellite sensing,
*Rhizosolenia* mats, surface currents, convergence zones

# 1. Introduction 

Large chlorophyll blooms, observable with ocean color satellite data,
frequently develop in the late summer in the oligotrophic subtropical
Pacific northeast of Hawaii (Wilson, 2003). The blooms are quite large,
sometimes reaching the size of California (400,000 km^2^). They have
been observed in 11 of 16 years of satellite ocean color data (CZCS,
OCTS and SeaWiFS) and can last up to 4-5 months. The blooms are as yet
uncharacterized, with no information available on either what species
form the blooms or on the mechanisms that supply nutrients to support
this elevated biomass. The two most consistent aspects of the blooms are
the timing of their development, occurring in late summer, and their
location, centered along 30°N between 135°-155°W. They do not appear to
be caused by topographic features or by subsurface mixing (Wilson,
2003). Drawing upon published results from studies near or within the
blooms, Wilson (2003) hypothesized that they are fuelled by either
nitrogen fixation or by new N brought in by the vertical migration of
*Rhizosolenia* diatom mats between the surface and the nutricline.

Here we expand upon the work of Wilson (2003) by (1) examining the
available biological data along 30°N in the Pacific, (2) presenting
analysis of the first biological samples taken near a bloom and (3)
evaluating the location of the chlorophyll blooms in a basin-scale
context. Most in situ biological observations from this part of the
Pacific come from the Hawaii Ocean Time-series (HOT, 23.75°N) and CLIMAX
(28°N) sites, neither of which is directly within the region where the
blooms develop. However, there have been a number of cruises that have
sampled *Rhizosolenia* diatom mats along \~30°N (see Fig 1 and Table 1).
These data are examined here, with a particular focus on data from a
2002 cruise (Montoya et al., 2004; Pilskaln et al., 2005; Singler and
Villareal, 2005) that serendipitously sampled the boundary of a small
chlorophyll bloom in this region. These are the only existing data from
a 30°N chlorophyll bloom, and they have not been previously examined in
the context of the blooms. The consistent position of the blooms within
the eastern Pacific basin suggests a strong link to large-scale physical
forcing. We discuss physical characteristics of the eastern Pacific that
could contribute to the formation of the chlorophyll blooms in this
localized area.

# 2. Data 

Satellite ocean color data from the Coastal Zone Color Scanner (CZCS),
the Ocean Color and Temperature Scanner (OCTS) and the SeaWiFS
(Sea-viewing Wide Field-of-view Sensor) sensors were used to identify
chlorophyll blooms between 1979-1985 and 1996-2004. For CZCS and OCTS,
monthly data were examined, for SeaWiFS, both 8-day and monthly
composite data were examined (using the 9 km, level-3 dataset). Blooms
are defined as regions with chlorophyll \> 0.15 mg/m^3^ in Jul.--Oct.,
when the ambient background level of chlorophyll is \~0.08 mg/m^3^.

A compilation of *Rhizosolenia* mat abundances from cruises between
1980-2003 is examined in relation to bloom location. In situ biological
data from a 2002 cruise that went through the southern edge of a small
chlorophyll bloom is compared with the satellite observations. This
cruise measured *Rhizosolenia* mats abundances (by SCUBA divers using
methods described in Villareal et al. (1996)), the abundance of diatoms
with *Richelia* endosymbionts (collected from a series of 64 µm mesh
MOCNESS tows), and rates of unicellular nitrogen fixation (using methods
described in Montoya et al. (2004)). Diatom abundance was collected
using a 1/4 m^2^ MOCNESS (64 µm mesh) system towed from 0-300 m. The
surface abundances presented here were calculated from the volume
filtered after enumerating an aliquot from the surface (0-15 m) sample.
*Hemiaulus* forms chains that are up to cm in size, and will be retained
by mesh sizes larger than the cell diameter. However, cell breakage is
an inevitable part of net sampling, as is the change in retention
efficiency of small particles as the net clogs. For this reason, we
consider these numbers to be useful only for relative comparison to
other MOCNESS samples collected on this cruise. Symbiont enumeration
from the 1993 cruise was conducted as described in Villareal (1994).

