Collaborative Research: MOTIV: Plankton Community Responses and Biogeochemical
Implications from a Tropical Instability Vortex in the HNLC Equatorial Pacific
Results from Current/Prior NSF Support
M.R. Landry: OCE-9022117, Microzooplankton Grazing Interactions in the Central Equatorial
Pacific, $365,000, 8/91-7/94. OCE-9218152, Zooplankton Variability and Particulate Fluxes at
Station ALOHA: A Contribution to the Hawaii Ocean Time-series (HOT) Program, $262,000,
8/93-7/98. OCE-9311246, Response of Microbial Community Structure and Protistan Grazing
Pathways to Environmental Forcing in the Arabian Sea, L. Campbell (co-P.I.), $519K, 6/94-5/98.
These projects were components of US JGOFS field investigations of plankton community
structure, production and biogeochemical cycling in tropical/subtropical open-ocean ecosystems.
In the equatorial Pacific, we studied the distributions and abundances of pico-phytoplankton, the
growth rates of phytoplankton and the grazing impact of microzooplankton along EqPac transects
(140°W) during El Niño and normal upwelling conditions. The emphasis of this work was on
understanding temporal and latitudinal variations and the dual regulatory influences of ironlimitation and grazing on phytoplankton standing stocks and production. As part of this effort, we
also participated in the IronEx II mesoscale enrichment, for which we synthesized the biological
responses (abundances and biomass of bacteria, phytoplankton, microzooplankton and
mesozooplankton; taxon-specific rates of phytoplankton growth and micro- and mesozooplankton
grazing) to iron fertilization. In the Arabian Sea, we investigated the influences of strong physical
(seasonal) forcing on the distributions, abundance and growth rates of phytoplankton and the
extent to which environmental regulation of community size structure determines grazing
pathways through micro- and mesozooplankton, and ultimately export fluxes. Lastly, in the HOT
Program, we have documented the seasonal cycle of mesozooplankton, its relation to the summer
period of enhanced nitrogen fixation, and the contribution of zooplankton vertical migrations to
carbon and nitrogen export fluxes. We have also conducted experimental studies of grazing
cascades (meso- and microzooplankton) on the structure and growth responses of lower trophic
levels (phytoplankton and bacteria). Data and insights from these combined projects provide a rich
base for developing and testing concepts relating to the structure and dynamics of tropical ocean
ecosystems. NSF support for these studies has been acknowledged in 43 papers to date (listed in
Reference section). These projects provided salary support and/or research opportunities for 14
graduate students, 2 post-docs and several undergraduates. Various aspects of this project were
included in 6 M.S. Theses and 5 Ph.D. Dissertations.
R.R. Bidigare: OCE-9022321, Photo-ecological Investigations of Phytoplankton of the Central
Equatorial Pacific Ocean, $252,865, 9/91-8/94.
During the 1992 U.S. JGOFS Equatorial Pacific (EqPac) study, we participated in survey and
time-series cruises to identify the factors controlling spatial and temporal variations in
phytoplankton biomass, community structure and productivity. We used the following pigments
as diagnostic markers: Chl a (phytoplankton biomass); divinyl chlorophyll a (Prochlorococcus);
phycoerythrin (cyanobacteria, including Synechococcus and Trichodesmium); 19'hexanoyloxyfucoxanthin (prymnesiophytes); 19'-butanoyloxyfucoxanthin (pelagophytes);
fucoxanthin (diatoms); and peridinin (dinoflagellates). Distributions of phytoplankton pigments
measured during early 1992 (El Niño conditions) were markedly different from late 1992
("normal" conditions). The El Niño event produced a significant reduction in eukaryotic
phytoplankton biomass, especially prymnesiophytes, pelagophytes and diatoms. Nitrate uptake
rates were positively correlated with diatom Chl a biomass during EqPac, suggesting that the
HNLC condition at our study site can be explained in part by factors that restrict the success of
diatoms. Pigment analyses were also performed in conjunction with dilution experiments to
determine growth and mortality rates for the dominant phytoplankton groups. A bio-optical model
was developed and used to estimate nearly continuous rates of primary production from moored
measurements. Time-series analysis of these data document that Kelvin waves and Tropical
Instability Waves (TIWs) are important in controlling the variability of primary production rates at
the equator. Pigment-specific carbon isotopic analyses (13Cchl and 13Cphytol) were used to develop
a new approach for estimating phytoplankton growth rates without the need for incubation.
Finally, our participation in IronEx I was supported via this project. This latter study demonstrated
that phytoplankton production rates in equatorial Pacific waters are regulated by the availability of
iron. In summary, our EqPac findings suggest that phytoplankton biomass and production rates in
the equatorial Pacific are controlled by a combination of physical (Kelvin waves, TIWs and
advection), chemical (iron limitation) and biological (growth-grazing imbalance) processes. This
project provided salary support and/or research opportunities for five graduate students and one
post-doctoral fellow. To date, 14 publications have been supported by this NSF research (listed in
Reference Section).
C. Measures: OCE-0117917, Collaborative research: Biogeochemistry of trace elements in the
western Pacific: atmospheric input and upper ocean cycling, $289,569; 08/01-07/04.
The 2002 IOC western Pacific cruise was designed to examine the role that the massive annual
outbreaks of dust, emanating from the desert regions of Asia, play in the addition of bioactive
materials to the surface of the western Pacific Ocean. Additionally, the cruise was designed to
investigate what synergistic role anthropogenic emissions of sulphur and nitrogen oxides from the
large population centres in Asia might play in promoting the solubility of this atmospheric dust
and delivering anthropogenic pollutants to the surface of the western Pacific Ocean. Initial
shipboard results using dissolved Al values indicate that despite the massive offshore atmospheric
transport of dust recorded by satellite imagery this winter, actual dust deposition to the oceanic
surface waters of this region was modest, in the range of 0.2 – 0.8 g mineral dust m-2 yr-1. These
values contrast sharply with computer deposition models for this region which predict ~10 g
mineral dust m-2 yr-1. These lower measured deposition values, and the accompanying low
dissolved surface water Fe values, are consistent with the High Nutrient Low Chlorophyll
(HNLC) status of the waters north of the Kuroshio Current. The initial Al:Fe ratios also indicate
that the surface waters of the NW Pacific region are similar to those of the Pacific section of the
Southern Ocean around Antarctica, despite that fact that the NW Pacific underlies one of the most
significant continent to ocean dust transfer paths. Data reduction is continuing as part of the
Master’s thesis of M. Brown it is expected that this will be completed in the Summer/Fall 2003
and the resulting data set will be published in a special volume with other participants in the
cruise.
C. Benitez-Nelson: OCE-9906634, Radioisotopic Measurements to Determine Plankton Nutrient
Sources and Carbon Export at Station ALOHA, $219,999, 9/99-08/03.
The primary goal of this research is to elucidate phosphorus (P) cycling and particulate organic
carbon export in the North Pacific Gyre using the short-lived radionuclides, 32P (t½_ = 14.3 d), 33P
(t½_ = 25.3 d) and 234Th (t½ = 24.1 d). Great strides have been made in elucidating the variability
of 234Th export and P cycling in this supposedly oligotrophic and steady state regime. A new,
small-volume technique was developed for the measurement of 234Th such that it is now possible
to obtain excellent depth resolution in particle formation and remineralization over the upper
water column. Based on results from this approach, 234Th export fluxes can vary dramatically
from cruise to cruise indicating that the regime is not in steady state over 30-day time scales.
