http://www.lodyc.jussieu.fr/gepco
SCIENTIFIC
AIMS
The process studies initiated by JGOFS have permitted
a better understanding of the coupling between the dynamics of the
ocean, and the main biogeochemical fluxes. They have also shown that
new production, that is the main, primary flux, is controlled not
only by nitrate availability, but also by iron, and by silica. In
addition, non-redfieldian processes such as biocalcification or diazotrophy,
have specific characteristics, and are important enough in some areas
to modify the ratios between chemical constituents involved in oceanic
biogeochemical fluxes. The phytoplankton species are responsible for
these variable biogeochemical capacities. A well-known example is
given by biocalcification, made by the Coccolithophorids that build
calcareous pieces in addition to their usual photosynthesis. Biocalcification
tends to decrease the alkalinity of seawater. The Trichodesmium (cyanophyceae)
are another example of a specific chemistry : they are able to fulfil
their nitrogen requirements with N2 instead of oxidized nitrogen forms,
thus contributing to increase the global reactive nitrogen stock in
the ocean. The priority for the JGOFS scientific community is
now to carry out the synthesis of results obtained in the field during
the preceding decade, and to improve the modelling of biogeochemical
fluxes. For this new challenge, data that document these specificities
are available only in those areas and time periods where JGOFS time
series or process studies took place, but they are lacking over most
of the ocean. The objective of GeP&CO is to describe and understand
the variability, in space and time, of phytoplankton, in wide oceanic
areas, from a biogeochemical point of view. GeP&CO is strongly
characterized by sampling along a merchant ship lane. This kind of
sampling is a low-cost solution to collect time series data on long
oceanic transects, but is limited to the surface mixed layer. This
layer however plays a dominant role : it exchanges energy and gases
with the atmosphere, and absorbs most of the light energy that is
necessary for photosynthesis. The systems encountered will be characterized
using their photosynthetic pigments, measured by HPLC and by spectrofluorimetry,
using quantitative inventories of the picoplankton by flow cytometry
en flux, and using some important chemical properties such as macronutrients
concentration, total CO2 and alkalinity, colored dissolved organic
matter. Bio-optical measurements (light absorption, seawater reflectances)
will also be made in order to validate on a regional basis the products
delivered from sea color sensors on satellites. These measurements
can be useful for the improvement of algorithms intended to detect
blooms of functional groups with important geochemical consequences
from satellite sea color data (blooms of coccolithophoridae or Trichodesmium).
These objectives make up the core of GeP&CO. Some additional measurements
will also be done, in order to optimize the observations on the merchant
ship lane. Part of them are easy to carry out, at low coast, and provide
information useful for the understanding of the observations (sea
surface temperature and salinity), while other ones have no or few
relationship with the GeP&CO objectives (atmospheric 13C).
The shipping ligne that has been selected starts from Le Havre every
three months, and goes to Nouméa, via Panama and Tahiti. It
samples the north Atlantic, the Sargasso Sea, the Gulf of Mexico,
the Gulf of Panama, the equatorial upwelling in the Pacific and the
south Pacific subtropical gyre. A scientific observer embarks on each
trip to carry out the sampling and to watch over the instruments.
GeP&CO started in October 1999. 12 cruises (one very three month)
were scheduled. The tenth cruise started on December 31 of 2001, in
conformity with the initial plan.
The analyses of samples have generally been made within a few months
after the end of each cruise. Counts of coccolithophorids however
necessitate much more time and, at present, are only available for
the first four cruises in the North Atlantic. Preliminary results
show that the variability of photosynthetic pigments ratios is low.
Pigments that are indicators for phytoplankton functional groups (fucoxanthin
for diatoms, or 19’hexano-yloxyfucoxanthin for coccolithophorids)
are found to be ubiquitous, with maximum abundance in the North Atlantic
during the spring bloom.
Two methodology studies are in progress. The first one shows that
the ancient fluorescence – acidification technique does not
lead to large errors in the estimate of chlorophyll a concentration
given the actual assemblages of pigments that emit red fluorescence
when excited by blue light by. The reason for this small error is
the scarcity of chlorophyll b and of pheophytin a in the phytoplankton
at the surface. The second one presents a new spectrofluorimetric
method for the routine measurement of phycoerythrin.