To characterize the physical environment of the bloom region,
climatological near-surface currents, calculated from a combined
analysis of drifter, satellite altimetry and wind data as in Niiler et
al. (2003) and Maximenko and Niiler (2005) are presented, as well as
output from a model of passive tracer concentrations (Maximenko,
manuscript in prep.). Hydrographic data is also analyzed from the P2
transect (http://whpo.ucsd.edu) across 30°N in the summer of 2004.
Although the 2004 bloom occurred a month prior to this transect, these
data reveal differences between the western and eastern Pacific basins.

# 3. Bloom Demographics 

The blooms can be identified as regions in the oligotrophic subtropical
Pacific gyre with chlorophyll \> 0.15 mg/m^3^ during Jul.-Oct. (Figure
2, images of the individual blooms can be seen at
http://www.pfeg.noaa.gov/\~cwilson/bloom). The location of the blooms is
relatively consistent from year-to-year between 135°-155°W at 30°N.
Additionally, there were two, month-long blooms that developed south of
this area, one near 165°W, 20°N in Aug. 1998, and one on the north side
of Hawaii in Jul. 2000. The focus of this paper is on the 30°N blooms.
While the analysis in Figure 2 used only data from Jul.-Oct., all months
were examined, and blooms do not develop outside of this time period.
The blooms develop \~10° south of the Transition Zone Chlorophyll Front
(TZCF), defined as the location of the 0.2 mg/m^3^ surface chlorophyll
across the basin (Polovina et al., 2001), although in the winter the
TZCF migrates into the bloom region and engulfs any blooms still
existing. The blooms are not forced by positive topography, occurring
over the Murray fracture zone (\> 5000 m depth) with no nearby
seamounts. The blooms do not appear to be associated with concurrent SSH
or SST anomalies, suggesting that processes other than subsurface mixing
are likely important (Wilson, 2003). While blooms have been seen near
Hawaii associated with mesoscale isopycnal doming (Brzezinski et al.,
1998; Vaillancourt et al., 2003), such features typically can be
identified by SSH or SST anomalies.

While the blooms have a consistent seasonality and location, there is
variability in their interannual development and magnitude (Figure 3).
The local chlorophyll variability in Jul.-Nov. illustrates the timing
and duration of the different blooms. They generally develop in Jul. or
Aug., and can sometimes last into Dec., although more commonly the
blooms last 4-6 weeks. In 2003, two separate blooms developed at
slightly different locations, each lasting about a month. The location
of the TZCF drives the winter chlorophyll variability, with chlorophyll
\> 0.1 mg/m^3^ indicative of proximity to the TZCF. In winter, the TZCF
penetrates into the western bloom region, but not into the eastern bloom
region.

Blooms have occurred in 11 of the 16 years of satellite ocean color
comprising the CZCS, OCTS and SeaWiFS datasets. This is a minimum count,
however, since some of the CZCS years did not have enough coverage in
this area to determine whether there was a bloom. There has been
sufficient spatial coverage between 1996-2004 to know that there were
only 2 years (1998 and 2001) without any significant bloom development.
The bloom's interannual occurrence does not appear to be tied to ENSO
variability (Figure 4). The frequency of the blooms is much higher than
that of strong El Niño or La Niña events, which occur every 5-7 years.
The two largest blooms observed with SeaWiFS occurred in 1997 and 2000,
which were El Niño and La Niña years, respectively.

# 4. Biological Forcing 

Possible sources of new nitrogen fueling the blooms are a biologically
mediated flux of nitrate by vertically migrating diatom mats and
nitrogen fixation (Wilson, 2003). These processes are not mutually
exclusive, and both could play a role in the formation and maintenance
of the observed chlorophyll blooms.