234
Th-derived POC export rates are 60% higher than measured by sediment traps, enhancing the
magnitude of carbon export from this oligotrophic regime to the global carbon total. Rain
collectors have been established on the islands of Kauai and Hawaii to obtain the ratios and fluxes
of 32P and 33P to Station ALOHA. Results indicate that the ratio is constant, but lower than that
measured in coastal New England. Ratios of 33P/32P in the total dissolved P pool indicate that
residence times vary markedly from month to month. Highest residence times during periods of
high productivity suggest that plankton are preferentially utilizing the bioavailable P pool when
stressed. 33P/32P ratios in zooplankton also show surprisingly high variability, with 2- to 4-fold
increases in P residence times from summer to winter. I am continuing 234Th sampling and have
recently completed separate measurements of 32P and 33P in dissolved inorganic and organic P
pools. Ancillary measurements of C, N and P in plankton are underway. To date, 4 publications
and 2 presentations have been supported by this NSF research (see separate listing in Reference
Section).
Introduction
The plankton biology of the surface oceans – its size and trophic structures, its productivity and
ultimately its role in global biogeochemical cycles – is intricately linked to ocean physics via
thermal constraints on rate processes, stratification and mixing processes, nutrient delivery
mechanisms, and circulation. The overall effects are complicated, however, by disproportionate
and non-linear biological responses to episodic phenomena, such that a system’s mean state often
poorly represents time-integrated rates at the regional scale. The present proposal focuses on the
dominant mode of mesoscale variability in the equatorial Pacific, tropical instability waves
(TIWs) and their associated vortices (TIVs). We seek to achieve a better understanding of
community dynamics, production and biogeochemical fluxes in the equatorial Pacific by
investigating ecosystem’s variability and responses in the cycloid flow field of a TIV.
TIWs and TIVs are dynamic and episodic, but seasonally predictable, phenomena that appear
in satellite images as spectacular north-south undulations of surface temperature and chlorophyll
(e.g., Legeckis 1977, 1986, Chavez et al. 1999). Upon examining such features, one is left with
many questions about their perturbations of the distributions and process rates of the entrained
communities and the physical and biological mechanisms that regulate them. In the Atlantic,
distributional fields of phytoplankton to small fish appear to be well organized at the vortex scale
(Menkes et al., 2002). Less is known about mesoscale relationships in Pacific TIVs, although
chance crossings of TIW fronts during JGOFS process studies have documented strong signals in
plankton stocks and rates (Murray et al. 1994, Yoder et al. 1994, Dam et al. 1996, Archer et al.
1997, Foley et al. 1997, Roman & Gauzens 1997, Le Borgne et al. 2002) as well as indices of
biogeochemical fluxes (Walsh et al. 1997, Dunne et al. 2002). Simulation and data assimilation
studies indicate that simple food web structures, which adequately represent the dynamics of the
equatorial plankton community under general system conditions, break down during active TIW
periods (e.g., Friedrichs 2002). Interpretations are further complicated by the fact that the HNLC
(High-Nutient, Low-Chlorophyll) equatorial Pacific is an iron-limited ecosystem, but neither Fe
concentrations nor the physiological and biomass responses linked to Fe enhancement have been
studied in a TIV frame of reference. Consequently, there is a considerable divergence of opinions
regarding the mechanisms (“eddy pumping” versus advective transport) that best characterize how
the limiting element becomes available and utilized within a TIV to produce the effects observed
(Morliere et al. 1994, Barber et al. 1996, Foley et al. 1997, Friedrichs & Hofmann 2001, Kennan
& Flament 2000, Strutton et al., 2001, Menkes et al., 2002). These are all issues that stimulate the
research proposed here.
We propose to address these issues as part of a focused interdisciplinary and international
collaboration (MOTIV - Multiple Observations in a Tropical Instability Vortex) to investigate the
physical, biological and biogeochemical characteristics of a TIV in the HNLC equatorial Pacific.
As detailed below, MOTIV has three components. The first is a French effort (C. Menkes, P.I.)
which provides both a research vessel and personnel for a well-rounded study of chemical and
biological distributions and process rates following the path of mass transport in the TIV circulation.
In a companion proposal, NSF funding and a U.S. ship are requested (S. Kennan, P.I.) for a
coordinated and complementary investigation of the large-scale physics and distributional fields of
the TIV in which the French process studies will be conducted. We propose to contribute to
MOTIV with measurements on both ships that will link the concentrations and sources of Fe to their
effects on microbial community composition, biomass structure, growth and grazing rates, cellular
Fe and Si concentrations, and ultimately to aggregate distributions and export fluxes.
Objectives
The overall objectives of this project are to determine how (mechanistically) and where
production, plankton community biomass and export flux are enhanced in the circulation field of a
TIV and to quantify their effects on trophic processes and carbon export. As a relatively small,
but central, component of MOTIV, we focus on a suite of measurements that will both answer
fundamental questions of community and flux responses and provide essential information for
modeling TIV influences on the regional scale. Our specific objectives are:
1. To elucidate the sources and mechanisms of Fe and nutrient delivery to the euphotic zone of
waters entrained in the TIV circulation;
2. To quantify the variability of microplankton community composition and biomass in the TIV
circulation and their relations to measured pigment concentrations (extracted and observed in
satellite imagery);
3. To assess phytoplankton growth, microzooplankton grazing, cellular Fe and Si contents and
other physiological indices of Fe responses in relation position in the TIV circulation and with
respect to concurrent studies of other plankton process rates and physiological responses (e.g.,
FRRF, primary production, 15N- and 30Si-uptake, TEP-production, mesozooplankton grazing);
4. To document TIV influences and spatial patterns with respect to particle distributions and 234Th
estimates of carbon fluxes.
Background
TIVCharacteristics from Previous Studies: In the alternating system of zonal
currents in the tropical Pacific, the surface westward South Equatorial
Current (SEC) straddles the eastward flowing Equatorial Undercurrent
(EUC) at depth at the equator, and the eastward flowing North Equatorial
Countercurrent (NECC) lies to the north. The southeast trades induce
upwelling at the equator, forming the North Equatorial Front (NEF)
between the cold equatorial tongue and warmer waters to the north. Each
year in early summer to fall, when the inter-tropical convergence zone
(ITCZ) migrates northward, southeast trades accelerate the current
systems, and meridional oscillations of the horizontal currents are
observed at periods of 15-35 d and zonal separations of 500-1500 km.
Since their first detection in current meter records as meanders of the Atlantic SEC (Düing et al.
1975) and as cusp-like deformations in satellite infrared images of the Pacific equatorial front
(Legeckis 1977), tropical instabilities have been found to have various manifestations: In the
tropical Pacific, they appear as oscillations of the EUC and SEC (Lukas 1987, Halpern et al. 1988,
Qiao & Weisberg 1995), meridional deformations of the equatorial front (Legeckis 1986, Pullen et
al. 1987), sea-level highs in SEC-NECC shear (Miller et al. 1985, Perigaud 1990), anticyclonic
vortices north of the equator (Hansen & Paul 1984, Chew & Bushnell 1990), and “lines in the sea”
visible from space (Yoder et al. 1994). Their observed meridional eddy fluxes of heat and
momentum are comparable to those associated with the annual mean circulation (Bryden & Brady
1989, Johnson & Luther 1994, Baturin & Niiler 1997).
The long spatial scales of the NEF meanders seen in early satellite infared images led to the
original name “equatorial long waves'', which later evolved to TIWs when it was learned that the
waves were generated from unstable mean currents. However, it has become increasingly evident
that the cold water cusps considered the foremost characteristic of tropical instabilities are
associated with the anticyclonic circulation of vortices (hence,
TIV). These vortices are approximately 500 km in diameter, half
the typical wavelength of the SST undulations, and they shape the
tracer distributions. In the Pacific TIV studied by Flament et al
(1996), cold tongue water entrained in the eddy moved northward
until it was subducted at the NEF. The same pattern was observed
in the PICOLO experiment in the tropical Atlantic (Menkes et al.
