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Ministry of Agriculture, Forestry and Fisheries
Public Relations Office, Ministry of Agriculture, Forestry
and Fisheries
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E-mail:koho_kaigai@nm.maff.go.jp
Editor.Tsuyoshi Arai
The iron dissolved in seawater has attracted the attention of ocean scientists in the current decade because this substance has recently been recognized as the "nutrient" that ultimately limits the increase of diatoms in high-nutrient low-chlorophyll (HNLC) oceans. To understand the detailed mechanism of this "iron limitation" process and examine the response of marine ecosystems when the iron limitation disappears as a result of iron addition, the subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS), a series of meso-scale in situ iron enrichment experiments, has been under way since fiscal 2001. The science plan of SEEDS is based on recommendations made by the Iron-Fertilization Experiment Panel (PICES-IFEP). Recently, the study has been recognized as a part of Japan's Surface Ocean-Lower Atmosphere Study (SOLAS) research initiative. SEEDS is a collaborative effort - the international component also involves the Fisheries Research Agency, the Institute of Ocean Science and Canada's SOLAS group, while the domestic component also involves the University of Tokyo, Central Research Institute of the Electric Power Industry, National Institute for Environmental Studies, Hokkaido University, National Institute of Radiological Science, Nagoya University, Kyoto University and Kochi University.
The first SEEDS field study was conducted in the western subarctic gyer of the North Pacific (48.5oN, 165oE) from July 18 through August 1, 2001. The field study consisted of a single addition of 350 kg of iron as FeSO4 with an inert tracer gas, sulphur hexafluoride (SF6), over an 8 x 10-km patch with a mixed layer depth of 10-15m. To understand the size of the amount of iron added in this operation, imagine adding a pinch of iron to a typical 25m swimming pool. The iron was added on July 18 and Day 1 was defined as the 24-hour period beginning July 19. Concentration of the dissolved iron rose to 1.88 nM immediately after the iron was injected and subsequently decreased rapidly. After this first phase of rapid decrease, the loss rate gradually slowed and remained at approximatrly 0.15 nM even after phytoplankton bloom development. Added iron stayed in the surface mixing layer throughout the period of observation, and biological and biochemical responses were observed exclusively in this layer. The first response of phytoplankton to iron input was observed at Day 2, in the form of increases in the photochemical quantum efficiencies of algal photosystem Ⅱ (Fv/Fm) of a fast repetition rate fluorometer (FRRF) and the growth rates of picoeukayotes as measured by dilution culture with flow cytometry measurements. Increase of phytoplankton biomass became significant from Day 6, rising exponentially to about 20 mg m-3 in chlorophyll-a concentrations until Day 10. After that, a relatively constant biomass of phytoplankton was observed from this point until observation concluded (Day 13). In addition, the iron supply led to floristic shifts that resulted in the dominance of chain-forming large centric diatoms. These two features - an extraordinary expansion of phytoplankton standing stocks and the dominance of large centric diatoms - had not been observed in the iron fertilization experiments held in the equatorial Pacific and southern oceans, suggesting that the western subarctic North Pacific is the ocean most sensitive to such an artificial iron injection.
The significant increase in phytoplankton standing stock accompanied large drawdowns of macronutrients, pC02 and dissolved inorganic carbon (DIC). Nitrate was abundant at 18 μM before the iron fertilization, as were phosphate and silicate. The absolute nitrate uptake rate at a depth of 5 m was 0.2 μmol l-1 d-1 on Day 0. This rate increased sharply, by approximately 20 times, after Day 7. A change in pCO2 inside the iron patch was observed after Day 5. The maximum differences in pCO2 and nitrate concentration between the surface water inside the patch and that outside the patch were 190 ppm and 11.7 μM, respectively, and were observed on Day 12. These results suggest that an iron injection in this particular oceanic area results in a significant depletion of oceanic carbon parameters, i.e., pCO2 and DIC, thereby generating a temporary oceanic CO2 sink.. The long-term fate of this CO2 sink is, however, very unstable, that is, if all the organic substances that increased in the iron patch do not settle down to the ocean subsurface, they will be decomposed in the mixed layer by biological activity and return to the atmosphere as CO2. In the 2001 SEEDS experiment, unfortunately, we were unable to determine what fraction of expanded organic substances in the iron patch ultimately settled down into the oceanic abyss. Wind-driven deviation of inside sediment-trap from the patch occurred occasionally, making it difficult to estimate export flux accurately. High export flux of carbon in the patch was observed between Day 10 and Day 12, but the export flux measured with drifting traps in the patch were not significantly different from that outside the patch. The export flux between Day 2 and Day 13 was 12.6% of the integrated primary production in the patch. Moreover, the increase of POC content in the surface mixed layer was 78% of the decrease of the DIC and influx of CO2 from the sea surface. These results suggest that a major part of the fixed carbon remained in the surface mixed layer as particulate matter at the end of our observation period. We had no data about whether this remaining organic matter ultimately settled down or was decomposed in the mixed layer following the conclusion of our observation.
The second SEEDS iron fertilization experiment was held in the eastern subarctic North Pacific in summer 2002. The main objectives of this experiment were to understand east-west differences in ecosystem response to iron fertilization and to obtain information on the most important problem remaining from the first experiment, that is, the fate of the expanded organic matter remaining in the fertilized patch. The second experiment was a cooperative effort involving three ships from Canada and Japan. The R/V Tulley, from Canada, injected FeSO4 twice over an 8x8-km patch, on July 8-9 and then again on July 14. This patch was initially monitored by two Canadian ships until July 28. Japan monitored from July 24 through August 5. When we arrived, on Day 16 - i.e., 16 days after the first injection - the patch was approximately 20 x 30 km wide and a slight increase in chlorophyll-a had been observed. Many properties were sampled during the experiment and are currently under measurement, although we can say at this point that we detected prominent POC flux in the in-patch sediment trap after Day 21, making this the first time scientists have observed remaining organic matter setting down to the oceanic subsurface. The SEEDS study is still underway and further experiments will be conducted to make quantitative assessment of oceanic iron-limitation process and evaluate the performance of iron fertilization in oceanic CO2 sequestration.
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