## 4.1 Nitrate transport by vertically migrating diatoms 

Vertically migrating *Rhizosolenia* diatom mats have been observed
extensively in the bloom region (Villareal and Carpenter, 1989;
Villareal et al., 1999; Villareal et al., 1996). *Rhizosolenia* mats
change buoyancy to descend below the nutricline to acquire nitrate and
return to the surface for photosynthesis. This dynamic is evidenced by a
variety of indirect measures: (1) the δ15N isotopic composition of the
mats is similar to the deep nitrate pool and distinctly heavier than the
surface particulate δ15N (Villareal et al., 1993) (2) the presence of
high internal nitrate pools found in diatoms only after exposure to
elevated external nitrate far higher than the \~ 5 nM nitrate
concentrations in surface waters in the gyres (Villareal et al., 1996)
(3) nitrate reductase (an inducible enzyme) activity sufficient to
provide for cellular N needs (Joseph et al., 1997) (4) the presence of
mats below the nutricline (Villareal et al., 1999) (5) *Rhizosolenia* is
capable of dark nitrate uptake (Richardson et al., 1996) and (6) C:N and
protein:carbohydrate ratios co-vary with buoyancy status in a manner
consistent with increasing N depletion along a gradient of positively to
negatively buoyant mats (Villareal et al., 1999). A variety of large
diameter *Rhizosolenia* species are capable of positive buoyancy at
rates of 3-5 m h^-1^ (Moore and Villareal, 1996; Villareal et al.,
1996). Carbohydrate ballasting has been suggested as the likely
mechanism of buoyancy regulation based on known patterns in limnetic
cyanobacteria, the above-mentioned co-variation of carbohydrate:protein
ratios with buoyancy status, and the relationship between carbohydrate
and buoyancy in *Rhizosolenia* (Moore and Villareal, 1996).
*Rhizosolenia* mats have been observed at nutricline depths (Pilskaln et
al., 2005; Villareal et al., 1993); however, attempts to collect them
from these depths (\~100-250 m) have generally been unsuccessful. The
mats leak nitrate at rates sufficient to replace the ambient nitrate
pool (\~5 nM) in as little as 3 days, although rates vary considerably
with environmental conditions and abundance (Singler and Villareal,
2005). Locally, new production resulting from vertical mat migration can
be nearly 50% of the export production measured at HOT (Villareal et
al., 1999) depending on mat abundance. Other buoyant species have many
of the same characteristics of *Rhizosolenia* mats, suggesting that
vertical migration as a life-history strategy is not limited to
*Rhizosolenia* spp. (Villareal and Lipschultz, 1995; Villareal et al.,
1999).

While the spatial distribution of *Rhizosolenia* mat data is limited,
there is a clear peak in abundance at 30°N and 140°-160°W (Figure 5),
the same region where the chlorophyll blooms develop. Most of these data
were collected in the June-Sept. period, again consistent with bloom
development. A 2002 cruise that sampled *Rhizosolenia* mats along a
transect near 30°N (Montoya et al., 2004; Pilskaln et al., 2005; Singler
and Villareal, 2005) fortuitously went through the southern edge of a
small chlorophyll bloom centered on 32°N, 150°W. The highest
*Rhizosolenia* mat abundance was observed near 155°W, although prior to
the bloom occurring in this region in 2002 (Figure 6a&b). Elevated
chlorophyll developed at the station with high *Rhizosolenia* mat
abundance at the end of July, several weeks after the mat measurements.
However, this does not rule out the mats as a nutrient source for the
chlorophyll bloom. For example, the maximum chlorophyll increase
associated with Fe enrichment experiments typically occurs 2-3 weeks
after the addition (Boyd et al., 2004; Gall, 2001). We also cannot rule
out other requirements for bloom development such as threshold
*Rhizosolenia* mat concentrations or coupling/uncoupling from grazers.