2002). Both the Pacific and Atlantic observations also exhibited
robust dipoles of convergent and divergent currents, which were
predicted numerically well before they were demonstrated
(Philander et al. 1986). Converging flow at the leading edge of
the TIV, leads to rapid downwelling there and sharply delineates
concentrations of buoyant plankton (e.g., Yoder et al. 1994,
Archer et al. 1997) –Indeed, in the picolo, there is no northward
flow right at the edge... Flows near the TIV center are divergent,
with implied vertical velocities at the pycnocline of ~ 50 m d-1
(Semtner & Chervin 1992, Harrison 1996, Kennan &
Flament 2000).
In June 1997, the observational program PICOLO
was conducted in the eastern tropical Atlantic to assess
the effect of a TIV on the distributions of nutrients,
plankton and nekton. Ocean currents were measured
with shipboard ADCP (acoustic doppler current
profiler) and 10 surface drifting buoys, and 75
hydrographic stations were conducted to 250 m with
20-km spacing. Because flows in the TIV were
coherent down to the pyclocline, the results of
PICOLO can be most easily illustrated with respect to
the depth-averaged circulation (Figure 1). Overall, cooler equatorial waters with the highest
nutrient concentrations were transported poleward in the TIV (west of 18°W). Areas of enhanced
productivity (~ 3 gC m-2 d-1) and concentrations of zooplankton and micronekton (dominated by the
small fish) follow sequentially in the flow field, with large tunas concentrated along the leading
edge of the front. Warmer, nitrate-, chlorophyll- and zooplankton-poor waters moved equatorward
in this circulation east of 18°W. As suggested by the pattern of surface convergence and divergence
(Figure 2), the distributions of biota were also influenced by vertical motions. In particular, the
intense convergence along the leading edge of the TIV led to subduction, creating sharp gradients in
all observed fields. The downwelled water at the front carried the near-surface community to a deep
chlorophyll maximum at the base of the euphotic zone of the overlying subtropical waters. These
waters returned to the vortex in the divergence area depicted in approximately the middle of the TIV
in Fig. 3. Thus, the anticipated path of transport within the TIV circulation is circular in a fixed
frame of reference, with a distinct SW to NE tilt which organizes the community responses into
distinct zones of nutrient input and uptake, primary production, grazing and presumably export
processes.
Issues for a TIV Study in the HNLC Equatorial Pacific: The two previous investigations of TIVs
(TIWE – Flament et al. 1996, Kennan and Flament 2000) and PICOLO – Menkes et al. 2002) have
successfully demonstrated an approach to their study in a moving frame of reference. They have also
provided, as summarized above, a coherent framework for their understanding as a coupled physicalbiological system. The biological and biogeochemical measurements to date, however, have been
relatively limited and, the observations from the Atlantic do not readily translate to the Pacific due to the
confounding influences of micro-nutrient limitation by iron. Fe distribution has never been mapped in a
TIV, nor do we know exactly where and how Fe is enhanced by the vortex circulation. Phytoplankton
responses are not simply those observed in chlorophyll concentrations, from extracted pigments or
satellite algorithms, but can involve major changes in community composition and size structure, in part
due to grazing activities which control some populations, but not others. Lastly, the biogeochemical flux
implications of TIVs have not been considered in past studies. These issues are central to our proposed
research and are considered further below.
In early observations of the enhancing effect of instability waves on chlorophyll
concentrations, “eddy pumping” was invoked as one of the potential mechanisms (e.g., Morliere
et al. 1994). According to this hypothesis, the eddy circulation should bring the pycnocline, and
consequently the nutrients residing in and below it, vertically upward so that the waters within the
euphotic zone are enriched (Falkowski et al. 1991). Such an eddy would be cyclonic with a raised
pycnocline in its center. That’s true if the physics is geostrophic but the upwelling in TIV is not
driven by this. Alternatively, the frictional decay of anticyclonic eddies may be associated with a
raising of the pycnocline (Franks et al. 1986). However, TIVs are anticyclonic with centrally
depressed pycnoclines, and they appear to maintain vigorous circulations as they progress
westward across the tropical Pacific. Moreover, the TIV circulation is trapped to the surface layer
above the pycnocline, and there is no evidence that upwelling near the center crosses the
pycnocline. I did not really understand the previous paragraph. The upwelling motion is really
peculiar to TIVs. Sean has specific ideas about that. May be you should state that the upwelling is
linked to a secondary circulation within the geostrophic flow. Thus, the physicists involved in the
MOTIV Program hypothesize that the main nutrient input to the TIV circulation is advective
transport from the equator. I think I partly disagree with Sean on this. My feelling is that there is
strong upwelling in the center that goes well within the thermocline. This brings in nutrients.
However, they are swept away immediately as they enter the vortex horizontal circulation and
moved back to the equator so it may be that this is not an efficient process but even if it was, the
horizontal circulation is so strong that the effect would be blurred by the oligotrophic conditions
of the returning flow. This is what happens to the SST actually. The source of cool SST is just
overwhelmed by the horizontal advection of warm waters from the north?. This is clearly on
opened question….This would link Fe input to the upwelling of Equatorial Undercurrent source
waters at the equator (e.g., Coale et al. 1996), a process that could be physically enhanced by the
observed trade wind anomalies coupled to the SST gradients at the TIV scale (Liu et al. 2000,
Chelton et al. 2001). I disagree with that. The winds actually slow down in the cool region and
accelerate in the warm region. So they tend to have a negative feedback on the TIV circulation, if
I am not mistaken. Or may be I don’t understand what you mean there ?
Although the effects of vortices on distributions of temperature, fresh water, and momentum
support the notion that horizontal advection is the dominant mechanism of instability-induced
bulk fluxes in the equatorial Pacific (Hansen & Paul 1984, Baturin & Niiler 1997, Kennan &
Flament 2000), iron inputs may be more subtle since so little is required for a biological response.
For example, EUC upwelling is clearly the most important source of new Fe immediately on the
equator, exceeding atmospheric input by about an order of magnitude on a daily rate basis (Coale
et al. 1996). Once that water is advected into the TIV circulation, however, the atmosphere input
continues in the absence of new EUC upwelling, such that both sources provide approximately the
same amount of Fe to the euphotic zone over the course of ~10 days. The near-surface niche of
the large, buoyant diatoms (Rhizosolenia) that concentrate in the front region (Yoder et al. 1994)
may be one indication that an atmospheric source could be important. Again I am unsure about
this, the wind diverges at the leading edge and converge in the trailing edge so they would actually
bring Fe in the trailing edge, no ? In this regard, regionally organized patterns of clouds and
precipitation are associated with the instabilities (Liu et al. 2000, Hashizume et al. 2001), and
might provide more Fe locally than the regional mean rate of atmospheric input. It is also possible
to consider that the collision of high-nutrient, Fe-deplete equatorial waters with low-nutrient, Fereplete subtropical waters at the TIV front could simulate production there by their mixing.
Lastly, it is reasonable to anticipate that the central divergence zone of the TIV itself could be a
source of Fe generated from remineralization of the organic matter downwelled at the front, or
enriched with Fe entrained from the deep euphotic zone of subtropical waters. Such sources
would presumably be possible to distinguish (or model), at least to first-order, from knowledge of
the Fe distribution and circulation of the vortex, using ratios of Fe:Al to infer rates of atmospheric
input (Measures REF).