The objectives and methods of GeP&CO, and access to the data,
are available at http://www.lodyc.jussieu.fr/gepco
1. Rationale
The oceanic
carbon cycle is mainly controlled by photosynthesis that occurs in
the photic layer. A simple representation of this process by models
uses chlorophyll, light, and nitrogen availability. Nitrogen is generally
available as nitrate, nitrite, ammonia or urea. However, since the
biological carbon sink is determined by new production, nitrate is
considered as the main limiting nutrient in the ocean, and this sink
is approximated using nitrate fixation and the C : N Redfield ratio.
More than ten years of oceanic studies coordinated by JGOFS lead to
a more complex scheme. The nitrate-based cycle cannot explain all
the details of the variability of the carbon cycle, that result from
processes whose importance was evidenced during the last decade. Some
phototrophs are thus able to fulfil their nitrogen requirements using
dissolved atmospheric N2 instead of nitrate or other more reactive
forms (Trichodesmium). Other species or groups of species are able
to build calcium carbonate pieces (coccolithophorids). These studies
have also shown that most phototrophs also necessitate iron in addition
to their nitrogen needs, and that this element, that has a low solubility,
plays an decisive role in biogeochemical cycles. Diatoms that can
grow rapidly need silica and are also strongly dependent on iron availability.
On the contrary, the procaryote Prochlorococcus sp. Is able to grow
at very low iron concentration. Knowledge of the specific biogeochemical
behavior of different phytoplankton groups, and of the factors that
control them, is thus an important challenge, which is at the core
of the EDOCC initiative (Ecological Determinants of Oceanic Carbon
Cycling).
GeP&CO intends to detect and to describe the variability of phytoplankton
populations, and to understand how the physics of the ocean controls
this variability. This is a prerequisite to realistic simulation of
the oceanic carbon cycle. Indeed, while all phytoplankton species
have a photosystem in which chlorophyll captures the energy of photons
to reduce carbon dioxide, the rate, and pathways for using this energy
vary according to phytoplankton groups. To improve the simulation
of biogeochemical cycles in the photic layer, Bisset et al. (1999)
for instance have developed a model with four phytoplankton compartments
: mixed layer Prochlorococcus, deep Prochlorococcus, Synechococcus
, and others. To summarize, while it is important to estimate primary
production using chlorophyll concentration and light, it is important
as well to know which groups of phytoplankton are doing phototsynthesis.
Examples are given below.
Diazotrophy
:
Some phototrophs species (the cyanobacteria Trichodesmium and Rickelia
intracellularis) have the capacity to use nitrogen in the N2 form.
Consequently, they are able to grow and make new production in places
where nitrate and ammonia are lacking. Thus, atmospheric nitrogen
can increase the stock of reactive nitrogen in the ocean, and this
flux must be considered for the long-term budget of nitrogen and carbon
in the ocean (Falkowski, 1997), the counterpart being denitrification
that occurs in oxygen depleted areas. According to Codispoti (1995),
25 1012g of atmospheric nitrogen are fixed each year in the ocean,
most of it by Trichodesmium (Capone et al. 1997). Many questions subsist
however if one wants to estimate the global role of diazotrophy in
the ocean (Hood et al., 2000). Diazotrophy makes it possible for ecosystems
to export carbon without any nitrate supply. This is a plausible explanation
to the summer decrease in the CO2 partial pressure at time series
stations BATS and ALOHA (Marchal et al., 1996 ; Karl et al., 1997
; Abell et al., 2000). Similarly, Walsh et al. (1999) had to introduce
nitrogen fixing cyanobacteria in their model of the Cariaco bassin
(Venezuela) in order to improve the simulation of biogeochemical variability
in this region. Cyanobacteria have in common specific pigments of
the phycobiliproteins group, that give to these species characteristic
optical properties of absorption (Subramaniam et al., 1999a). The
best known are Trichodesmiums. When they form blooms at the surface,
they have a strong absorption in the near infra-red (in the same way
as terrestrial plants), and a strong retrodiffusion caused by gas
vacuoles. These properties may provide the possibility to detect them
from satellites (Subramaniam et al., 1999b).