## 4.2. Nitrogen Fixation 

Nitrogen fixation is common in the bloom area and is associated with
several distinct prokaryote taxa including *Trichodesimium* (Karl et
al., 1992), endosymbiotic cyanobacteria (*Richelia)* contained within
*Hemialus* and *Rhizosolenia* diatoms (Mague et al., 1974; Scharek et
al., 1999a; Scharek et al., 1999b; Venrick, 1974) and unicellular
cyanobacteria (Falcón et al., 2004; Montoya et al., 2004; Zehr et al.,
2001). While large *Trichodesimium* blooms have been observed near
Hawaii (Karl et al., 1992), *Trichodesimium* appears less common further
north in the bloom vicinity (Venrick, 1997, Villareal, unpublished
observation). Summer diatom blooms have been observed and sampled at the
HOT and CLIMAX sites (Brzezinski et al., 1998; Heinbokel, 1986; Mague et
al., 1974; Scharek et al., 1999a; Scharek et al., 1999b; Venrick, 1974),
but never at times with available satellite chlorophyll data. Some of
these events are quite significant with *Hemiaulus* and *Rhizosolenia*
spp. densities reaching 10^3^ cells/L, or 1-2 orders of magnitude above
winter background. Both of these genera can contain the nitrogen-fixing
endosymbiont *Richelia intracellularis* at times (Mague et al., 1974;
Villareal, 1991). Recent work has suggested that unicellular
cyanobacteria could fix as much nitrogen in the Pacific as
*Trichodesimium* and *Richelia* (Falcón et al., 2004; Montoya et al.,
2004; Zehr et al., 2001)*.*

Since almost all of these observations come from either the HOT or
CLIMAX study area, they provide little information about the spatial
distribution of their abundance, especially as neither site is within
the main area where the blooms occur (see Figure 2). During the 2002
cruise to this area, the highest abundances of the
*Rhizosolenia-Richelia* symbiosis were found within the chlorophyll
bloom (Figure 6c&d). Of the two stations with elevated levels of
*Rhizosolenia-Richelia*, one was within the chlorophyll bloom, and one
sampled an area several weeks before the development of the chlorophyll
bloom at that location. There are two other stations within the bloom
region that do not have elevated levels of the *Rhizosolenia-Richelia*
association. Similar small-scale (tens of kms) heterogeneity was also
observed within diatom blooms at the CLIMAX site (Venrick, 1974). The
highest rates of nitrogen-fixation values (\<100 µm size fraction,
Montoya et al. (2004)) are also associated with the region of the bloom
(Figure 6e&f). However, the spatial distribution of elevated
nitrogen-fixation rates are spread further away from the bloom than the
high values of *Rhizosolenia* mats and *Rhizosolenia-Richelia*. The
areal rate of nitrogen fixation measured at 30°N, 520 μmol N m^-2^
d^-1^, is much higher than rates observed at HOT, which are less than
100 μmol N m^-2^ d^-1^ (Montoya et al., 2004).

Another diatom that can contain the *Richelia* symbiont in the Pacific
is *Hemiaulus*. The symbiont is not always present, and the level of
symbiosis may be a proxy for allochthonous nutrient injection. For
example, in an eddy off of Hawaii subsurface nutrient injection reduced
the percentage of symbiosis to near zero (Vaillancourt et al., 2003).
Data from the 1993 cruise showed a large increase in the percentage of
*Hemiaulus* with symbionts between 135°-145°W, from less than 10% to
45-65% (Figure 7). While it is not clear what would cause this regional
change in percent symbiosis, the size and location of this feature is
consistent with nitrogen-fixation playing an important role in the
chlorophyll bloom region. One interpretation is that *Hemiaulus*
populations outside the bloom area may be supported by N sources other
than nitrogen fixation. Unfortunately, there are no satellite data from
this time period to determine whether the variation in symbiosis
percentage was associated with a chlorophyll bloom.

## 4.3 Nutrient Limitation 

While both biological processes, mat migration and N~2~ fixation,
alleviate nitrogen limitation, potential limitation by Fe, Si and P must
be considered. The biological pathways involved in both processes
utilize more Fe than those in regular phytoplankton (Raven, 1988; Raven,
1990; Singler and Villareal, 2005). Fe concentrations are low in this
part of the Pacific, but there is an aeolian flux from Asian dust storms
in the springtime (Duce and Tindale, 1991). Fluorescence values of
phytoplankton in this area indicate that they are not Fe-stressed,
(Johnson et al., 2003), and the same result has been seen for
*Rhizosolenia* mats in the bloom region (Singler and Villareal, 2005).
However, mats west of 160°W (outside of the bloom region) have lower
fluorescence values (F~v~:F~m~), a response consistent with Fe-stress
(Singler and Villareal, 2005). It is possible that Fe limitation, and
the interannual variability in aeolian deposition, could play a role in
the development of the observed chlorophyll blooms.