In describing the effect of the TIW sampled at 140°W during the JGOFS EqPac Program,
Barber et al. (1996) likened it to a natural analog of a small-scale Fe-enrichment experiment. We,
thus, take the results from the IronEx II experiment as a reasonable basis for comparison and/or
prediction. The IronEx experiment showed clearly that there is first a physiological response of
phytoplankton to added Fe, representing a shift-up in photosystem efficiency as measured by FastRepetition Rate Fluorometry (Behrenfield , Kolber ). On the time-scale of a day or less, the
chlorophyll content per cell also responded rapidly, decreasing cellular C:Chl ratio by
approximately a factor of 4 (Landry et al. 2000a). Actual phytoplankton growth rate, measured as
a specific rate of cell increase, lagged these physiological responses by a day or two, but
ultimately all components of the community, including the photosynthetic bacterium
Prochlorococcus, responded with higher rates of cell division (Landry et al. 2000b, Chisholm
). Nonetheless, grazing by protistan microzooplankton efficiently controlled the abundances and
biomass of the smaller phytoplankton cells (approximately everything < 10 µm), such that their
concentrations remained comparable to those in the ambient environment, even at the peak of the
fertilization bloom (Landry et al. 2000b, Landry 2002). Thus, larger pennate diatoms grew to
dominate the phytoplankton bloom assemblage. Lastly, both the abundance and the grazing
impact of larger protistan microzooplankton increased in response to the diatoms, such that ~6
days after the initial Fe fertilization the community had shifted up to a new dynamic equilibrium
of balanced growth and grazing and 5-fold higher phytoplankton biomass (Landry et al. 2000b).
As a first approximation, a water parcel coming from vigorous equatorial upwelling and being
entrained into a TIV would likely exhibit such a succession.
There may be differences, however, in the downstream consequences for such a parcel
compared to observations in IronEx. For one, IronEx II did not elicit a substantial response of the
mesozooplankton community (Rollwagen Bollens & Landry 2000), in contrast to direct
observations of elevated zooplankton stocks for both Atlantic and Pacific TIVs. During the EqPac
study, for example, Roman & Gauzens (1997) reported 6-fold increases in copepod biomass,
weight-specific filtration rates and grazing impacts associated with the passage of an instability
wave. One might surmise from this that the zooplankton community may be able to exploit
circulation features of the TIV to maintain themselves in a concentration center downstream of the
area of highest phytoplankton production and biomass. The alternative, that they achieve high
mass entirely during the several days of transit from the equator to the front seems less realistic,
particularly given the evidence for substantially less than optimal growth conditions for
zooplankton in ambient equatorial waters (Rollwagen Bollens & Landry 2000). Either way, the
presence of a significant mesozooplankton grazing filter in the in the TIV circulation has
implications for export via their production of sinking fecal pellets. Both the concentration of
particulates in the TIV convergent front (aggregate formation) and the rapid sinking of fresh
organic particulates at the front could also have important implications in this regard. It is logical
to expect that excess TIV production downwelled at the front might be efficiently exported
through the base of the subtropical euphotic zone during the return loop to surface waters at the
TIV divergence. Furthermore, the transit time at depth in the northern half of the TIV circulation
should represent a distinct zone of enhanced export linked temporally to the production and
grazing processes in the southern portion. This may illustrate another important difference
between the IronEx results and TIV dynamics. Export, measured by the 232Th method, increased
in the IronEx II bloom but only in proportion to increased production (Bidigare et al. 1999). This
reflected the new balance among production, microzooplankton grazing and remineralization
processes in which the essential features of a microzooplankton-dominated, low export system
were maintained (Landry et al. 2000b). In the TIV circulation system, it seems likely that there is
a more efficient coupling between production and export processes.
Proposed Research
MOTIV Overview, Status and Contingencies: The MOTIV Program has been designed optimally
as a collaborative, two-ship (one French, one US) experiment for late-summer, early-autumn 2004.
The French ship (R/V l’Atalante) will be responsible for conducting the main biological and
biogeochemical process-oriented component of the research plan. As more fully detailed below, this
will be done following the path of mass transport in the moving frame-of-reference of a TIV. While
the French ship will have a sufficient physical component to function as a stand-alone project if need
be, its limited design provides no opportunity for assessing the distributional fields of relevant
physical, chemical and biological variables. The US ship (nominally, R/V Revelle) will therefore
provide the broad-scale context for the process-oriented studies while elucidating physical rates and
mechanisms. Although we have sampling responsibilities on both ships, the present proposal
emphasizes our primary involvement as the US component of the process studies. Accordingly, we
have clearly delineated, in the Budget Justifications for each P.I., a core request for participating on the
French ship from the additional resources needed for measurements proposed on the US ship.
Prior to submitting the present proposal and its PO complement (Kennan et al.), l’Atalante had
been cleared for studies in the Pacific Ocean, and MOTIV (C. Menkes, P.I.) had successfully
passed its first and second scientific reviews. Funding has also been provided for modeling efforts
that will both support and incorporate results from the proposed field experiment. All that
remains is the formal scheduling. While neither US nor French commitments are thus entirely
firm at this point, the French plans are quite far along. The long lead-times required for ship
scheduling purposes on both sides necessitate semi-parallel, rather than serial, evaluations of the
proposed science.
TIVs occur reliably in our target study area (135-145°W) in the equatorial Pacific from August
to November. Their frontal features are particularly developed from 2-3°N and most tractable for
study during non-El Niño years. For instance, the strong TIW impacts on property distributions
and temporal spatial interpretations for the JGOFS EqPac (Aug-Sept ’92) and EBENE (Oct-Nov
’97) studies were both for non-Niño, fall cruises. The ITCZ is in its northern position during this
season, with atmospheric conditions most favorable for obtaining clear satellite images. We have
thus chosen September-November 2004 as an optimal time for scheduling the MOTIV
experiment. The largest uncertainty for planning purposes is whether 2004 will be an El Niño
year or not. Given the prevailing, and now declining, El Niño of 2002-03, we do not expect this
to be a problem. Based on early forecasts, however, MOTIV could slip to 2005 on l’Atalante’s
schedule and we would exercise funding flexibilities (start date or extension) to accommodate the
experiment in that year. We do note, however, that 2004 and 2005 represent the only viable
window in the foreseeable future for a US-French collaborative study as envisioned here, with full
French resources and participation. French ships were unavailable for research in the Pacific from
1998 to 2003, and l’Atalante will again be reassigned for priority research in other oceans after
2005.
Study Area: The MOTIV experiment is proposed for the area bounded by approximately 0-8°N, 135145°W. This area is slightly to the west of the region of the maximum variability associated with TIW
signals (centered at 130°W, 2°N; Chelton et al. 2000), but it is a reasonable choice for a variety of
practical and historical reasons. EqPac process studies were conducted along longtitude 140°W, and
travel times from the logical departure ports of both French (Tahiti) and US (Hawaii) ships would be
minimized by a site in this vicinity. A line of TAO moorings is located along 140°W, with daily
transmissions of ADCP (acoustic dopler) current profiles at the equator, and moorings in this area have
been instrumented with biologically relevant sensors (Foley et al. 1997, Chavez et al. 1999, Strutton et
al. 2001). The relatively rich history of measurements around 140°W will therefore provide valuable
perspective for planning the experiment, and ultimately interpreting and modeling its results. Given
the typical 1000-km wave length of a TIW, the 135-145°W study region should contain portions of 1-2
such features, and their positions should be apparent from satellite SST and chlorophyll imagery and
TAO current meter velocities in the weeks and days prior to ship departure.
US Ship: Large-scale Sampling: It is useful to consider first a summary of the research plan for the
US ship Revelle, since it provides the large-scale context and the optimal strategy for the full MOTIV
experimental design. Details of the sampling rationale and strategy for the Revelle are fully developed
in the companion proposal, Collaborative Research; MOTIV: Observations of the Physical and
Biochemical Environment (S. Kennan, P. Flament & K. Richards).