Biocalcification
:
Another group, the coccolithophorids, builds calacareous coccoliths
in addition to fixing carbon in the organic form. This has consequences
on the equilibrium of carbonates that differ from those of photosynthesis,
resulting in a decrease of seawater valkalinity (Balch and Kilpatrick,
1996). This process is named biocalcification. It is difficult to
measure, and is only represented in an approximate and empirical way
in models of the oceanic carbon cycle (Yamanaka and Tajika, 1996,
Le Quéré, 1999). Biocalcification was estimated to be
about 3 to 12 % of the primary production flux in the equatorial Pacific
during the EqPac cruises (Balch and Kilpatrick, 1996). In the Arabian
Sea, biocalcification amounted to 1 to 5 % (Balch et al., 2000). Coccolithophorids
blooms have characteristic optical properties caused by the strong
retrodiffusion by coccoliths (Brown and Yoder, 1994 ; Balch et al.,
1999 ; Tyrell et al., 1999), and can be detected from satellite. The
coupled effects of photosynthesis and biocalcification decrease alkalinity,
which generally leads to a rise of CO2 partial pressure (Holligan
et al., 1994). As a feed back, a rise of pCO2 seems to stimulate photosynthesis
in two species of coccolithophorids, but decreases their biocalcification
rate (Riebesell et al., 2000). Coccolithophorids dominate in some
areas, especially in frontal zones at temperate latitude (Eynaud et
al., 1999). They are abundant in the diet of filter feeders (Gorsky
et al., 1999).
Biocalcification is also worked out by zooplanktonic foraminifers
and pteropods that seem to be controlled by the occurrence of phytoplankton
blooms (Schiebel et al., 2001).
Iron
limitation :
While iron is more and more considered as a key element for phytoplankton
growth (Martin and Fitzwater, 1988; Price et al. 1994), species have
specific responses to iron limitation : diatoms are very sensitive.
Oppositely, Prochlorococcus sp. Seem to be able to grow fast in the
absence of iron (Di Tullio et al., 1993). High Prochlorococcus growth
rates indeed have been observed in the iron poor central equatorial
Pacific, (one doubling each day) demonstrating that these organisms
are not or few affected (Vaulot et al., 1995). Other cyanobacteria
such as Synechococcus produce siderophores that help them to compete
against other species when iron is lacking (Whilhelm and Trick, 1994).
Iron limitation does not seem to affect the C / N assimilation ratio,
but it decreases the rate of photosynthesis. The field data on iron
concentration at sea begin to be abundant enough for the study of
global trends (Lefevre and Watson, 1999; Fung et al., 2000), and models
have been developed that consider the iron demand by the phytoplankton
(Loukos et al., 1997; Hurtt and Armstrong, 1999). The effect of iron
however varies according to phytoplankton groups : the IRONEX II experiment
showed that the diatoms have the fasted growth rate when their iron
requirements are fulfilled, while the Prochlorococcus do not react
to addition of iron (Cavender-Bares et al., 1999). The consequences
of iron limitation however are not fully understood. For instance,
it has been observed in the equatorial Pacific that iron addition
did increase the size of Prochlorococcus cells, and the amount of
chlorophyll per cell, but that the number of Prochlorococcus in seawater
did not change significantly, as cell mortality probably counterbalanced
their division rate (Mann and Chisolm, 2000). At the ocan surface,
iron has Aeolian or upwelling origin (Fung et al., 2000), and its
availability is thus controlled by physical processes. In addition,
there seems to exist a biological control, as some phytoplankton groups
seem to have the capacity for pre-conditionning seawater : prokaryotic
algae indeed seem to block iron for themselves by emitting siderophores
that absorb iron and that they only can assimilate. A similar process,
with de porphyrins instead of siderophores, would benefit to eukaryotes
only (Hutchins et al., 1999). Attempts to represent the role of "ligands"
produced bys phytoplankton groups in models have been made (Archer
et al., 2000).