Silicate concentrations show a pronounced westerly decrease across the
eastern basin of the central N. Pacific. Brzezinksi et al. (1998) noted
high rates of particulate production in the bloom region in 1995 with
production rates declining in the water west of 160°W in 1996. Although
additional silicate always stimulated higher particulate Si production
rates, elevated concentrations of *Hemiaulus* did not appear to require
high ambient concentrations. For example, a bloom of \>15,000 cell L^-1^
was noted at 26°N,159°W and was not coincident with elevated Si
concentrations. However there did appear to be a regional difference in
the Si dynamics within *Rhizosolenia* mats. The western transect Si
concentrations were sufficiently low that *Rhizosolenia* mats could only
acquire sufficient Si for doubling at depth; whereas, in the eastern
transect, Si concentrations were high enough at the surface to permit
continuous uptake (Shipe et al., 1999).

While the two potential sources of N (mat migration and N~2~ fixation by
*Richelia*) are similar in their needs for Fe and Si, they are
distinctly different in their P needs. *Rhizosolenia* mats show no
buoyancy related differences in their N:P ratios, and probably acquire P
at depth as well (Villareal, unpublished analysis). *Hemiaulus* and
*Rhizosolenia* diatoms with endosymbiotic *Richelia* would require a P
supplement to maintain Redfield stoichiometry. Since the relatively
small *Hemiaulus* and *Rhizosolenia* species are unlikely to vertically
migrate due to size requirements (Villareal, 1988), and there is not
data to suggest unusual advective or diffusive sources of P, the source
of additional P remains problematic. The sources and roles of these
potentially limiting nutrients on both processes needs to be studied
further.

# 5. Physical Forcing 

Previous analyses have not revealed any obvious linkages between the
blooms and local physical forcing. The blooms are not associated with
SST or SSH anomalies that would be indicative of mixing or breaking
internal waves (Wilson, 2003). However, we cannot reject the possibility
that physical processes may be responsible directly, or indirectly, for
the blooms. In fact, their consistent location at 30°N, 135°-155°W, and
their absence in the western part of the Pacific Ocean (Figure 2),
suggest that large-scale physical forcing may provide necessary
conditions for bloom development.

If the biological processes discussed above are responsible for the
chlorophyll blooms, then the physical requirements of these processes
could dictate when and where they develop. For example, since cell
buoyancy is crucial to mat migration, this process would be most
effective under calm conditions that permit sea surface accumulations of
*Rhizosolenia* mats (Villareal and Carpenter, 1989; Villareal et al.,
1996). The relationship of physical forcing with N~2~-fixation is less
clear. *Hemiaulus* is preferentially associated with the shallow mixed
layer typical of late summer, calm conditions (Venrick, 1988). Calm
conditions may minimize the vertical flux of NO~3~, and thus increase
the selective advantage of N~2~-fixation as a strategy. Both habitat
patterns are consistent with the timing of the chlorophyll blooms, which
develop in late summer when wind speeds are lowest.

## 5.1 Latitudinal Position 

In the summer, there is a zone of minimum wind speed and shallow mixed
layer depths along 30°N (not shown). In fact the minimal wind speed at
30°N dictated the latitude of the biological cruises as their sampling
procedures (getting SCUBA divers into and out of small inflatable boats
launched off of research vessels) required the winds \<10-15 knots. 30°N
is also the approximate position of the subtropical front (STF, Roden,
1974). It has been suggested that there is summer upwelling associated
with the front that does not have a surface expression (Niiler and
Reynolds, 1984), in which case it is possible that a subsurface nutrient
injection drives the blooms. The surface signature of the STF is
seasonal, being most intense in winter, when it has a clear SST
signature, and weaker in summer due to strong frontolysis (Kazmin and
Rienecker, 1996). In summer, the front is manifest mainly by a salinity,
rather than a temperature, gradient (Roden, 1974; Roden, 1975), and
hence is not distinguishable by satellite data. Thus, it is not possible
to evaluate the relationship between the blooms' appearance and
location, and the strength and position of the front. However, the STF
is a relatively uniform feature across the Pacific basin, and the
presence of blooms only in the eastern half of the Pacific suggests
other physical dynamics are responsible.