Given the overall objective of MOTIV to follow, as closely as possible, a parcel of water moving
in the cycloidal path of a TIV, the Revelle will make three major contributions – 1) an initial
synoptic survey of the entire vortex structure, 2) release and continuous tracking of drifting buoys,
and 3) SeaSoar mapping during the process studies. To account for the observations in the vortex
circulation, information is needed about the entire
vortex structure and especially the characteristic
convergent and divergent dipole flow patterns of such
instabilities. To obtain this information, Revelle will
arrive in the area 9 days before l’Atalante to perform a
synoptic survey of the currents and to deploy drifting
buoys and APEX floats. The survey work will involve
contour mapping with a SeaSoar towed vehicle and
surface sampling (as below) along the zigzag pattern
depicted in Figure 3a. It is unavoidable that the water
column above the pycnocline at any position in the
vortex will not retain its shape over time due to vertical
motions and shears in the horizontal flow. Therefore,
the experimental design accounts for observing the
three-dimensional variations of the circulation in a true
Lagrangian sense using buoys drogued to both 15 and
75 m, as well as isobaric APEX floats. Lagrangian
data, as in TIWE-2 (Flament et al. 1996), is essential
for determining the translation speed of the vortex over
the duration of the experiment. The APEX floats, in particular, will not only provide information on
the flow at depth, but will yield hydrographic profiles twice daily over a region of the vortex that
cannot be sampled regularly by the SeaSoar.
The overall survey plan for the Revelle during the period of process studies is shown in Figure
3b. Using SeaSoar and surface sampling, the R/V Revelle will map the physical and bio-optical
environment orthogonal to the direction of mass transport on an ~80-km scale. The two-ship
experiment occurs over the 22-day period needed to follow the cycloidal transport. During this time
both ships will be positioning based on the real-time trajectories of drifters deployed in the vortex
circulation. L’Atalante will emulate as closely as possible the drifter trajectories in a cycloidal
pattern, and its sampling strategy will repeat every two days (see below). The Revelle will
anticipate l’Atalante’s position by 12 hours and repeat 4 transverse sections across the cycloid path,
followed by transit to the next day's anticipated location - all the while towing SeaSoar. Pass-by
sampling during the surveys will be also be useful in relating (calibrating) sensor measurements
from the SeaSoar to samples collected from l’Atalante (e.g, nitrate measurements and zooplankton
net tows).
To provide basic information on the distributional fields of key variables, the Revelle’s SeaSoar
vehicle will be outfitted with sensors for CTD, nitrate, PAR (photosynthetically active radiation),
chlorophyll fluorescence, beam-cp transmissometry and an OPC (Optical Plankton Counter). Of
these, we are responsible for the OPC data collection and analyses, which will be used to assess
the distributions of mesozooplankton (calibrated by SeaSoar passes in close proximity to net
collections from the French ship) and particle aggregates (from gross differences between total
OPC counts and size distributions relative to those attributable to mesozooplankton). We also
propose to conduct continuous surface (mixed layer) sampling of Fe and Al from a trace-metal
clean water stream pumped onto the ship from a small, independent towed sampler (Measures
REF ???). Lastly, we will sample the mixed layer at approximately hourly intervals during
Revelle SeaSoar surveys for discrete estimates of phytoplankton pigments (HPLC) and population
assessments by flow cytometry (FCM). Microscopical samples will be taken on a less rigorous
schedule, but as needed to complement the pigment and FCM analyses, and particularly to
determine community compositional changes across environmental gradients.
French Ship Plan: Process Study: In presenting the MOTIV plan for the French ship, we take the
conservative position that it might be the only participating research vessel and therefore dependent
entirely on its own resources, including a dedicated land-based operation to transmit relevant daily
satellite images, to complete its mission. If that were the case, the TIV location procedure would
involve steaming north from Tahiti to nominally 2.5°N, 150°W (~4 days), then turning east to intersect
the leading edge of a westward propagating wave. This would be evident, using shipboard
thermosalinograph and ADCP systems, from decreasing temperature, increasing salinity and marked
northwest velocity. Once encountered, the ship would continue east to the trailing edge of the TIV,
where the front is detected by a sharp rise in SST, a drop in SSS and westward to south-westward
currents. Having located both edges, the ship would steam back ~100 km to begin operations (Stn. 1)
in rich, cool equatorial waters entrained in the vortex. This procedure should take about 3.5 days in
the worst case (assuming the trailing edge at 135°W when it starts), which amounts to a total 7-8 days
of positioning from the ship’s departure.
Figure 4: Modeled trajectory (dots) of a neutrally
buoyant drifter in a TIV from the CLIPPER
simulation, transposed to Pacific longitudes. The
drifter is followed for 20 days, each dot
representing a position at 6-h intervals. Numbers
correspond to the planned MOTIV stations (every 2
days, as described below). Colors are indicative of
the depths of water mass transport due to upwelling
and downwelling processes in the vortex
circulation. The orange line represents typical
leading and trailing fronts of SST and chlorophyll
at the initial ship position. The red line represents
the initial ship tracking for the fronts, as explained
above. The longitude axis is a variable.
Model experiments (Dutrieux, 2002) based on previous TIW studies (TIWE2 -- Flament et al.
1996, PICOLO2 -- Menkes et al. 2002) suggest that water at the initial ship position will follow a
vortex cycloid trajectory with ~20 d required to complete the loop depicted in Fig. 4. Central to
the experiment is the capacity to follow a water parcel in this "typical" vortex loop. SVP-WOCE
type telemetering buoys will be deployed from l’Atalante to complement water mass tracking
efforts on the US ship and to better define convergence and divergence features and velocities.
Twenty (20) drifters will be released when strong northward currents are intersected during the
initial eastward transit through the TIW.
Sampling and process studies are planned for stations of 1.5-d duration (detailed below), and
ship transit time between stations will be about 11-12 h, with some flexibility for adapting
positions guided by daily drifter trajectories and satellite images. This strategy allows 10 stations
to be occupied at 2-d intervals over the ~20-d circulation time of the vortex loop (Kennan &
Flament 2000), Menkes et al. 2002). A cluster of 10 drifters will be released when approaching
Stn. 2 (Fig. 4) to assess convergence patterns at the front (Kennan & Flament 2000). When the
front is reached (between Stns. 2 & 3), two extra days of effort will be devoted to studying the
physical and ecological characteristics of this region. Another cluster of 10 drifters will be
released at Stn. 4 to estimate surface divergence patterns of the vortex (Kennan & Flament 2000),
and 10 drifters will be released between Stns. 6 and 8 to close of the cycloid circulation. The
velocities of surface currents in previous TIW experiments and model simulations rarely exceed
1m/s. Thus, the drifters should never travel more than 200 km in two days and should always be
within reach with 11 h of steaming time (13 kts) between stations. X-CTDs will be launched
every 4 h (2 between each station) during these transits to assess thermohaline structure at the
same frequency as on-station CTD sampling.
Table 1. Summary of station measurements, methods and responsible principal investigators for
biological and biogeochemical studies on the R/V l’Atalante. US participants covered by
this proposal are highlighted in bold font.
Measurements
Methods
Temp, salinity, density, particulates
Currents, positioning
Radars
Air-sea fluxes
Major nutrients (N, P, Si)
Primary prod., N-uptake, Chl a
Biogenic Si, Si-uptake & dissol.
