LOHAFEX
provides surprising insights on plankton ecology that dampen hopes of
using the Southern Ocean to sequester atmospheric CO2
(Official LOHAFEX Press Release)
Goa/Bremerhaven/Berlin, 23 March 2009. The Indo-German team of
scientists from the National Institute of Oceanography (NIO), Goa, of
the CSIR and the Alfred Wegener Institute for Polar and Marine Research
in the Helmholtz Association (AWI), Bremerhaven, Germany, together with
colleagues from 5 other countries, have returned home from an
expedition with the research vessel Polarstern. They spent an arduous 2
½ months in the notorious Roaring Forties of the southern
Atlantic where they carried out the ocean iron fertilization experiment
LOHAFEX. The scientists fertilized a 300 square kilometre patch of
ocean inside an eddy (a clockwise rotating water column with a diameter
of 100 kilometres) with 4 tonnes of dissolved iron and were able to
follow its effect on the plankton and on ocean chemistry including
concentrations of carbon dioxide and other radiatively important gases
continuously for 39 days. The experiment has yielded new insights on
how ocean ecosystems function but has dampened hopes on the potential
of the Southern Ocean to sequester significant amounts of carbon
dioxide from the atmosphere and thus help mitigate global warming.
Response of the plankton
As expected, iron addition stimulated growth of the planktonic algae
(phytoplankton) which doubled their biomass within the first two weeks
by taking up carbon dioxide (CO2) from the water. “The initial uptake
depressed partial pressure of CO2 by up to 15 micro-atmosphere” said
co-Chief Scientist Dr. Wajih Naqvi of NIO. However, contrary to
expectation, further growth of the phytoplankton bloom was stopped by
increasing grazing pressure of abundant, small crustacean zooplankton
(copepods), after which the planktonic ecosystem entered a recycling
mode. As a result, further CO2 uptake declined and only a modest amount
of carbon sank out of the surface layer by the end of the experiment.
Hence, the transfer of CO2 from the atmosphere to the ocean to
compensate the deficit caused by the LOHAFEX bloom was smaller than in
most previous experiments.
The larger blooms stimulated by earlier experiments were due to a group
of algae known as diatoms which are protected against grazers by shells
made of glass (silica) and are known to sink to great depths after
blooming. Diatoms could not grow in the LOHAFEX experiment because all
the silicic acid (the raw material of diatom shells) had already been
extracted by previous, natural blooms. These were presumably fertilized
by natural sources of iron such as melting icebergs and dust blown off
Patagonia. Hence a major finding was that other algal groups, although
stimulated by iron fertilization, are unable to make blooms equivalent
to those of diatoms. Since the silicic acid content of surface waters
in the sub-Antarctic zone (the region located between the Polar Front
and the Subtropical Convergence that accounts for half of the total
area of the Southern Ocean) is low, iron fertilization in this vast
region is unlikely to result in removal of significant amounts of CO2
from the atmosphere.
Predators: Amphipods instead of krill
“To our surprise, the iron-fertilized patch attracted large numbers of
zooplankton predators belonging to the crustacean group known as
amphipods” explains Professor Dr Victor Smetacek, co-chief scientist
from the AWI. These shrimp like crustaceans are between two and three
centimetres long and feed indiscriminately on other zooplankton
including copepods but also organisms much larger than themselves such
as salps and chaetognaths. The dominant species (Themisto gaudichaudii)
is the main food of squid and fin whales in the south-western Atlantic,
so this finding is of particular interest because the amphipods’
biomass in the patch approached that of the better-known krill in
productive waters further to the south. Indeed, before decimation of
the populations of great whales by the 1960s, there were many more fin
whales feeding on amphipods in the northern zone of the Southern Ocean,
than there were blue whales feeding on krill in the southern zone.
However, much less is known about the biology of Themisto than of
krill, although it provides the food base of the extensive squid
fishery in the south-western Atlantic. LOHAFEX thus provided unexpected
insights into the ecology of this neglected key species.
A second fertilization of the patch after 3 weeks had no further effect
on the phytoplankton indicating that the ecosystem was already
saturated with iron. Algal species which regularly make blooms in
coastal regions including the Antarctic, were most heavily grazed and
replaced by tiny algal cells, only slightly larger than bacteria, that
are too small to be gathered by copepods. Up to 20 million cells per
litre of these minute algae were recorded. Interestingly, the plankton
community in the unfertilized water surrounding the patch developed in
the same way and had the same species diversity but was operating at a
lower level of biomass. “The bacterial community both inside and
outside the patch was very similar and their cell numbers remained
unusually low throughout”, said Dr. Bernhard Fuchs of the Max Planck
Institute for Marine Microbiology (Bremen). This implies that most of
the nutrient recycling (including iron) was carried out by the
zooplankton. This is a particularly surprising finding because the
microbial food web based on bacteria is believed to be the base of all
recycling planktonic ecosystems.
Concentrations of gases other than CO2 produced by the plankton, some
of which are potent greenhouse gases such as nitrous oxide and methane
and others, such as halogenated hydrocarbons which contribute to
stratospheric ozone depletion, either did not change or increased
negligibly in the bloom. By the end of the experiment, chlorophyll
concentrations were in decline and the patch will by now have merged
with its surroundings leaving behind no trace other than swarms of
well-fed amphipods that will probably disperse to feed in less
productive waters.
Outlook
These preliminary findings will be supported by additional measurements
to be accomplished in the home labs on the many frozen and preserved
samples collected during the cruise. The data will be refined and
interrelationships between the organisms quantified in the coming
months. These will be intensely discussed and prepared for joint
publication in scientific journals at workshops to be held in Goa by
the end of the year.
Participants of LOHAFEX are extremely satisfied with the results. “It
was a strenuous cruise, full of anxiety and hope as we chased our patch
around the collapsing eddy. On one occasion we expected it to be sucked
out and dispersed by the strong currents surrounding it, but the patch
was stayed as if by a miraculous hand at the exit and hovered there for
two weeks until we had to leave”, said Victor Smetacek. Strong winds
were almost the rule, and the vessel had to leave the area to avoid big
storms twice, although shorter storms with wind speeds of over 120 km
per hour were weathered on site. Nevertheless, despite the hard work
under difficult circumstances, LOHAFEX has been an exciting experience
laced with the spirit of adventure and haunted by uncertainty quite
unlike other scientific cruises. “Despite coming from seven different
countries and having diverse scientific backgrounds, scientists on
LOHAFEX worked for a common cause and lived like a big family. The
experiment thus provides an excellent example of international
collaboration in interdisciplinary ocean sciences”, said Wajih Naqvi.”
“The officers and crew of the Polarstern did a fantastic job in
providing technical support under testing conditions without which the
expedition could not have achieved its objectives”, he added. Spicy
Indian curries were prepared at each meal by a Goan cook specially
engaged for this cruise.