## 5.2 Longitudinal position 

The chlorophyll blooms occur in the eastern gyre of the North Pacific.
This separate gyre was first identified by Sverdrup et al. (Sverdrup et
al., 1942) from analysis of hydrographic data. Both Munk (1950) and
Kenyon (1975) presented theoretical calculations demonstrating how wind
patterns could create a closed anticyclonic gyre in the eastern Pacific.
The gyre can be seen in the temperature and salinity data from the P2
line at 30°N as the clear distinction between profiles in the eastern
(135°-155°W) and western (west of 155°W) Pacific (Figure 8). Water
between 10°-20°C is significantly fresher in the eastern gyre than in
the western Pacific. This temperature range corresponds to water depths
between 50-400 m. The depth of the nitricline also changes between the
eastern and western Pacific, as seen in the vertical section of nitrate
from the P2 line in Figure 9. While there is no longitudinal gradient at
100 m depth, at 200 m the average nitrate (and phosphate, not shown)
value east of 180° is double that in the western Pacific (Figure 9). It
is possible that the higher nutrients levels closer to the surface in
the east support more abundant *Rhizosolenia* mat populations (Figure
5), resulting in blooms only in the eastern gyre. The depths with higher
nutrient levels are within the range exploited by *Rhizosolenia* mats.
Video plankton recorder imagery mounted on an ROV during the 2002 cruise
observed mats down to 300 m depth (Pilskaln et al., 2005).

There are other physical differences between the eastern gyre and the
rest of the Pacific. There is an abrupt decrease in eddy activity east
of 170°W (Bernstein and White, 1977), and the surface currents are weak
and more convergent than the same latitude in the western Pacific
(Figure 10). This convergence is clearly evidenced by surface drifters
that tend to collect within the eastern gyre. Since direct analysis of
these Lagrangian data is biased toward the sparse locations of massive
deployments of drifters, a model has been developed using the statistics
of drifter trajectories to describe the horizontal advection of a
passive tracer (Maximenko, manuscript in preparation). As seen in Figure
11 a passive tracer advected in the same way as drifters will become
heavily concentrated in the eastern gyre relative to the rest of the
North Pacific. The calm and convergent conditions in the eastern gyre
would allow buoyant organisms such as *Rhizosolenia* to accumulate in
large numbers at the surface, and would favor N~2~-fixation. The
development of surface patches of *Rhizosolenia* diatoms in tropical
Pacific instability waves has been associated with areas of convergence
(Yoder et al., 1994), and this has been noted for other particles
(Dandonneau et al., 2003) including plastic debris and *Velella* (Smith
and Hyrenbach, 2003).

# 6. Potential Impacts 

The biological forcings hypothesized for the blooms are important since
both nitrogen fixation and mat migration introduce new nitrogen into the
euphotic zone independently of vertical mixing processes. A
phytoplankton bloom responding to upwelled nitrate can produce elevated
export rates, but there can be no net carbon flux to depth unless the
upwelling of nitrate and CO~2~ are uncoupled, whereas a bloom driven by
nitrogen fixation is necessarily uncoupled from upward CO~2~ flux.
Transport of nitrate by phytoplankton vertical migration also uncouples
nitrate from subsurface carbon pools, and permits net export of carbon
from the euphotic zone. Diatom blooms in particular are important
mechanisms for sequestering carbon into the deep ocean (Goldman, 1988),
as they can settle rapidly and are capable of highly episodic export
from the surface (Scharek et al., 1999a; Scharek et al., 1999b).
Locally, new production resulting from vertical mat migration could be
nearly 50% of that from the diffusive flux of nitrate (Villareal et al.,
1999).

The blooms could also have an impact on higher tropic levels in the
ecosystem. For example, seabirds use the subtropical convergence as a
migratory highway and can smell phytoplankton blooms (Hay and Kubanek,
2002; Nevitt, 2000). The bloom region is also within important habitat
for highly migratory species such as albacore, bigeye tuna and
billfishes (Anda-Montañez et al., 2004; Laurs et al., 1984; Musyl et
al., 2003; Sibert et al., 2003), and within the target area of important
longline fisheries (Bigelow et al., 1999; Seki et al., 2002). The blooms
may represent a highly localized food source for prey species, and could
be important congregation points at various parts of these predators'
life histories. However, currently there are insufficient fisheries data
coincident within the chlorophyll blooms to ascertain their impact
(Wilson, unpublished analysis).