Photosynthetic efficiency
DOC, POC, PON
TEP concentration & production
Dissolved & total iron (Fe)
Microbial community
CTD, X-CTD, transmissometer
Drifters, satellites, radars
Investigators
C. Menkes, J. Vialard
& M.Radenac
P. Flament ?
Inertial dissipation, eddy correlation L. Eymard
Auto-analyzer
Y. Montel
In situ 14C, 15N, fluorometry
Y. Dandonneau
30
Si, Leynaert et al. (2001)
A. Leynaert
Fastracka FRRF on CTD
M. Babin
Shimadzu & CHN analyzers
X. Mari
Mari et al. (2001)
X. Mari
Flow injection
C. Measures
Microscopy, flow-cytometry
M. Landry, K. Selph
Taxon-specific pigments
Cell-specific Fe & Si contents
Phytopl. µ, microzoo. grazing
Carbon export
Mesozooplankton stocks
Gorsky
Mesozooplankton grazing
Micro-nekton
HPLC analysis
X-ray fluorescence microscopy
Dilution – deck & in situ
232
Th, pump casts for POC/234Th
Hydrobios net, video profiler
R. Bidigare
B. Twining/M. Landry
M. Landry
C. Benitez-Nelson
C. Champalbert, G.
Gut fluorescence
ADCP, large net trawls
G. Champalbert
E. Marchal
The proposed sampling and experimental studies for l’Atalante comprise a relatively compact
but coherent program of relevant physical measurements, chemical concentrations, biological
stocks and process rates that are likely to vary according to their position in the TIV. These are
summarized in Table 1. Under NSF funding to P. Flament, high-frequency radar transmitters (16
and 27 MHz with real-time interfacing to the ship's navigation system) will be installed to
measure surface currents continuously in a 200-km swath. These will give real-time vector maps
of the small-scale current field and will help in locating fronts and upwelling zones critical to the
biological sampling. An instrumented mast will provide basic measurements for air-sea flux
estimates (Weill et al., 2002). CTD operations will provide continuous depth profiles for
temperature, salinity, density, light (PAR), particulates (transmissometer) and photosystem
photosynthetic efficiency (FRRF) and discrete depth profiles (24 bottles) for major nutrients
(nitrate, nitrite, ammonium, phosphorus, silicate), dissolved and total iron, dissolved and
particulate organic carbon, biogenic silica, TEP (Transparent Exopolymeric Particles associated
with phytoplankton nutrient stress and aggregation processes), phytoplankton chlorophyll and
accessory pigments, abundances and biomass of auto- and heterotrophic pico-, nano- and
microplankton, and 232Th. Phytoplankton production and nutrient uptake rate, as well as
production rates of extracellular TEP and DO14C, will be determined from 24-h in situ
incubations with 14C-bicarbonate, 15N-labelled nitrate and ammonium, and 32Si-silicate (Morel et
al. 1996; Leynaert et al. 2001, Mari et al. 2001). Taxon-specific rates of phytoplankton growth
and microzooplankton grazing will be estimated in shipboard and in situ dilution experiments,
and additional experimental studies will be conducted to determine the physiological parameters
(kinetics) of silicic acid uptake by diatoms in the presence/absence of added Fe. Standing stocks
of mesozooplankton and micronekton and zooplankton gut pigment contents (for grazing
estimates; Gaudy et al. 2002) will be assessed within defined depth strata from net collections,
video profiling and ADCP backscatter. In frontal regions of surface current convergence,
neuston nets will be used to estimate organism concentrations in the surface film. The tentative
schedule for 36-h station operations is as follows:
Day 1: 1800 - CTD water-column sampling and set-up for shipboard evening experiments
1900 - zooplankton and micronekton sampling (~6-h intervals thereafter)
2200 - CTD cast
0200 - CTD casts - plankton community & in situ incubations (14C, 15N, 30Si, grazing)
deployment of in situ incubation lines
0600 - 10h00, 14h00 CTD casts (and 4-h intervals thereafter)
Day 2: ~0500 - recovery of in situ incubation lines
0600 - CTD cast
0700 - depart to next station
Specific Contributions of this Component to the MOTIV Experiment: As outlined above, our specific
component of the MOTIV experiment will measure a suite of biological and biogeochemical properties to
determine their broad-scale distributions in a TIV and how they respond dynamically to local forcing along
the typical circuit of water transport in the vortex circulation. Our measurements include dissolved and
total concentrations of the limiting micro-nutrient Fe, the spatially variable inputs of which should be a key
to understanding the stimulatory impact of TIVs on phytoplankton growth rates and community biomass
accumulation. Plankton community structure and biomass, from bacteria to mesozooplankton, will be
determined using approaches appropriate for different sizes and functional groups -- pico- (< 2 m), nano(2-20 m) and microplankton (20-200 m). Flow cytometry (FCM), taxon-specific pigments (HPLC) and
microscopy will complement one another in representing the different functional and taxonomic
components of the microbial community. HPLC-determined accessory pigments provide class-specific
differentiation for phytoplankton (Bidigare & Trees 2000). FCM analyses allow high precision
enumeration of the heterotrophic and photosynthetic bacteria, the typical dominants in these waters
(Campbell et al. 1994, 1997). Total, size- and taxon-grouped biomass estimates of phytoplankton and
protist grazers will be derived from image-enhanced epifluorescence and Flow-CAM analyses (Brown &
Landry 2001a). Furthermore, the combination of biomass estimates from FCM and microscopy with
pigment measurements from fluorometry and HPLC will allow the assessment of cellular changes in
pigment concentrations and C:Chl a ratios at various locations around the TIW vortex circulation and with
respect to ambient HNLC waters (Brown et al. 2003). These measurements will be used in combination
with seawater dilution experiments to estimate rates of phytoplankton growth and microzooplankton
grazing (Landry et al. 2003). Growth-grazing imbalances within the vortex provide an upper limit for
carbon export, and various locations within the vortex may show higher concentrations of
mesozooplankton and/or particle aggregates, both associated with enhanced export flux. Flux estimates by
a refined, small-volume 234Th method will quantify the magnitude of export rates variations within the
circulation feature. In summary, our component of the MOTIV research plan will deal with the coupling
of iron inputs to the vortex, their stimulatory impacts on microbial community stocks and rates, and their
ultimate implications for export flux. The various elements are related logically as a sequence of processes
and effects. Thus, we expect that their distributional patterns in the vortex frame of reference should show
their relative magnitudes to be spatially separated but linked by the time-scales of water transport.
One particularly novel aspect of the proposed methodologies is the use of synchrotron-based
X-ray fluorescence microscopy (XRF) to assess the trace element contents of individual
phytoplankton cells. Such analyses will be done, at nominal cost to the project and emphasizing
Fe and Si to provide a direct indication of Fe input effects on the physiologies of representative
protist species and to reveal the coupled effects of Fe- and Si-limitation for diatoms (e.g.,
Hutchins & Bruland 1998). In XRF, individual cells are probed with X rays, which when
absorbed result in the emission of inner, core-level electrons. When outer shell electrons fill these
core-level vacancies, either Auger electrons or fluorescence photons are emitted, with
fluorescence photons strongly favored for the heavier elements. The energies of fluorescence
photons are determined by electron orbitals, so they are precisely the same for any atom of a given
element. Spatial distributions of elements are determined by scanning a sample in raster fashion
relative to a highly focused beam, collecting X-ray emission spectra at each point, and then
analyzing these spectra relative to standards. Figure 5 shows the elemental fluorescence maps for
a centric diatom (~16 m) collected from the Southern Ocean during the Southern Ocean Fe
Experiment (SOFeX). In addition to strong fluorescence signals from the major elements Si, K,
and Ca, there are clear signals from the trace metals Mn, Ni, Cu, Zn, and the limiting nutrient
metal Fe, which occurred at sub-nanomolar concentrations at the collection site (K. Johnson,
unpubl.).
Methods
Iron and Aluminium Analyses: Vertical profiles of Fe will be
determined from discrete samples collected in Go-Flo bottles
and transferred to acid-leached teflon bottles. Samples for total
dissolved Fe will be filtered through acid-leached filters and
processed on shipboard by Flow Injection Analysis (Measures
et al. 1995). The method is sufficiently sensitive that the preconcentration of ~3 mL of sea water onto the resin (a 1-minute
load time) yields an analytical detection limit of 0.025 nM
(based on three standard deviations of the analytical blank).
Typical precision of the method under at sea conditions has
been found to be 2.5% at 0.35 nM. The technique has been
used at sea on several occasions, including the US JGOFS
Arabian Sea Process Study, the 1996 IOC baseline cruise, and
the US JGOFS Southern Ocean, and has produced data that is
both precise and consistent with shore-based data collected
Figure 5. Light, epifluorescence, and
false-color XRF element maps of a
centric diatom collected from the
from the Central Pacific (Landing and Bruland, 1987). The Figure 5 legend does not appear well, it
seems to be cut…Currently, it takes approximately 8 min to run each sample in duplicate.