Limitation
by silica :
The phytoplankton group that can grow at fatest rate is the diatoms
(Goldman, 1993; Nelson and Brzezinski, 1997). Their growth is limited
both by iron, which they need at high concentration, and by silica,
which they need to build their skeleton. Silica is abundant in cold
seas, and becomes rapidly limitant in tropical regions where new production
thus must be done by other phytoplankton groups, which have a smammer
growth rate (Nelson and Tréguer, 1992). Consequently, silica
that is needed by diatoms controls the ocean’s biological carbon
sink (Tréguer and Pondaven, 2000). When iron is lacking, the
diatoms tend to build thicker à fabriquer des skeletons (Takeda,
1998), because the time between cells division is greater. Each year
after the spring bloom, the diatoms produce the largest part of carbon
export to depth in the North Atlantic (Lochte et al., 1993). They
are less numerous in tropical seas (except in coastal upwellings),
but here too, they are thought to be responsible for most of the new
production (Bender and McPhaden, 1990 ; Goldman, 1993). Still, silica
is limitant in tropical regions, even in the equatorial Pacific upwelling
( Blain et al., 1997 ; Dugdale and Wilkerson, 1998). The diameter
of diatoms is generally between 20 and 200 µ, so that they cannot
be detected by routine flow cytometry analysis. However, they contain
fucoxanthin that is rare in other groups. Considering the important
role of diatoms in new production, as well as their high fuxocanthin
content, Claustre (1994) proposed that the ratio (fuxocanthin + péridinin)
/ (total pigments) be a proxi of the f ratio (new production to total
production ratio).
The production of dimethylsulfide :
Some algae give a remarkable example of the role that biological processes
may have on climate : as a protection against grazers, they have an
enzyme (DMSP-lyase) that produces a repellant derived from dymethyl
sulfonium proponiate (DMSP) that some grazers (especially protozoans)
avoid (Wolfe et al. 1997), and that is finally degraded into dimethylsulfide
(DMS). The later is an important source of aerosols and condensation
nuclei for the formation of clouds. This is accounted for in some
atmosphere models for climate change, but the variability of DMS sources,
and what causes that variability, are poorly known. While it is clearly
established that primary producers are at the origin of marine dimethylsulfide
emission, the phytoplankton groups contribute in different manners.
The coccolithophorids are known to be the main producers (Malin, 1997).
In addition to the phytoplankton composition, its physiological state
would also be important. Fuhrman (1999) indeed shows a direct effect
of viruses on DMS production. Consequently, it is not possible to
forecast the DMS concentration using only the chlorophyll concentration.
The abnormally high abundance of DMS, while correlated to chlorophyll
concentration, that was observed by Gui-Peng et al. (1999), may be
explained by such physiological factors.
Thus, in addition
to the ocean’s variability in terms of chlorophyll and macronutrients
concentration, there also exists an enormous variability in the phytoplankton
composition that results in a variety of biogeochemical processes.
An illustration can be given, starting from iron, silica and nitrate
rich waters. Under such conditions, diatoms will grow first, occasioning
an intense biological sink of carbon (this can be seen during the
North Atlantic spring bloom : Lochte et al., 1993). When silica shortage
occurs, followed by iron, nitrate, and phosphorus shortage, there
is a succession of different groups of algae, and finally, in ultra
oligotrophic areas, populations are dominated by picoplanckon, and
especially by the genus Prochlorococcus (Landry et al., 1997). Between
these two extremes, the capacity of the ocean to export organic carbon
to depth decreases by several orders o f magnitude.
JGOFS has defined oceanic provinces that encompass the diversity of
biological systems. Process studies carried out in these provinces
led to considerable improvement of knowledge. However, these studies
are limited in space and/or time, and observations are lacking in
many parts of the ocean. At present, 3D ocean biogeochemical models,
as well as satellite sea color data, are the only tools to enable
global studies and interpolation. These tools however still ignore
most of the biological variability, and GeP&CO aims to document
this variability over large space and time scales.
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