# 6. Summary 

Our analysis of existing data does not rule out either of the
hypothesized biological mechanisms for the blooms. Climatologically, the
bloom region has higher abundances of vertically migrating
*Rhizosolenia* mats, and high mat numbers were observed near the 2002
chlorophyll bloom. More nitrogen fixation was also observed in general
proximity to the 2002 bloom, both directly measured, and inferred from
higher levels of endosymbiotic diatoms. The blooms occur in the eastern
gyre of the North Pacific, a region characterized by weak, yet
converging, surface currents, a physical environment conducive for
aggregating buoyant organisms in large numbers. More direct in situ
sampling of the chlorophyll blooms is needed to better understand their
composition and the mechanisms behind their formation. Better
understanding of the physical conditions under which blooms develop will
be important for efforts to model the global effects of diazotrophy on
the carbon cycle (LaRoche and Breitbarth, 2005).

# Acknowledgements 

Thanks to the SeaWiFS Project and the DAAC at NASA/GSFC for the
satellite chlorophyll data, to the CLIVAR program for data from the P2
line along 30°N, and to Xuemei Qiu for assistance with data analysis and
figure generation. This paper benefited from stimulating discussions
with Don Olsen and Yvette Spitz about the dynamics of the eastern
Pacific, and from constructive comments from two anonymous reviewers.
Support for this work was provided in part by a NASA grant
OCEAN-0046-0204 to CW and by NSF OCE 0094591 to TAV. NM was partly
supported by NASA grant NNG04GH81G and by the Japan Agency for
Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of
IPRC. Contribution number XXX from the Marine Science Institute and
IPRC/SOEST contribution xxx/yyyy.

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  ------------------- ------------ --------------------------------------
  **Cruise**          **Dates**    **References**

  R/V *Cayuse*        Oct 1980     (Alldredge and Silver, 1982)

  R/V *Cayuse*        Sep 1981     (Martínez et al., 1983; Martínez,
                                   1982)

  R/V *Thomas         Mar/Apr 1989 (Villareal and Carpenter, 1989)
  Washington*                      

  R/V *Wecoma*        Aug 1992     (Villareal et al., 1996)

  R/V *Wecoma*        Sep 1992     (Villareal et al., 1993)

  R/V *New Horizon*   May/Jun 1993 (Villareal et al., 1996)

  R/V *Moana Wave*    Aug 1995     (Brzezinski et al., 1998; Shipe et
                                   al., 1999)

  R/V *New Horizon*   Jul/Aug 1996 (Brzezinski et al., 1998; McKay et
                                   al., 2000; Shipe et al., 1999;
                                   Villareal et al., 1999)

  R/V *Melville*      Jul/Aug 2002 (Montoya et al., 2004; Pilskaln et
                                   al., 2005; Singler and Villareal,
                                   2005)

  R/V *New Horizon*   Aug/Sep 2003 (Pilskaln et al., 2005; Singler and
                                   Villareal, 2005)

  R/V *Melville*      Jun/Jul 2004 CLIVAR P2, leg 1

  R/V *Melville*      Jul/Aug 2004 CLIVAR P2, leg 2
  ------------------- ------------ --------------------------------------

Table 1. Summary of the cruises that collected the data presented in
this paper.

![](media/image1.pct){width="6.294444444444444in"
height="2.1458333333333335in"}

Figure 1. Map showing the location of cruises whose biological data are
presented in this paper. References resulting from these cruises are
summarized in Table 1. The gray area indicates the region where
chlorophyll blooms are observed from satellite data (see Fig 2).

![](media/image2.png){width="6.290277777777778in"
height="3.2111111111111112in"}

![](media/image3.png){width="6.291666666666667in"
height="0.7236111111111111in"}

Figure 2. Percentage of time (using monthly images) that SeaWiFS
chlorophyll is above 0.15 mg/m^3^ during July-October, of 1997-2004,
showing the integrated region of the episodic chlorophyll blooms at 30°N
between 140°-155°W. A bloom lasting a single month has an associated
value of \~3%. White dots indicate the locations of the HOT, Climax and
"M" stations.