The Flow Injection Analysis method of Resing and Measures (1994) will be used to determine
Al concentrations. This fluorometric method, which is based on the batch method of Hydes and
Liss (1976), has a detection limit of ~0.15 nM and a precision of 1.7% at 2.4 nM. The method
has been used successfully onboard ship on numerous occasions including the US JGOFS
Arabian Sea Process Study, the IOC 1996 baseline cruise and the US JGOFS Southern Ocean.
As with the Fe method, a sample can be run in duplicate in ~8 min.
Total particulate Fe, and Al will be determined by analyses of material collected onto preweighed
0.2-µm Nuclepore filters. The filters are digested using HNO3 in Teflon bombs in a microwave
digestion system. The digests are analysed by the FIA methods detailed above. Standards for these
determinations will be made up in DIW, acidified with HNO3 in order to match matrices (Measures et.
al. 1995).
Pigment Analyses: For routine Chl a analyses, 250 mL of seawater will be filtered onto 25 mm
Whatman GF/F filters, extracted in 10 mL 90% acetone for 24 h (dark, 0C), and analyzed using a
TD700 fluorometer. For HPLC analyses, 2-L samples will be filtered through 25 mm Whatman GF/F
filters and stored under liquid nitrogen until analysis. In the laboratory, pigments are extracted in 3
mL acetone for 24 h (dark, 0C). Centrifuged acetone extracts are analyzed for chlorophylls and
carotenoids with the HPLC method of Bidigare & Trees (2000). Pigment concentrations are
calculated using internal and external standards.
Photosynthetic pigments will be measured routinely by HPLC (High Pressure Liquid
Chromatography) and spectrofluorometry on samples collected on each CTD cast. Samples for flow
cytometry counts will be taken from the same bottles, fixed, frozen, and analysed after the cruises.
Experimental studies of phytoplankton plankton growth and microzooplankton grazing will also
include HPLC pigment analyses by the method of Bidigare & Ondrusek (1996). Chl contributions by
the major phytoplankton groups will be calculated from the concentrations of class-specific accessory
pigments using the CHEMTAX pigment algorithm (Mackey et al., 1996).
Microbial Community: Composition and biomass of the microbial community, including
autotrophic and heterotrophic bacteria and protists, will be determined from flow cytometric and
microscopical methods comparable to those used successfully during the French JGOFS EBENE
study at 8°S to 8°N, 180°) (Brown et al. 2003). Depth profiles will be sampled at each l’Atalante
station, corresponding to the approximate times of the daily minima and maxima in community
biomass (0600 and 1800, respectively), and coordinated with relevant sample collections and
experimental studies.
Picoplankton samples (1 ml) will be preserved (paraformaldehyde, 0.5% final) and frozen in
liquid nitrogen. In the laboratory, the samples will be thawed and stained with Hoescht 33342 (0.8
µg mL -1 final concentration) for 30 min (Monger & Landry 1993) before analysis on a BeckmanCoulter Altra flow cytometer equipped with multiple lasers. The lasers are aligned colinearly with
the first laser tuned to the UV range to excite Hoescht-stained DNA. The blue fluorescence from
the DNA stain distinguishes cells from non-living particulate matter. The second laser is tuned to
1.3 W to excite the pigments of autotrophic cells. Cell populations (Prochlorococcus,
Synechococcus, heterotrophic bacteria, picoeukaryotic algae) are distinguished from one another by
differences in light scatter and fluorescence emission. All FCM samples will be spiked with a
mixture of Polysciences Fluoresbrite YG 0.57- and 0.98-µm visible beads and 0.46-µm UV beads
for the normalization of fluorescence and scattering properties.
Seawater samples (50 mL) for filtered (>5 psi) onto black 0.8-µm black polycarbonate filters,
overlaying 10-µm Millipore filters for uniform cell distributions. The samples are drawn down until
approximately 2 mL remain in the filtration tower and DAPI (50 µg mL-1) is added and allowed to
sit for 30 sec. The filters are mounted onto glass slides with non-fluorescent immersion oil. Slides
will be viewed at 400 or 1000X magnification with an Olympus IX71 inverted microscope
configured with an epifluorescent kit, 100-W power supply, and a MagnaFire digital camera with
1280 x 1024 pixel resolution. At le0 random images per slide will be captured and digitized. For
qualitative purposes and to minimize the fading of Chl a over time, all slides will be viewed and
imaged on the ship within 24 h of collection before archival storage at -85°C.
Microplankton samples (500 mL) will be preserved with 0.5% glutaraldehyde and/or acid Lugols
and enumerated fresh on shipboard (French ship) or ashore (US ship samples) using a Flow
Cytometer And Microscope (FlowCAM) system (Sieracki et al. 1998). Particles flow through this
instrument to an optical sensing volume where their silhouettes are measured using frame-grabbing
technology. Image-analysis software then estimates cell nanoplankton analyses will be preserved
with 2 mL of 10% paraformaldehyde followed by 25 µL of proflavin (0.033% w/v). The preserved
samples will be stored in the dark at 4C for 1-6 h, then equivalent spherical diameters (ESD) and
normalized particle size distributions (log ESD vs. log size; ~1000 particles/analysis). The
glutaraldehyde samples will be viewed under transmitted light for the identification and
enumeration of diatoms and dinoflagellates, and epifluorescence will be used to distinguish autoand heterotrophic dinoflagellates. If a difference is found between glutaraldehyde and Lugol’s
samples on site, the latter may be preferable for the identification and enumeration of delicate
ciliated protozoa. Particle volume will be converted to cellular C using the equations of Verity et al.
(1992) and Menden-Deur & Lessard (2000). Realistic resolution of the Flow-CAM system is
nominally to cells of ~ 5 µm.
Cellular Iron and Silica Contents:. Several steps are required to prepare samples for XRF. Water
samples collected using ‘clean’ techniques (Bruland et al. 1979; Cullen and Sherrell 1999) are
preserved with Chelexed, buffered glutaraldehyde (EM-grade, pH 8, 0.25% final conc.), and the
plankton cells are concentrated immediately on carbon/Formvar-coated Au EM finder grids (Electron
Microscopy Sciences) with gentle centrifugation. The grids are then removed with Teflon-coated
forceps and gently rinsed with Milli-Q water to remove salts before drying in a darkened laminar flow
hood. After the dried cells are examined and photographically mapped (transmitted light and
epifluorescence microscopy, dry objectives), the grids can be stored in a dessicator indefinitely.
Prior to XRF analysis, the grid is placed on a kinematic specimen holder and mounted on a highresolution Leica DMXRE microscope with state-of-the-art optics to allow operation in several
different modes with a Rayleigh resolution up to 0.2 µm. The orientation of the mount in the holder
corresponds to that in the microprobe. The sample is precisely located on the grid relative to a
reference point using a high spatial resolution motorized x/y stage (Ludl Bioprecision) with 0.1-µm
resolution, 0.75-µm repeatability and 3.0-µm accuracy for a 77 x 51 mm travel range. Digital brightfield and differential interference contrast images of the target are acquired at this time for later
comparison to the XRF image.
In the microprobe, the same cells are relocated and scanned in a 2-D raster fashion by the focused
x-ray beam, and the entire x-ray fluorescence spectrum is recorded at each pixel. The spectra from the
pixels covering the cell are then summed to generate a single spectrum for each cell, which is
corrected for background fluorescence. This spectrum is fit to a summed exponentially modified
gaussian (EMG) peak model with a sigmoidal baseline, using custom software. Element
concentrations (g area-1) are then calculated from peak heights using peak height:concentration ratios
determined from NIST thin-film standards and integrated over the area of the cell to produce cellular
element quotas. Finally, these are normalized to estimates of cellular C which are calculated by
applying published C:vol ratios to cell sizes estimated from the x-ray scans. Total carbon mass in
smaller plankton cells (<5 m) can also be measured directly using the Stony Brook soft X-ray
scanning transmission microscope (STXM) at Brookhaven National Laboratory.
