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.

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.