![](media/image4.png){width="5.833333333333333in"
height="2.1013888888888888in"}

Figure 3. The annual chlorophyll cycle from the region of each year's
bloom as observed by SeaWiFS data (1997-2004). The black lines are the
average chlorophyll concentration from two boxes west (160-162°W,
29-33°N) and east (160-162°W, 29-33°N) of the bloom region to
demonstrate how the meridional change in the location of the TZCF
affects the winter chlorophyll variability in the bloom region.

![](media/image5.png){width="5.935416666666667in"
height="1.5298611111111111in"}

Figure 4. Interannual occurrence of the chlorophyll blooms (shown in
yellow) superimposed on the NINO3 index. Large positive (negative)
anomalies indicate El Niño (La Niña) events. The gray box indicates a
period with no available satellite data.

![](media/image6.png){width="6.2965277777777775in" height="3.7125in"}

Figure 5. Spatial distribution of the abundance of *Rhizosolenia* mats
in the North Pacific, compiled from cruises listed in Table 1. The
concentration of samples near 30°N is due to weather conditions; the
trade winds increase north and south of 30°N, impeding sampling. The
shaded green area indicates the bloom region.

![](media/image7.jpeg){width="2.6979166666666665in"
height="0.6055555555555555in"}![](media/image8.jpeg){width="2.6979166666666665in"
height="0.6055555555555555in"}![](media/image9.jpeg){width="2.6909722222222223in"
height="0.9125in"}![](media/image10.jpeg){width="2.6909722222222223in"
height="0.9215277777777777in"}![](media/image11.jpeg){width="2.6909722222222223in"
height="0.9215277777777777in"}![](media/image12.jpeg){width="2.6909722222222223in"
height="0.9215277777777777in"}![](media/image13.jpeg){width="2.6979166666666665in"
height="1.0027777777777778in"}![](media/image14.jpeg){width="2.6979166666666665in"
height="1.0027777777777778in"}

Figure 6. SeaWiFS image of the 2002 chlorophyll bloom overlaid with the
relative abundances of (top) *Rhizosolenia* mats, mats/m^3^ (middle)
*Rhizosolenia* diatoms containing the symbiotic *Richelia*, cell/m^3^
and (bottom) unicellular rates of nitrogen fixation, nmol L^-1^ hr^-1^.
On the left is the weekly image from July 4-11, 2002 (most concurrent
with the in situ samples), on the right is the monthly composite from
August, demonstrating changes in the bloom's shape and location. The in
situ data were collected between June 24-July 10.

![](media/image15.jpeg){width="2.1805555555555554in"
height="1.6347222222222222in"}

Figure 7. *Hemiaulus* cells in surface samples containing symbiotic
*Richelia* as a percentage of all *Hemiaulus* against longitude. The
transect extended from 27.5°-31.1°N in May-June, 1993.

![](media/image16.jpeg){width="6.291666666666667in"
height="4.113888888888889in"}

Figure 8. Temperature-salinity diagram from the P2 line in summer 2004
along 30°N. Data from the western Pacific (west of 155°W) are in red,
the eastern Pacific (135-155°W) are in green, and the fresher water from
the California Current (east of 135°W) are in blue.

![](media/image17.png){width="6.294444444444444in"
height="4.863888888888889in"}

Figure 9. Nitrate section along 30°N from the P2 line in summer 2004
showing the longitudinal nutrient gradient below 150 m.

![](media/image18.jpeg){width="6.288888888888889in"
height="4.715972222222222in"}

Figure 10. 1992-2002 mean currents at 15 m depth from Maximenko and
Niiler (2004). Colors show velocity magnitude. The bloom region is
outlined in red.

![](media/image19.jpeg){width="6.297916666666667in"
height="4.863194444444445in"}

Figure 11. The relative increase of a passive tracer after 10 years of
evolution from an initially uniform state. Output derived from a model
using the statistics of drifter trajectories (to remove biases
associated with irregularly spaced drifter deployments) to describe the
horizontal advection of a passive tracer (Maximenko, manuscript in
preparation).