Phytoplankton Growth and Microzooplankton Grazing: Dilution experiments will be run at each
station to resolve instantaneous rates of phytoplankton growth (µ) and the mortality (m) attributable to
microzooplankton grazing. Full 5-treatment experiments will be run for three standard light depths
(mixed layer, intermediate and lower euphotic zone) and incubated in shipboard light boxes for the
natural 24-h photocycle (Landry et al., 1995a,b, 2000, 2003). Abbreviated experiments consisting of
replicated bottles of 2 treatments (20% and 100% natural seawater) at each depth will be incubated on
the in situ line (e.g., Landry et al. 1984, Brown et al. 1999). Mean community rate estimates will be
determined from shipboard analyses of chlorophyll a. Rate estimates for component populations will
be determined by flow cytometry and taxon-specific pigments. The disappearance rates of FLBs,
added as tracers in these experiments, are used to assess potential non-linearities in the rate
determinations (Landry et al. 1995b). In addition, initial and final FCM analyses of phytoplankton
cellular fluorescence and side scatter, as well as microscopical assessments of the ratios of
C:accessory pigments for specific groups (e.g., fucoxanthin:Cdiatom, hexfucoxanthin:Cprymnesio,
peridinin:Cdinoflag allow for growth rate adjustments to correct for physiological responses (Chla cell-1,
size) to day-to-day differences in light level or ambient versus incubator conditions (Landry et al.
2003). Taxon-specific rates of production (PP = µ * Cm) and grazing losses (G = m * Cm), both in
terms of µg C L-1 d-1, are computed according to Landry et al. (2001, 203) from taxon-specific
estimates of µ and m. For these computations, Cm is the group mean carbon concentration during the
incubation [= Cphyto[e(µ-m)t – 1]/(µ-m)t], where Cphyto is the initial carbon biomass determined from
microscopical and/or flow cytometric analyses.
Optical Plankton Counts: An OPC instrument (Focal Technologies) will be mounted on the
Revelle’s SeaSoar to measure particulate distributions and size composition. The OPC measures the
cross-sectional areas of targets in the range of 0.3 to 20-mm equivalent spherical diameter at a rate
of up to 200 counts s-1. These are generally mesozooplankton, but could include clumps of phytoaggregates; hence the measured size-frequencies will be cross-checked against net-collected
samples from the French ship. For routine data collection, OPC counts will be binned into 128 size
categories and integrated for 1-2 m depth intervals. At tow speeds of about 7 kt, the vehicle will
cycle between the near-surface and 200 m in about 6-7 min, providing depth-resolved profiles at ~1km spacing.
Export production: 234Th (thorium-uranium disequilibrium) has been used successfully to study
particle export on time-scales of days to weeks using both steady-state and non-steady state
assumptions (Buesseler 1998). In the present study, the application of a new 2-L 234Th technique
(Benitez-Nelson et al., 2001b; Buesseler et al., 2001) will allow fine-scale depth resolution of the
heterogeneities in export flux. At each station, depth profiles of 234Th activity will be obtained using
MnO2 precipitation from the small-volume samples and non-destructive low-level beta counting. For
carbon and elemental flux estimates, we will estimate the C and N ratios relative to 234Th on larger
sinking particles using a large-volume pump samples (250-500 L) collected on 53-µm particles filters
(Buesseler et al. 1995, 1998, 2001, Benitez-Nelson et al. 2001). At Stn. ALOHA in the oligotrophic
subtropical Pacific, C export derived from C/234Th ratios in sediment traps and in situ pumps are,
within error, identical (Benitez-Nelson et al. 2001). By measuring 234Th profiles at all stations in the
TIV and interpolating between them with measured and modeled estimates of the horizontal and
vertical circulation field, we will be able to quantify the physical influences on 234Th distributions.
Vertical processes that bring 234Th-rich deep waters closer to the surface can result in significant
underestimates of actual particle fluxes (Sweeney 20001). We will correct for this upwelling effect
using the physical trajectories of water mass movement in the TIV circulation.
Project Organization and Responsibilities
As lead PI, Mike Landry has overall responsibility for this project, including the timely
completion of field work and manuscript preparation. He will serve as the liaison with l’Atalante
and Revelle Chief Scientists to coordinate the sampling plans and cruise schedules. In addition, he
is responsible for micro-plankton community analyses by microscopy and FlowCAM and growth
and grazing assessments on l’Atalante as well as OPC operations on the Revelle. Ben Twining
will participate on the l’Atalante as part of the Landry contingent and will contribute XRF
analyses of cellular Fe and Si contents. Bob Bidigare is lead P.I. for the University of Hawaii
component and has responsibility for all pigment analyses. Chris Measures is responsible for all
Fe and Al measurements on both ships. Karen Selph will participate in experimental
determinations of growth and grazing on l’Atalante and has the lead role in all population
assessments by flow cytometry. Sue Brown will lead group operations on Revelle and will be
responsible for coordinating all population assessments from that that ship’s sampling. Claudia
Benitez-Nelson is responsible for pump sampling and 234Th-based export estimates.
Significance of Proposed Research
Tropical instability vortices, with their associated fronts and undulations, represent the largest
signal in tropical ocean variability on time scales less than a season. They are major mechanisms
for the transfer of momentum, heat and freshwater away from the equator, and they are strongly
linked to enhanced levels of biological activity from phytoplankton to tuna fisheries. The research
proposed here is part of an interdisciplinary and international collaboration to elucidate the
biological responses and biogeochemical implications of a TIV within a rigorously constrained
physical experiment in the HNLC equatorial Pacific. Our specific component is central to linking
the sources and inputs of the limiting element (Fe) to downstream physiological and ecological
responses of the plankton community and ultimately to export flux. The results of this experiment
will yield not only new understanding of the dynamics and biological-physical coupling of these
important features, but will provide critical data for structuring and testing biogeochemically and
ecologically relevant, eddy-resolving models for the tropical oceans.
Broader Impacts of the Project
This project will directly support the education and research development of one post-doctoral
scholar (B. Twining), 3 graduate and at least 2 undergraduate students. Research opportunities will be
available for 2 students in the University of Hawaii’s.newly established undergraduate Global
Environmental Science program (http://www.soest.hawaii.edu/oceanography/GES/), which
requires each student to complete a Senior Thesis before graduation. In addition, funding to BenitezNelson will underwrite continuing opportunities for undergraduate research for students at Benedict
College and South Carolina State University as part of the South Carolina Alliance for Minority
Participation (SCAMP).
Results from this research will be made widely available to the oceanographic community such that
the data may be used to develop oceanographic models of the tropical Pacific with realistic physical
and biological coupling at the eddy scale. Two such synergistic activities are already planned. First,
Landry is a named collaborator for the development of French models that will implement the
biological rates and responses observed in MOTIV into a physical model of a TIV (C. Menkes, P.I.).
Second, Landry, Measures and Selph are also involved in a proposed “Biocomplexity” project (D.
Nelson. P.I.) on the ecological dynamics of different functional groups of phytoplankton (diatoms,
coccolithophores, picoplankton) in the equatorial Pacific, which has a well-developed modeling
component (F. Chai, U. Maine). The results of the MOTIV experiment would strongly complement
the goals of that project, and would be relevant and available for modeling as part of that effort. The
Biocomplexity project has a strong educational program, involving direct field participation by K-12
teachers, a user-friendly website for downloading relevant information on the equatorial Pacific, and a
formal course of study for teachers, with summer workshops on both the west (San Francisco) and east
coasts (Bigelow Labs, Maine). We are committed to participating in this educational program, and the
MOTIV results would make a superb contribution to illustrating how physical forcing can impact the
plankton and biogeochemical fluxes.